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This article writter by MN Editors on July 25, 2022

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· 12 min read >

Food Microbiology

Foodborne Microorganism – Bacteria

To identify bacteria in food, microscopic examination is the first step. It can determine the structure, shape, size, aggregation and staining reactions....

Foodborne Microorganism - Bacteria
Foodborne Microorganism - Bacteria

Morphological Characteristics Important in Food Bacteriology

To identify bacteria in food, microscopic examination is the first step. It can determine the structure, shape, size, aggregation and staining reactions. These characteristics could be particularly important.

Encapsulation

  • Sliminess and ropiness may be caused by capsules or slime.
  • Capsules can also be used to improve the resistance to heat and chemicals.
  • They may be a source of reserved nutrients.
  • The majority of capsules contain polysaccharides containing dextrin or dextran.

Formation of Endospores

  • Bacteria from the genera Bacillus and Clostridium, Desulfotomaculum (rods), Sporolactobacillus (rods) and Sporosarcina(cocci), all have the ability to form endospores.
  • The spore-forming species in the genera Bacillus and Clostridium are of primary importance to food microbiologists.
  • Endospores, which are formed at intracellular sites, are highly refractile and resistant to heat, UV light and desiccation.
  • The free endospore is released by lysis of the vegetative cells. This can be left dormant for many years with no discernable metabolism.
  • It is very complex to follow the complete cycle from the vegetative cells through sporulation to the free spore, possible extended sleep, and then germination to the spore.
  • Often, sporulation occurs in the later logarithmic phases of growth. This could be due to nutrient depletion and product accumulation.
  • The spore’s transition from vegetative to spore is marked by a large uptake of Ca2+ and the production of dipicolinic acid, which is absent in vegetative cells.
  • The formation of DPA or Ca 2+ is closely related to the acquisition of heat resistance through the forming of spore.
  • Germination is generally favored by conditions that favor vegetative cell growth. However, it can also occur under conditions that don’t permit such growth, such as low temperatures.
  • It can be triggered by a mixture of amino acids, Mg 2+ and Mn 2+ions, glucose, dipicolinic acid, plus Ca 2+ions, or heat activation which activates dormant enzymes.
  • The ideal temperature and time to heat shock depends on the type of spore. For example, spores that are thermophiles will need heat treatment more than those of mesophiles.
  • Germination can be inhibited with sorbic acid at an acid pH.
  • “Dormancy” refers to delayed germination and outgrowth under conditions that are favorable for it.
  • However, the spores failed to germinate due to unfavorable conditions, such as inhibitors or a lack of essential nutrients (e.g. amino acids).
  • Some spores might not germinate and others may not grow out. Others may be damaged by radiation or heat so they require a more complicated or specialized growth medium than their ancestors.
  • Reports have shown delayed spore germination ranging from a few weeks to several months. For example, dormancy for a few days up to 3 to 4 months with Bacillus megaterium spores and 15 to 72 months for Clostridium botulinum spores.

Formation of Cell Aggregates

  • Some bacteria are able to form long chains, while others can clump under certain conditions.
  • It is much more difficult to kill all bacteria within intertwined chains and large clumps than it is to kill individual cells.

Cultural Characteristics Important in Food Bacteriology

The amount of bacteria that grows in and on food can often make it unattractive or objectionable. Pigmented bacteria can cause discolorations to the surfaces and liquids of food. The growth of the bacteria may leave the surface of the liquids slimy or cloudy.

Physiological Characteristics Important in Food Bacteriology

  • The role of the bacteriologist involves the study of bacteria growth and activity in food and the associated chemical changes.
  • These include hydrolysis from complex carbohydrates to simpler ones, hydrolysis proteins to polypeptides and amino acids, ammonia or other amines, and hydrolysis fats to glycerol, fatty acids and the hydrolysis hydrolysis of proteins.
  • O-R reactions are used by bacteria to extract energy from food (carbohydrates and other carbon compounds, simple nucleo-carbon compounds, etc.). These reactions can produce organic acids, alcohols and aldehydes as well as ketones and gases.
  • Understanding the factors that promote or inhibit the growth of bacteria is crucial to understanding the principles of food preservation. 

