Bacteriology is the study of bacteria—microscopic unicellular prokaryotes lacking a membrane-bound nucleus. Bacteria are among the most versatile organisms on Earth, thriving in environments ranging from deep-sea hydrothermal vents to arctic ice.
Note: The human body harbors approximately 38 trillion bacterial cells, roughly equal to the number of human cells—making our microbiome a critical component of health.
Gram staining is a fundamental classification technique based on cell wall composition:
| Feature | Gram-Positive | Gram-Negative |
|---|---|---|
| Peptidoglycan | Thick (20–80 nm) | Thin (2–7 nm) |
| Outer membrane | Absent | Present (LPS) |
| Gram stain result | Purple/blue | Pink/red |
| Antibiotic sensitivity | Penicillin-sensitive | More resistant |
- Peptidoglycan layer — Provides structural rigidity and shape; target of beta-lactam antibiotics.
- Plasma membrane — Phospholipid bilayer housing transport proteins and respiratory enzymes.
- Flagella — Locomotion organelles; H-antigens used in serotyping.
- Pili (fimbriae) — Attachment structures; critical in biofilm formation and conjugation.
Warning: In clinical settings, antibiotic resistance can emerge via plasmid transfer during bacterial conjugation—always consider resistance profiling before prescribing.
Bacterial populations follow a characteristic growth curve:
- Lag Phase — Metabolic adaptation; no net growth.
- Exponential (Log) Phase — Rapid binary fission; generation time is species-specific.
- Stationary Phase — Growth rate equals death rate; nutrient depletion triggers stress responses.
- Death Phase — Population declines as toxic metabolites accumulate.
# Standard 4-step Gram stain procedure
1. Apply crystal violet (primary stain) — 60 seconds
2. Apply Gram iodine mordant — 60 seconds
3. Decolorize with 95% ethanol — 10–20 seconds
4. Apply safranin counterstain — 60 seconds
Tip: Always use a culture in log-phase growth (18–24 h) for reliable Gram staining. Older cultures may show gram-variable results due to cell wall degradation.