Microbial identification relies on staining techniques, biochemical tests, and specialized culture media. The oxidase test detects cytochrome c oxidase, differentiating Pseudomonas (positive) from Enterobacteriaceae (negative). The catalase test detects the catalase enzyme that breaks hydrogen peroxide into water and oxygen, distinguishing Staphylococcus (positive) from Streptococcus (negative). The coagulase test identifies Staphylococcus aureus (positive) by detecting an enzyme that clots plasma, separating it from coagulase-negative species like S. epidermidis. The oxidative-fermentative (O-F) test distinguishes fermenters, which produce acid under anaerobic conditions, from strict oxidizers.
Culture media can be selective, differential, or both. Selective media such as MacConkey agar inhibit unwanted organisms while permitting growth of Gram-negatives. Differential media allow visual distinction based on biochemical reactions; MacConkey agar also differentiates lactose fermenters (pink colonies) from non-fermenters (colorless). Blood agar reveals hemolytic patterns: beta-hemolysis produces a clear zone of complete red blood cell lysis, as seen with Streptococcus pyogenes, while alpha-hemolysis causes partial lysis with a greenish discoloration, characteristic of Streptococcus pneumoniae and viridans streptococci. The Quellung reaction, a capsule-swelling test, is used to serotype encapsulated bacteria such as S. pneumoniae.
Controlling microbial growth requires distinguishing between sterilization, which kills all microbial life including spores, and disinfection, which reduces pathogens to a safe level but may spare spores. Autoclaving uses pressurized steam, typically at 121 degrees C and 15 psi for 15-20 minutes, reliably destroying endospores. Pasteurization employs milder heat, such as 63 degrees C for 30 minutes or 72 degrees C for 15 seconds, to kill most pathogens without sterilizing the product. Antiseptics are chemicals safe for living tissue, while disinfectants are used on inanimate surfaces and may be too harsh for skin. Alcohols at 60-90% concentration denature proteins and disrupt lipid membranes, while chlorine-based agents like bleach oxidize proteins and nucleic acids through hypochlorous acid formation. HEPA filtration removes airborne bacteria in operating rooms and biosafety cabinets, while ionizing radiation such as gamma rays damages DNA and generates destructive free radicals to sterilize disposable medical supplies. Ethylene oxide gas sterilizes heat-sensitive equipment by alkylating proteins and nucleic acids.