Genera of Bacteria Important in Food Bacteriology

  • Genus Acetobacter – These bacteria oxidize alcohol to acetic acids. They are motile, rod-shaped, and can be found on fruits, vegetables and souring fruits as well as in alcoholic beverages. They can cause serious spoilage in alcoholic beverages.
  • Genus Aeromonas – These are gram negative rods that have a temperature for growth between 22 and 28 C. They are often isolated from aquatic environments. Hydrophila can cause illness in humans, but it can also be fatal to fish and other mammals.
  • Genus Alcaligenes : This is the alkaline reaction that occurs in the medium of growth. A. viscolactis* causes milk ropiness, while A. metalcaligenes* results in slimy cottage cheese. These organisms are found in manure, feed, soil, water and dust. This genus also includes organisms that were previously classified under the genus Achromobacter.
  • Genus Alteromonas – Several extinct species of Pseudomonas have been reclassified as Alteromonas. These are important marine organisms for seafoods.
  • Genus Arthrobacter: Arthrobacter is a dominant soil organism and is inert in many foods. Some species, however, can grow at 5 C. These species would be considered psychrotrophs.
  • Genus Bacillus – Endospores from species in this genus, which ranges from an aerobic to a facultative genus, usually don’t swell the rods they are attached to. Different species can be mesophilic, thermophilic, active proteolytic or moderately proteolytic. They may also be nonproteolytic and gas-forming. The spores from mesophiles (e.g. B. subtilis) are generally less heat-resistant than those of thermophiles. Spores from obligate thermophiles (e.g. B. subtilis) are more resistant to heat than those from facultative thermophiles (e.g. B. coagulans). B. cereus, an active proteolytic species, is one example of such a species. B. polymyxa (the main acid- and gas-forming species) and B. macerans (the “aerobacilli”) are sometimes called “aerobacilli.” However, many mesophiles can form acids from glucose or other sugars, but only a small quantity that is often neutralized by ammonia from the nitrogenous food. A thermophilic flat-sour bacteria can make large amounts of lactic acids from sugar. This culture, B. coagulans may be used to produce lactic acid. Bacillus species can be found in soil.
  • Genus Brevibacterium : B. linens may be synonymous with Arthrobacter globiformis. B. linens could be an important component of the surface smear in certain cheeses (e.g. brick or Limburger), where the culture produces orange-red pigmentation that aids in ripening.
  • Genus Brochotrix:  Brochotrix is a gram-positive rod that can form long filamentous chains, which may then be twisted into knotted masses. Growth is best at 20-25 C. However, depending on the strain, growth can take place at any temperature between 0 and 45 C. Growth can occur at pH 5.0 to 9.0 (optimum pH, 7.7) and with 6.5 to 10% NaCl. Heating organisms at 63 C for 5 minutes will cause them to die. The D value at 63 C has been determined to be 0.1 minutes. They can spoil many meats and meat products if they are kept at 63 C or vacuum packed and refrigerated. B. thermosphacta, however, is the only one.
  • Genus Campylobacter : These bacteria were first classified under the genus Vibrio. They are oxidase positive, catalase-positive and gram-negative. They prefer low oxygen tension. There are several strains of C. Fetus subsp. Jejuni have been linked to gastroenteritis in humans.
  • Clostridium genus: Endospores from species in this genus are known to swell the rods they form. All species are catalase negative. Many species ferment carbohydrates actively, producing acids (usually including butyric acid) and gases (usually hydrogen and carbon dioxide). Different species can be mesophilic, thermophilic, and proteolytic or not. C. thermosaccharolyticum is an example of a saccharolytic obligate thermophile; this organism causes gaseous spoilage of canned vegetables. Mesophilic, proteolytic species such as C. lentoputrescens* or C. putrefaciens are often responsible for food putrefaction. C. perfringens and similar species can cause a violent disruption of the milk curd, leading to lactatefermenting. Late gas is caused by C. butyricum in cured cheese. Clostridium species are primarily found in the soil, but they can also be found in bad silage, feeds and manure.
  • Genus Corynebacterium: The diphtheria organism C. diptheriae can be transmitted by food. C. bovis is characterized by the clubbed, barred or slender rods that are characteristic of the genus. It can be found in aseptically distilled milk and could be a source of bovine mastitis.
  • Genus Desulfotomaculum: A rod that is gram-negative and swells when there is an endospore. These are common in soil, freshwater, and rumen. Sulfur compounds can be used as terminal electron acceptors in respiration, and thereby reduce to hydrogen sulfide. C. nigrificans is responsible for the sulfide stinker spoilage of canned foods.
  • Genus Enterobacter: Some of them were once classified as Aerobacter. They are widespread in nature and a member the coliform group.
  • Genus Erwinia – This genus includes plant pathogens that cause necrosis and galls in plants, as well as soft rots. These plants can also damage vegetable and fruit products. E. carotovora can be associated with “bacterial soft rot”, a market disease. E. carotovora subsp. carotovora is responsible for rotting of many plants. E. carotovora subsp. atroseptica causes black rot in potatoes. E. carotovora subsp. betavasculorum* causes soft-rot in sugar beets.
  • Genus Escherichia – A predominant gram-negative rod that is found in the feces of warm-blooded mammals and widely distributed in nature. The genus Escherichia is one of the “coliform groups” and can be pathogenic to people. It is broken into several biotypes or serotypes.
  • Genus Flavobacterium : This genus’ yellow-to-orange-pigmented species can cause discoloration on meat surfaces and may also spoil eggs, milk, and shellfish. Some of these organisms can be psychrotrophic, and they have been observed growing on frozen vegetables. Some Flavobacterium species were included in the genus Halobacterium.
  • Genus Gluconobacter (Formerly Acetomonas): Species can oxidize alcohol to acetic acids. G. oxydans causes ropiness in beer following viscous growth in beer or wort.
  • Genus Halobacterium – Bacteria from this genus (e.g. H. salinarium) are obligate halosophiles and usually chromogenic. They can grow on foods high-salt, such as salty fish, and cause discoloration. This genus now includes many bacteria that were previously classified as Flavobacterium.
  • Genus Klebsiella: Many Genus of Klebsiella are capsulated. Commonly found in the respiratory and digestive tracts of people. K. pneumoniae is responsible for bacterial pneumonia in people.

