Yersinia

Cause disease in animals and are important zoonoses
10 species of which Y. pestis, Y. pseudotuberculosis and Y. enterocolitica are pathogenic to animals and humans; Y. pestis is the most pathogenic
Rodents provide a reservoir of Y. pestis, which is the cause of human plague; fleas transmit the infection to other animals and humans
Y. pseudotuberculosis and Y. enterocolitica reside in the intestine of domestic and wild animals and birds
Birds may cause mechanical transfer of the organisms

Enterobacteria, but grow more slowly and at lower temperatures than other enterobacteria
Gram negative, non-spore forming, facultative anaerobes - rods or colibacilli
Non-lactose fermentors
Facultative intracellular pathogens
Show bipolar staining in Giemsa-stained smears from animal tissue
Pathogenic strains identified by serotyping and biotyping

Y. enterocolitica and Y. pseudotuberculosis enter the intestinal mucosa via M cells of the Peyer's patches
Engulfed by macrphages in the mucosa
All three invasive species are facultative intracellular organisms and grow inside macrophages
Plasmid and chromosomal-encoded virulence factors required for survival and multiplication in macrophages
Survive in phagolysosomes and do not interfere with degranulation or lysosomal fusion
Resistant to macrophage killing mechanisms
Antiphagocytic proteins secreted by the organisms interfere with host neutrophils
Y. pestis is more invasive than the other species and also possesses and antiphagocytic capsule and a plasminogen activator which aids systemic spread; endotoxin also contributes to its pathogenicity
Transport within macrophages to mesenteric lymph nodes
Replication in lymph nodes and development of necrotic lesions, with neutrophil invasion
The bacteria destroy the macrophages causing septicaemia

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