Salmonella - pathogens of food toxic infections
Last reviewed: 23.04.2024
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Key signs of the genus Salmonella
Key signs of the genus Salmonella are as follows: short Gram-negative rods with rounded ends, 1.5-4.0 microns long, in most cases mobile (peritrichi), spores and capsules do not have, form during the fermentation of glucose (and a number of other carbohydrates), acid and gas (with the exception of S. Typhi and some other serotypes), have lysine and ornithine decarboxylases, do not have phenylalanine deaminase, form H2S (some do not form), give a positive reaction with MR, grow on starved agar with citrate (except S. Typhi), not ferment lactose (except S. Arizonae and S. Diarizonae), and form indole, urease, and have given a negative reaction Voges-Proskauer. The content of G + C in DNA is 50-52%. Cultural properties of these bacteria are the same as that of typhoid and paratyphoid A and B.
Resistance of Salmonella
The resistance of salmonella to certain physical and chemical factors is quite high. Heating at 70 ° C is maintained for 30 minutes. Resistance to high temperatures increases when salmonella is found in food, especially in meat. When cooking for 2.5 h, meat contaminated with salmonella and stored in cold water becomes sterile in pieces weighing not more than 400.0 g at a thickness of 19 cm pieces; and when laying boiling water, sterility is achieved in the same cooking period only in pieces weighing up to 200.0 g, with a thickness of 5.0-5.5 cm. Pickling 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 to 1.5-2 months. Usual chemical disinfectants kill Salmonella in 10-15 minutes.
Pathogenicity factors of salmonella
Salmonella has factors of adhesion and colonization, factors of invasion; they have endotoxin and, finally, they, at least S. Typhimurium and some other serotypes, can synthesize two types of exotoxins:
- thermolabile and thermostable enterotoxins of the LT and ST type;
- shigapodobnye cytotoxins.
A specific feature of toxins is intracellular localization and isolation after the destruction of bacterial cells. LT Salmonella has a structural and functional similarity with LT enterotoxigenic E. Coli and with cholerogen. Its molecular weight is 110 kD, it is stable in the pH range 2.0-10.0. Toxin formation in Salmonella is combined with the presence of two factors of skin permeability:
- high-speed - it is produced by many strains of salmonella, it is heat-stable (at 100 ° C it persists for 4 hours), it acts for 1-2 hours;
- delayed - thermolabile (it breaks down at 75 ° C for 30 min), causes the effect (condensation of the rabbit's skin) after 18-24 h after injection.
Molecular mechanisms of diarrhea caused by LT and ST salmonella, apparently. Are also associated with a violation of the function of adenylate and guanylate cyclase systems of enterocytes. Cytotoxin produced by salmonella, thermolabil, its cytotoxic effect is manifested in inhibition of protein synthesis by enterocytes. It was found that individual strains of salmonella can simultaneously synthesize LT, ST and cytotoxin, while others - only cytotoxin.
The virulence of salmonella also depends on the plasmid found in them with mm. 60 MD, its loss significantly reduces the virulence of bacteria. It is assumed that the emergence of epidemic clones of salmonella is associated with the acquisition of plasmids of virulence and R-plasmids.
Postinfectious immunity
Postinfectious immunity is not well understood. Judging by the fact that mainly children are affected by salmonellosis, postinfectious immunity is quite intense, but it appears to be type-specific.
Epidemiology of salmonellosis
Of the known Salmonella, only S. Typhi and S. Paratyphi A cause the disease only in humans - typhoid and paratyphoid A. All other Salmonella are pathogenic also for animals. The primary source of salmonella are animals: cattle, swine, waterfowl, chickens, sinantropic rodents and a large number of other animals. Diseases of animals caused by salmonella are divided into 3 main groups: primary salmonellosis, secondary salmonellosis and enteritis of cattle. Primary salmonellosis (paratyphoid calves, typhoid pigs, chicken typhus, dysentery of chickens, etc.) are caused by certain pathogens and occur with a characteristic clinic. Secondary salmonellosis occurs under conditions when the animal's organism is sharply weakened as a result of some causes (often of various diseases); they are not associated with specific types of salmonella in certain animals, are caused by their various serotypes, but most often S. Typhimuriwn.
Cattle enteritis is characterized by a specific clinical picture and in this respect is similar to primary salmonella. However, enteritis in this case is a secondary manifestation, 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 foodborne toxic infections are animals suffering from secondary salmonella and enteritis of cattle. A large role in the epidemiology of salmonellosis is played by waterfowl and their eggs, as well as chickens, their eggs and other poultry products. Salmonella can enter the egg directly during its development, but can easily penetrate through intact shells. Outbreaks of toxic infections are most often associated with eating meat contaminated with salmonella, up to 70-75%, including up to 30% of slaughtered meat. Forced slaughter is often subjected to animals that are in an agonizing state. In weakened animals, salmonella easily penetrate from the intestine into the blood, and through it into the muscles, causing lifetime infection of the meat. The share of eggs and poultry products accounts for more than 10%, milk and dairy products - about 10%, and fish products - about 3-5% of all outbreaks of salmonella.
Modern epidemiology of salmonellosis is characterized by a constant increase in the incidence of people and animals and an increase in the number of serotypes of salmonella that cause these diseases. From 1984 to 1988, the number of cases of salmonellosis in England increased 6-fold. However, WHO experts believe that the true number of cases of salmonellosis remains unknown. In their opinion, no more than 5-10% of infected people are detected. One of the main reasons for the increase in the incidence of salmonellosis is the infection of food products during their production as a result of the widespread distribution of salmonella in environmental objects and in processing plants, where animals come in whose salmonellosis flows in a latent form. One of the main reasons for the wide circulation of Salmonella among animals is the use of food containing processed by-products of animal origin and very often infected with Salmonella.
