Salmonella enterica serovar Typhimurium

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University of Oklahoma Study Abroad Microbiology in Arezzo, Italy[1]

Scanning electron microscope image of Salmonella Typhimurium. From: Nbcnews.com [2]

Etiology/Bacteriology

Taxonomy

| Domain = Bacteria | Phylum = Proteobacteria | Class = Gammaproteobacteria | Order = Enterobacteriales | Family = Enterobacteriaceae | Genus = Salmonella | Species = S. enterica | serotype = Typhimurium

Pathogenesis

Transmission/Reservoirs

Salmonella Typhimurium is generally thought to have a wide range of animal hosts, including birds, cattle and many other domesticated animals. However, recent research as led many to believe that the individual variants have a much more narrow range of possible hosts [1] . The invasive African variant has only been found to infect humans; specifically people with compromised immune systems. Since the beginning of the HIV epidemic through Sub-Saharan, invasive African Salmonella Typhimurium has been found to transfer among the most severe HIV victims as a human-to-human pathogen [2].

The most common modes of infection for Salmonella Typhimurium are by food-borne transmission especially in hospital settings [3]. Many of the most severe infections are believed to be hospital-acquired. Besides patients with severe onset HIV, malnourished children are the next most susceptible group [3].

As well as the above susceptibility there have other links found between these suffering from malaria, sickle cell anemia, and people recently treated with gastric acid suppression and the aqisition of Salmonella Typhimurium.

Incubation/Colonization

Invasive, African Salmonella Typhimurium does not typical cause disease in healthy individuals, but those that are immunocompromised due to other disease (eg HIV) or just for lack of overall nutrition are much more likely to be infected by the pathogen [4]. There is no conclusive data on whether the invasive or gastrointestinal forms of Salmonella Typhimurium are actually two different pathogens or if their modes of transition, colonization, or incubation differ [1].

Epidemiology

Incidence of Salmonella Typhimurium occur largely in Sub-Saharan Africa, with the highest incidence during or just after that areas rainy season. The median ages for those infected are 32 for adults and 22 months for children [5]. Overall there has been no in-depth study into the epidemiology of either invasive and non-invasive non-typhoidal Salmonella strains [1].

Virulence Factors

The most alarming virulence factor of invasive Salmonella Typhimurium is the rapid acquisition of drug-resistance, which hassled to multidrug-resistant “super” strains unaffected by common antibiotics such as ampicillin, cotrimoxazole, and chloramphenicol [4]. Invasive Salmonella Typhimurium also has many similar virulence factors to the traditional variant, these include type III secretion systems, Vi antigen, many surface polysaccharides, and flagella [5].

Clinical Features

Acute gastroenteritis is the most common symptom in infected patients. This causes diarrhea, abdominal cramping, fever, and vomiting. Fever will usually subside in 72 hours, with bloody diarrhea lasting between three and seven days. These effects can be more severe or prolonged in children and the elderly. Bacteremia, or the spread of the pathogen into the blood stream, generally occurs in 5-10% of cases and can lead to more severe symptoms such as meningitis and infections of the bones and joints. This can be especially dangerous in immunocompromised patients such as those suffering from HIV or Malaria [6].

Diagnosis

The classical method for confirming a salmonella infection was by testing for the presence of antibodies using a Widal aggulation test. This method is highly unreliable and in an experiment only tested positive on 38.5% of confirmed salmonella infections [7]. Since then, a new method for detecting antibodies has been developed using an enzyme immunoassay for IgM, IgG, and IgA antibodies. This method uses a commercially available lipopolysaccharaide for S. Typhimurium and S. enteritidis. This method produced a much better 88.5% positive rate of confirmed cases. An alternative culturing method can be used to confirm diagnosis by tablet diffusion on Danish Blood Agar with Rosco Neosensitabs. If this test identifies a nalidixic acid resistance, an E-test can also be used to confirm diagnosis [8].

References

1. Feasey NA, Dougan G, Kingsley RA, Heyderman RS, Gordon MA: Invasive non-typhoidal salmonella disease: an emerging and neglected tropical disease in Africa. Lancet 2012. 379:9835::2489-99.

2. Okoro CK, Kingsley RA, Connor TR, Harris SR, Parry CM, Al-Mashhadani MN, Kariuki S, Msefula CL, Gordon MA, de Pinna E, Wain J, Heyderman RS, Obaro S, Alonso PL, Mandomando I, MacLennan CA, Tapia MD, Levine MM, Tennant SM, Parkhill J, Dougan G: Intracontinental spread of human invasive Salmonella Typhimurium pathovariants in sub-Saharan Africa. Nature Genetics 2012. 44:1215–1221.

3. Morpeth SC, Ramadhani HO, Crump JA. Invasive Non-Typhi Salmonella Disease in Africa. Clin Infect Dis. (2009) 49:4:606-611.

4. Gordon MA, Graham SM, Walsh AL, Wilson L, Phiri A, Molyneux E, Zijlstra EE, Heyderman RS, Hart CA, Molyneux ME: Epidemics of Invasive Salmonella enterica Serovar Enteritidis and S. enterica Serovar Typhimurium Infection Associated with Multidrug Resistance among Adults and Children in Malawi. Clin Infect Dis. 2008. 46:7:963-969.

5. de Jong HK, Parry CM, van der Poll T, Wiersinga WJ: Host–Pathogen Interaction in Invasive Salmonellosis. PLoS Pathog 2012. 8:10.

6. Chen HM, Wang Y, Su LH, Chiu CH: Nontyphoid Salmonella Infection: Microbiology, Clinical Features, and Antimicrobial Therapy. Pediatrics & Neonatology 2013, 54:3:147-152.

7. Isomaki O, Vuento R, Granfors K: Serological Diagnosis of Salmonella Infections by Enzyme Immunoassay. Lancet 1989, 333:8652:1411-1414.

8. Helms M, Vastrup P, Gerner-Smidt P, Mølbak K: Excess Mortality Associated with Antimicrobial Drug-Resistant Salmonella Typhimurium. Emerging Infectious Diseases 2002, 8:5:490-495.