Salmonella are members of the Enterobacteriaceae. Their normal habitat is the intestine of animals.
There are over 2000 different antigenic types of Salmonella.
Salmonella species cause enteric fever such as typhoid fever, enterocolitis, septicemia which causes metastatic diseases such as osteomyelitis.
Salmonella were originally classified into separate species.
However, now they have accepted that all salmonella represent serovars of a single species known as Salmonella enterica.
Certain serotypes cause food-borne infections. They are the major cause of food-borne infections worldwide.
Most of the infections caused by Salmonella are benign and restricted to intestinal tract and not causes major symptoms.
They causes short-lived diarrhea like symptoms.
However, some serotypes such as Salmonella typhi (S. Typhi) and Salmonella paratyphi causes systemic diseases which may be fatal.     Â
They are facultatively anaerobic Gram-negative bacilli.
They can be grown on many culture media and can be confirmed by using biochemical tests and antigen detection tests.
For the identification of the organism, specimen collection should be done according to the disease condition.
For the diagnosis of enteric fever blood, faeces, urine specimens can be cultured.
In diarrhoeal disease blood and faeces are collected.
Specimens should be collected when in fever times.
In bacteraemia conditions blood is collected for culture.
When arthritis or abscess is suspected, pus or joint fluid should be used for the culture as specimens. Â
Microscopy
In gram stained slides Salmonella are gram negative bacilli.
All Salmonella are actively motile and they are non-capsulated.
Salmonella are none spore forming except Salmonella typhi.
Culture
They are aerobic or facultatively anaerobic.
They grow optimally at 37°C and they can live between 15–45ºC temperatures.
To isolate Salmonella from faecal specimens a selective medium is necessary.
For the enrichment of these organisms to detect small number of Salmonella, selective enrichment broths such as Rappaport-Vassiliadis (RV) broth, is used when S. typhi is not suspected.
For the isolation of S. Typhi from blood 10% ox-gall in distilled water is used as enrichment media. Tetrathionate or Selenite broth are also used for the enrichment.
Blood Agar
Salmonella produce 2–3 mm diameter, grey-white colonies.
They are non haemolytic. Some strains appear as mucoid colonies.
MacConkey agar
Non lactose fermenting pale color colonies are produced on MacConkey agar.
Xylose–lysine desoxycholate agar (XLD agar)
Pink color colonies with black centers are produced on XLD agar by Hydrogen sulphide (H2S)-producing Salmonellae. The colonies are 3–5 mm in diameter.
Salmonella which do not produce Hydrogen sulphide will give pink color non lactose fermenting colonies without black centers, which are similar to Shigella colonies.
Deoxycholate Citrate Agar (DCA agar)
Salmonella produce pale colour non lactose fermenting colonies and H2S producing Salmonella produce black centers.
Salmonella Shigella agar (SS agar)
Selective and differential medium to isolate Salmonella and Shigella. Salmonella that produce Hydrogen sulphide will give black color colonies on SS agar.
Biochemical Identification
Salmonella give different Kligler iron agar (KIA) patterns.
Salmonella gives pink colour slope and yellow colour butt indicating fermentation of glucose but not lactose.
Cracks in the KIA medium will helpful to identify the Salmonella which produce gases by fermenting glucose.
Blackening of the medium give clues to identify H2S producing Salmonella and H2S pattern will helpful to differentiate the different species according to the visible pattern of H2S.
Most Salmonella serotypes are,
- Urease negative
- Indole negative
- Non lactose fermenting
- Citrate positive (S. Typhi and S. Paratyphi A are citrate negative)
- Lysine decarboxylase (LDC) positive (S. Paratyphi A is LDC negative)
- Beta-galactosidase (ONPG test) negative
Serotyping also can be done for the differentiation of the organisms. Based on the composition of somatic antigen (O) and the Flagella antigen (H) serotyping is done.
For full serotyping, commercially available polyvalent and monovalent O and H antisera will be used.
For the diagnosis of typhoid fever serologically Widal test and Ig M antibody immunoassays are used.
Treatment
Self-limiting Enterocolitis like diseases caused by Salmonella are resolves without any treatment.
However, Fluid and electrolyte replacement should be done as diarrhoea is present.Â
Ceftriaxone or ciprofloxacin, chloramphenicol, co-trimoxazole, and ampicillin are useful for the
treatment of systemic infections.
Prevention
Adhering to public health measures and following personal hygienic measures are the main prevention methods of Salmonella infection.
Proper disposal and treatment of sewage, timely monitoring of the quality of chlorinated water supply, monitoring of status of contamination of coliform bacteria, time to time culturing of stool samples from the food handlers to detect carriers, hand washing prior to food handling are major preventive measures.
Moreover, pasteurization of milk, usage of properly cooked eggs, meat, poultry and other risk foods also can be done.
There are two vaccines available for the protection against S, Typhi. One vaccine is given orally and other vaccine is given intramuscularly.
They give 50%–80% protection against infection caused by S. Typhi.
One vaccine is live, attenuated vaccine. It is given orally and the other one contains the Vi capsular polysaccharide of S. typhi.
It is given by intramuscularly. These vaccine are found as very effective and mainly recommended to the travelers to the high risk areas. Â
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