MicrobiologyTier 1Medical Sciences concept

Organism prediction from clinical vignette (endocarditis, line infection)

Core concept

Exam vignettes describe a clinical context (valve type, line duration, patient population) and expect the most likely causative organism to be named, not a broad differential
Endocarditis organism by context: native valve, community -- viridans streptococci (subacute, dental procedure history) or Staph aureus (acute, more virulent, IVDU/skin source); prosthetic valve, early (<2 months post-op) -- coagulase-negative staphylococci (e.g. S. epidermidis, implanted at surgery); prosthetic valve, late -- similar organisms to native valve endocarditis
IV line infection: coagulase-negative staphylococci most common overall; Staph aureus for more aggressive/higher-severity presentations; Candida in patients on prolonged broad-spectrum antibiotics, TPN, or significant immunosuppression

Context	Most likely organism
Native valve, subacute, dental history	Viridans streptococci
Native valve, acute, IVDU	Staph aureus (often tricuspid valve in IVDU)
Prosthetic valve, <2 months post-op	Coagulase-negative staphylococci
Culture-negative endocarditis	HACEK organisms, Coxiella, Bartonella (fastidious/atypical)
Central line, general	Coagulase-negative staphylococci
Central line + TPN/prolonged antibiotics	Candida species
IVDU, tricuspid valve	Staph aureus

IVDU endocarditis classically involves the tricuspid valve (right-sided), unlike most other endocarditis which is left-sided (mitral/aortic) -- a frequently tested discriminator

Empirical antibiotic choice while awaiting cultures is directly guided by this pattern-matching: e.g. early prosthetic valve endocarditis empirical cover must include activity against coagulase-negative staphylococci (often methicillin-resistant), unlike typical native valve empirical regimens
Trap: assuming all endocarditis is subacute/streptococcal -- an acute, rapidly progressive presentation with embolic phenomena points to Staph aureus and changes both urgency and empirical antibiotic choice
Culture-negative endocarditis should prompt specific serology/PCR testing for fastidious organisms (Coxiella burnetii -- Q fever, Bartonella) rather than simply broadening standard antibiotic empirical cover

Prosthetic valve timing (early vs late) parallels the post-transplant infection timeline logic -- organism likelihood shifts with time since a foreign material/surgical event, not randomly
Candida line infection risk factors overlap with those for invasive fungal infection generally (neutropenia, broad-spectrum antibiotics, central access, TPN)
HACEK organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) are the classic fastidious Gram-negative endocarditis causes requiring extended culture incubation to identify

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