Bacterial Culture Calculators

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Bacterial culture is the process of growing bacteria under controlled laboratory conditions in nutrient media that provide the elements and energy needed for growth. Bacteria grow in liquid broth (allowing large-volume growth and OD₆₀₀ monitoring) or on solid agar (enabling colony isolation and pure culture work). Culture conditions — temperature, aeration, pH, and media composition — must be matched to the organism. Common media include LB broth, blood agar, MacConkey agar, and selective/differential media. Aseptic technique prevents contamination of cultures and protects operators from potentially hazardous organisms.

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Common Media

  • LB (Luria-Bertani): rich medium for E. coli; 10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCl
  • Blood agar: 5% sheep blood; supports fastidious organisms; shows hemolysis (alpha/beta/gamma)
  • MacConkey agar: selective for Gram-negatives; differential for lactose fermentation (pink vs. colorless colonies)
  • Minimal medium: carbon + nitrogen + salts only; for auxotroph and metabolic studies

Liquid vs. Solid Culture

Broth: large volumes; monitor OD₆₀₀; shake 200–250 rpm for aeration. Agar plates: colony isolation; streak plate creates isolated single colonies from dilution.

Growth Monitoring

OD₆₀₀ correlates with cell density. E. coli: OD₆₀₀ ≈ 1.0 ≈ 8×10⁸ cells/mL. Log phase: OD₆₀₀ doubles every generation time. Growth rate μ = slope of ln(OD₆₀₀) vs. time.

Glossary

LB Broth
Lysogeny (Luria-Bertani) broth: 10 g/L tryptone + 5 g/L yeast extract + 10 g/L NaCl; standard rich medium for E. coli and most lab bacteria; supports rapid growth (generation ~20 min at 37°C).
Selective Media
Media containing inhibitors allowing only certain organisms to grow; examples: MacConkey (Gram-negatives only); mannitol salt agar (staphylococci); XLD agar (Salmonella).
OD₆₀₀
Optical density at 600 nm; monitors bacterial growth in liquid culture; proportional to cell density; E. coli OD₆₀₀ = 1.0 ≈ 8×10⁸ cells/mL; μ = slope of ln(OD₆₀₀) vs. time.

Frequently Asked Questions

Bacterial culture grows bacteria under controlled conditions to study properties or produce biomass. Requirements: Nutrients — carbon and energy source; nitrogen source; minerals. Physical conditions — temperature (37°C for most human pathogens; varies by organism). Aeration — shake flasks for aerobes; anaerobic chamber for strict anaerobes. Sterility — aseptic technique prevents contamination. E. coli optimal: 37°C, aerobic, pH 7.0, LB broth; generation time ~20 min.

Selective media contains inhibitors that allow only certain organisms to grow: MacConkey agar (bile salts + crystal violet) inhibits Gram-positives → selects Gram-negative enterics. Mannitol salt agar (7.5% NaCl) selects salt-tolerant staphylococci. Differential media allows multiple organisms to grow but produces distinct colony appearances: MacConkey pH indicator — lactose fermenters produce acid → pink colonies; non-fermenters = colorless. Blood agar: beta-hemolysis (clear halo) vs. alpha (greening) vs. gamma (none). Many media are both selective and differential simultaneously.

Streak plate method: use an inoculation loop to streak sample across a plate in 4 quadrants, each streaked from the last to progressively dilute bacteria. In the final quadrant, individual cells are separated enough that each forms a discrete isolated colony descended from a single cell. Pick isolated colonies with a sterile loop and restreak on fresh agar to confirm purity. Alternatively: serial dilution and pour/spread plate — dilute sample 10⁻³ to 10⁻⁷; spread 0.1 mL on agar; isolated colonies emerge after incubation.

OD₆₀₀ (optical density at 600 nm) is the standard method: light scattering at 600 nm is proportional to cell density; OD₆₀₀ ≈ 1.0 ≈ 8×10⁸ E. coli cells/mL. Growth curve phases: Lag — cells adapt to medium; no OD increase. Log (exponential) — OD doubles every generation time; linear on semi-log plot; calculate μ from slope of ln(OD) vs. time. Stationary — OD plateau; nutrient depletion or waste accumulation. Death — OD decline. OD is quick but measures all cells (live + dead); CFU/mL (plate count) is needed to measure viability.