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Microbiology · Other clinically important bacteria

Haemophilus ducreyi !

  • Bacterium
  • Gram-negative
  • For completeness

A sexually transmitted bacterium that causes chancroid: painful genital ulcers with inguinal lymph nodes that suppurate.

Updated 2026-11-01

Also called: soft chancre

At a glance

Gram
Gram-negative
Shape
Coccobacillus
Oxygen
Facultative anaerobe, grows best with CO2
Family
Pasteurellaceae
Stain
Coccobacilli in parallel chains ("school of fish")

Morphology

  • Small Gram-negative coccobacillus
  • Lines up in parallel rows on smears
  • Fastidious: needs X factor (haemin) and enriched media

Transmission

  • Sexual contact with the ulcer
  • Small breaks in genital skin or mucosa

Reservoir: Humans only

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Cytolethal distending toxin damages epithelial cells and delays healing
  • Adhesins fix it to damaged skin and ulcer tissue
  • Resists phagocytosis and complement

Diseases it causes

Clinical picture

  • One or more deep, painful genital ulcers with ragged edges and a dirty base
  • Tender inguinal bubo that may drain; present in fewer than half of cases
  • Facilitates HIV transmission

Diagnosis

  • Culture of the ulcer base on special enriched media (rarely available, low sensitivity)
  • Validated PCR in some laboratories
  • Probable diagnosis: typical painful ulcer, syphilis ruled out and herpes testing negative
  • Test everyone with chancroid for HIV and syphilis

Treatment

Macrolides (azithromycin or erythromycin), third-generation cephalosporins (ceftriaxone) or fluoroquinolones (ciprofloxacin). People with HIV and uncircumcised men respond less well. Sex partners are evaluated and treated.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Condoms
  • Treatment of sex partners
  • HIV and syphilis screening

Test yourself

Think of the answer, then tap the question to see it.

What key clinical difference separates chancroid from the syphilitic chancre?

Chancroid hurts and has ragged edges; the syphilitic chancre is painless with a clean, indurated base.

How does H. ducreyi look on a Gram stain of the ulcer base?

As Gram-negative coccobacilli in parallel rows, described as a "school of fish".

Which other tests are ordered for a person with chancroid?

HIV and syphilis tests, because they share the route and the ulcer facilitates HIV.

Which antibiotic families are used against chancroid?

Macrolides, third-generation cephalosporins and fluoroquinolones.

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← Back to the group: Other clinically important bacteria

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Chancroid - STI Treatment Guidelines (CDC, 2021); Jawetz, Melnick & Adelberg's Medical Microbiology, 26th ed., chapter 18: Haemophilus, Bordetella, Brucella, and Francisella (McGraw-Hill, 2013).

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Representative model, for educational purposes only.