ACVM Veterinary Microbiology cheat sheet

Bacterial cell wall: Gram-positive has thick peptidoglycan plus teichoic acids; Gram-negative has thin peptidoglycan plus an outer membrane with lipopolysaccharide (LPS).

LPS lipid A is the endotoxin moiety; it triggers TLR4 signaling and drives fever and septic shock in Gram-negative infections.

Spore formers of veterinary relevance: Bacillus and Clostridium; spores resist heat, desiccation, and many disinfectants, surviving until favorable conditions return.

Facultative intracellular pathogens include Brucella, Salmonella, Listeria, and Mycobacterium; their intracellular niche shields them from antibody and many drugs.

Mycoplasma lack a cell wall, so beta-lactams are inactive; they also stain poorly Gram-negative and require enriched media for culture.

Obligate anaerobes (e.g., Fusobacterium, Clostridium, Dichelobacter) require anaerobic transport and culture; culture failure is common in mixed infections.

Capsule (e.g., Bacillus anthracis poly-D-glutamate, Pasteurella, Klebsiella) impedes phagocytosis and is a major virulence and typing determinant.

Selective media examples: MacConkey for Gram-negative enterics, blood agar for hemolysis patterns, and tellurite or selective agars for Corynebacterium.

Viral classification uses genome type (dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA), capsid symmetry, envelope presence, and replication strategy.

RNA viruses generally have higher mutation rates (error-prone RNA-dependent RNA polymerase), favoring quasispecies formation, antigenic drift, and vaccine escape.

Enveloped viruses are more vulnerable to lipid solvents, detergents, and drying; non-enveloped viruses (e.g., parvovirus, calicivirus) persist longer in the environment.

Antigenic shift (reassortment of segmented genomes, e.g., influenza) produces major antigenic change; antigenic drift is gradual point-mutation accumulation.

Viral replication steps: attachment, penetration, uncoating, genome replication and protein synthesis, assembly, and release (budding or lysis).

Latency: herpesviruses establish latent infection in neurons or lymphoid cells, with reactivation under stress or immunosuppression.

Reverse-transcribing retroviruses (e.g., BLV) integrate proviral DNA into host genome via reverse transcriptase and integrase.

Viral diagnosis: PCR detects nucleic acid (best for active or latent detection), serology detects antibody (paired sera showing fourfold rise indicates recent infection).

Fungi are eukaryotes with ergosterol in the cell membrane and chitin in the cell wall; these are targets for polyenes (bind ergosterol) and azoles (inhibit ergosterol synthesis).

Molds grow as branching hyphae; yeasts reproduce by budding; dimorphic fungi switch between mold (environment) and yeast (tissue) forms with temperature.

Dermatophytes (Microsporum, Trichophyton, Epidermophyton) digest keratin; they cause ringworm and are identified by macroconidia and colony morphology.

Malassezia is a lipophilic yeast requiring lipid supplementation; it causes dermatitis and otitis in animals.

Aspergillus and Mucorales are opportunistic environmental molds; invasive disease follows immunosuppression or massive spore exposure.

Cryptococcus neoformans has a polysaccharide capsule that resists phagocytosis; capsule antigen can be detected by latex agglutination or lateral flow.

Systemic dimorphic fungi (Blastomyces, Histoplasma, Coccidioides) are primary pathogens; culture requires biosafety precautions for infectious conidia.

Antifungal classes: polyenes (amphotericin B), azoles (itraconazole, fluconazole), and echinocandins (beta-glucan synthase inhibitors, ineffective against Mucorales).

Innate immunity acts within hours via pattern recognition receptors (TLRs, NLRs) recognizing PAMPs such as LPS, flagellin, and dsRNA.

Adaptive immunity: B cells produce antibody (humoral), T cells mediate cell-mediated immunity; CD4 helper T cells orchestrate, CD8 cytotoxic T cells kill infected cells.

Antibody classes: IgM is the primary-response and surface B-cell receptor; IgG dominates secondary responses and crosses the placenta in some species; IgA is mucosal; IgE drives type I hypersensitivity.

Fourfold or greater rise in paired serum antibody titers between acute and convalescent samples supports recent infection.

Vaccine types: modified-live (strong, durable response, possible reversion), killed (safe, requires boosters), subunit/toxoid (targeted antigen), and recombinant vectored vaccines.

Maternal antibody transfer: via colostrum in ruminants and pigs (no placental transfer), transplacental in primates; interference with vaccination is a key consideration.

Hypersensitivity types: I (IgE anaphylaxis), II (antibody-mediated cell destruction), III (immune complex deposition), IV (delayed, T-cell mediated, e.g., tuberculin reaction).

