Air microbiology

• Air microbiology is the study of suspended microorganisms in the air. Also known as aero microbiology.
• The study of microbes and their airborne spores that are invisible to the naked eye.
• Aero-spoliation, airborne transmission, and biological materials research.
• In addition, respiratory illnesses are studied.
• Important to the subject of aero-microbiology is the formation of aerosols.
• Bioaerosols are aerosolized microorganisms.
• Microorganisms often found in the atmosphere between 300 and 10,000 feet above the surface.
• There are much fewer microorganisms in the atmosphere than in seas and soil.
• These bacteria have the potential to move large distances with the aid of wind and precipitation, hence increasing the prevalence of infectious diseases they cause.
• Important microorganism-affecting gases include hydrogen sulphide, sulphur dioxide, carbon monoxide, chlorine, hydrogen fluoride, ozone, and others.
• Bacteria, fungi, actinomycetes, algae, pteridophyte spores, pollen grains, micro insects, and viruses are microbiological forms.
• In the 1930s, aero-microbiology was employed to describe airborne spores (e.g. fungi and other microorganisms ).
• In 1951, the phrase was expanded to cover insect population dispersal, fungal spores, bacteria, and viruses.
• In 1964, the term covered the study of biologically significant airborne particles.

Indoor air microbiology 

• It deals with microorganisms present in air in an indoor  environment. 
• They are the microorganisms which are responsible for biodeterioration of storage materials, equipment, library materials and archives. 
• Microbes can survive for extended period in indoors as they have relatively less exposure to radiations. 
• It involves home and work place environments in which air borne microbes create major public health  concerns. 
• Microbes found in different sites:  Aeromicrospora of pharmacy, Aeromicroflora of hospitals, Houses Aeromicroflora of storage materials.
• Indoor air contains more disease causing agents that has higher chances of infections than outdoor air  especially in large gatherings like schools and theaters. The commonest genera of fungi in indoor air are Penicillium, Aspergillus. 
• The commonest genera of bacteria found in indoor air are Staphylococci, Bacillus and Clostridium. 

1. Aeromicrospora of Pharmacy

• The preparation of Ayurvedic medications occurs in a pharmacy. Sharma and Navneet (1996) documented the presence of aerofungi in the fermentation unit of the Gurukul Kangri Pharmacy in Hardwar. They isolated fungus between February and March of 1994.
• Temperature and relative humidity were environmental elements that influenced their incidence. In the diurnal cycle, fungi exhibited a preference for the evening.
• The main species were Cladosporium cladosporioides, Alternaria sp., Penicillium cyclopium, Epicoccum nigrum, etc.

2. Aeromicroflora of Hospitals

• A hospital is a significant indoor setting for the transmission of airborne diseases. It works as a reservoir for diseases that are then transmitted to other individuals, such as patients, hospital staff, visitors, etc. In turn, it is transmitted to patients.
• Even coughing and sneezing contribute to the spread of infectious germs and viruses.
• The infections transferred by the hospital are Mycobacterium tuberculosis. Staphylococcus aureus, influ­enza virus, Aspergillus flavus, A. fumigatus, Can­dida albicans, etc.
• Aspergillus species are not natural human flora, although they frequently cause lung infections in hospital settings.
• C. albicans-caused candidiasis is the most prevalent type of hospital infection. C. albicans typically contaminates hospital wards through direct finger contact.
• In the operating room, squames from the skin of patients spread harmful bacteria. There are numerous dangerous microorganisms in squames that are communicated to patients.
• Therefore, efforts must be made to prevent primary and secondary infections by preventing the transmission of microorganisms in hospitals.

3. Other Houses

• Indoor air always contains spores. Even in clean environments, approximately 25 spores/m3 have been detected. In homes equipped with air conditioners and coolers (humidifiers).
• Humidity and low temperature (about 25 °C) produce a favourable environment for the proliferation and dissemination of microorganisms, which increases the likelihood of airborne microorganisms.
• In general, fungus (such as Aspergillus, Geotrichium, Penicillium, Phialophora, etc.), yeasts, bacteria, etc. colonise cold mist humidifiers. In addition, insects serve as vectors for human infections and deposit their eggs in stagnant water.

