Table of Contents
Salmonella, a genus of bacteria, belongs to the family Enterobacteriaceae. These are rod-shaped, gram-negative bacilli known for their significant role in various diseases affecting both humans and animals. Therefore, understanding the characteristics and functions of Salmonella is crucial in the field of microbiology and medicine.
The genus Salmonella is characterized by its enterobacterial properties. These bacteria are fermentative, facultative anaerobes, which means they can grow in both the presence and absence of oxygen. They are oxidase-negative, gram-negative rods, and are catalase positive. Besides, they are generally motile and are known to be aerogenic, non-lactose fermenting, urease-negative, citrate positive, Acetyl methyl carbinol-negative, and KCN-sensitive.
Salmonella encompasses over 2,000 serotypes, all of which have the potential to be pathogens. The most notable member of this genus is Salmonella Typhi, the causative agent of typhoid fever. This specific bacillus was first observed by Eberth in 1880 and was later isolated by Gaffky in 1884. It was initially known as the Eberth-Gaffky bacillus or Eberthella Typhi. However, in 1885, it was renamed as S. Typhi by Salmon and Smith.
For practical purposes, the genus Salmonella can be divided into two primary groups based on their effects on humans:
- Enteric Fever Group: This group consists of typhoid and paratyphoid bacilli. These are exclusively human parasites, causing diseases like typhoid fever.
- Food Poisoning Group: These are primarily animal parasites but can also infect humans, leading to conditions such as gastroenteritis, septicemia, and other localized infections.
Salmonella species are non-spore-forming and predominantly motile. They have cell diameters between 0.7 and 1.5 μm and lengths ranging from 2 to 5 μm. These bacteria are chemotrophs, deriving their energy from oxidation and reduction reactions using organic sources. They are also facultative anaerobes, meaning they can function both aerobically and anaerobically.
Certain serotypes of Salmonella are intracellular pathogens. Most infections caused by these bacteria result from the consumption of food contaminated with feces. Typhoidal Salmonella serotypes are exclusive to humans and can lead to foodborne illnesses, typhoid, and paratyphoid fever. On the other hand, nontyphoidal Salmonella serotypes are zoonotic, meaning they can be transferred between animals and humans. These serotypes primarily affect the gastrointestinal tract, causing salmonellosis.
In conclusion, the genus Salmonella plays a pivotal role in various diseases affecting humans and animals. Their ability to adapt and survive in diverse conditions makes them a significant concern in public health. Understanding their characteristics, functions, and modes of transmission is essential for effective prevention and treatment.
Characteristics of Salmonella
Salmonella is a genus of bacteria that is commonly associated with foodborne illnesses. Understanding its characteristics is crucial for its identification, prevention, and treatment. Here’s a detailed and objective description of the characteristics of Salmonella:
- Cellular Morphology: Salmonella bacteria are rod-shaped (bacilli) and typically measure between 2-4 μm in length and approximately 0.6 μm in diameter.
- Gram Staining: Salmonella is a Gram-negative bacterium. This means that its cell wall structure consists of a thin peptidoglycan layer located between an inner cytoplasmic cell membrane and an outer bacterial membrane.
- Acid Fastness: Salmonella is non-acid fast. This characteristic differentiates it from certain other bacteria, like Mycobacterium, which retain dye even after being washed with an acid-alcohol solution.
- Capsulation: The bacterium is non-capsulated, indicating it lacks an outer protective polysaccharide layer.
- Spore Formation: Salmonella does not produce endospores. Endospores are a dormant, tough, and non-reproductive structure produced by certain bacteria.
- Motility: A distinguishing feature of Salmonella is its motility. Most of its serotypes possess peritrichous flagella, which allow them to move. However, certain serotypes, such as S. gallinarum and S. pullorum, are exceptions and are non-motile.
- Fimbriae Production: Many Salmonella strains produce type-I fimbriae. These hair-like structures facilitate the bacterium’s attachment to host cells, aiding in colonization.
- Biochemical Reactions: Salmonella can ferment glucose, producing acid and gas. However, it does not ferment lactose, distinguishing it from some other enteric bacteria.
- Pathogenicity: Salmonella is known for its pathogenicity in humans and animals. It can cause diseases ranging from mild gastroenteritis to more severe conditions like typhoid fever.
- Reservoirs: Salmonella has a wide range of natural reservoirs, including poultry, cattle, and other animals. Contaminated food, especially undercooked chicken, eggs, and meat, are common sources of Salmonella infections in humans.
- Temperature Tolerance: Salmonella can survive in a range of temperatures but thrives at human body temperature (37°C). It can also survive for short periods in refrigerated conditions, emphasizing the importance of proper food storage.