Groups of Bacteria Important in Food Bacteriology

Food-safe bacteria are often grouped based on one common characteristic, without considering their systematic classification. Some bacterial species may be included in more than one of these artificial groups. We will show you examples of commonly used groupings.

Lactic Acid-Forming Bacteria, orLactics

  • The most important characteristic in the lactic acid bacteria are their abilities to convert sugars into lactic acid.
  • This can be useful for making sauerkraut or cheese, but it is not good for spoiling wines.
  • They produce acid quickly and often in large quantities, so they tend to eliminate most of the competition from microorganisms.
  • Major genera are Leuconostoc and Lactobacillus and Streptococcus.

Acetic Acid-Forming Bacteria, orAcetics

  • Acetic acid bacteria consists mainly of one of two genera: Acetobacter or Gluconobacter.
  • Both oxidize the ethyl alcohol into acetic acids, but Acetobacter can oxidize acetic as further to carbon dioxide.
  • These are the characteristics that make the Acetic Acid bacteria so important
    1. Their ability to oxidize alcohol to acetic acids makes them useful in vinegar manufacturing and dangerous in alcoholic beverages.
    2. Their strong oxidizing powers may cause the oxidation by undesirable species or desirable species under unfavorable circumstances of acetic acid. This oxidizing potential may prove useful in the oxidation D-sorbitol-to-L-sorbose conversion in synthetic ascorbic acid preparation.
    3. Excessive sliminess in some species (e.g. Acetobacter aceti Subsp, Suboxydans*), can clog vinegar generators.

Butyric Acid-Forming Bacteria, orButyrics

  • The majority of bacteria in this group are spore-forming anaerobes from the genus Clostridium.

Propionic Acid-Forming Bacteria, or Propionics

  • Although most bacteria in this group belong to the genus Propionibacterium they have also been known to produce propionic cocci.

Proteolytic Bacteria

  • This heterogeneous group is made up of active proteolytic bacteria that produce extracellular proteinases. The enzymes diffuse beyond the cells and are called “extracellular proteinases”.
  • Although all bacteria contain proteinases within the cell, only a few types have extracellular proteins.
  • Proteolytic bacteria can be divided into aerobic and facultative, which may be sporeforming or non-sporeforming.
  • Bacillus cereus, an aerobic, spore forming, proteolytic bacteriaium, Pseudomonas Fluorescens, is non-sporeforming and aerobic to facultative. Clostridiumsporogenes, however, is spore forming and anaerobic.
  • Proteolytic enzymes are found in many species, including Clostridium and Bacillus, Bacillus and Pseudomonas.
  • Acid-proteolytic bacteria is a type of bacteria that simultaneously performs acid fermentation and proteolysis.
  • Streptococcus flaccis var, micrococcus caseolyticus* and liquefaciens* are acid-proteolytic.
  • Some bacteria can be putrefactive. They decompose proteins anaerobically and produce foul-smelling compounds like hydrogen sulfide or mercaptans,amines, amines, amines and indole.
  • Clostridium proteolytic species are most putrefactive. Proteus, Pseudomonas and other nonsporeformers species are also putrefactive.
  • It is possible to putrefy split proteins.
  • Pseudomonas species have been known to produce proteinase capable of surviving ultrahigh heat treatment.

