Bacteriology Revision Framework

This closing section provides a single classification tool to apply to any bacterium. Work through these thirteen questions in order for every organism; the order itself reflects the logical dependency between them, each answer narrowing and informing the next.

1. Gram reaction. Positive or negative, or, for the organisms that fall outside this binary, specifically note why: acid fast for mycobacteria, no cell wall at all for Mycoplasma and Ureaplasma, too thin to Gram stain reliably for spirochetes. This is the first and most fundamental branch point, determining which major group the organism belongs to.

2. Shape. Cocci, bacilli, coccobacilli, vibrios or curved, spiral, filamentous, or pleomorphic. Combined with Gram reaction, this narrows the organism to one of the major organism groups.

3. Oxygen requirement. Obligate aerobe, facultative anaerobe, obligate anaerobe, microaerophile, or aerotolerant. Directly determines expected culture conditions and, for several organisms, is itself a key diagnostic or identifying feature, Campylobacter's microaerophilic requirement, the strict anaerobes.

4. Spore formation. Present only in Bacillus and Clostridium. Determines required disinfection or sterilisation approach, autoclaving required, and is itself a major virulence-adjacent property given endospore environmental resistance.

5. Motility. Present or absent, and, where present, the specific pattern, peritrichous, polar, or the unusual temperature-dependent motility of Listeria and Yersinia enterocolitica, is often independently diagnostic.

6. Capsule. Present or absent; where present, note whether it is the dominant virulence factor, the classic encapsulated organisms, and whether it is vaccine-targeted.

7. Important biochemical tests. The specific differentiating tests relevant to this organism's immediate taxonomic neighbours: catalase, coagulase, novobiocin for staphylococci, haemolysis pattern, Lancefield grouping, optochin, bacitracin for streptococci, oxidase for the Enterobacterales-versus-nonfermenter split, urease, indole, H2S, lactose fermentation across the Gram negative bacilli.

8. Reservoir. Human-restricted, animal or zoonotic, or environmental. This single fact predicts an enormous amount about epidemiology, and directly determines whether person-to-person transmission is even possible, contrast Legionella, environmental, never person-to-person, against Neisseria meningitidis, human-restricted, efficiently person-to-person.

9. Transmission. The specific route, respiratory droplet, airborne, faecal-oral, sexual, vector-borne, vertical, direct contact, directly follows from the reservoir and determines the applicable infection control precautions.

10. Virulence factors. Adhesins, invasins, capsule, intracellular survival, secretion systems, iron acquisition, biofilm formation, enzymes, antigenic variation, and/or toxin production.

11. Major diseases. The specific clinical syndrome or syndromes this organism causes.

12. Diagnostic method. The specific specimen and test most likely to establish the diagnosis for this organism: microscopy, culture, and its specific requirements, selective media, extended incubation, specific temperature, antigen detection, PCR, or serology.

13. Resistance pattern. Known intrinsic resistance, a structural or inherent property, such as Stenotrophomonas's intrinsic carbapenem resistance, or Enterococcus's intrinsic cephalosporin resistance, and commonly encountered acquired resistance phenotypes, MRSA, VRE, ESBL, AmpC, carbapenemases.

These thirteen points are not independent trivia to memorise separately; each genuinely constrains and predicts the others, which is the entire reason this framework is worth internalising rather than simply reading once.

Gram reaction and shape together narrow an unfamiliar organism to roughly the right category before a single additional fact is known.

Reservoir predicts transmission route, which predicts the applicable infection control precautions, which is a direct, practical clinical decision, not an abstract classification exercise.

Virulence mechanism predicts the clinical disease pattern: an organism with a potent, specific A-B exotoxin, diphtheria, cholera, tetanus, botulism, produces a distinctive, often single-organ-system clinical picture even at a comparatively low local organism burden, whereas an organism relying predominantly on capsule-mediated immune evasion and invasion, S. pneumoniae, N. meningitidis, produces a more generalised, invasive, multi-site disease pattern.

Resistance pattern is the final, most practically urgent point, but only becomes genuinely useful once the preceding twelve points have correctly identified the organism in the first place; resistance data attached to a misidentified organism is worse than useless.

Applying the framework to a hypothetical unfamiliar Gram negative, oxidase-positive, motile bacillus isolated from a wound in a patient recently exposed to seawater illustrates how the points build on each other.

Gram reaction and shape place it among the Gram negative bacilli rather than the Enterobacterales, since oxidase positivity already excludes that family. Oxygen requirement, if aerobic, further narrows toward the nonfermenting or curved Gram negative groups rather than the strict anaerobes. Spore formation and capsule are quickly excluded as irrelevant to this picture.

Motility, polar flagellum, and the specific biochemical pattern narrow the identification further. Reservoir and transmission, seawater exposure, point strongly toward a marine Vibrio species rather than an enteric or zoonotic organism.

Virulence factors and major disease, a rapidly progressive wound infection, are consistent with Vibrio parahaemolyticus or a related halophilic Vibrio species rather than Vibrio cholerae, which does not typically cause wound infection. Diagnostic method, culture on thiosulfate-citrate-bile salts-sucrose agar, confirms the identification, and resistance pattern is then checked last, once the organism itself is correctly identified.

This sequence, working from the broadest, most structural facts toward the narrowest, most clinically specific ones, is the same logical path this framework applies to any organism, familiar or not.