4. Aeromicroflora of Storage Materials

A number of important goods are held in houses, and staff members are assigned to protect them. However, aeromicroflora degrade these substances. Several instances are provided below:

(i) Library

• There are thousands to millions of valuable, common and rare, printed and handwritten volumes in a library. The primary component of paper is cellulose.
• Therefore, microbes that breakdown cellulose colonise and disintegrate paper.
• Alternaria, Aspergillus, Curvularia, Bispora, Chaetomium, Cladosporium, Fusarium, Helminthosporium, Periconia, Nigrospora, Rhizopus, Stemphilum, Trichoderma, etc. are common cellulose-degrading fungus.
• These fungi decompose cellulose at a quicker pace when exposed to high humidity and low temperature. In addition, despite the creation of a large quantity of enzymes, the degradation of rexin and leather has been seen to be minimal.

(ii) Wall Paintings

• Wall paintings are the cultural heritage of a country’s region. The wall murals at Ajanta and Ellora caves, which are of international renown, have revealed signs of biodeterioration.
• Tilak and Kulkarni (1972) investigated cave aerospora for the first time.
• Tilak (1972) identified fungal spores from Ajanta and Ellora paintings and caves. Almost certainly, aerofungi have degraded the wall paintings.
• Bat faeces likely serves as a substrate for the growth of aerofungi, which is supported by bat faeces. Moreover, meteorological conditions contribute to the growth and biodeterioration of wall murals.

Outdoor air microbiology 

• Outside airoutdoor aero microbiology refers to the study of air in the atmosphere that is found outside of buildings.
• In an outdoor or extramural setting, the movement of bio aerosols is governed by the vastness of space and the existence of air turbulence.
• Depending on human population density, the number and type of microorganisms may vary from place to location.
• Among the microorganisms are algae, protozoa, yeasts, and moulds. Mold spore are prominent, e.g clasdosporium.
• Species of bacteria are either spore-forming or non-spore-forming.
• Fungi dominate the microflora of outdoor air.
• The two most prevalent fungal genera are cladosporiul and sporobolomyces.
• Aspergillus, Alternaria, Phytophthora, and Erysiphe are other aerosals produced by fungi.
• Also contains besidispores, ascopres of yeast, mycelium pieces, and mould canidia.
• Bacillus and clostridium, sarcina, mirococcus, coryneb acterium, and Achromobacter are the bacterial genera.

Sources of air microbes 

Various sources are listed below;

• Soil: Wind-agitated soil bacteria are released into the air and remain suspended there for an extended amount of time. Human acts such as digging or tilling the soil may also discharge germs from the soil into the air.
• Water: Microorganisms that live in water release water droplets or aerosols. The splashing of water caused by wind or tide can also produce droplets or aerosols.
• Wind and tides: Air currents may transport germs from plant or animal surfaces into the atmosphere. For instance, Puccinia graminis spores
• Human beings: Human beings are the primary source of airborne germs. Activities such as coughing, sneezing, speaking, and laughing release pathogenic flora from the upper respiratory tract and mouth into the air.

Forms of discharge of air microbes 

Microorganisms are expelled in three distinct forms, which are categorised according to their relative size and moisture content.

• Droplets, 
• Droplet nuclei and 
• Infectious dust. 

Droplets 

• Typically, droplets are produced by sneezing, coughing, or speaking. Both are composed of saliva and mucus.
• It has been estimated that a single sneeze may contain between 10,000 and 100,000 germs.
• The length of time droplets can remain suspended is determined by their size.
• In calm air, the majority of droplets are rather big and tend to settle swiftly.
• The pathogenic microorganism-containing droplets may be a source of infectious illness.

Droplet nuclei

• In a warm, dry atmosphere, little droplets tend to evaporate quickly and become droplet nuclei.
• The leftovers of solid material remaining after a droplet has dried out are known as droplet nuclei.
• These are 1-4um in size and lightweight.
• Can remain airborne for hours or days and traverse great distances.
• If the germs remain viable when dried, it may serve as a continued source of illness.
• Viability is governed by a number of complicated parameters, including atmospheric conditions such as humidity, sunshine, and temperature, the size of the particles carrying the organisms, and the sensitivity or resistance of the specific microbial species to the new physical environment.

Infectious Dust 

• Large aerosol droplets quickly settle from the air onto various surfaces and become dry.
• A patient’s nasal and throat discharges can potentially contaminate surfaces and become dry.
• In the patient’s room, disturbing this dried material by changing the bed, touching a handkerchief with dried secretions, or sweeping the floors might generate dust particles that add bacteria to the circulating air.
• Depending on their size, two types of droplets spread airborne illnesses.
• Droplet infection proper applies to droplets with a diameter more than 100 um.
• The other form is referred to as an airborne infection and pertains to droplet remains that have dried.

