Bacteriology: Core Concepts for the ACVM Candidate
What you must know
Bacteriology questions reward a structural framework: cell wall architecture (Gram-positive thick peptidoglycan with teichoic acids; Gram-negative thin peptidoglycan plus an outer membrane bearing lipopolysaccharide), growth and metabolism (aerobes, facultative anaerobes, microaerophiles, obligate anaerobes; fastidious organisms requiring enriched media), and taxonomic grouping by morphology, staining, hemolysis, and biochemical profile. Know the classic veterinary pathogens by host and syndrome: Staphylococcus (pyoderma, mastitis), Streptococcus (strangles, mastitis), Escherichia coli and Salmonella (enteric and systemic disease), Pasteurella/Mannheimia (bovine and ovine respiratory disease), Clostridium (toxin-mediated disease), Mycobacterium (chronic granulomatous disease), Leptospira (spirochete, darkfield or PCR), and Mycoplasma (no cell wall, pleomorphic).
Traps
Questions love cell wall exceptions: Mycoplasma lacks peptidoglycan and is intrinsically resistant to beta-lactams; Mycobacterium has a lipid-rich mycolic acid wall that resists Gram staining (acid-fast instead). Another trap is confusing hemolysis patterns (beta = complete clearing, alpha = greenish partial, gamma = none) or assigning a capsule to the wrong organism. Watch for questions that conflate colonization with infection, or that ask which organism is zoonotic versus strictly host-adapted. Spore-formers are Bacillus and Clostridium; do not attribute spores to Staphylococcus.
Rule of thumb
When a stem describes a toxin-mediated, rapid, often fatal disease with a Gram-positive rod and spores, think Clostridium before anything else; when it describes chronic, slowly progressive granulomatous disease, think Mycobacterium.
Worked example
A question states: "A Gram-negative, oxidase-positive, curved rod is recovered from aborted fetal tissue and requires microaerophilic conditions." Curved morphology, oxidase positivity, and microaerophilia point to Campylobacter (or a related organism), not E. coli (oxidase negative, straight rod, facultative anaerobe). The discriminator is oxidase plus morphology, not the Gram reaction alone.