The genus Yersinia includes three species which can cause infectious diseases in humans: Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica.
The genus is named after researcher Alexandre Yersin who discovered the plague pathogen Y. pestis in 1894. Whereas these species are transmitted typically by the bite of fleas which are contaminated with this pathogen and cause the fulminant infectious disease referred to as plague, Y. pseudotuberculosis and Y. enterocolitica are transmitted by the uptake of contaminated food and cause gastrointestinal infections. After Salmonelle and Campylobacters they are today the third most frequent pathogen causing bacterial intestinal diseases (enterititis pathogen) in Germany and Europe.
Significance of Yersinias as pathogens
The medical significance of pathogenic Yersinias in humans has considerably changed over time. Whereas plague influenced the history of mankind through devastating pandemics for many centuries, this infectious disease has played a minor role worldwide as an epidemic cause and no role in Europe. On the other hand, Y. pseudotuberculosis and Y. enterocolitica have become increasingly important since the 1950s in respect of intestinal diseases.
The clinical pictures in humans caused by Y. pseudotuberculosis and Y. enterocolitica are referred to as yersiniosis and can cause a broad range of clinical symptoms which depend on the age and resistance of the patients. After an infection infants have as a rule a self-limiting acute gastrointestinal inflammation (gastroenteritis), whereas school children and teenagers suffer in most cases from a swelling of the lymph nodes (mesenterial lymphadenitis) with unspecific stomach pains. Adults can have “flu infections” with pharyngitis. If there are basic diseases, liver abscesses as well as inflammations of the endocardium, the pericardium or the pleura etc. can occur. Other sequelae without a direct proof of pathogen can be reactive arthritis, pseudo-Crohn’s disease and erythema nodosum (an acute inflammation of the subcutaneous fatty tissue).
Y. pseudotuberculosis is widely spread in the environment but most yersinioses are caused by Y. enterocolitica. The transmission of Y. enterocolitica to humans occurs through contaminated meat, milk or water. Pigs are considered as the main reservoir of Y. enterocolitica and raw or insufficiently cooked pork meat is the main source of infections.
Yersinia are gram-negative, facultatively anaerobic short rods which are allocated to the family of Enterobacteriaceae and can grow in a temperature range of 4°C to 43°C.
In particular the species Y. enterocolitica is highly heterogeneous and is subdivided into several biovars and serovars. Through the reaction of antisera to the O- and H-antigens in the cell wall of the bacteria a subdivision is carried out into more than 70 serovars, whereas the subdivision into the biovars 1 to 5 is carried out through biochemical characteristics. Strains of the biovar 1A and Y. enterocolitica usually lack most of the virulence factors and are therefore considered as non-pathogenic. The strains of the biovars 1B and 2 to 5 have, however, a conserved virulence plasmid and other chromosomally encoded virulence factors such as invasin, ail etc. and therefore are considered as pathogenic.