Factors Affecting Microbes

1. Relative Humidity

• It has been demonstrated that the relative humidity or relative water content of the air is crucial for the survival of airborne microbes.
• In general, it has been found that the majority of Gram-negative bacteria associated with aerosols tend to survive for longer periods at low to moderate relative humidity levels, with accelerated decomposition at relative humidity levels more than 80%.
• High relative humidity has the opposite effect on Gram-positive bacteria, which tend to remain alive for longer periods of time. Thus, the surface biochemistry of a bacterium is connected to its capacity to survive in a bioaerosol. A structural alteration in the lipid bilayers of the cell membrane is one reason for the loss of viability associated with extremely low relative humidity.
• As a cell loses water, its bilayer membrane transitions from its characteristic crystalline structure to a gel phase. This structural phase change impacts the protein configurations on the cell surface and ultimately culminates in cell death.
• Gram-negative bacteria generally react negatively to desiccation, whereas Gram-positive bacteria are more tolerant of desiccation stress.
• An rise in relative humidity was similarly detrimental to the influenza virus. Recent research indicates that viruses with enclosed nucleocapsids (such as the influenza virus) have a longer airborne life when the relative humidity is below 50%, whereas viruses with naked nucleocapsids (such as enteric viruses) are more stable when the relative humidity is above 50%.
• Notably, viruses with enclosed nucleocapsids have a higher survival rate in aerosols than those without. Some viruses are also persistent in the AMB route over wide ranges of relative humidity, making them extremely effective airborne pathogens.

2. Temperature

• Inactivating microorganisms is mostly dependent on temperature. High temperatures promote inactivation, which is mostly connected with desiccation and protein denaturation, whereas low temperatures promote prolonged survival periods.
• However, as temperatures near freezing, the development of ice crystals on the surfaces of certain organisms causes them to lose viability.
• There is a strong relationship between the impacts of temperature and numerous other environmental parameters, including relative humidity.

3. Radiation

• Shorter UV wavelengths and ionising radiation, such as X-rays, are the primary sources of radiation damage to microorganisms, such as bacteria, viruses, fungus, and protozoa.
• The primary target of UV irradiation damage is the DNA nucleotides. Multiple types of DNA damage are caused by ionising radiation or X-rays, including single strand breaks, double strand breaks, and modifications in the structure of nucleic acid bases.
• UV radiation causes damage mostly by intrastrand dimerization, which distorts the DNA helix by drawing thymidines closer together. In turn, this inhibits biological processes such as genome replication, transcription, and translation.
• Several ways have been demonstrated to prevent radiation damage to organisms. All of these factors, such as the interaction of microorganisms with bigger airborne particles, the presence of pigments or carotenoids, high relative humidity, and cloud cover, tend to absorb or protect bioaerosols from radiation.
• Numerous types of animals possess systems for repairing UV-induced DNA damage. Dienococcus radiodurans is an organism that possesses a radiation resistance mechanism. D. radiodurans is a soil bacterium that is regarded as the most radiation-resistant organism yet discovered.
• Its ability to enzymatically repair chromosomal DNA damage is an essential component of its radiation resistance.
• The repair mechanism utilised by these bacteria is so efficient that a significant portion of the cell’s metabolic energy is devoted to this activity alone.

4. Oxygen, OAF and Ions

• Oxygen, open air factors (OAFs), and ions are difficult to analyse environmental components of the atmosphere.
• In general, it has been demonstrated that these three components inactivate numerous types of airborne microorganisms.
• Oxygen toxicity is not associated with the dimolecular form of oxygen (O2), but rather plays a significant role in the inactivation of microorganisms when O2 is transformed to more reactive forms. Included in this category are superoxide radicals, hydrogen peroxide radicals, and hydroxide radicals. These radicals are produced naturally in the environment by lightning, ultraviolet radiation, pollution, etc. These reactive forms of oxygen cause DNA damage by generating mutations that can accumulate over time.
• Controlling the detrimental effects of reactive forms of oxygen are the repair mechanisms outlined in the preceding section.
• Similarly, the open air factor (OAF) refers to an environmental influence that cannot be recreated in laboratory settings.
• It is defined as a mixture of components formed when ozone and hydrocarbons (usually related to ethylene) react. It is closely related to oxygen toxicity.
• For instance, high concentrations of hydrocarbons and ozone can raise the inactivation rates of numerous species, most likely due to their destructive effects on enzymes and nucleic acids. Therefore, OAFs have been closely associated with airborne microbial survival.
• Numerous natural processes result in the creation of other ions, such as those containing chlorine, nitrogen, and sulphur.
• These include the activity of lightning, the shearing of water, and the action of other forms of radiation that shift electrons from gas molecules, so producing a vast array of anions and cations unrelated to oxygen radicals.
• These ions exhibit a broad spectrum of biological function. Positive ions have only physical effects on microorganisms, such as the deactivation of cell surface proteins, whereas negative ions have both physical and biological consequences, such as DNA damage on the inside of the cell.