Lipolytic Bacteria

  • This heterogeneous group includes bacteria that produce lipases enzymes, which catalyze the hydrolysis fats into fatty acids and/or glycerol.
  • Many aerobic, active proteolytic bacteria are also lipolytic.
  • For example, Pseudomonas Fluorescens is a strongly lipolytic species. Lipolytic species are found in Pseudomonas and Alcaligenes as well as Serratia, Micrococcus, Staphylococcus and Staphylococcus.
  • Many of the microbial lipsases are resistant even to standard processing methods.
  • It is not a sign that the product has no microbial lipases if there are no viable lipolytic bacteria in spoiled foods.

Saccharolytic Bacteria

  • These bacteria convert disaccharides and polysaccharides into simpler sugars.
  • Only a few bacteria types are amylolytic. This means that they have amylase which can cause starch to be hydrolyzed outside of the cell.
  • Clostridium andyricum and Bacillus subtilis are both amylolytic.
  • Only a few bacteria are capable of hydrolyzing cellulose.
  • Clostridium species can be classified as either proteolytic, which may or may not target sugars, or saccharolytic, which attack sugars but do not attack proteins.
  • C lentoputrescens*, which is proteolytic, does not normally ferment carbohydrates. C. butyricum, on the other hand is nonproteolytic and ferments sugars.

Pectinolytic Bacteria

  • Complex carbohydrates called pectins can be found in fruits and vegetables. They are responsible for rigidity of the cell walls.
  • Commercial products can use pectic substances from citrus fruits as gelling agents.
  • Pectinase, a variety of pectolytic enzymes, may cause softening of plant tissues and loss of gelling power in different foods.
  • Pectinolytic may occur in some species of Erwinia and Bacillus, Clostridium as well as those of Flavobacterium, Aeromonas or Arthrobacter.

Thermophilic Bacteria, orThermophiles

  • Foods that are heated at high temperatures require these bacteria to thrive.
  • B. stearothermophilus is responsible for the thermophilic flat spoilage of low-acid canned food.
  • Gaseous thermophilic spoilage of canned foods is a result of growth by C thermosaccharolyticum.

Thermoduric Bacteria

  • Thermoduric bacteria is defined as bacteria that can withstand heat treatments such as pasteurization.
  • Pasteurization can be done on liquid eggs for Bacillus species, micrococci* and enterococci.
  • Foods are often contaminated with bacteria from the genera Clostridium and Bacillus, Micrococcus. Streptococcus. Lactobacillus.
  • Sometimes molds like Byssochlamys Fulva, Penicillium, and Aspergillus are thermoduric.
  • Some thermoduric bacteria such as Bacillus or enteroccocci can also be psychrotrophic. (See the next section).
  • These psychrotrophs are often found in milk where there is more pasteurization and longer cooling times.

Psychrotrophic Bacteria, or Psychrotrophs

  • These bacteria can grow at temperatures below freezing.
  • Psychrophiles are not able to grow at the optimal temperature of refrigeration, but they can do so with psychrotrophs.
  • The majority of bacteria that causes loss of quality in nonsterile refrigerated foods (except seafood) are psychrotrops.
  • Psychrotrophic bacteria is found mainly in the genera Pseudomonas Flavobacterium and Achromobacter. However, Micrococcus and Lactobacillus may also contain psychrotrophic organisms.
  • Many yeasts and molds can also grow in cold temperatures.

Halophilic Bacteria, or Halophiles

  • For true halophilic bacteria to grow, they need very low concentrations of dissolved salt chloride.
  • These bacteria include Pseudomonas and Moraxella, Flavobacterium and Acinetobacter species. They grow well in media containing 0.5 to 3.0% salt.
  • These microorganisms have been isolated from many types of shellfish and fish.
  • Moderate halophiles are bacteria that can be isolated from salted fish, salted meats, and salted vegetables. They grow best in mediums with 3.0 to 15% salt.
  • These bacteria can be found in the genera Bacillus and Micrococcus, Vibrio and Acinetobacter.
  • Sometimes, in foods with high salt content, such as food that contains 15-30 percent salt, extreme Halophiles like Halobacterium or Halococcus can be isolated.
  • They are often also colored pink or red.
  • Other bacteria can tolerate salt; i.e., Halotolerant bacteria can grow without or with salt.
  • They are usually capable of growing in foods with 5.0 percent salt or higher.
  • Sarcina, Pediococcus and Pediococcus are all examples of halophilic and halotolerant bacteria that is important for food.