- Antibiotic Resistance: Some strains of Salmonella have developed resistance to commonly used antibiotics, making treatment more challenging.
Morphology of Salmonella
- Cell Shape and Size: Salmonella bacteria are rod-shaped, known as bacilli. They typically measure between 2-4 μm in length and approximately 0.6 μm in diameter.
- Gram Staining: Salmonella is classified as a Gram-negative bacterium. This designation is based on its cell wall composition and its reaction to the Gram staining procedure. The Gram-negative classification indicates that the bacterium has a thin peptidoglycan layer sandwiched between an inner cytoplasmic cell membrane and a bacterial outer membrane.
- Acid Fastness: The bacterium is non-acid fast. This means that it does not retain the primary dye when subjected to acid-fast staining, a technique primarily used for the differentiation of Mycobacterium species.
- Capsule: Salmonella is non-capsulated, indicating that it does not have an outer protective polysaccharide layer surrounding its cell wall.
- Spore Formation: The bacterium is non-sporing, which means it does not produce endospores. Endospores are highly resistant structures formed by some bacteria to survive in adverse environmental conditions.
- Motility: Most serotypes of Salmonella are motile. They achieve this movement through peritrichous flagella. These are long, whip-like structures that protrude from all around the bacterial cell, allowing it to move in liquid environments. However, some serotypes, such as S. gallinarum and S. pullorum, are non-motile.
- Fimbriae: Most strains of many Salmonella serotypes produce type-I fimbriae. Fimbriae are hair-like projections on the bacterial surface that play a crucial role in adhesion. They help the bacterium attach to surfaces, especially host cells, facilitating colonization and invasion.
Antigenic structure of Salmonella
Salmonella, a genus of bacteria known for its pathogenicity, exhibits a complex antigenic structure. This structure plays a pivotal role in its identification and understanding its interaction with the host immune system. Therefore, a detailed and sequential explanation of its antigenic components is essential.
Salmonella possesses several antigens, which include:
- Flagellar Antigen (H): These are determinant groups present on the flagellar protein. Being heat and alcohol labile, they are well preserved in 0.04-0.2% formaldehyde. Heating at 60°C detaches the flagella from the bacteria, and complete detachment is achieved by heating for 30 minutes at 100°C.
- Somatic Antigen (O): This antigen is a complex of phospholipid, protein, and polysaccharide that forms an integral part of the bacterial cell wall. They are hydrophilic, allowing the bacteria to form stable suspensions in saline. Over 60 different O-antigens have been identified. These antigens are heat stable and alcohol stable. The O-antigen, also known as the Boivin antigen, is identical to endotoxin. It is less immunogenic than the H-antigen, resulting in a generally lower antibody titer.
- Surface Antigen (Vi): Found in some species, this antigen is believed to be a virulence factor. Most isolated Salmonella forms have the Vi-antigen as an outer layer. This antigen is an acidic polysaccharide that, when fully developed, reduces the bacteria’s agglutinability. Many strains of Salmonella, particularly S. Typhi, do not agglutinate with O-antiserum due to the presence of this surface polysaccharide antigen. The Vi-antigen tends to be lost on serial subculture. It acts as a virulence factor by inhibiting phagocytosis and resisting complement activation. Strains possessing the Vi-antigen tend to cause clinical diseases more consistently.
Besides these primary antigens, Salmonella also carries fimbriae. The fimbrial antigens are not crucial for identification but can cause confusion due to their non-specific nature and widespread sharing among enterobacteria.
Then, it’s essential to note that the Vi-antigen is heat labile. It can be removed from bacteria by heating the suspension for 1 hour at 100°C. However, it remains unaffected by alcohol.
Molecular mechanisms of infection of Salmonella
Salmonella, a pathogenic bacterium, employs a variety of molecular mechanisms to establish infection in its host. The strategies adopted by Salmonella vary depending on the specific serotype, with distinctions observed between typhoidal and nontyphoidal serotypes.
Entry into the Host: Both typhoidal and nontyphoidal serotypes of Salmonella must first breach the intestinal cell wall to gain entry into the host. While they share this initial step, their subsequent strategies differ significantly. As Salmonella travels to its target tissue in the gastrointestinal tract, it encounters various challenges, including stomach acid, bile’s detergent-like activity, decreasing oxygen levels, competition from the normal gut flora, and antimicrobial peptides on the intestinal wall’s surface. In response to these stresses, Salmonella activates virulence factors, facilitating the switch from normal growth to a virulent state.
