©Copyright 1999 Microbiology Department, Mount Sinai Hospital, Toronto, ON, Canada.
All rights reserved. Last Modified
 
 
Salmonella enterica
  The Bug | Disease | Prevention | Links     

The Bug


What is it?

Salmonella typhimurium DT104 (S.t. DT104) is a member of the Enterobacteriaceae family, species Salmonella. The first strain of Salmonella was discovered and reported in 1885 by D.E. Salmon, a veterinary surgeon. Today there are 2213 known serotypes, grouped in one species Enterica, and divided in six subspecies including Enterica. Some of them infect only specific hosts (e.g. as single animal species or group), but serotype Typhimurium can infect a wide variety of animals as well as humans.

S. t. DT104 is one of the clonal groups of Typhimurium. The formal scientific name for this organism is Salmonella enterica subspecies enterica serotype Typhimurium DT104. Identification of this particular strain is a multi-step process which may include serogrouping, serotyping, phage typing, antibiograms, plasmid profile and plasmid compatibility grouping, PFGE (pulsed field gel electrophoresis), and FAME (fatty acid methyl ester) analysis. The first known human S. typhimurium DT 104 isolate was identified in Canada in 1970 and was fully sensitive to all antimicrobial agents tested. It is a gram negative rod, not motile, aerobic and facultatively anaerobic. Present strains have become resistant to ampicillin, chloramphenicol, streptomycin, sulfonamides and tetracycline. There are reports of increasing numbers of isolates with additional resistance to trimethoprim and ciprofloxacin. This will further complicate the options available in the treatment of Salmonella infections. This antibiotic resistance appears to have resulted from the widespread use of antibiotics for treatment, prophylaxis and growth promotion in farm animals.

The genes encoding antibiotic resistance in S.t. DT104 are chromosomally integrated (as opposed to being plasmid-mediated). Chromosomal integration is a mechanism by which bacteria can retain resistance to certain antibiotics permanently, even when there is no longer the selective pressure from specific antibiotics. Therefore, withdrawing the use of the antibiotic agents will not reverse the resistance in a specific bacterial strain. Also the presence of two class I integrons makes possible the simultaneous dissemination of resistance to multiple antibiotics into different species of gram negative bacteria.

Salmonella typhimurium DT104 is the second most common Salmonella species isolated from humans, second only to Salmonella Enteritidis phage type 4.


Where is it found?

The first report of epidemics caused by Salmonella typhimurium DT104 in livestock and humans came from Great Britain in 1984. In 1990, there were 259 cases of human disease caused by S.T.DT104. In 1996, there were 4006. S. t. DT104 is emerging as a foodborne pathogen, having been detected in many areas of the world including the United States, Canada, Germany, France, Italy, Austria, Denmark, Saudia Arabia and Southern Asia. Although precise information on the incidence of S.t. DT104 is not available, several countries have reported increases similar to the United Kingdom occurring at approximately the same time . Antibiotic resistant organisms do not respect national boundaries. The ease and rapidity of modern travel and the expanding global food market allows pathogens to be disseminated from their point of origin very quickly. In Canada, the first strain of penta-resistant S.t. DT104 was identified in 1989 and the numbers of isolates have increased since then.


How is it transmitted?

Humans acquire infection primarily through consumption of contaminated animal food products. Multi-drug resistant Salmonella typhimurium DT104 can be found in a broad range of foodstuffs. Outbreaks in the United Kingdom and Northern Ireland have been linked to poultry, a variety of meats and meat products, and unpasteurized milk. Foods may be contaminated in different ways: meat by feces during the slaughter process, animals with bacteria in its blood at the time of slaughter; milk by feces during the milking process, or by mastitis in the milked cow.

In addition to acquiring infection from contaminated food, human cases have also occurred where individuals have had contact with infected cattle. A small proportion of cases may have contracted infection from pets such as cats and dogs, which can also be infected with this strain of Salmonella. Pets probably acquire the infection like humans, in other words through consumption of contaminated raw meat, poultry or poultry-derived products. Cats have been reported to shed S. typhimurium DT104 in feces for twelve weeks or longer after recovery from an acute illness. The cycle may be perpetuated in a barn setting where cats contaminate animal feeds.

Finally, humans can get the disease by fecal-oral transmission from another infected person.


Who/what is at risk of infection?

The persons most at risk of disease caused by Salmonella typhimurium DT104 are persons at the extremes of age; the young and the elderly. The very young have poorly developed microbial gut flora (which is protective in older children and adults) while the elderly may have waning immune systems or weak gastric acidity. Most cases are reported in children less than four years old. However, immunosuppressed individuals (e.g. those taking anticancer/cytotoxic drugs or high-dose steroids, or those with HIV infection) are at increased risk of infection. In addition, people who have consumed raw and undercooked animal products, unpasteurized milk, and people caring for infected pets with this strain of Salmonella are at greater risk of infection with this organism. In the UK in 1995, it was also demonstrated that persons in close contact with farm animals infected with S. t. DT104 (e.g. food producers, livestock handlers) were at greater risk of acquiring S.t. DT104 infection than the general population.


 

© Copyright 1999-2007 Department of Microbiology, Mount Sinai Hospital, Toronto, Canada. All rights reserved.
This website has been made possible through an unrestricted educational grant from Pfizer Canada Inc.
© Copyright 1999-2007 Department of Microbiology, Mount Sinai Hospital, Toronto, Canada. All rights reserved.