Microbiology
Microbiology tests bacterial structure and staining, culture and identification, sterilization and disinfection, antimicrobial principles, virology, mycology, immunology, and oral microbiology. Dental items often focus on caries and periodontal pathogens and on correct sterilization practice.
Core rules
- Gram stain sequence: crystal violet, iodine, alcohol decolorization, safranin. Gram-positive cells retain the purple crystal violet-iodine complex; gram-negative cells take up the pink counterstain.
- Autoclave sterilization uses saturated steam at 121 degrees Celsius, 15 psi, for about 15 minutes; it kills spores. Dry heat and ethylene oxide are alternatives for heat- or moisture-sensitive items.
- Streptococcus mutans is a key cariogenic organism; Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans are associated with periodontal disease.
- Viruses require host cells for replication; antibiotics do not treat viral infections.
- Innate immunity is immediate and nonspecific; adaptive immunity is specific and creates memory through B and T cells.
Worked example
A wound swab grows gram-positive cocci in clusters that are catalase positive and coagulase positive. The Gram reaction gives purple cocci; the cluster arrangement suggests Staphylococcus; coagulase positivity identifies Staphylococcus aureus. If the organisms were instead gram-positive cocci in chains, catalase negative, the likely identification would shift toward Streptococcus.
Traps
- Alcohol decolorization is the critical Gram stain step; over-decolorizing makes gram-positive cells appear gram-negative.
- 70 percent alcohol is more effective than 100 percent because water aids protein denaturation.
- Spores are more resistant than vegetative cells, so sterilization must be verified with biological indicators.