Other clinically important bacteria !
- Bacteria
Eight bacteria outside the big groups that still show up in exams: anaerobes, bites, seafood, the hospital, Whipple and the non-venereal treponemes.
Updated 2026-11-01
What they are
A practical group, not a taxonomic one: bacteria a student meets in one very specific clinical setting. Each has a classic clue (foul-smelling abscess, painful genital ulcer, cat bite, raw oysters, cystic fibrosis, ICU resistance, PAS-positive macrophages or skin lesions in tropical children).
What they share
- Most are Gram-negative; Tropheryma whipplei is Gram-positive and treponemes do not show well on Gram stain
- Almost all are recognised by context: a bite, seawater, the hospital, cystic fibrosis or rural poverty
- Several carry intrinsic resistance that changes the antibiotic choice (AmpC in Enterobacter, Burkholderia against aminoglycosides and polymyxins)
- None has a vaccine for human use
Microorganisms in this group
- Bacteroides fragilisAn anaerobe of the colon that, once it escapes through a perforation or surgery, forms foul-smelling abdominal and pelvic abscesses.Exam priority: medium
- Haemophilus ducreyiA sexually transmitted bacterium that causes chancroid: painful genital ulcers with inguinal lymph nodes that suppurate.For completeness
- Pasteurella multocidaA bacterium from the mouths of dogs and cats that infects bites and scratches: the skin turns red, swollen and painful within hours.For completeness
- Burkholderia cepacia complex and Burkholderia pseudomalleiSoil and water bacteria: B. cepacia infects the lungs in cystic fibrosis; B. pseudomallei causes tropical melioidosis.For completeness
- Vibrio vulnificus and Vibrio parahaemolyticusSeawater vibrios: diarrhoea from raw shellfish and wound infections that can kill people with liver disease.For completeness
- Enterobacter, Serratia and CitrobacterHospital Enterobacterales that infect patients with urinary catheters, lines or ventilators and pile up antibiotic resistance.For completeness
- Tropheryma whippleiA rare bacterium that causes Whipple disease: joint pain, diarrhoea with malabsorption and weight loss; untreated, it is fatal.For completeness
- Non-venereal treponemes (yaws, bejel and pinta)Relatives of the syphilis agent spread by skin contact among children in poor tropical areas: yaws, bejel and pinta.For completeness
Test yourself
Think of the answer, then tap the question to see it.
Which bacterium of this group is the Gram-negative anaerobe most often isolated from abdominal abscesses?
Bacteroides fragilis, part of the normal colonic flora.
How is chancroid told apart clinically from primary syphilis?
Chancroid gives a painful, ragged ulcer with tender inguinal nodes; the syphilitic chancre is painless with a clean base.
In which patients does Burkholderia cepacia complex cause severe respiratory infection?
People with cystic fibrosis or other chronic lung disease and immunocompromised hospital patients.
What do yaws, bejel and pinta share with syphilis?
They are caused by treponemes that cannot be told apart from Treponema pallidum by morphology or by syphilis serology.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Mixed Anaerobic Infections - Infectious Disease - MSD Manual Professional Edition (MSD Manual Professional Edition, 2025); About Carbapenem-resistant Enterobacterales (CRE) | CRE | CDC (CDC, 2025); Chancroid - STI Treatment Guidelines (CDC, 2021).
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Representative model, for educational purposes only.