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Salmonellae - causative agents of food toxic infections
Last reviewed: 04.07.2025
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Key features of the genus Salmonella
The key features of the genus Salmonella are as follows: short gram-negative rods with rounded ends, 1.5-4.0 µm long, mostly motile (peritrichous), do not have spores or capsules, form acid and gas during fermentation of glucose (and some other carbohydrates) (except for S. typhi and some other serotypes), have lysine and ornithine decarboxylase, do not have phenylalanine deaminase, form H2S (some do not), give a positive reaction with MR, grow on starvation agar with citrate (except for S. typhi), do not ferment lactose (except for S. arizonae and S. diarizonae), do not form indole, do not have urease and give a negative Voges-Proskauer reaction. The G + C content in DNA is 50-52%. The cultural properties of these bacteria are the same as those of the causative agents of typhoid and paratyphoid A and B.
Salmonella resistance
The resistance of salmonella to some physical and chemical factors is quite high. Heating at a temperature of 70 °C is maintained for 30 minutes. Resistance to high temperatures increases when salmonella is present in food products, especially in meat. When boiled for 2.5 hours, meat contaminated with salmonella and placed in cold water becomes sterile in pieces weighing no more than 400.0 g with a thickness of pieces of 19 cm; and when placed in boiling water, sterility for the same cooking time is achieved only in pieces weighing up to 200.0 g, with a thickness of 5.0-5.5 cm. Salting and smoking meat have a relatively weak effect on salmonella. With a NaCl content of 12-20% in salted and smoked meat, salmonella survive at room temperature for up to 1.5-2 months. Conventional chemical disinfectants kill salmonella in 10-15 minutes.
Pathogenicity factors of salmonella
Salmonella have adhesion and colonization factors, invasion factors; they have endotoxin and, finally, they, at least S. typhimurium and some other serotypes, can synthesize two types of exotoxins:
- heat-labile and heat-stable enterotoxins of the LT and ST types;
- Shiga-like cytotoxins.
The peculiarity of toxins is intracellular localization and release after destruction of bacterial cells. LT salmonella has structural and functional similarity with LT enterotoxigenic E. coli and with choleragen. Its m. m. is 110 kDa, it is stable in the pH range of 2.0-10.0. Toxin formation in salmonella is combined with the presence of two factors of skin permeability:
- fast-acting - produced by many strains of salmonella, heat-stable (at 100 °C it remains stable for 4 hours), acts for 1-2 hours;
- delayed - thermolabile (destroyed at 75 °C within 30 minutes), causes an effect (thickening of the rabbit skin) 18-24 hours after administration.
Molecular mechanisms of diarrhea caused by LT and ST salmonella are apparently also associated with dysfunction of the adenylate and guanylate cyclase systems of enterocytes. The cytotoxin produced by salmonella is thermolabile, its cytotoxic effect is manifested in the inhibition of protein synthesis by enterocytes. It has been found that individual salmonella strains can simultaneously synthesize LT, ST and cytotoxin, while others can synthesize only cytotoxin.
The virulence of salmonella also depends on the plasmid with mm. 60 MD found in them, the loss of which significantly reduces the virulence of the bacteria. It is assumed that the emergence of epidemic clones of salmonella is associated with the acquisition of virulence plasmids and R-plasmids.
Post-infectious immunity
Post-infection immunity has not been studied sufficiently. Judging by the fact that salmonellosis mainly affects children, post-infection immunity is quite intense, but is apparently type-specific.
Epidemiology of salmonellosis
Of the known salmonella, only S. typhi and S. paratyphi A cause disease only in humans - typhoid fever and paratyphoid A. All other salmonella are also pathogenic for animals. The primary source of salmonella are animals: cattle, pigs, waterfowl, chickens, synanthropic rodents and a large number of other animals. Animal diseases caused by salmonella are divided into 3 main groups: primary salmonellosis, secondary salmonellosis and enteritis in cattle. Primary salmonellosis (paratyphoid fever in calves, typhoid fever in piglets, typhoid fever in chickens, chicken dysentery, etc.) is caused by certain pathogens and occurs with characteristic clinical symptoms. Secondary salmonellosis occurs when the animal's body is sharply weakened as a result of some reasons (often various diseases); They are not associated with specific types of salmonella in specific animals and are caused by various serotypes, but most often by S. typhimuriwn.
