Bartonella henselae
Fastidious Gram-negative facultative intracellular bacillus transmitted by cat scratches or bites, causing cat-scratch disease, bacillary angiomatosis, and peliosis hepatis.
Organism Card
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Exam Intelligence
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Bartonella henselae is explicitly listed as a cause of culture-negative endocarditis in GC 101 lecture handout alongside Coxiella burnettii, Chlamydia, Brucella, and Tropheryma [1]. The lecture specifically instructs students to "contact microbiologists if special organisms are considered" and request serology + PCR for these agents when blood cultures are negative in suspected endocarditis.
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In the IE / Fever-and-a-murmur lecture notes, Bartonella is grouped under "fastidious organisms" as a cause of culture-negative endocarditis, alongside HACEK, Mycoplasma, and Abiotrophia [2]. This is the framing most likely to appear in exam questions — a stem describing subacute endocarditis with negative blood cultures and a history of cat contact.
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GC 101 handout emphasises that ≥ 3 sets of blood cultures should be taken for suspected endocarditis, and that for culture-negative cases, serology and nucleic acid amplification assay (PCR) are the key next-step investigations [1].
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Ryan Ho Ophthalmology notes list Bartonella henselae as a cause of posterior uveitis under "bacterial (atypical)" infections [3]. This is an important cross-specialty hook — a vignette with cat scratch + visual loss + macular star = Bartonella neuroretinitis.
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The Modified Duke's Criteria minor criterion "atypical organisms in blood culture" can include Bartonella when serological evidence is obtained — functionally Bartonella endocarditis often requires the serological criterion to meet Duke's criteria [2].
- Culture-negative endocarditis differential: The top organisms to list are Bartonella, HACEK group, Coxiella burnetii (Q fever), Brucella, Chlamydia, Tropheryma whipplei, and fungi. The most common reason for culture-negative endocarditis overall is prior antibiotic therapy (partially treated) [2]. Among true fastidious organisms, Bartonella and Coxiella are the most commonly tested.
- Bartonella vs HACEK: HACEK organisms will eventually grow on prolonged culture (5–7 days on enriched media); Bartonella requires specialised lysis-centrifugation culture or serology/PCR. Both cause large vegetations and subacute presentation.
- BA vs Kaposi sarcoma: Both present as red-purple vascular papulonodular lesions in AIDS. Key discriminator: BA is caused by Bartonella and responds to erythromycin/doxycycline; KS is caused by HHV-8 and requires chemotherapy/antiretrovirals. Histologically BA shows neutrophilic inflammation and bacilli on Warthin-Starry stain; KS shows spindle cells.
- Cat scratch disease vs lymphoma: CSD causes regional (not generalised) lymphadenopathy, usually axillary or cervical, with a history of cat exposure. A lymph node biopsy showing granulomatous inflammation with microabscesses favours CSD over lymphoma.
No past paper question directly examining Bartonella henselae as the primary answer was identified in the indexed Fourth Summative papers (2016–2025). However, Bartonella is examinable as a listed answer in culture-negative endocarditis stems or Modified Duke's Criteria questions, which appear repeatedly in the IE/cardiology question pool. Students should be prepared to list it among fastidious causes of culture-negative endocarditis if such a question arises.
Brucella
Facultative intracellular, small Gram-negative coccobacillus transmitted from animals to humans, causing brucellosis—a systemic febrile illness characterized by undulant fever, hepatosplenomegaly, and osteoarticular complications.
Bacteroides fragilis
Encapsulated obligate anaerobic Gram-negative bacillus that is the most common anaerobic pathogen in humans, causing intra-abdominal abscesses, peritonitis, and bacteremia.