The molecular mechanisms of Salmonella infection are complex and multifaceted, involving a series of steps that allow the bacteria to invade, replicate, and spread within the host. The process is characterized by the bacteria’s ability to respond to various environmental stresses and to regulate the expression of virulence factors accordingly. This regulatory capacity is crucial for Salmonella to switch from a non-virulent state to a virulent one, thereby initiating the infection process. The following is an expository account of the molecular mechanisms of Salmonella infection:
Switch to Virulence: Upon entering the gastrointestinal tract, Salmonella faces a series of challenges, including stomach acid, bile, a decreasing oxygen supply, competition from normal gut flora, and antimicrobial peptides on the intestinal cells. In response to these stresses, Salmonella expresses virulence factors that facilitate the transition from benign colonization to active infection.
Stages of Infection: The infection process can be sequentially outlined as follows:
- Approach: Salmonella moves toward host cells using intestinal peristalsis and active swimming via flagella. It penetrates the mucus barrier and positions itself near the intestinal epithelium.
- Adhesion: The bacteria adhere to host cells using adhesins and a type III secretion system (T3SS), which is essential for the subsequent invasion.
- Invasion: Salmonella employs variant mechanisms to enter host cells, including bacterial-mediated endocytosis, particularly in M cells of the intestinal wall.
- Replication: Once inside, Salmonella can replicate within the host cells, creating a niche for bacterial multiplication.
- Spread: The bacteria can disseminate to other organs using cells in the blood, avoiding immune defenses, or return to the intestine to re-seed the population.
- Re-invasion and Replication: Secondary infections can occur at systemic sites, leading to further replication.
Mechanisms of Entry: Nontyphoidal Salmonella serotypes often induce intestinal inflammation and diarrhea by entering M cells through bacterial-mediated endocytosis and disrupting tight junctions between intestinal cells. Typhoidal serotypes, however, may utilize a stealthier approach by invading CD18-positive immune cells, facilitating a more systemic infection with fewer bacteria required for infection.
Intracellular Life: Salmonella can enter macrophages via macropinocytosis and survive within a specialized compartment known as the Salmonella-Containing Vacuole (SCV). The acidification of the SCV triggers the expression of T3SS-2, which is crucial for bacterial survival in the host cytosol and systemic disease establishment.
Type III Secretion Systems: Salmonella’s success in causing infection is largely due to two T3SSs. T3SS-1 is responsible for injecting bacterial effectors into the host cytosol, leading to membrane ruffling and bacterial uptake by nonphagocytic cells. T3SS-2 is essential for survival within the host and is associated with the induction of intestinal inflammatory responses and diarrhea.
AvrA Toxin: The AvrA toxin, delivered by the SPI1 T3SS of S. Typhimurium, inhibits the innate immune system through its serine/threonine acetyltransferase activity. This interaction with the host’s eukaryotic cells diminishes the host’s ability to mount an effective immune response, increasing susceptibility to infection.
Clinical Symptoms: Salmonellosis can present in various clinical forms, including gastrointestinal infection, enteric fever, bacteremia, localized infection, and a chronic carrier state. Initial symptoms are often nonspecific, such as fever and weakness, but can progress to more severe localized infections or systemic spread. Chronic infections can reactivate, leading to secondary infections in various body parts, including bones, after the acute phase has resolved.
Immune System Disruption: Salmonella has evolved mechanisms to disrupt specific arms of the immune system, such as certain NF-kappaB proteins, while leaving others intact. This targeted disruption allows Salmonella to evade certain immune responses, contributing to its pathogenicity.
In summary, the molecular mechanisms of Salmonella infection involve a coordinated series of steps that enable the bacteria to invade host cells, evade the immune system, and establish infection. These processes are mediated by a complex interplay of bacterial virulence factors, including T3SSs and specific toxins like AvrA, which manipulate host cell functions and immune responses to facilitate bacterial survival and dissemination.
Cultural characteristics of Salmonella
The cultural characteristics of Salmonella are a key aspect in the identification and study of these bacteria. Salmonella species are known for their ability to grow under both aerobic and facultative anaerobic conditions. They can proliferate on simple media across a pH range of 6 to 8 and at varying temperatures, with an optimum temperature of 37°C, which is the human body temperature.
Upon cultivation, Salmonella colonies are typically large, with a diameter of 2-3 mm. They are circular, low convex, and smooth, with a more translucent appearance than coliform colonies. The colonies’ morphology can vary depending on the medium used for growth:
- On Nutrient Agar (NA) and Blood Agar (BA): After 24 hours at 37°C, colonies of most Salmonella strains are moderately large, measuring 2-3mm in diameter. They are grey-white, moist, and have a smooth convex surface with an entire edge. The size and opacity may vary with different serotypes.