Enteritis in cattle is characterized by a specific clinical picture and in this respect is similar to primary salmonellosis. However, enteritis in this case is a secondary manifestation, while the primary role is played by various predisposing circumstances. Its causative agents are most often S. enteritidis and S. typhimurium.
The most dangerous sources of food toxic infections are animals suffering from secondary salmonellosis and enteritis of cattle. Waterfowl and their eggs, as well as chickens, their eggs and other poultry products play a major role in the epidemiology of salmonellosis. Salmonella can get into an egg directly during its development, but can easily penetrate through an intact shell. Outbreaks of toxic infections are most often associated with the consumption of meat infected with salmonella - up to 70-75%, including up to 30% of meat from forced slaughtered cattle. Forced slaughter is often subjected to animals in a moribund state. In weakened animals, salmonella easily penetrates from the intestines into the blood, and through it - into the muscles, causing lifetime infection of meat. Eggs and poultry products account for more than 10%, milk and dairy products account for about 10%, and fish products account for about 3-5% of all salmonella outbreaks.
Modern salmonellosis epidemiology is characterized by a constant increase in the incidence of humans and animals and an increase in the number of salmonella serotypes that cause these diseases. From 1984 to 1988, the number of salmonellosis cases in England increased 6-fold. However, WHO experts believe that the true number of salmonellosis cases remains unknown. In their opinion, no more than 5-10% of infected individuals are identified. One of the main reasons for the increase in salmonellosis cases is the contamination of food products during their production as a result of the widespread distribution of salmonella in the external environment and in processing plants where animals with latent salmonellosis are received. One of the main reasons for the wide circulation of salmonella among animals is the use of feed containing processed by-products of animal origin and very often contaminated with salmonella.
Despite the constant increase in the number of salmonella serotypes isolated from humans and animals, up to 98% of all cases of salmonellosis are still caused by salmonella groups A, B, C, D and E, primarily S. typhimurium and S. enteritidis (up to 70-80% of cases of disease).
Another important feature of modern salmonellosis epidemiology is the establishment of the role of humans as a source of salmonella infection. Human infection from a sick person or a carrier of the bacteria is possible not only through food, in which salmonella find good conditions for reproduction, but also through contact and household contact. This method of infection leads to the widespread distribution of asymptomatic bacterial carriage.
A large waterborne salmonella epidemic in 1965 in Riverside (USA), caused by S. typhimurium (about 16 thousand people fell ill), showed that salmonella infection is possible not only through food, but also through water.
The peculiarities of salmonellosis epidemiology in recent years also include the increase in the etiological role of S. enteritidis, the activation of the food route of transmission of infectious agents with a predominant role of poultry and poultry products, an increase in the number of group diseases, including nosocomial ones, and an increase in morbidity among children under 14 years of age (more than 60% of all cases of disease).
Symptoms of salmonellosis
Salmonellosis can occur with various clinical pictures: in the form of food toxicoinfection, salmonella diarrhea and generalized (typhoid) form - everything depends on the size of the infectious dose, the degree of virulence of the pathogens and the immune status of the organism. Massive seeding of a food product with salmonella causes food toxicoinfection, in which the main symptoms are associated with the entry of the pathogen into the blood in large quantities, its decay and the release of endotoxin. Salmonella diarrhea is based on the colonization of enterocytes by salmonella. After attaching to the glycocalyx of the small intestine, salmonella penetrate between the villi and, attaching to the plasma membrane of enterocytes, colonize it, damage the microvilli, cause desquamation of enterocytes and moderate inflammation of the mucous membrane. The released enterotoxin causes diarrhea, and the cytotoxin causes cell death. Salmonella multiply on the plasma membrane, but not in enterocytes, and their invasion occurs through the epithelium into the underlying tissues of the mucous membrane, they are transported through it in macrophages, enter the lymph and blood, causing bacteremia and generalization of the infectious process.
