Plague, caused by Y. pestis, was one of the most devastating diseases in history. Plague has caused large-scale epidemics, thereby changing the course of history in many nations. It has been estimated that up to 200 million people have died from this disease. In the early 20th century, plague epidemics were responsible for about 10 million deaths in India.
Y. pestis shows the following features:
· Y. pestis is a Gram-negative coccobacillus that measures1.5 3 0.7 m in size, with rounded ends and convex sides.
· The bacilli are arranged in single, in pairs, or in short chains. Pleomorphism is very common. Involution forms, such as coccoid, club-shaped, and filamentous forms are seen in old cultures or when grown on an unfavorable medium, such as nutrient agar containing 3% NaCl.
· Giemsa or methylene blue stained smears of clinical speci-mens show characteristic bipolar staining (safety pin appear-ance) with both ends densely stained and a clear central areaof the bacteria.
· Y. pestis is a nonmotile, nonsporing, and non–acid fast butcapsulated bacterium.
Y. pestis is aerobic and facultatively anaerobic. It grows at atemperature range of 2–45°C with an optimum temperature of 27°C (unlike most bacteria), but it grows better at 37°C in culture. The bacteria grow at a wide range of pH (5–9.6), with an optimum pH of 7.2.
Solid media: Y. pestiscan grow on a variety of media includ-ing Mueller–Hinton agar, nutrient agar, blood agar, and MacConkey agar. On nutrient agar, Y. pestis produces small, delicate, and transparent colonies becoming opaque on pro-longed incubation. Blood agar containing sodium azide (7 g/mL) is a selective medium for Y. pestis. Colonies on blood agar are dark brown due to the absorption of the hemin pig-ment. The bacteria grow poorly on MacConkey agar and deoxycholate citrate agar, producing pinpoint reddish colonies after 24 hours of incubation.
Liquid media: In broth,Y. pestisproduces a flocculent growthwith granular deposit at the bottom and along the sides of the tube, with little or no turbidity. On prolonged incubation, a delicate pellicle may form at the surface. Y. pestis produces a characteristic growth when grown in a flask of broth with oil or ghee (clarified butter) floated on top (ghee broth). The growth in the medium appears to hang down into the broth from the surface, resembling stalactites (stalactite growth).
pestis shows following:
· Y. pestis is catalase positive, aesculin positive, and MR positive.
· It is coagulase positive.
· It ferments glucose, maltose, and mannitol with production of acid but no gas. Lactose, sucrose, or rhamnose are not fermented.
· It is oxidase negative, urease negative, and citrate negative.
· It does not reduce nitrate and does not liquefy gelatin. Indole is not produced.
Susceptibility to physical and chemical agents: Y. pestisissensitive to heat, sunlight, drying, and chemical disinfectants. The bacilli are killed at a temperature of 55°C in 30 minutes. They are also killed rapidly by disinfectants, such as 0.5% phe-nol in 15 minutes. The bacilli remain viable for long periods in sealed agar slopes, in frozen tissue, and in cold and moist envi-ronments, such as in the soil of rodent burrows. All strains are lysed by a specific antiplague bacteriophage at 22°C.