Osmophilic or Saccharophilic Bacteria

  • Different species of yeasts are the most common osmophilic microorganisms found in food.
  • Osmophilic bacteria is bacteria that grows in high sugar concentrations; however, most bacteria known as osmophiles can withstand sugar, such as species of Leuconostoc.

Pigmented Bacteria

  • Pigmented bacteria that grows on or in food can produce colors that span the visible spectrum, including black and white.
  • In the next discussion on spoilage of food, there will be many examples.
  • Some species are pigmented in all but one of their genera, such as Flavobacterium (yellow-orange) and Serratia(red).
  • Many species can be found as pigmented species, such as many Micrococcus species. Some species also have pigmented varieties, such as the rust-colored Lactobacillus Plantarum, which discolors cheese.
  • The pigmentation of Halobacterium species is pink, red or red to orange. Halococcus species can be pigmented red, or red to orange.

Slime- or Rope-Forming bacteria

  • These bacteria include Alcaligenes viscolactis* and Enterobacter aerogenes. They cause ropiness in milk and Klebsiellaoxytoca slimes in sucrose solutions.
  • Some species of Lactobacillus and Streptococcus may have milk that is slimy or ropy.
  • A micrococcus makes meat curing solutions ropy.
  • Lactobacillus plantarum strains and other lactobacilli can cause ropiness in various fruits, vegetables, and grain products. Roping in bread can be caused by some Bacillus species.

Gas-Forming Bacteria

  • Many bacteria species produce small amounts of gas, and it is so slow that it is difficult to detect.
  • Sometimes, this is true for hetero-fermentative lacs. However, gas evolution can be evident under other conditions.
  • Among the genera that contain gas-forming bacteria are Leuconostoc, Lactobacillus (heterofermentative), Propionibacterium, Escherichia, Enterobacter, Proteus, Bacillus (the aerobacilli), and Clostridium. The first three genera’ bacteria produce carbon dioxide only, while the others produce both carbon dioxide as well as hydrogen.

Coliform and Fecal coliform group

  1. Coliforms, which are short rods, are Gram-negative, aerobic, and facultative anaerobic, non-spore-forming bacteria that ferment lactose with gas production.
  2. Escherichiacoli and Enterobacter aerogenes are the most common coliform bacteria species. However, up to twenty species could be included in this group, which includes species of other Enterobacteriaceae or Aeromonas species.
  3. The fecal group of coliforms includes those that can grow at elevated temperatures (44.5 C or 45 C).
  4. The elevated incubation test was originally designed to distinguish coliforms from fecal origin and those of nonfecal.
  5. Taxonomically, the term “fecalcoliform” and “coliform” are not valid. They refer to bacteria groups that can grow under certain conditions.
  6. There are many methods for determining coliform count, fecalcoliform count, and E.coli counts on food. These indicators are accepted and widely used.
  7. E.coli testing in water was used as an alternative to Salmonella typhi testing and the beginning of the “indicator” microorganisms.
  8. This concept was inspired by Shardingen’s 1892 suggestion that E.coli species be used as an indicator or index of fecal contamination, since they are easier to recover than Salmonella species.
  9. Other indicators or tests that can be used are the Enterobacteriaceae or fecal streptococci, the Enterobacteriaceae and staphylococci. These indicate the possibility of S. aureus Enterotoxin, or abuse, and Geotrichum candidum which is a mold in machinery.

The coliform bacteria is important for food spoilage because of its many characteristics.

  1. Their ability to grow in a variety substrates, to use a few carbohydrates and other organic compounds for energy, and to obtain nitrogen from a small number of simple nitrogenous compounds.
  2. Their ability to synthesize the majority of the vitamins they require.
  3. The ability of the group’s ability to grow well at a wide range of temperatures (from below 10 C to 46 C),
  4. Their ability to make significant amounts of acid from sugars.
  5. Their ability to produce off-flavors that are often called “unclean” and “barny” and (6) their ability to cause sliminess, or ropiness in foods.
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