Isolating a pure bacterial culture from a mixed sample is the first step in any microbiological workflow — whether for clinical diagnosis, food safety testing, or research.

Culture Media

Bacteria are grown in nutrient-rich formulations that provide carbon, nitrogen, minerals, and energy sources. Media are classified by composition:

• Defined (synthetic) media: every chemical component is known. Used for metabolic studies.
• Complex media: contain digests of animal or plant material (peptone, tryptone, yeast extract). LB (Luria-Bertani) broth is the most common complex medium for E. coli.
• Selective media: contain inhibitors (antibiotics, bile salts, dyes) that suppress unwanted bacteria while allowing the target organism to grow. Examples: MacConkey agar (selects Gram-negative bacteria), Mannitol salt agar (selects staphylococci).
• Differential media: contain indicators that distinguish bacterial types. MacConkey agar is both selective and differential — lactose fermenters turn pink, non-fermenters remain colorless.
• Enriched media: supplemented with blood, serum, or growth factors to support fastidious organisms. Blood agar (5% sheep blood) is the most common enriched medium.

Streak Plating

The quadrant streak method isolates single colonies from a mixed culture:

1. Sterilize an inoculating loop by flaming to red hot, then cool by touching the sterile agar edge.
2. Pick a small amount of the bacterial sample.
3. Streak the first quadrant in a tight zigzag pattern.
4. Flame the loop, cool, and drag through the first quadrant into the second.
5. Repeat for the third and fourth quadrants, flaming between each.
6. Incubate inverted at the appropriate temperature (37 °C for most pathogens, 30 °C for environmental isolates).

After incubation, isolated colonies appear along the streaks of the final quadrants where the cell density is lowest.

Spread Plate and Pour Plate

The spread plate technique distributes a diluted bacterial suspension evenly across the surface of an agar plate using a sterile glass spreader. It is used for quantifying bacterial counts (CFU/mL).

The pour plate technique mixes the diluted sample with molten agar before pouring into the plate. Colonies grow both on the surface and within the agar. It is useful for counting heat-tolerant organisms and for detecting low numbers of organisms.

Incubation Conditions

Standard incubation is at 37 °C in air for 18–24 hours. Variations include:

• Capnophilic: 5–10% CO₂ (some pathogens, e.g., Neisseria).
• Anaerobic: oxygen-free atmosphere using an anaerobic jar or chamber (e.g., Clostridium, Bacteroides).
• Microaerophilic: low oxygen, elevated CO₂ (e.g., Campylobacter).
• Thermophilic: 55–60 °C (Thermus aquaticus).