Microbiology Disease Report: Salmonella Typhi

Salmonella, is a genus of bacteria known to result in major foodborne illnesses throughout the world. The genus, “Salmonella” was originally named after Dr. Daniel Elmer Salmon, a veterinary pathologist who worked for The United States Department of Agriculture (USDA). He is most notably known for finding Salmonella in hogs that carried the disease hog cholera in the 1800s. Conversely, Karl Joseph Eberth, a student of well-known scientist Rudolf Virchow, was actually the individual who discovered Salmonella. He discovered the bacillus in the abdominal lymph nodes and spleen in 1879. Robert Koch, another well-known scientist, amongst other German and English bacteriologists, later confirmed his discoveries. The most common species, which are pathogenic to humans, is Salmonella enterica with more than 2,500 serotypes being identified within the species (Goay et al. 1).

Salmonella is a member of the family Enterobacteriaceae. The genus Salmonella is divided into two species, S. enterica and S. bongori. Furthermore, Salmonella enterica can be broken down into two sub-classifications based on the serotype. This is done by separating two groups between the species based on the microorganism having the same set of antigens on the surface: either typhoidal or nontyphoidal. Salmonella typhi is a subspecies of Salmonella enterica that causes typhoid fever, a life-threatening systemic infection in humans. S. typhi is a multi-organ pathogen whose main survival reservoir is humans. It affects the liver, spleen, bloodstream and the lymphatic tissues of the small intestine. Individuals commonly affected by typhoid fever are travelers visiting developing countries with poor sanitary systems and lack of antibiotics, such as countries in Asia, Latin America and Africa. Infection by S. typhi leads to the development of typhoid (enteric) fever as previously stated.

According to the Centers for Disease Control and Prevention (CDC), symptoms include a sudden and sustained fever, severe headache, nausea, and loss of appetite in addition to constipation or diarrhea and abdominal cramps. Worldwide, typhoid fever causes 269,000 deaths from 26. 9 million new cases each year, with the bacteria mainly targeting travelers, children, immune-compromised individuals and the elderly (Goay et al. 1). Upon infection, symptoms generally appear within 12-72 hours with an approximate time of 7-14 days of treatment. S. typhi is a gram-negative, rod-shaped bacillus, that is also motile and a non-lactose fermenter.

Additionally, this gram-negative bacillus is a facultative anaerobe and is susceptible to many antibiotics. Over 100 strains of this bacillus have been discovered and isolated. Unfortunately, not all strains have susceptibility as some have varying metabolic characteristics, multidrug resistance genes and levels of virulence that make it harder to treat strains in which resistance is prevalent. As a result, it is important to correctly identify and diagnose typhoid fever to reduce morbidity and mortality as well as to prevent transmission of the disease. S. typhi carries many different characteristics that make it an effective pathogen and one that is difficult to kill. S. typhi produces and secretes a protein called “invasin” that allows non-phagocytic cells to carry the bacterium. By doing so, the bacterium is able to live intracellularly.

In addition, a common trait amongst Gram-negative organisms is that it carries an endotoxin as well as the Vi capsular antigen, which reduces inflammatory responses in the intestinal tissues (Tran et al. 527). Identification of S. typhi can be obtained via gram staining, motility test and MacConkey/EMB agar plates. Drinking or eating contaminated food or water containing the bacteria develops typhoid fever. After ingesting the contaminated food or water, the S. typhi bacterium enters the small intestine and blood stream temporarily. As a result, symptoms of diarrhea, constipation and abdominal cramps can develop. Additionally, S. typhi is carried by white blood cells to the liver, spleen and bone marrow where it multiplies. Following this attack by the bacteria, many individuals will develop a fever amongst other fever-related symptoms.

Finally, the bacterium invades the gallbladder, biliary system and lymphatic tissue of the bowels where it multiplies again. There are two vaccines available to prevent typhoid fever, an inactivated vaccine (shot) and a live vaccine (oral pills). An inactivated vaccine is given in two doses. The first dose provides protection and should be given two weeks prior to travel, as traveling is when most individuals are susceptible to the S. typhi bacteria, and a booster shot should be given every two years for those who remain at risk. The live typhoid vaccine is given orally and consists of four doses, each dose consisting of one capsule every other day of the week. A booster dose is given every five years if the individual remains at risk. The most common method of acquiring S. typhi is drinking water that has been tainted with the feces and urine of infected individuals. Typically, it requires an approximate inoculum size of 100,000 to cause an infection (Tran et al. 529).

In addition, an encounter with an infected individual of poor hygiene may also lead to transmission of the bacteria. Furthermore, any consumption of shellfish from polluted bodies of water may also result in acquisition of the bacteria. The best method of avoiding infection by S. typhi is prevention of fecal contamination of food and drinking water. Proper hygiene, water purification, waste management and proper treatment of the sick can control this since the only source of this agent is infected humans. Due to proper methods instilled to avoid food and water contamination, outbreak of typhoid fever has gone down tremendously in countries with proper sanitary systems.

Additionally, the production of a vaccine has aided and attributed to the low prevalence of typhoid fever in developed societies, whereas lack of a sanitary system and proper antibiotics in developing countries has resulted in more cases of S. typhi transmission. Typhoid fever was a major player in outbreaks during the 1900s, however with proper methods of sanitation there has been a decline in the number of substantial outbreaks and epidemics. The development of vaccines as well as sanitary procedures has helped to eradicate, or at least control, Salmonella as a whole. Although, incidences in the United States remain low, there is always a possible chance of an outbreak due to worldwide traveling and carriers of the disease. As a result, it is important to assume that typhoid fever is a threat and to always implement preventative measures wherever possible.