Significance of air microbes 

• When compared to microorganisms in other habitats, the number of air micro flora is quite tiny; yet, they serve a crucial role.
• Because the air is in contact with nearly all living and nonliving things.
• Since 1799, when Lazaro Spallanzani attempted to deny spontaneous generation, the relevance of air flora has been the subject of study.
• In 1837, Theodore Schwann conducted an experiment in favour of Spallanzani’s theory.
• Where he injected fresh, hot air into a sterile pork stew and demonstrated the absence of microbial development.
• These two scientists laid the foundation for forced aeration fermentations in the present era.
• Pasteur demonstrated in 1861 that bacteria can exist as airborne pollutants.

Significance in Human Health 

• The transmission of infectious pathogens occurs through the air.
• A man inhales approximately 5m^3 of air every day.
• Less than 1% of airborne bacteria are pathogens; the majority of airborne germs are innocuous saprophytes and commensals.
• Even though the amount of contamination is relatively low, the likelihood of a person becoming infected is greatest if he is exposed to a high concentration of airborne germs.

Staphylococcus aureus 

• The quantity of S. aureus in the air might range from 0 to 1/mo to 50/m3.
• However, outdoor air quality is a vital component of man’s surroundings.
• In the case of indoor air, the risk of infectious illness transmission is heightened during gatherings.

Significance in Hospitals

• There are instances in which extra infectious diseases can be acquired during hospitalisation. Hospitals are the battlegrounds in the campaign against infectious diseases.
• The hospital’s air may serve as a reservoir for harmful bacteria transferred by patients.
• Nosocomial infections are infections acquired during hospitalisation, and the pathogens involved are known as nosocomial pathogens.
• Haemophilus influenzae, Streptococcus pneumonia, Streptococcus pneumonia, Staphylococcus aureus, Pseudomonas aeruginosa, members of the Enterobacteriaceae, and respiratory viruses are frequent microorganisms associated with hospital infections.

Microorganisms in Industries 

Food manufacture

• Various fermentation products involve microorganisms that have been transported by the air and have settled on or in the substance.
• Often, microbial activity is responsible for the production of alcoholic drinks, vinegar, sauerkraut, ensilage, and dairy products, among others.

Spoilage of foods and fermentation products 

• In industrial operations where specific organisms have to be produced, it is a significant challenge to provide sterile air devoid of contaminating organisms.

Bioaerosol Control

Controlling airborne microorganisms can be accomplished in a number of ways. The airborne dissemination of diseases can be regulated at the stages of launching, transit, and deposition. Ventilation, filtering, UV treatment, biocidal chemicals, and physical isolation are some of the methods used to regulate bioaerosols. These are covered in the sections that follow.

1. Ventilation

• Ventilation is the most prevalent approach for preventing the collection of airborne particles. This approach includes producing a flow of air through contaminated locations.
• This can be accomplished by simply opening a window and letting outside air to circulate within, or by utilising air conditioning and heating systems that pump outside air into a room.
• Ventilation is one of the least effective strategies for controlling airborne infections, although it remains crucial.
• Ventilation relies on the mixing of intramural and extramural air to minimise the particle concentration.
• In certain instances, however, the injection of extramural air can increase airborne particulates. For instance, one study revealed that hospitals in Delhi, India that relied solely on ventilation had greater airborne fungal loads inside the hospital than outside.
• This suggests that ventilation alone may not be adequate to reduce bioaerosols sufficiently.
• Thus, for the majority of public buildings, particularly hospitals, it is necessary to deploy alternative bioaerosol management methods.

2. Filtration

• Unidirectional airflow filtration is a straightforward and effective method for controlling airborne contaminants. Reportedly, certain filters, such as high-efficiency particulate air (HEPA) filters, remove practically all pathogenic particles.
• These filters are frequently utilised in biological safety hoods. However, due to their high cost, they are rarely employed in the construction of filtration systems. Instead, various filtration methods that rely on baghouse filtration are utilised (a baghouse operates similarly to a vacuum cleaner bag).
• Higher percentages indicate greater filtration efficiency.
• The standard grade for filters used in buildings is between 30 and 50 percent. A 97% dust-spot rating is required to efficiently remove virus particles from the air, according to studies.
• Other elements that influence filtration efficiency include the type of circulation system and how well it mobilises air throughout the building, the type of baghouse system utilised, the filter material selected (nylon wound, spun fibreglass, etc.), and the filter’s nominal porosity (1 m–5 m).
• All of these parameters influence the effectiveness of air filtration and particle removal, including bioaerosols.
• Despite the great level of efficiency that can be reached with filtration, many systems cannot prevent the spread of airborne germs, particularly viruses, and further treatments may be necessary to ensure that the air is safe to breathe.