Classification of Salmonella
The genus Salmonella includes the following species: Salmonella bongori, Salmonella subterranea, S. enteritica (formerly S. choleraesuis) with six main subspecies: S. salamae, S. arizonae, S. diarizonae, S. houtenae, S. indica, S. enterica which differ in a number of biochemical characteristics.
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Serological classification of salmonella according to White and Kauffmann
Salmonella have O-, H-, and K-antigens. Sixty-five different O-antigens have been identified. They are designated by Arabic numerals from 1 to 67. Based on O-antigen, salmonella are divided into 50 serological groups (AZ, 51-65). Some O-antigens are found in salmonella of two groups (Ob, 08); antigens 01 and 012 are found in representatives of many serogroups, but representatives of each serogroup have one main O-antigen common to all, according to which they are divided into serogroups. The specificity of O-antigens is determined by the polysaccharide LPS. All salmonella polysaccharides have a common internal core to which O-specific side chains are attached, consisting of a repeating set of oligosaccharides. Differences in the bonds and compositions of these sugars provide the chemical basis for serological specificity. For example, the specificity of 02 antigen is determined by the sugar paratose, 04 by abequose, 09 by tyvelose, etc.
Salmonella have two types of H-antigens: phase I and phase II. More than 80 variants of phase I H-antigens have been identified. They are designated by lowercase Latin letters (az) and Arabic numerals (Zj-z59). Phase I H-antigens are found only in certain serotypes, in other words, serogroups are divided into serotypes by H-antigens. Phase II H-antigens have common components, they are designated by Arabic numerals and are found in different serovariants. Nine phase II H-antigens have been identified.
Salmonella K-antigens are represented by different variants: Vi- (S. typhi, S. paratyphi C, S. dublin), M-, 5-antigens. The significance of Vi-antigen was discussed above.
The modern serological classification of salmonella already includes over 2500 serotypes.
For serological identification of salmonella, diagnostic adsorbed mono- and polyvalent O- and H-sera are produced, containing agglutinins to O- and H-antigens of those salmonella serotypes that most often cause diseases in humans and animals.
Most salmonella (about 98%) are sensitive to salmonella phage 01. In addition, a scheme for phage typing of the most common pathogen of salmonellosis, S. typhimurium, has been developed; it allows differentiation of more than 120 of its phage types.
Laboratory diagnostics of salmonellosis
The main method of diagnosing salmonella infection is bacteriological. The material for the study is feces, vomit, blood, gastric lavage, urine, and the products that caused the poisoning. Features of bacteriological diagnostics of salmonellosis:
- use of enrichment media (selenite, magnesium), especially when examining feces;
- To detect salmonella, samples should be taken from the last, more liquid part of the stool (the upper part of the small intestine);
- maintain a ratio of 1:5 (one part feces to 5 parts medium);
- due to the fact that S. arizonae and S. diarizonae ferment lactose, not only Endo medium but also bismuth sulfite agar should be used as a differential diagnostic medium, on which salmonella colonies acquire a black (some - greenish) color;
- For blood culture use Rapoport medium;
- use for preliminary identification of colonies of 01-salmonella phage, to which up to 98% of salmonella are sensitive;
- For the final identification of isolated cultures, polyvalent adsorbed O- and H-sera are first used, and then the corresponding monovalent O- and H-sera.
Polyvalent immunofluorescent serums can be used for rapid detection of salmonella. RPGA with polyvalent erythrocyte diagnosticums containing polysaccharide antigens of serogroups A, B, C, D and E is used to detect antibodies in the blood serum of patients and those who have recovered.