Salmonella enterica is a gram-negative bacterium, belonging to the Enterobacteriaceae family. It is one of the two known species of its genus, along with Salmonella bongori.
Six subspecies of S. enterica (S. e. Enterica, S. e. Arizonae, S. e. Diarizonae, S. e. Houtenae, S. e. Indica, and S. e. Salamae) are recognized, including more of 2,500 identifiable serotypes through different antigenic formulas.
Salmonella enterica. Colonies of pathogenic bacteria growing on an agar culture plate
S. enterica is a facultative intracellular pathogen that inhabits the gastrointestinal system of animals and humans. It is the most common etiological agent of diseases transmitted by contaminated food and is one of the four main causes of diarrheal diseases worldwide.
A serotype of the subspecies S. e. enterica produces typhoid fever, identified by the World Health Organization as a serious public health problem, with 11 to 20 million people infected and 128,000 to 161,000 deaths each year. Southwest Asia, Central Asia, some countries in South America, and Sub-Saharan Africa are the most affected regions.
Morphology
S. enterica
The life cycle of S. enterica is fecal - oral. This bacteria mainly inhabits the intestinal tract of humans and other animals. The different serotypes can be specific to a particular host or they can be ubiquitous.
Through the excreta of sick individuals, salmonellae can spread on living surfaces (soil, plants) or inert (water, glass, polymers, metals, etc.), forming biofilms.
These biofilms are made up of aggregations of microorganisms surrounded by a matrix of extracellular polymeric substances and fatty acids that protects them from antimicrobial agents, biocides, chelators and toxins.
This allows them to survive for several weeks in aqueous media and for longer periods in the soil, even if the temperature, humidity and pH conditions are not the most favorable.
A healthy person can be contaminated with S. enterica through the consumption of contaminated water or vegetables irrigated with contaminated water, or by ingestion of food from infected animals, mainly poultry and their eggs, beef or pig meat, dairy products.
Metabolism
These bacteria have a fermentative and oxidative metabolism. They develop optimally in pH conditions between 6.6 and 8.2. They do not tolerate high concentrations of salt.
They are capable of fermenting glucose and other carbohydrates, thereby producing ATP, CO 2, and H 2. They also feed on maltose and maltodextrins.
They are capable of reducing nitrates to nitrites, obtain carbon from citrate, produce H 2 S, and break down hydrogen peroxide into water and oxygen.
They produce colonies of 2 to 3 um in diameter (after 18 to 24 hours), with the exception of some serotypes that produce dwarf colonies.
Pathology
Once S. enterica enters a new host, it begins its cycle of infection through lymphoid tissue. The bacteria adhere to the intestinal epithelial cells of the ileum and the M cells, inducing in them a rearrangement of their cytoskeleton that triggers the formation of large undulations on the surface allowing non-selective endocytosis, for which the bacteria manage to enter the cell.
Likewise, it produces cytotoxic effects that destroy M cells and induce apoptosis in activated macrophages and phagocytosis in non-activated macrophages, for which they are transported to the liver and spleen, where they multiply.
Disease and symptoms
In humans S. enterica can cause two diseases: typhoid fever, caused by S. enterica sub. enterica Paratyphi serotypes or salmonellosis caused by other serotypes.
Typhoid fever is caused by an oral ingestion of at least 10 5 cells of the Paratyphi serotype, which specifically infects pigs. The symptoms of typhoid fever are a constant high fever of 40ºC, profuse sweating, gastroenteritis and diarrhea.
In this type of condition, bacteria attack the mesenteric lymph nodes where they reproduce and lysis of a part of the bacterial population occurs.
Thus, viable bacteria and endotoxins are released through the ganglia, through the bloodstream, generating septicemia and producing inflammatory and necrotic phenomena.
Non-typhoid salmonellosis is caused by ingesting at least 10 9 cells of the ubiquitous serotypes of S. enterica, producing symptoms of diarrhea, vomiting, stomach cramps, and fever.
These symptoms occur 12 to 72 hours after ingestion of contaminated food, last between 4 and 7 days, and most people recover spontaneously.
Treatment
Non-typhoid salmonellosis cases in which symptoms do not resolve spontaneously may require hospitalization. In these cases, hydration of the patient and the replacement of electrolytes lost due to vomiting and diarrhea is recommended.
Antibiotic therapy is not recommended in mild or moderate cases in healthy people, due to the increase in resistance and multi-resistance to antibiotics in Salmonella in recent years.
However, in patients at risk, such as infants, the elderly, immunosuppressed patients and those affected with blood diseases, they may require treatment with antibiotics.
Typhoid fever cases require treatment with antibiotics. Ceftriaxone (a cephalosporin) or ciprofloxacin (a quinolone) is currently prescribed, because resistance to ampicillin, amoxicillin, cotrimoxazole, streptomycin, kanamycin, chloramphenicol, tetracycline, and sulfonamides have commonly developed.
Quinolone resistant varieties have even been reported. In cases of septicemia, dexamethasone has been used.
WHO recommends fine-tuning preventive measures at all stages of the food chain, both in the cultivation, rearing, processing, manufacturing and preparation of food and in commercial establishments and in households, to prevent contamination by S. enterica.
References
- Barreto, M., Castillo-Ruiz, M. and Retamal P. (2016) Salmonella enterica: a review of the agent, host and environment trilogy, and its importance in Chile. Chilean Journal Infectology 33 (5): 547-557.
- Figueroa Ochoa, IM and Verdugo Rodríguez, A. (2005) Molecular mechanisms of pathogenicity of Salmonella sp. Latin American Journal of Microbiology 47 (1-2): 25-42.
- Parra, M., Durango, J. and Máttar, S (2002). Microbiology, pathogenesis, epidemiology, clinic and diagnosis of infections caused by Salmonella. Journal of the Faculty of Veterinary Medicine and Zootechnics of the University of Córdoba 7: (2), 187-200.
- Tindall, BJ, Grimont, PAD, Garrity, GM & Euze´by, JP (2005). Nomenclature and taxonomy of the genus Salmonella. International Journal of Systematic and Evolutionary Microbiology 55: 521–524.
- Todar, K. (2008). Todar's Online Textbook of Bacteriology. Wisconsin, USA. Taken from www.textbookofbacteriology.net/salmonella.html