3. Biocidal Control

• Biocidal control is an additional treatment that can be used to eliminate all airborne germs, guaranteeing that they are no longer viable and infectious. There are numerous eradication techniques available, including superheating, superdehydration, ozonation, and UV irradiation.
• UVGI, or ultraviolet germicidal radiation, is the most widely used of these treatments. UVGI has been demonstrated to be effective against numerous types of infections, while some microorganisms exhibit varying degrees of resistance.
• In a hospital ward for tuberculosis (TB), UV irradiation was tested for its ability to prevent the spread of disease. Through a split ventilation duct, contaminated air was evacuated from the TB ward and routed into two animal holding pens containing guinea pigs.
• One pen received air treated with ultraviolet radiation, while the other received untreated air.
• The guinea pigs in the untreated air compartment contracted tuberculosis, however none of the animals in the UV-treated air compartment contracted the disease.
• According to the American Hospital Association (1974), UV radiation may effectively eliminate practically all pathogenic pathogens, although the efficacy is largely dependent on UV intensity and exposure period.
• Thus, significant elements that affect survival (temperature, relative humidity, UV radiation, and ozone) in the extramural environment can be employed to regulate the transmission of an infectious disease in the intramural environment.

4. Isolation

• Isolation is the process of enclosing an environment using positive or negative air pressure gradients and airtight seals.
• When accumulated airflow enters an isolated zone, negative pressure exists.
• Negative-pressure environments include the isolation chambers of hospital tuberculosis wards, which are meant to protect those outside the TB wards from the infectious agent created within these regions.
• This type of technology is intended to protect other hospital patients from the Mycobacterium tuberculosis germs present in the isolation area.
• After passing through a HEPA filter and biocidal control chamber, air from these rooms is discharged into the atmosphere.
• Positive-pressure isolation chambers work by forcing air out of the room, thereby protecting the room’s inhabitants from outside contaminants.
• The TB ward is presumably a negative-pressure isolation chamber, whilst the remainder of the hospital, or at least the adjacent anterooms, are under positive-pressure isolation.
• Other examples are hospitals’ intensive care units for immunocompromised patients, such as organ transplant, human immunodeficiency virus (HIV), and chemotherapy patients.
• These locations are shielded against exposure to infections and opportunistic pathogens of any kind.
• The air circulating in these intensive care wards is filtered with HEPA filters, producing nearly pathogen-free air.

Airborne Diseases

The diseases that are caused by airborne microorganisms or that are transmitted through breathing are known as airborne diseases. An infected person can transfer airborne sickness through coughing, sneezing, or speaking. Certain viruses and bacteria are able to fly and land on other people or surfaces.

Symptoms 

Symptoms of airborne infections typically include inflammation of the nose, throat, and lungs, coughing, sneezing, a runny nose, a sore throat, and swollen glands. Symptoms include headache, appetite loss, fever, and weariness.

Types of airborne diseases

Many infections, including Influenza, Tuberculosis (TB), Measles, and Mumps, are transmitted through the air.

1. Influenza 

• The majority of people have experienced influenza. It spreads rapidly since it is contagious approximately one day before the onset of symptoms.
• It remains contagious for five to seven additional days. If you have a compromised immune system for whatever reason, you can spread the disease for longer.
• There are several flu strains, and they are constantly evolving. This makes it tough for your immune system to develop.

2. Tuberculosis (TB)

• TB is an airborne infection. However, this bacterial illness spreads slowly. Generally, prolonged close contact with an infected person is required. One can be infected without becoming unwell or spreading the disease to others.
• About 25 billion people are infected with tuberculosis worldwide. Most are healthy. Approximately 9.6 million persons have active TB worldwide.
• Those with a compromised immune system are most susceptible to contracting the disease.

