Yersinia pestis :: Biology Biological Health Medical Essays

Yersinia pestis Life History Yersinia pestis is the causative agent of the systemic invasive infectious disease often referred to as the plague. The Y. pestis is an extremely virulent pathogen that is likely to cause severe illness and death upon infection unless antibiotics are administered. In the past, Y. pestis has caused devastating epidemics during three periods of modern history; the Justinian Plague spread from the Middle East to the Mediterranean during the 6th-8th centuries AD and killed approximately 25% of the population below the Alps region. Perhaps the most famous incidence of any disease was the devastating Black Plague of 8th-14th century Europe that eradicated 25 million people (nearly 25% of the population) and marked the end of the Dark Ages. The third endemic began in 1855 in China and was responsible for millions of deaths. Microbiological Characteristics Yersinia pestis is a Gram-negative, bipolar-staining coccobacillus member of the Enterobacteriaceae family, and is an obligate intracellular pathogen that must be contained within the blood to survive. It is also a fermentative, motile organism that produces a thick anti-phagocytic slime layer in its path. Transmission Y. pestis has the ability to cause disease in rodents, insects and humans. The primary carriers of the pathogen are the Oriental rat flea, Xenopsylla cheopis, and infected rodents. The path of transmission to humans usually involves a flea feeding on an infected rodent and becoming a carrier of infection. Once internalized, the bacteria will continue to reproduce until a large blockage is formed in the midgut of the flea, causing digestion and other gastrointestinal functions to cease. When the flea attempts to feed on humans, the blockage inhibits any blood from entering the stomach cavity; instead, portions of the blockage, often containing 11,000-24,000 bacilli, are regurgitated into the mammalian host. Virulence Factors Yersinia pestis encodes two antigenic molecules: Fraction 1 (F1) capsular antigen, and VW antigen. Both of these molecules are needed for pathogenicity, and are not expressed at temperatures lower than 37Â °C. This requirement is the main reason why Yersinia is not virulent in fleas, since their body temperature normally levels around 25Â °C. Yersinia is a model for studying Type III Secretion Systems (TTSS) that inject bacterial proteins into a host cell. In Y. pestis, it is the translocation of Yersinia outer proteins (Yop’s) that blocks the host cell’s ability to communicate with immune system cells and down-regulates the response of phagocytic host cells to infection.