Yersinia Species
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Direct answer
Three species, three syndromes — plague, mesenteric adenitis and pseudoappendicitis — make Yersinia a genus of three separate exam questions. Yersinia pestis, the plague bacillus, is a non-motile, pleomorphic gram-negative coccobacillus showing bipolar safety-pin staining with Wayson or methylene blue; it cycles between rodents and the rat flea Xenopsylla cheopis, whose blocked proventriculus regurgitates thousands of bacilli at each bite, producing bubonic, septicaemic and person-to-person pneumonic plague. Yersinia enterocolitica and Y. pseudotuberculosis are food- and water-borne enteric pathogens famous for mesenteric adenitis and terminal ileitis that mimic appendicitis, for growth at refrigerator temperature (cold enrichment) and for post-infectious erythema nodosum and reactive arthritis. India's reference point is the 1994 Surat pneumonic plague outbreak, a modern demonstration of how fast foci can re-emerge.
What you must remember
- Y. pestis morphology: ovoid gram-negative coccobacillus, marked bipolar staining (safety-pin) with Wayson stain; non-motile at both 25 and 37 degrees — the species-level differentiator from other yersiniae, which are motile at 25 degrees but not at 37.
- Culture characters: slow growth, pinpoint colonies at 48 hours on blood agar, colourless on MacConkey, better at 28 degrees; a laboratory should suspect plague from rough, fried-egg-like colonies and notify immediately.
- Virulence determinants: F1 capsular antigen (antiphagocytic, expressed at 37 degrees), V and W antigens, the plasmid-encoded Yop type III secretion system injecting anti-phagocytic effectors, plasminogen activator and coagulase — the machinery for the blocked flea and for overwhelming intracellular evasion.
- Plague epidemiology: enzootic rodent cycles with epizootic spread through rats; the oriental rat flea Xenopsylla cheopis; human bubonic plague from flea bite (inguinal or femoral bubo), pneumonic plague by droplet spread with near-universal fatality untreated.
- Clinical forms: bubonic (fever with exquisitely tender bubo), septicaemic (DIC, purpura — the "black death" name), pneumonic (high fever, haemoptysis, rapid death) — treatment is streptomycin (alternatives gentamicin, doxycycline, chloramphenicol), with isolation and prophylaxis of pneumonic contacts.
- Y. enterocolitica biology: psychrotrophic — multiplies at 4 degrees, enabling cold enrichment in phosphate-buffered saline and transfusion-associated sepsis from refrigerated red cells; selective CIN (cefsulodin-irgasan-novobiocin) agar produces bullseye colonies; iron-dependent virulence worsens in iron overload and desferrioxamine therapy.
- Y. enterocolitica disease: children get watery or bloody diarrhoea with fever and abdominal pain; adolescents get pseudoappendicitis (mesenteric adenitis, terminal ileitis) — a normal appendix at surgery; post-infectious erythema nodosum and HLA-B27-linked reactive arthritis follow.
- Indian anchor: the Surat 1994 pneumonic plague outbreak — hundreds of cases, national panic and trade losses — remains the country's modern plague event, with natural foci historically recognised in the western states.
The appendicectomy that was not needed
An eighteen-year-old presents in winter with right iliac fossa pain, low-grade fever, vomiting and diarrhoea starting days earlier — and is listed for appendicectomy, which finds a normal appendix, enlarged mesenteric nodes and an inflamed terminal ileum. This is Yersinia enterocolitica mesenteric adenitis, the classic pseudoappendicitis, and the recognition saves the appendix — the surgeon samples a node, closes, and the laboratory (informed in advance, cold-enriching the stool and plating on CIN agar) confirms the diagnosis days later. The story often continues: erythema nodosum over the shins weeks later, or reactive arthritis in an HLA-B27-positive young man.
The cold-growth biology explains the second vignette: a patient transfused with red cells stored for weeks develops fatal sepsis with a gram-negative rod that grew unnoticed at 4 degrees — Y. enterocolitica, whose psychrotrophy turns the blood bank into an incubator.
Where students slip
The bipolar-staining organism is reported "on Gram stain" — the safety-pin look needs Wayson or Loeffler methylene blue, a viva correction worth remembering alongside the fact that Burkholderia and Pasteurella share bipolar morphology. Second slip: mobility — students forget that Y. pestis is non-motile while the enteric yersiniae are motile at 25 but not 37 degrees, the differential test that separates the genus internally. Third, the flea mechanism gets glossed: blocking of the flea proventriculus by a fibrinoid biofilm (linked to the hms locus and murine toxin) makes the flea regurgitate rather than swallow, which is why blocked fleas transmit efficiently and starve; "flea bites therefore plague" earns no marks without the block. Finally, on Surat 1994, keep facts conservative — pneumonic plague with substantial mortality and massive disruption; contested case counts are not examinable.
Frequently asked questions
Which stain demonstrates the bipolar safety-pin appearance of Yersinia pestis?
Wayson stain (or Loeffler methylene blue) shows the dark-staining poles against a pale centre that give the plague bacillus its safety-pin look, best seen in material from a bubo or sputum.
How does the rat flea transmit plague efficiently?
Yersinia pestis forms a biofilm that blocks the flea's proventriculus; the starving flea regurgitates bacilli into the bite wound while trying to feed, inoculating thousands of organisms.
Why is Yersinia enterocolitica linked to pseudoappendicitis?
It causes mesenteric lymphadenitis and terminal ileitis that clinically mimic acute appendicitis, so surgery not infrequently finds a normal appendix — the diagnosis is then made on node culture and serology.
What is cold enrichment, and when is it used?
Incubating stool or food specimens in buffer at 4 degrees Celsius exploits the psychrotrophy of Yersinia enterocolitica, suppressing other flora while the pathogen multiplies over one to three weeks.
What was significant about the Surat 1994 outbreak?
A pneumonic plague outbreak in Gujarat — person-to-person respiratory spread of Yersinia pestis — causing deaths, mass migration and international trade restrictions, and reawakening Indian plague surveillance.