3. Measles 

• The measles is highly contagious, especially in crowded situations. Up to two hours, the virus might remain active in the air or on surfaces.
• The incubation period for measles is four days before and four days after the rash emerges. Most individuals experience measles just once.
• Worldwide, measles is the top cause of death among children. It is predicted that between 2000 and 2015, the measles vaccine avoided 20,3 million deaths.
• Typically begins on the face and neck, then spreads over the next few days.
• The following are severe complications of measles:
  • ear infections 
  • diarrhea 
  • dehydration
  • severe respiratory infection 
  • blindness 
  • swelling of the brain, or encephalitis 

What is an aerosol?

Aerosols are technically a suspension of small solid particles or liquid droplets in a gas, such as smoke, air pollution, smog, and CS gas. The name aerosol is derived from the fact that airborne matter is a suspension, i.e. a mixture in which solid or liquid or combination solid-liquid particles are suspended in a fluid.

In order to distinguish suspensions from solutions, the term sol initially referred to dispersions of submicroscopic particles in a liquid. Aerosol is a term that was derived from research of air dispersion and currently refers to liquid droplets, solid particles, and mixtures of these.

What is phylloplane?

• Phylloplane is the habitat of microorganisms right on the leaf’s surface. Numerous bacterial species, fungal hyphae and spores, and unicellular yeast cells reside in the phylloplane.
• Additionally, the presence of viral organisms on the leaf surface cannot be ruled out. Phylloplane fungi can manufacture the cellular enzyme. In addition, they are known to produce pectinase, cutinase, and protease enzymes.
• On the surfaces of the leaves are coloured colonies of yeasts and bacteria. It is hypothesised that the pigments of the microbial population provide protection against UV radiation and direct sunlight on the leaf surface.
• Pine tree phylloplane communities are better able to utilise sugars and alcohol as carbon sources than bacterial populations in the litter (decomposing leaves and twigs) layer.
• In the month of May, Xanthomonads and pink chromogens characterise rye phylloplane bacterial communities. July is characterised by Xanthomonads and Pseudomonads, September by Xanthomonads, and September by listeria and staphylococci. As phylloplane invaders, populations of Alternaria, Epicoccus, and Stremphylium have been seen 9.

Characteristics of Phylloplane Microflora

Given the phylloplane microflora is exposed to the environment, it is continuously impacted by meteorological conditions. Therefore, microflora develop specific identifying characteristics so that they can adapt to their environment:

1. Morphological Characteristics

• The occupants of phylloplanes exhibit morphological features for survival. These include the pigmentation of their mycelia, spores, pycnidia, apothecia, and cleistothecia for defence against intense light and dehydration.
• The dark pigments function as a light blocker.
• These pigments are commonly known as melanin.

2. Physiological Characteristics

Phylloplane fungus have a variety of physiological properties, which are outlined below.

(a) Nutrition

• Phylloplane microfungi are capable of decomposing cellulose by the production of cellulases. In addition, the pectinases, cutinases, and proteases of numerous fungi, such as Alternaria alternata, Aureobasidium pullulans, Botrytis cineria, and Cladosporium herbarum, have been estimated.

(b) Radiations

• Light of typical intensity is harmless to phylloplane fungus.
• UV component of the spectrum has an important impact. Compared to fungi with hyaline mycelia, those containing melanin pigment are more resistant to UV radiation.
• UV exposure for five minutes kills hyaline spores of Aureobasidium and Sporobolomyces, whereas dark spores of Alternaria and Epicoccum survive even after 35 minutes of exposure. UV radiation with a high intensity is deadly to microorganisms.

(c) Relative humidity

• During rain and dew production, the leaf surface has a high relative humidity. But when there is wind, the microclimate is diminished.
• Therefore, low relative humidity is advantageous for phylloplane fungus.
• At 100 percent relative humidity, the germ tube and mycelium of most fungi grow more rapidly. Even some could not develop at relative humidity levels below 93%.

(d) Temperature

• Phylloplane fungi are typically mesophilic, growing between 20 and 25 degrees Celsius. Some fungi (such as Alternaria alternata, Aureobasidium pullulans, Cladospo­rium herbarum, and Botrytis cinerea) may even grow at temperatures below 0°C.

Aeromicrobiological Pathway/Dispersal of microbes

The aeromicrobiological route depicts (1) the release of bioaerosols into the air, (2) the subsequent movement of these particles through diffusion and dispersion, and (3) their deposition. This pathway is illustrated by liquid aerosols containing the influenza virus that are released into the air by coughing, sneezing, or even speaking. A cough or sneeze disperses these virus-associated aerosols, which are then carried through the air, inhaled, and deposited in the lungs of a nearby individual, where they can establish a new infection. The deposition of live bacteria and the resulting infection have traditionally received the greatest attention, although comprehending the aerobiological route requires an appreciation of all three processes (launch, transport, and deposition).

