What is Salmonella?

Salmonella is a ubiquitous human and animal pathogen which causes almost a hundred million cases of gastroenteritis each year throughout the world. Salmonellosis in humans usually presents as self-limiting food poisoning (gastroenteritis), albeit it can occasionally manifest as a severe systemic infection (enteric fever) that requires swift antibiotic treatment.

Most cases of salmonellosis in humans occur after the consumption of contaminated food products such as poultry meat, pork, beef, eggs, vegetables, juices, and other types of foods. Therefore, prudent use of antimicrobials in human medicine and agriculture is pivotal in minimizing the emergency aspects and spread of resistant Salmonella.

3D illustration of Salmonella Bacteria. Image Credit: urfin / Shutterstock3D illustration of Salmonella Bacteria. Image Credit: urfin / Shutterstock

Structure and virulence factors

Salmonellae are Gram-negative, flagellated, facultative anaerobic bacilli that contain three significant antigens: H or flagellar antigen, O or somatic antigen, and Vi antigen (possessed by only a few serovars). In addition, the cell envelope contains a complex lipopolysaccharide (LPS) structure that is released upon lysis of the cell.

Salmonella is a prime example of an intracellular bacterium; thus, it can be found within various phagocytic and non-phagocytic cells in the human body. After successful intestinal colonization, this organism enters enterocytes, M cells and dendritic cells in the intestinal epithelium, subsequently reaching the submucosa and disseminating through the bloodstream.

Virulence factors of salmonellae are encoded on so-called pathogenicity islands found on the chromosome. Invasion of host cells and intracellular survival are dependent on two type III secretion systems. Still, other virulence factors (such as ion transporters and superoxide dismutase) also play a role in exploiting the intracellular niche.

Salmonella can also form complex surface-associated communities (also known as biofilms), which contribute to its persistence in host and non-host environments and is especially important in food processing environments. Fimbriae, flagella, cellulose, colanic acid, anionic O-antigen capsule, and fatty acids are important structural components for biofilm formation.

Clinical presentation

Species and serovars of the Salmonella genus are responsible for two distinct syndromes. Nontyphoidal salmonellae are common causes of food-borne gastroenteritis in the community. Typhoidal Salmonella causes enteric fever, which represents an important differential diagnosis in any traveler returning from a tropical country with a fever.

Nontyphoidal salmonellosis is caused by ingesting contaminated food or water or occasionally by contact with an infected person. Symptoms occur 6-72 hours after ingestion of pathogenic organisms, and the illness lasts between 2 and 7 days. Most cases are self-limiting, and the focus of management is adequate rehydration.

Although antibiotic therapy is seldom warranted, some important exceptions include neonates, immunocompromised individuals, and (in certain instances) the elderly. Enteric fever always necessitates antibiotic treatment, with the notification to the appropriate health authority to investigate possible outbreaks.

Mayo Clinic Minute: Know the symptoms of salmonella infection

Diagnosis

Identification of salmonellae in the microbiological laboratory is not demanding. Direct plating and inoculation of standard enrichment broths are employed to recover Salmonella species from fecal samples. Rectal swabs are not used, as they are considered inferior to fecal specimens.

Many selective agar plates are available for Salmonella; MacConkey agar has low selectivity, whereas xylose lysine deoxycholate (XLD) agar or Hektoen enteric agar have higher selectivity. More-selective chromogenic agars are sometimes used, and salmonellae grow well on automated systems for the culture of blood samples.

The biochemical reactions of suspicious colonies are determined on triple sugar iron agar and lysine-iron agar, with a presumptive identification being made. Such biochemical identification can then be confirmed by antigenic analysis of O and H antigens with the use of polyvalent and specific antisera.

Polymerase chain reaction (PCR) and plasmid profiling can be used for the identification and typing of Salmonella isolates. In addition, phage typing and restriction fragment length polymorphism analyses are convenient tools to investigate Salmonella outbreaks and conduct case-control studies.

Sources

  1. http://cid.oxfordjournals.org/content/36/1/112.full
  2. http://cid.oxfordjournals.org/content/32/2/263.long
  3. http://www.who.int/mediacentre/factsheets/fs139/en/
  4. http://onlinelibrary.wiley.com/doi/10.1016/j.femsim.2005.02.002/pdf
  5. www.oie.int/.../2.09.09_SALMONELLOSIS.pdf
  6. Cooke FJ, Threlfall EJ, Wain J. Current Trends in the Spread and Occurrence of Human Salmonellosis Molecular Typing and Emerging Antibiotic Resistance. In: Rhen M. Salmonella: Molecular Biology and Pathogenesis. Horizon Scientific Press, 2007; pp. 1-30.

Further Reading

Last Updated: Apr 22, 2022

Dr. Tomislav Meštrović

Written by

Dr. Tomislav Meštrović

Dr. Tomislav Meštrović is a medical doctor (MD) with a Ph.D. in biomedical and health sciences, specialist in the field of clinical microbiology, and an Assistant Professor at Croatia's youngest university - University North. In addition to his interest in clinical, research and lecturing activities, his immense passion for medical writing and scientific communication goes back to his student days. He enjoys contributing back to the community. In his spare time, Tomislav is a movie buff and an avid traveler.

Citations

Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    Meštrović, Tomislav. (2022, April 22). What is Salmonella?. News-Medical. Retrieved on August 14, 2022 from https://www.news-medical.net/health/What-is-Salmonella.aspx.

  • MLA

    Meštrović, Tomislav. "What is Salmonella?". News-Medical. 14 August 2022. <https://www.news-medical.net/health/What-is-Salmonella.aspx>.

  • Chicago

    Meštrović, Tomislav. "What is Salmonella?". News-Medical. https://www.news-medical.net/health/What-is-Salmonella.aspx. (accessed August 14, 2022).

  • Harvard

    Meštrović, Tomislav. 2022. What is Salmonella?. News-Medical, viewed 14 August 2022, https://www.news-medical.net/health/What-is-Salmonella.aspx.

Comments

  1. AFFS Agrotech AFFS Agrotech Taiwan says:

    Where are the antibiotics from?

    It is the panacea to bacterial disease when first introduced in here. However, the phenomenon of antibiotic abuse is devastated everywhere in the world now. Around 50% of antibiotics are used in raising livestock and poultry.

    The pollution-free agricultural mechanism is to establish antibiotic-free animal farming system, and then use its clean animal’s feces as the fertilizer for agricultural planting. In this way, we can stop continuing pollution from the wastes of farm animals and farm crops. And moreover it can let the environment and ecology restored.

    AFFS means Antibiotic-Free Farming System which is an innovative animal farming system without using any antibiotics and synthetic drugs during the entire farming process.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
Post a new comment
Post
You might also like...
Bacteria causing typhoid fever are highly developing antibiotic resistance