Microbiology: Organism Identification, Staining, and Susceptibility
Core concepts
Know the Gram stain interpretation: gram-positive (purple, thick peptidoglycan) vs gram-negative (pink, thin layer with outer membrane), and morphology (cocci, bacilli, clusters, chains). Know classic organism-stain associations: Staphylococcus aureus (gram-positive cocci in clusters), Streptococcus pyogenes (chains), Neisseria (gram-negative diplococci), Escherichia coli (gram-negative bacilli). Understand culture media: blood agar (general), MacConkey (gram-negative, lactose fermentation), chocolate agar (fastidious organisms like Haemophilus and Neisseria), and selective media.
Identification and susceptibility
Know biochemical tests: catalase, coagulase (S. aureus positive), oxidase (Pseudomonas, Neisseria positive), urease, and indole. Understand the difference between intrinsic and acquired resistance, and the concept of MIC (minimum inhibitory concentration) and breakpoints. Know that MRSA reflects the mecA gene (altered penicillin-binding protein) and that ESBL-producing organisms resist most beta-lactams.
Traps
Questions test the Gram stain color logic (decolorization step is critical) and confuse coagulase-negative staphylococci with S. aureus. Beware assuming a positive oxidase means Pseudomonas only - Neisseria is also oxidase-positive.
Worked example
Gram-positive cocci in clusters, catalase positive, coagulase positive → Staphylococcus aureus. If coagulase negative, it is a coagulase-negative staphylococcus (e.g., S. epidermidis). Rule of thumb: cluster + catalase + coagulase = S. aureus.