1. Launching

• Launching is the process by which particles become suspended in the Earth’s atmosphere. Because bioaerosols must be released into the atmosphere in order to be transported, it is essential to comprehend this procedure.
• Bioaerosols are primarily launched from terrestrial and aquatic sources, with terrestrial sources being associated with higher airborne concentrations or atmospheric loading than aquatic sources.
• Diverse mechanisms, including but not limited to: air turbulence caused by the movement of humans, animals, and machines; the generation, storage, treatment, and disposal of waste material; natural mechanical processes such as the action of water and wind on contaminated solid or liquid surfaces; and the release of fungal spores due to natural fungal life cycles.

Types of Launching Source

Sources of airborne particles can be either point, linear, or area-based.

a. A point Source

• A point source is an isolated and well-defined launching site, such as a mound of biosolids before they are spread across a field.
• Dispersion from point sources has a basic conical shape. Point sources can be further classified according to the launching phenomenon: (1) instantaneous point sources, such as a single occurrence like a sneeze; or (2) continuous point sources, from which launching happens over extended time periods, such as a biosolid pile.

b. Linear sources

• In contrast to point sources, linear and area sources involve expansive, less precisely defined regions. On the same size scale, linear and area sources have greater particle wave dispersion compared to point sources, which exhibit conical wave dispersion.
• In addition to linear and area sources, there are also instantaneous and continuous launching sites of origin. An immediate linear source could be a passing aircraft discharging a biological warfare agent, for instance.
• An example of a continuous area source could be the emission of bioaerosols from a broad field that has been fertilised with biosolids or animal manures.

2. Transport

• Transport or dispersion is the transmission of kinetic energy from the movement of air to airborne particles, resulting in their movement from one location to another.
• This “energy of motion” gained by airborne particles is significant and can result in the long-distance spread of airborne germs. Transport of bioaerosols can be described by distance and time.

Types of Transport based on Time and distance

a. Submicroscale transport

• Submicroscale movement involves brief durations, less than 10 minutes, and short distances, less than 100 m. This mode of transportation is prevalent within buildings and other enclosed areas.

b. Microscale transport

• Microscale travel, spanning 10 minutes to one hour and 100 metres to one kilometre, is the most prevalent sort of transport phenomenon.

c. Mesoscale transport

• Mesoscale transport refers to transportation that occurs on a daily basis and covers distances of up to 100 kilometres.

d. Macroscale

• Transport at the macroscale extends time and distance even further. Due of the limited ability of most microorganisms to survive in the atmosphere, the sub macroscale and microscale are the most commonly regarded scales.
• Some viruses, spores, and spore-forming bacteria have been proven to enter mesoscale and even macroscale transport, it should be highlighted.

As bioaerosols migrate through time and space, several forces, including diffusion, inactivation, and finally deposition, work upon them.

Diffusion is the scattering and/or dissipation of bioaerosols in response to a concentration gradient and gravity, and is facilitated by airflow and atmospheric turbulence in general. The method of Osbert Reynolds can be used to determine the amount of turbulence associated with airflow, and consequently the relative amount of diffusion that may occur in combination with particles such as bioaerosols.

3. Deposition

• The last step in the aeromicrobiology route is deposition. An airborne bioaerosol will eventually depart the turbulence of the suspending gas and will ultimately be deposited on a surface by one or a combination of connected mechanisms.
• The next sections examine these mechanisms: gravity settling, downward molecular diffusion, surface impaction, rain deposition, and electrostatic deposition.
• These processes are related in numerous ways, and even when viewed separately, they all combine to form a constant, if not steady, deposition of particles.

a. Gravitational Settling

• The primary mechanism involved with deposition is particles’ interaction with gravity. Gravity exerts a downward push on all particles denser than air, effectively limiting the spatial and temporal distribution of airborne particles.
• In the absence of air movement, steady-state gravitational deposition can be represented relatively simply by Stokes’ law, which takes into account gravitational attraction, particle density, particle diameter, and air viscosity.

b. Downward Molecular

• Diffusion As suggested by its name, downward molecular diffusion is a random process driven by natural air currents and eddies that stimulate and intensify the downward flow of airborne particulates.
• These random movements occurs even in relatively quiet air and have a tendency to occur downward due to gravitational influences. Consequently, measured rates of gravitational deposition are typically higher than those anticipated by the Stokes equation.
• The increased rate of deposition is a result of the additional impacts of molecular diffusion moving downhill.
• The force of the wind also affects molecular diffusion. Molecular diffusion-enhanced deposition rates tend to rise as wind velocity and turbulence increase.

