Salmonella enterica typhi
Worldwide, typhoid fever affects roughly 17 million people annually, causing nearly 600,000 deaths. The causative agent, Salmonella enterica typhi (referred to as Salmonella typhi from now on), is an obligate parasite that has no known natural reservoir outside of humans. Little is known about the historical emergence of human S. typhi infections, however it is thought to have caused the deaths of many famous figures such as British author and poet Rudyard Kipling, the inventor of the airplane, Wilbur Wright, and the Greek Empire’s Alexander the Great. The earliest recorded epidemic occurred in Jamestown, VA where it is thought that 6,000 people died of typhoid fever in the early 17th Century. This disease is rare in the United States and developed nations, but always poses the risk of emergence.
Originally isolated in 1880 by Karl J. Erberth, S. typhi is a multi-organ pathogen that inhabits the lympathic tissues of the small intestine, liver, spleen, and bloodstream of infected humans. It is not known to infect animals and is most common in developing countries with poor sanitary systems and lack of antibiotics, putting travelers to Asia, Latin America, and Africa in a high risk group. Of the 266 people infected in the United States in 2002, approximately 70% had traveled internationally within 6 weeks of the onset of disease.
This gram-negative enteric bacillus belongs to the family Enterobacteriaceae. It is a motile, facultative anaerobe that is susceptible to various antibiotics. Currently, 107 strains of this organism have been isolated, many containing varying metabolic characteristics, levels of virulence, and multi-drug resistance genes that complicate treatment in areas that resistance is prevalent. Diagnostic identification can be attained by growth on MacConkey and EMB agars, and the bacteria is strictly non-lactose fermenting. It also produces no gas when grown in TSI media, which is used to differentiate it from other Enterobacteriaceae.
Typhoid/ Enteric Fever:
Infection of S. typhi leads to the development of typhoid, or enteric fever. This disease is characterized by the sudden onset of a sustained and systemic fever, severe headache, nausea, and loss of appetite. Other symptoms include constipation or diarrhea, enlargement of the spleen, possible development of meningitis, and/or general malaise. Untreated typhoid fever cases result in mortality rates ranging from 12-30% while treated cases allow for 99% survival.
S. typhi has a combination of characteristics that make it an effective pathogen. This species contains an endotoxin typical of Gram negative organisms, as well as the Vi antigen which is thought to increase virulence. It also produces and excretes a protein known as “invasin” that allows non-phagocytic cells to take up the bacterium, where it is able to live intracellularly. It is also...