Salmonella Infection

The Salmonella genus has more than 2500 different serotypes [1], which cause a variety of symptoms. The set of syndromes resulting from Salmonella infection (salmonellosis) are gastroenteritis, septicemia, focal disease, and enteric fever, in addition to the carrier state, which is asymptomatic.

Gastroenteritis, of which there are more than one million cases per year in the USA, is brought on by salmonellae belonging to the non-typhoidal group, most commonly by serotypes Enteritidis, Typhimurium, Newport, Heidelberg, and Javiana of Salmonella enterica subspecies enterica. Most non-typhoidal Salmonellae have broad host ranges, and animals are their main reservoir. Salmonella is transmitted to humans mainly by contaminated foods: meat, milk, eggs, vegetables, or any food not carefully handled. However, direct contact with animals, and with products consumed or used by them may also lead to infections [2]. Children under the age of 5, infants who are not breastfed [3], the elderly, those with gastrectomy or reduced stomach acid [4], the immunocompromised, and people with hemolytic conditions have an increased risk for salmonella infection.

The symptoms of salmonella enteritis are abdominal pain, diarrhea, fever, nausea, and vomiting. The episode may last from a few days to a week and usually does not require antimicrobial treatment, although in some cases, the diarrhea may cause severe dehydration.

Salmonellae invade the enterocytes in the gut, and in some rare cases continue to spread through the lymph nodes to other organs, among them the liver, spleen, and bones. Invasive infections can cause bacteremia, osteomyelitis, meningitis and septic arthritis. Bacteremia occurs infrequently, mainly in the very young and in the elderly population [5] [6], after infection with a few particular serotypes, for example, Salmonella enterica subspecies enterica serotype Typhimurium (commonly, although inaccurately referred to in an abbreviated form as S.Typhimurium). A strain of Salmonella typhimurium is causing serious bacteremic infections in tropical Africa [7].

Salmonella typhi and paratyphi give rise to typhoid fever, which is a serious systemic disease with relatively high mortality rates. There are only a few hundred cases in the USA per year, mainly in travelers, but there are about 21 million infections per year in developing countries [8].

Enteritis is diagnosed by culturing the organisms from the stool, whereas bacteremic samples are obtained from blood or other extraintestinal sites. Samples are plated on several media (both nonselective and selective), often with initial growth on enrichment media. Metabolic characteristics of the organism are further examined in various media such as triple sugar iron agar. Group assignment of Salmonellae is based on antigenic analysis. S. Typhimurium, for example, belongs to group B. Pulsed-field gel electrophoresis (PFGE) and multiple locus variable number tandem repeat analysis are also used to determine the infecting organism.

Culturing and then examining the bacteria by immunological and biochemical methods is work-intensive. Identifying the bacterial species based on their proteome using mass spectrometry methods [9] is a newer, simpler method. Syndromic panels, using multiplexed nucleic acid tests, are also in use [10] [11]. Whether these methods will completely replace bacterial cultures is questionable, because without cultures no biological testing (for example antimicrobial resistance assays) can be performed. Ultrasonography may serve to distinguish salmonellosis – or other types of enteritis – from Crohn's disease.

The diagnosis of typhoid fever is difficult, as blood cultures and the standard immunological test, the Widal test, are not sufficiently sensitive or specific [12]. Newer immunological assays, and nested polymerase chain reactions [13] may offer faster and more specific diagnosis.

In the USA, Salmonella infections are reportable. Clinical laboratories submit samples to state agencies from where questionable strains are sent to the Centers for Disease Control and Prevention (CDC).

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