Immunodeficiency (e.g., feline leukemia virus, bovine viral diarrhea virus) increases susceptibility to secondary and opportunistic infections.

Koch's postulates establish causation: organism present in all diseased hosts, isolated in pure culture, causes disease when inoculated, and re-isolated from the experimental host.

Virulence factors include adhesins (pili, fimbriae), invasins, capsules, siderophores for iron acquisition, and toxins (endotoxin and exotoxins).

Exotoxins are proteins, often heat-labile and immunogenic (toxoids); examples include tetanus neurotoxin, botulinum toxin, and clostridial alpha toxins.

Endotoxin (LPS lipid A) is heat-stable and non-toxoidable; it causes fever, hypotension, and disseminated intravascular coagulation.

Carrier state: an infected host sheds the pathogen without overt disease, sustaining transmission (e.g., Salmonella, Streptococcus equi).

Vertical transmission routes: transplacental, via birth canal, or via colostrum; each route shapes control strategies and diagnostic timing.

Biofilm formation (e.g., Staphylococcus, Pseudomonas) enhances adherence and confers marked resistance to antibiotics and host defenses.

Pathogen dose, route of exposure, and host immune status jointly determine outcome (subclinical, acute, or chronic disease).

Aseptic technique and appropriate transport media (e.g., Amies, Cary-Blair) preserve viability; cold-sensitive organisms (Campylobacter, some fastidious bacteria) need specific handling.

Gram stain: crystal violet, iodine, decolorizer, safranin; Gram-positive retain purple, Gram-negative take the counterstain pink.

Culture: blood agar for fastidious organisms and hemolysis typing; MacConkey for lactose fermentation; selective and differential media for specific pathogens.

Antimicrobial susceptibility testing methods: disk diffusion (Kirby-Bauer, zone sizes vs. breakpoints), broth microdilution (MIC), and gradient strips (Etest).

MALDI-TOF mass spectrometry identifies isolates by protein mass fingerprints; PCR and sequencing provide rapid species-level and resistance-gene detection.

Serologic tests: ELISA, agglutination, and virus neutralization; interpretation requires paired sera or known vaccination status.

Cytology and histopathology with special stains (Gram, acid-fast, PAS, silver) aid rapid presumptive diagnosis before culture results.

Quality control: reference strains (e.g., ATCC organisms), internal and external controls, and documented validation ensure reliable results.

Beta-lactams (penicillins, cephalosporins) inhibit cell wall synthesis via penicillin-binding proteins; resistance arises via beta-lactamase production, altered PBPs, or efflux.

Aminoglycosides bind the 30S ribosomal subunit; they are concentration-dependent killers with nephrotoxicity and ototoxicity risks.

Tetracyclines bind the 30S subunit and are bacteriostatic; resistance is common via efflux pumps and ribosomal protection proteins.

Fluoroquinolones inhibit DNA gyrase and topoisomerase IV; resistance arises from target mutations and efflux.

Macrolides and lincosamides bind the 50S subunit and inhibit protein synthesis; cross-resistance occurs via methylation of the 23S rRNA target.

Resistance mechanisms: enzymatic inactivation, target modification, decreased permeability, efflux, and horizontal gene transfer via plasmids, transposons, or integrons.

MIC is the lowest antimicrobial concentration inhibiting visible growth; susceptibility depends on comparing MIC to established breakpoints.

Minimum inhibitory concentration interpretation requires species-specific breakpoints; intrinsic resistance (e.g., Mycoplasma to beta-lactams) is not acquired resistance.

DNA replication is semiconservative; PCR requires template, primers, dNTPs, thermostable polymerase (Taq), buffer, and Mg2+ cofactor.

PCR cycle: denaturation (~95°C), primer annealing (~50-65°C), extension (~72°C); product detection by gel electrophoresis or real-time fluorescence.

Real-time PCR (qPCR) quantifies target via cycle threshold (Ct); lower Ct indicates higher starting template amount.

Sanger sequencing uses dideoxynucleotide chain termination; next-generation sequencing enables high-throughput whole-genome and metagenomic analysis.

Whole-genome sequencing supports outbreak tracing, virulence-gene detection, and resistance-gene identification (e.g., mecA, bla genes).

Plasmids, transposons, and integrons mediate horizontal gene transfer of antimicrobial resistance genes between bacteria.

Restriction enzymes recognize specific palindromic sequences; CRISPR-Cas systems provide adaptive bacterial immunity and are repurposed as gene-editing tools.

Bioinformatics tools: BLAST for sequence similarity, phylogenetic trees for strain relationships, and multilocus sequence typing (MLST) for epidemiology.