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Salmonella: Medical Microbiology


Salmonella is a type of gram-negative bacteria that can lead to typhoid fever, food poisoning and paratyphoid fever. These species can cause the production of hydrogen sulfide and are motile. Their name is derived from that of an American veterinary pathologist, Daniel Elmer Salmon. Along with Theobald Smith, who was also a renowned pathologist and epidemiologist, Salmon discovered the organism which causes hog cholera, Salmonella enterica var. Choleraesius (Ryan & Ray, 2004).

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A research published in 2008 provided documented proof about the presence of Salmonella antibodies in Malawi children as researchers succeeded in identifying an antibody which acts as a protecting agent against bacterial infections of the blood caused by Salmonella. Results of research involving 352 children at Blantyre’s Queen Elizabeth hospital showed that infants and children till the age of two develop antibodies that help in eliminating the abovementioned bacteria. This was positive news for researchers as with this knowledge, they are one step closer to the prospect of a Salmonella vaccine (MacLennan et al., 2008)

Salmonella is present in many turtles and a large number of other reptiles as well. The culture mediums on which it is isolated in clinical laboratories are MacConkey agar, XLD agar, XLT agar or DCA agar. Salmonella can cause infections in the intestine and in a healthy bowel, the number of bacteria normally present is much greater than their amount. Because of these reasons, a selective medium is necessary for primary isolation, and CLED agar or another type of comparatively non-selective medium is generally not used (Ryan & Ray, 2004).

Often, the quantity of salmonella is so low in clinical samples that enrichment culture has to be carried out, in which selenite broth or Rappaport Vassiliadis soy peptone broth is used as a selective broth medium in which a small volume of stool is incubated in, overnight. These media have properties that hinder the growth of the microbes which are generally present in the healthy human bowel, while at the same time they make it possible for salmonellae to increase their quantity. Then, salmonellae can be recovered by immunizing the enrichment broth on either one or more than one of the primary selective media (Ryan & Ray, 2004).

Salmonellae from moist colonies on blood agar, their diameter being about 2 to 3 mm. if the cells are grown for a longer period of time at a temperature ranging between 25 and 28°C, they will produce a biofilm, which is a group of cellulose, proteins and complex carbohydrates. This characteristic, that is the ability to produce the said biofilm, is a sign of dimorphism. Dimorphism is the name given to the ability of one genome to produce numerous phenotypes when exposed to certain external environmental conditions. Salmonellae generally produce hydrogen sulfide, and do not ferment lactose. The hydrogen sulfide they produce when exposed to media containing ferric ammonium citrate, reacts to turn into a black spot in the center of the creamy colonies (Ryan & Ray, 2004).

As far as classification of salmonella is concerned, there are two species namely S. bongori (previously subspecies V) and S. enterica (formerly called S. choleraesuis) which are further classified into six subspecies. These are: I: enterica, II: salami, IIIa: Arizona, IIIb: Arizona, IV: houtenae, V: obsolete (now designated S. bongori), VI: India (Ryan & Ray, 2004).

Within both these species, S. bongori and S. enterica, there are also over 2500 serotypes or serovars, that are found in many different types of environments and are linked with a disparate variety of diseases. More than 99.5% of human isolates are from the subspecies S. enterica. To reduce complexity, the Centers for Disease Control and Prevention recommends that the serovar Salmonella Typhi is used to refer to Salmonella species, instead of the technically correct, but longer name, Salmonella enterica subspecies enterica serovar Typhi (Giannella, 1996).

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In terms of serology classification, the main division in salmonella is first by the somatic O antigen, then by flagellar H antigens. The latter are further sub-categorized into phase 1 and phase 2, and H antigens belonging to both phases are required to completely identify an isolate but the norm has become such that routine labs will leave this part to Reference Laboratories (Giannella, 1996).

Aside from typhoid and paratyphoid, salmonellosis is an infection is not related to blood, contrary to what is popular belief regarding it. Examples include Salmonella Enteritidis (1,9,12:g,m) – where the O antigens found are 1, 9 and 12; with the H antigens being g and m. Another example is Salmonella Typhi (9, 12, Vi;d ) where the O antigens are 9, 12; with the H antigen being d and the Vi antigen is called so because of its link with the bacterial capsule and its role as a Virulence factor. Recommendations by the Health Protection Agency include regular testing of the following antigens: O antigens: 2 4 6.7 8 9 and 3.10; phase 1 H antigens: a b d E G i r Vi; phase 2 H antigens: 1,2 1,5 1,6 1,7 (Ryan & Ray, 2004).

To guard against Salmonella as the food disease and illness it brings with it, it is crucial that food and food contact surfaces are clean and sanitary. Alcohol serves as an effective topical sanitizer. Quaternary ammonium is also often used with alcohol because it acts as a sanitizer that provides long-lasting protection. To prevent salmonella, surfaces are also kept clean by using non-flammable alcohol vapor in chlorine bleach or by using the safe and environmentally method developed in Japan i.e. carbon dioxide NAV-CO2 systems (Giannella, 1996).

Salmonellosis is the name given to an infection as a result of salmonella bacteria. The salmonella species which cause diseases are today known as a single species, names Salmonella enterica. Subtypes include salmonella typhi which is responsible for typhoid fever. Food poisoning which is often the result of salmonellae is caused by their presence in poultry, and raw eggs (because of which it is crucial that foods containing raw eggs are thoroughly cooked before consumption).

Often, food which is not eaten fresh but is frozen for extended periods of time also results in foodborne illness. Diarrhea, vomiting, fever, and cramps are often indications of this possible condition. In a large number of cases, 3 to 7 days is the time span of the infection. The need for antibiotic treatment and/or hospitalization does usually not exist in the majority of non-typhoidal strains linked with food-borne infection, however, complications may occur in cases of certain serovars (for example, Typhi) in which case the need arises for serious treatment.

Dehydration caused by diarrhea might require administration of intravenous fluids and medications to remove the symptoms such as fever. A problem arises in the common uses antibiotics in beef as well as poultry industries, which has led to the development of antibiotic resistance in a strain of salmonella (Ryan & Ray, 2004).


Another way of catching Salmonellosis is by physically coming into contact with reptiles like turtles or iguanas. Cases of gastrointestinal illness as a result of salmonella were also reported in 2003 and 2004 as a result of physical handling of pet rodents (Ryan & Ray, 2004).

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Ryan K. J. & Ray C. G. (eds). (2004). Sherris Medical Microbiology. 4th ed. McGraw Hill.

Giannella, R. A. (1996). “Salmonella” in Baron S. et al (eds.): Baron’s Medical Microbiology. 4th ed. Univ of Texas Medical Branch.

MacLennan, C. A. et al. (2008). The neglected role of antibody in protection against bacteremia caused by nontyphoidal strains of Salmonella in African children. The Journal of Clinical Investigation, 118 (4), 1553-1562.

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"Salmonella: Medical Microbiology." StudyCorgi, 24 Oct. 2021,

1. StudyCorgi. "Salmonella: Medical Microbiology." October 24, 2021.


StudyCorgi. "Salmonella: Medical Microbiology." October 24, 2021.


StudyCorgi. 2021. "Salmonella: Medical Microbiology." October 24, 2021.


StudyCorgi. (2021) 'Salmonella: Medical Microbiology'. 24 October.

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