SALMONELLA TYPHIMURIUM: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2025

Salmonella Typhimurium is a flagellate, Gram-negative, bacillus-type bacterium whose full name is Salmonella enterica subspecies enterica serovar Typhimurium. It is a facultative anaerobic flagellated unicellular organism, which causes the disease known as salmonellosis, a disease that attacks both humans and other animal species. The epithet Typhimurium means mouse typhus. This bacterium causes a typhus-like disease in mice.

It is included in the Bacteria domain, Proteobacteria phylum, Gammaproteobacteria class, Enterobacteriales order, Enterobacteriaceae family, Salmonella genus, and like other Enterobacteriaceae, it produces an enterotoxin that causes damage to the intestinal mucosa.

SEM colorized photograph showing Salmonella typhimurium (red) invading human cells. Author: Rocky Mountain Laboratories, NIAID, NIH By US gov (File: SalmonellaNIAID.jpg), via Wikimedia Commons

General characteristics

Lifecycle

Salmonella Typhimurium reproduces by bipartition or binary fission. That is, the only cell that makes up this organism divides in two by constriction. In this way, two genetically identical individuals (clones) originate. Bacterial reproduction occurs in the small intestine of the host animal.

Once reproduction has occurred, the bacteria are released into the external environment mixed with the feces. The action of the enterotoxin of the bacterium promotes diarrhea with it providing a means of dispersal for itself.

Outside, feces contaminate various surfaces. Other animals of the same species or others, ingest the bacteria, either by consuming contaminated food or having contact with contaminated matter that is taken to the mouth. In this way, the bacteria reach the small intestine again and continue its life cycle.

Habitat

Salmonella Typhimurium is distributed worldwide. It inhabits various animal hosts: birds, cattle and pigs, rodents, dogs, cats, iguanas, turtles and humans.

It survives in areas contaminated by the feces of these animals. These include water, soil, ceramic, stainless steel, fruit and vegetable surfaces such as tomatoes, chili peppers, melons, among others.

Its optimum temperature is between 35 and 43 ºC. However, it is able to survive low temperatures down to 5.2 ºC and die at temperatures above 46.2 ºC. On the other hand, for their survival they require moisture on the surface where they live.

Disease

Salmonella Typhimurium causes the disease known as salmonellosis or more generically gastroenteritis. The disease attacks humans and other animal species.

It occurs due to the action of the bacteria that live in the small intestine. There it invades the intestinal epithelium and secretes an enterotoxin that penetrates the cells of the intestine. This enterotoxin is 98% similar to the Vibrio cholerae (CT) enterotoxin.

epidemiology

It is estimated that more than 1.3 billion cases of gastroenteritis occur each year in the world. In an average of 3 million cases the disease is fatal. Likewise, the losses in the poultry industry are important.

Pathogenicity

The sequence of events that determine the pathogenesis of Salmonella Typhimurium includes adherence to the epithelial cell surface. Later the invasion of the bacterium into the host cell.

Once inside, it causes metabolic imbalances that cause the activation of the enzyme adenylate cyclase, destruction of villi, secretion of pro-inflammatory cytosines, decreased intestinal absorption capacity, among other effects.

The action of the enzyme adenylate cyclase generates the production of intracellular cyclic adenosine monophosphate, triggering the pumping of large amounts of water and electrolytes through the cell membranes of epithelial cells. The epithelial mucosa of the intestine is also damaged and inflammation occurs. All of this produces diarrhea.

The bacteria are spread in two ways. One occurs when cellular self-destruction mechanisms (apoptosis) are activated, thus freeing the bacteria. It can also move inside macrophages that have been invaded.

Macrophages are cells of the immune system that move through the circulatory system. They are an important part of the immune system with the function of swallowing malignant bacteria. However, Salmonella Typhimurium and other pathogenic bacteria have mechanisms that allow them to be swallowed, but not destroyed.

Salmonella Typhimurium can reach the liver and spleen by an alternate route, which does not require intestinal colonization or invasion of intestinal epithelial cells. This is possible thanks to its ability to colonize macrophages.

Symptoms and development of the disease

From 6 to 72 hours after exposure to the bacteria, symptoms appear. Nausea, vomiting, headache, fever, abdominal pain, and diarrhea occur.

These symptoms last 4 to 7 days. The development of the disease depends on the conditions of the host or patient, the dose ingested and the specific strain of bacteria.

In children, the elderly and immunosuppressed people, special care must be taken, since dehydration can occur that can cause death.

In some cases there may be complications. Infection can become generalized when bacteria invade the blood (septicemia), or an autoimmune reaction can develop and cause reactive arthritis 3 to 4 weeks after the onset of acute symptoms.

Prevention

It is necessary to maintain surveillance and control measures on farms in order to avoid the commercialization of sick animals. Proper hygiene practices must be followed in production structures.

Eating raw or undercooked foods should be avoided. In the case of fruits and vegetables, wash them properly with boiled or filtered water. Consume pasteurized milk and derivatives.

The main thing to prevent contagion is personal hygiene and the areas of food production or consumption. Wash your hands well before and after handling food or before consuming it. Likewise, keep all kitchen work tools clean.

The bacterium is sensitive to heat and to disinfectants commonly used in the food industry. If chlorine is applied for 5 minutes it is enough to eliminate it.

References

1. Ahmer BM, M Tran and F. Heffron (1999) The Virulence Plasmid of Salmonella typhimurium Is Self-Transmissible. Journal of Bacteriology. 181 (4): 1364–1368.
2. Figueroa IM and A Verdugo (2005) Molecular mechanisms of pathogenicity of Salmonella sp. Latin American Journal of Microbiology. 47 (1-2): 25-42.
3. McClelland M, KE Sanderson, J Spieth, S Clifton, P Latreille, L Courtney, S Porwollik J Ali, M Dante, F Du, S Hou, D Layman, S Leonard, C Nguyen, K Rotter, A Holmes, N Grewal, E Mulvaney, E Ryan and R Wilson (2001) Complete genome sequence of Salmonella enterica serovar Typhimurium LT2. Nature. 413: 852-856.
4. Popoff MY and LE LeMinor (2015) Salmonella. In: Whitman WB (editor) Bergey's Manual of Systematics of Archaea and Bacteria. John Wiley & Sons, Inc., in association with Bergey's Manual Trust.
5. National Food Protection Network and National Administration of Medicines, Food and Medical Technology. Ministry of Health of Argentina. Foodborne Diseases. Technical Sheet Nº9: Salmonellosis. Retrieved 2018-11-10. Taken from anmat.gov.ar.
6. Rosenshine I, S Ruschkowski, V Foubister and BB Finlay (1994) Salmonella typhimurium Invasion of Epithelial Cells: Role of Induced Host Cell Tyrosine Protein Phosphorylation. Infection and Immunity. 62 (11): 4969-4974.