Microbiology Calculators

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Microbiology is the scientific study of microorganisms — organisms too small to be seen with the naked eye, including bacteria, viruses, fungi, protozoa, algae, and prions. It encompasses the study of their structure, physiology, genetics, ecology, and interactions with hosts and the environment. Microbiology has applications across medicine (infectious disease, clinical diagnostics), food science, environmental monitoring, biotechnology, and agriculture. Key techniques include culture and isolation on solid or liquid media, microscopy, staining (Gram, acid-fast, spore staining), and molecular methods (PCR, 16S sequencing).

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Major Microorganism Groups

  • Bacteria: Prokaryotes; no nucleus; peptidoglycan cell walls; diverse metabolism; classified by Gram stain (positive/negative), shape (cocci, bacilli, spirilla), and 16S rRNA phylogeny
  • Viruses: Non-cellular; require host cells for replication; DNA or RNA genome; obligate intracellular parasites; classified by genome type, Baltimore classification
  • Fungi: Eukaryotes; cell walls with chitin and glucan; include yeasts (unicellular) and molds (filamentous hyphae); some cause human disease (Candida, Aspergillus)
  • Protozoa: Single-celled eukaryotes; some parasitic (Plasmodium, Trypanosoma, Giardia)
  • Archaea: Prokaryotes; distinct from bacteria; many extremophiles; key in biogeochemical cycling

Key Laboratory Techniques

Culture: grow organisms on selective, differential, or enrichment media. Gram stain: crystal violet → iodine → decolorizer → safranin; G+ = purple; G− = pink. PCR and 16S rRNA sequencing for identification. Antimicrobial susceptibility testing (Kirby-Bauer disc diffusion, MIC by broth microdilution). Electron microscopy for virus morphology.

Microbial Growth

Binary fission (bacteria): each cell divides into two; exponential growth described by N = N₀ × 2^n; generation time (td) typically 20 min (E. coli in LB) to 24 hours (M. tuberculosis).

Glossary

Gram Stain
The primary bacterial differential stain: Gram-positive (G+) bacteria retain crystal violet (purple); Gram-negative (G−) bacteria are counterstained pink with safranin; reflects cell wall peptidoglycan thickness.
Koch's Postulates
Four criteria establishing causation between a microorganism and a disease; foundational to the germ theory of disease; basis of modern infectious disease microbiology.
Sterilization
Complete elimination of all microorganisms including spores; methods include autoclaving (121°C, 15 min), dry heat, filtration (0.22 μm), gamma irradiation, and ethylene oxide gas.

Frequently Asked Questions

Major microorganism groups: (1) Bacteria — prokaryotes; peptidoglycan cell walls; diverse metabolic strategies; divided by Gram stain into G+ and G−; classified by 16S rRNA. (2) Viruses — non-cellular obligate intracellular parasites; DNA or RNA genomes; require host cell machinery for replication. (3) Fungi — eukaryotes; chitin cell walls; include unicellular yeasts and multicellular molds; important saprotrophs and pathogens. (4) Protozoa — single-celled eukaryotic organisms; include important parasites (Plasmodium malaria, Giardia). (5) Archaea — prokaryotes distinct from bacteria; many extremophiles; key participants in nitrogen and methane cycling.

The Gram stain differentiates bacteria by cell wall structure: Gram-positive (G+) bacteria have a thick peptidoglycan layer that retains crystal violet dye → purple/violet. Gram-negative (G−) bacteria have a thin peptidoglycan layer sandwiched between inner and outer membranes → decolorized by acetone-alcohol → counterstained pink with safranin. Steps: crystal violet (primary stain) → Gram's iodine (mordant) → acetone-alcohol (decolorizer) → safranin (counterstain). G+ retain crystal violet; G− do not. Clinical significance: G− bacteria have lipopolysaccharide (LPS) endotoxin in their outer membrane; different antibiotics work against G+ vs. G− organisms.

Koch's postulates (1884) define the criteria for establishing a causal relationship between a microorganism and a disease: (1) The organism must be present in all cases of the disease and absent from healthy individuals. (2) The organism must be isolated from diseased tissue and grown in pure culture. (3) The pure culture organism must cause the disease when introduced to a healthy susceptible host. (4) The organism must be re-isolated from the experimentally diseased host and shown to be identical to the original. Koch's postulates established the germ theory of disease and are still the conceptual basis of infectious disease microbiology, though molecular Koch's postulates now extend them to genetic and molecular levels.

Sterilization completely eliminates ALL microorganisms including spores — the most resistant form. Methods: autoclaving (121°C, 15 psi, 15–20 min — the standard method); dry heat (160–180°C, 2 hours); ethylene oxide gas (heat-sensitive materials); gamma irradiation (implants, disposables); filtration (0.22 μm membranes for heat-sensitive liquids). Disinfection kills most but not necessarily all microorganisms — usually does not destroy spores. Methods: chemical disinfectants (bleach, quaternary ammonium compounds, glutaraldehyde); boiling (kills vegetative cells but not all spores); pasteurization. Antiseptics are disinfectants safe for use on skin/mucous membranes.