- In Peptone Water and Nutrient Broth (NB): In these liquid media, most strains exhibit abundant growth with uniform turbidity. A surface pellicle usually forms upon prolonged incubation. R variants may produce a granular deposit and sometimes a thick pellicle.
- On MacConkey Bile Salt Lactose Agar: After 24 hours at 37°C, Salmonella colonies are pale yellow or nearly colorless, with a diameter of 1-3 mm.
- On Brilliant Green MacConkey Agar: This medium, which contains Brilliant Green at a concentration of 0.004gm/liter, is inhibitory to E. coli, Proteus, and other commensal Enterobacteria. It serves as an excellent selective and differential medium for S. Typhi, where Salmonella colonies appear as low convex, pale green translucent colonies measuring 1-3 mm in diameter. Lactose fermenters produce purple colonies.
- On Deoxycholate Citrate Agar (DCA): Salmonella colonies on DCA are similar or slightly smaller than those on MacConkey Agar. They are pale or colorless, smooth, shiny, and translucent. Some may have a black center or be surrounded by a clear zone, but these characteristics typically require 48 hours of incubation to develop. DCA is also selective for Shigella, which produces similar colonies.
- On Wilson and Blair’s Brilliant Green Bismuth Sulfide Agar (BBBA): This medium is particularly useful for isolating S. Typhi. Crowded colonies may take up the dye from the medium and appear green or pale brown. Larger discrete colonies may have a black center due to hydrogen sulfide (H2S) production, which also imparts a metallic sheen to the colonies.
- On Xylose Lysine Deoxycholate (XLD): Developed as a selective medium for Shigella, XLD distinguishes Salmonella and Shigella by the red color of the colonies due to the alkaline reaction to phenol red. Salmonella, and some related genera like Edwardsiella, are distinguished from Shigella by their H2S production, which reacts with ferric ammonium citrate to produce a black center in the colonies.
In addition to these solid media, enrichment broths like Tetrathionate broth, Selenite F-broth, Strontium chloride broth, and Malachite green magnesium chloride broth are used to enrich Salmonella species, including S. Typhi, from specimens that may contain these bacteria.
Biochemical tests for Salmonella
Biochemical testing is a fundamental procedure in microbiology used to identify bacterial species, including those within the genus Salmonella. These tests are designed to detect specific enzymatic activities and metabolic capabilities that are characteristic of Salmonella species. The following is a detailed explanation of the biochemical tests used for Salmonella identification:
- Salmonella species are known to ferment various carbohydrates, producing acid and sometimes gas as byproducts.
- Salmonella typhi, S. gallinarum, and some anaerogenic variants of other serotypes, such as S. typhimurium, typically produce acid only, without gas.
- Commonly, Salmonella can ferment glucose, mannitol, arabinose, maltose, dulcitol, and sorbitol. However, they do not ferment lactose, sucrose, salicin, or adonitol.
- The ONPG test, which detects β-galactosidase activity, is negative for Salmonella, indicating that they do not possess this enzyme.
- Salmonella species have the ability to decarboxylate certain amino acids, which means they can remove a carboxyl group from the amino acids, resulting in the production of amines.
- Salmonella can decarboxylate lysine, ornithine, and arginine, but not glutamic acid.
- There are exceptions within the genus; for instance, S. Typhi does not decarboxylate ornithine, and S. Paratyphi A does not decarboxylate lysine.
Other Biochemical Tests:
- The majority of Salmonella species are indole negative, meaning they do not produce the compound indole from the amino acid tryptophan.
- They are methyl red (MR) positive, which indicates they perform mixed acid fermentation.
- They are Voges-Proskauer (VP) negative, showing they do not produce acetoin during glucose fermentation.
- Most Salmonella species can utilize citrate as a sole carbon source, except S. Typhi and S. Paratyphi A.
- Salmonella species do not utilize malonate or gluconate and are urease negative.
- Hydrogen sulfide (H2S) production is a notable trait of Salmonella and is observed in ferrous chloride gelatin medium, except for S. Paratyphi A.
- Salmonella species do not grow in potassium cyanide (KCN) medium and do not liquefy gelatin.
These biochemical tests are part of a systematic approach to identify Salmonella species by their metabolic fingerprints. Each test reveals a specific aspect of the bacteria’s enzymatic machinery and, when combined, provides a profile that is often unique to the species or serotype. It is through this detailed and sequential analysis that microbiologists can distinguish Salmonella from other enteric bacteria and confirm its presence in clinical and environmental samples.