Despite the constant increase in the number of serotypes of salmonellae isolated from humans and animals, up to 98% of all cases of salmonellosis are due to Salmonella of groups A, B, C, D and E, primarily S. Typhimurium and 5. Enteritidis (up to 70- 80% of cases of diseases).
Another important feature of the modern epidemiology of salmonellosis is the establishment of the role of man as a source of infection with salmonella. Infection of a person from a sick or bacterial carrier is possible not only through food in which salmonella finds good conditions for reproduction, but also by contact-household means. This method of infection leads to a wide spread of asymptomatic bacterial transport.
A major water epidemic of salmonella infection in Riverside (USA) in 1965, caused by S. Typhimurium (ill about 16 thousand people), showed that infection with salmonella is possible not only through food, but also through water.
The increase in the etiological role of S. Enteritidis, the activation of the food pathway for transmission of infectious agents with the predominance of the role of poultry and poultry products, the increase in the number of group diseases, including nosocomial infections, the increase in the incidence among children under 14 years (more than 60 % of all cases of diseases).
Symptoms of salmonella
Salmonellosis can occur with a different clinical picture: in the form of food poisoning, salmonella diarrhea and generalized (typhoid) form, all depends on the amount of the infecting dose, the degree of virulence of the pathogens and the immune status of the organism. Massive contamination of food products with salmonella causes foodborne disease, in which the main symptoms are associated with the introduction of the causative agent into the blood in large quantities, its disintegration and the release of endotoxin. At the heart of salmonella diarrhea is the colonization of salmonella enterocytes. After attaching to the glycocalyx of the small intestine, salmonellae are introduced between the villi and, attaching to the plasmolemma of the enterocytes, colonize it, damage the microvilli, cause the sloughing of the enterocytes and mild inflammation of the mucous membrane. The released enterotoxin causes diarrhea, and cytotoxin - cell death. Salmonella reproduce on the plasmolemma, but not in the enterocytes, but their invasion 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 major subspecies: S. Salamae, S. Arizonae, S. Diarizonae, S. Houtenae, S. Indica, S. Enterica which differ in a number of biochemical features.
[12], [13], [14], [15], [16], [17]
Serological classification of salmonella according to White and Kauffmann
Salmonella has O-, H- and K-antigens. 65 different O-antigens were detected. They are designated by Arabic numerals from 1 to 67. According to the O-antigen Salmonella are divided into 50 serological groups (AZ, 51-65). Some O-antigens are found in salmonella of two groups (Oh, 08); antigens 01 and 012 - in representatives of many serogroups, but representatives of each serogroup have one main, common for all O-antigen, according to which they are divided into serogroups. Specificity of O-antigens is determined by polysaccharide LPS. In all salmonella, polysaccharides have a common internal core, to which O-specific side chains, consisting of a recurring set of oligosaccharides, are attached. Differences in the relationships and compositions of these sugars provide a chemical basis for serological specificity. For example, the specificity of 02-antigen is determined by the sugar paratase, 04 - by an abovesv, 09 - tyvelose, etc.
Salmonella distinguishes two types of H-antigens: I phase and II phase. More than 80 variants of H-antigens of the I phase were detected. They are denoted by lowercase Latin letters (az) and Arabic numerals (Zj-z59). H-antigens of the I phase are found only in certain serotypes, in other words, according to the H-antigens, the serogroups are divided into serotypes. H-antigens of the II phase have common components in their composition, they are denoted by Arabic numerals and occur in different serovariants. 9 H-antigens of the II phase were detected.
Salmonella K antigens are represented by different variants: Vi- (S. Typhi, S. Paratyphi C, S. Dublin), M-, 5-antigens. The meaning of Vi-antigen is mentioned above.
The modern serological classification of Salmonella counts more than 2500 serotypes.
For the serological identification of salmonella, diagnostic adsorbed mono- and polyvalent O and H serums containing agglutinins to the O and H antigens of those salmonella serotypes that most often cause diseases of humans and animals are released.
Most of Salmonellae (about 98%) is sensitive to Salmonella phage 01. In addition, a scheme for phagotyping the most common causative agent of salmonellosis, S. Typhimurium, has been developed, it allows differentiating more than 120 of its phagotypes.
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, which caused food poisoning. Features of bacteriological diagnostics of salmonellosis:
- use of enrichment media (selenite, magnesium), especially in the study of stool;
- To detect salmonella, samples should be taken from the last, more liquid, part of the bowel movement (upper part of the small intestine);
- observe a ratio of 1: 5 (one part of the stools in 5 parts of the environment);
- in connection with the fact that S. Arizonae and S. Diarizonae ferment lactose, use not only the Endo medium but also bismuth-sulfitagar as a differential-diagnostic medium, on which the salmonella colonies acquire black (some - greenish) color;
- for the sowing of blood use the medium Rapoport;
- use for pre-identification of colonies of 01-salmonella phage, to which up to 98% of salmonella are susceptible;
- For the final identification of the isolated cultures, polyvalent adsorbed O- and H-sera were first used, and then the corresponding monovalent O- and H-sera were used.
For rapid detection of salmonella, polyvalent immunofluorescent sera can be used. Serum groups A, B, C, D, and E. Are used for the detection of antibodies in the blood serum of patients and those who have recovered from RPAH using polyvalent erythrocyte diagnosticums containing polysaccharide antigens.