# Yersinia Species > Yersinia species for MBBS Microbiology: plague Wayson stain, flea vector, Surat 1994, pseudoappendicitis, cold enrichment and CIN agar. - Canonical URL: https://prepelephant.com/topics/mbbs/microbiology/yersinia-species-micro - Exam / course: MBBS · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Yersinia Species", PrepElephant, https://prepelephant.com/topics/mbbs/microbiology/yersinia-species-micro ## 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.