Yersinia pestis
Introduction
Yersinia pestis is a gram-negative, non-motile, rod-shaped coccobacillus with no spores. It is a facultative anaerobic organism that can infect humans via the oriental rat flea. It causes the disease plague, which caused the Black Death in the middle ages. Yersinia pestis is a very dangerous organism, requiring safety precautions to be taken when being handled.
Biology
Yersinia pestis is a member of the family Enterobacteriaceae. Its genus, Yersinia, is named after Alexandre Yersin, a Swiss-French physician and bacteriologist, who first isolated the bacterium in 1894 during an outbreak of plague in Hong Kong. The species name, pestis, is derived from the Latin word for "plague" or "pestilence".
The bacterium is a facultative anaerobe, meaning it can grow in both aerobic and anaerobic conditions. It is a coccobacillus, meaning it is rod-shaped but with rounded ends, and is gram-negative, meaning it does not retain the violet crystal violet stain used in the Gram staining method of bacterial differentiation.
Pathogenesis
Yersinia pestis is primarily a rodent pathogen, with humans being secondary hosts. The bacteria form aggregates in the gut of infected fleas, preventing the flea from feeding, but not killing it. When the flea bites a human or another mammal to feed, these aggregates are regurgitated into the wound, causing infection.
Once in the body, Yersinia pestis can evade the immune system and establish infection. It does this by producing several virulence factors that interfere with the normal function of the immune system. These include a type III secretion system that injects proteins into immune cells, disrupting their function, and a protein capsule that protects the bacteria from phagocytosis.
Disease
Yersinia pestis is the causative agent of plague, a disease that has caused several major pandemics throughout history. The most well-known of these is the Black Death of the 14th century, which killed an estimated 75-200 million people in Europe.
There are three forms of plague: bubonic, septicemic, and pneumonic. Bubonic plague is the most common form and is characterized by the sudden onset of fever, chills, weakness, and swollen and painful lymph nodes (buboes). If the bacteria spread to the bloodstream, they can cause septicemic plague, which can be fatal if not treated promptly. If the bacteria spread to the lungs, they can cause pneumonic plague, the most severe form of the disease, which can be transmitted from person to person through respiratory droplets.
Treatment and Prevention
Plague is a serious illness, but it can be treated with antibiotics. The earlier treatment is started, the better the chances of a full recovery. Antibiotics such as streptomycin, gentamicin, doxycycline, or ciprofloxacin are commonly used.
Prevention of plague relies on controlling rodent populations and fleas. In areas where plague is common, people are advised to avoid contact with rodents and to use insect repellent to prevent flea bites. There is also a vaccine available for high-risk individuals, such as those working in laboratories with the bacteria.