c. Surface Impaction

• Surface impaction is the process by which particles, such as leaves, trees, buildings, and computers, come into contact with surfaces. Impaction is accompanied by a loss of kinetic energy.
• In nature, it is uncommon to see unrestricted wind currents on flat, smooth surfaces. Therefore, surface impaction is a crucial element affecting transit and deposition, particularly for bioaerosols.
• The relative possibility that an airborne object will collide with another object in its course is its impact potential. However, impaction does not inevitably lead to persistent deposition.
• After colliding with an object, a particle has the ability to rebound. When a particle bounces off a surface, it reenters the air current at a slower rate, which can have one of two outcomes: (1) it can allow subsequent downward molecular diffusion and gravitational settling, resulting in deposition on another nearby surface; or (2) it can allow the particle to escape the surface and reenter the air current. According to studies, impaction is influenced by the particle’s velocity and size, as well as the size and form of the surface it is approaching.

d. Rain and Electrostatic

• Deposition Additionally, precipitation and electrostatic charge can impact deposition. Rainfall deposition is the result of a condensation interaction between two particles (raindrop and bioaerosol), which combine to form a bioaerosol with a higher mass that settles more quickly.
• This can be mathematically represented using the Stokes equation. In the example described in Information Box 5.1, the calculated terminal velocity of a clostridial spore is 0.016 cm/sec.
• If the same spore (bioaerosol) condenses with another particle, such as a water droplet, its mass and, consequently, its terminal velocity increase.
• For example, if the clostridial spore condensed with a water droplet that quadrupled the bioaerosol density from 1.3 to 2.6 g/cm3, the terminal velocity would increase from 0.016 to 0.032 cm/s.
• The extent of the particle plume’s dispersion affects the overall effectiveness of rain deposition. Larger, more dispersed plumes are impacted more strongly than smaller, more concentrated plumes.
• Rainfall intensity also affects precipitation deposition. The heavier the precipitation, the greater the overall condensation reaction rates and numbers, and the bigger the subsequent increase in rain deposition.
• Bioaerosols are also condensed through electrostatic deposition, which is based on electrovalent particle attraction. The majority of particles have an associated charge. At neutral pH, the majority of microorganisms have an overall negative charge associated with their surfaces.
• These negatively charged airborne particles can form associations with positively charged airborne particles, leading to electrostatic condensation.
• There may be a coagulation effect between particles (similar to the condensation of clostridial spores with water droplets), which would increase bioaerosol mass and accelerate deposition.
• Electroattractive or electrorepulsive influences may also be present when an electromagnetically charged bioaerosol approaches an electromagnetically charged surface.

What is the Phyllosphere?

• The phyllosphere is the environment around the flat leaf. In other terms, it is the habitat surrounding a plant’s leaf.
• Phyllosphere, like phylloplane, contains different bacterial and fungal communities. Pseudomonas, and even fluorescent Pseudomonas, are the predominant populations in the phyllosphere region of some Pinus species’ green needles.
• Sporobolomyces roseus, Rhodotorula glutinis, R. mucilaginosa, Cryptococcus laurentu, Torulpsis ingeniosa, and Aureobasidium pullans populations are frequently observed in the phyllosphere.

What is antagonism?

Antagonism refers to the suppression, damage, or destruction of one type of microorganisms by another. It is an inter-population relationship in which one population bears a deleterious or negative effect over another population of microorganisms.

References

• Paul, Dipak & Biswas, Karabi & Sengupta, Chandan & Sinha, Sankar. (2015). Studies on Environmental Monitoring of Aeromicroflora in a Hospital at Kalyani, West Bengal, India. Frontiers in Environmental Microbiology. 1. 47-50. 10.11648/j.fem.20150103.13. 
• Pepper IL, Gerba CP. Aeromicrobiology. Environmental Microbiology. 2015:89–110. doi: 10.1016/B978-0-12-394626-3.00005-3. Epub 2014 Oct 10. PMCID: PMC7149531.
• https://www.slideshare.net/amjadkhanafridi4all/aeromicrobiology-185366127
• https://www.slideshare.net/zakiakhatoon2/aeromicrobiology
• http://www.jetir.org/papers/JETIR1503069.pdf
• https://redox-college.s3.ap-south-1.amazonaws.com/kmc/2020/May/07/BxDJaa4Hh8JZCegt87tn.pdf
• https://microbewiki.kenyon.edu/index.php/Aeromicrobiology