Overview
Bacteria are cultured in the laboratory on artificial media to diagnose infectious diseases, study morphology, obtain antigens for vaccines/serology, perform genetic manipulation, estimate viable counts, and separate mixed bacterial populations.
Robert Koch pioneered solid media using potato pieces before agar became the standard solidifying agent. Today, culture media are classified by consistency, nutritional component, and functional use — each type engineered for a specific diagnostic purpose.
Learning Objectives
After this lesson you will be able to…- Enlist the common ingredients of culture medium
- Describe a brief history of culture media
- Classify culture media by consistency, nutritional component, and functional use
- Describe the preparation and storage of culture media
- Differentiate selective, enrichment, and differential/indicator media
Clinical Story
Why This MattersA patient with suspected urinary tract infection submits a urine sample. The technician plates it on MacConkey agar, where pink colonies appear — indicating lactose-fermenting coliforms such as E. coli. Choosing the right culture medium, in this case a selective/differential agar, was the deciding factor in reaching a rapid, accurate diagnosis.
Core Concepts
Liquid media show turbidity, pellicle, or granular deposits but cannot distinguish mixed bacteria. Solid media use agar (2–4%) as a solidifying agent — agar melts at 95°C, solidifies at 42°C, and is not nutritive or hydrolysed by most bacteria. Semi-solid media (0.2–0.5% agar) demonstrate motility. Biphasic media combine liquid and solid phases in one bottle (e.g. Castaneda system for blood culture).
Simple media (peptone water, nutrient agar) support non-fastidious bacteria. Complex media (blood agar) have ingredients of uncertain exact composition. Synthetic/defined media (Davis & Mingioli medium) have every component precisely known and are used for research purposes.
Basal media (peptone water, nutrient agar/broth) support most non-fastidious bacteria. Enriched media add blood, serum or egg yolk to grow fastidious organisms (blood agar, chocolate agar, Loeffler's serum slope). Selective/enrichment media inhibit unwanted commensals using antibiotics, dyes, or altered pH (e.g. Thayer Martin agar for N. gonorrhoeae, MacConkey agar with bile salts). Differential/indicator media distinguish organisms by biochemical reaction visible as a colour change (e.g. lactose fermentation on MacConkey agar).
Transport media (Stuart's, Amies, Cary Blair) prevent desiccation and preserve viability of organisms during transit without altering concentration; charcoal neutralizes inhibitory factors. Anaerobic media (Robertson cooked meat, thioglycollate broth) are reduced by boiling or adding reducing agents such as thioglycollate, ascorbic acid, or cysteine, with methylene blue or resazurin used as redox indicators.
Laboratory Principle
Selective media suppress unwanted organisms using inhibitory agents (bile salts, antibiotics, dyes) so only target bacteria can grow, while differential media use a substrate and a pH indicator so that bacteria able to metabolize that substrate produce a visible colour change — allowing rapid presumptive identification directly from a mixed specimen.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Agar (New Zealand / Japanese) | 2% (solid), 0.2–0.5% (semi-solid) | Solidifying agent | Room temperature, dry |
| Peptone | Standard grade | Nitrogen/amino acid source | Room temperature, sealed |
| Sheep blood | 5–10% by volume | Enrichment for fastidious bacteria (blood agar) | 2–8°C, aseptically collected |
| Potassium tellurite | 0.04% | Selective for C. diphtheriae | Room temperature, dark |
Step-by-Step Procedure
Weigh the correct amount of dehydrated medium powder and dissolve in distilled/demineralised water per manufacturer instructions.
Use a pH indicator (phenol red, neutral red, bromothymol blue) to verify and adjust pH before sterilization.
Autoclave the medium; heat-labile components (glucose, antibiotics, serum, blood) are filtered and added separately after autoclaving.
Add blood, serum, or selective agents (antibiotics, dyes) at the correct temperature just above the agar solidifying point.
Pour into plates or tubes, allow to solidify, and store at 4–5°C for 1–2 weeks or as appropriate for the medium type.
Flow Diagram
Quality Control
A representative sample from each lot of media must be tested for performance (growth of known reference organisms) and sterility/contamination before clinical use.
Culture media manufacturers and laboratories participate in EQA schemes verifying that batches consistently support/inhibit the correct organisms.
Reference Values
Key Working Parameters⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Pink colonies on MacConkey agar | Lactose-fermenting coliform (e.g. E. coli, Klebsiella) | Proceed to biochemical identification |
| Colourless colonies on MacConkey agar | Non-lactose fermenter (e.g. Salmonella, Shigella) | Further biochemical/serological testing |
| Black colonies on Wilson & Blair's medium | H₂S production suggestive of Salmonella typhi | Confirm with biochemical and serological tests |
Common Errors & How to Avoid Them
Cause: Adding glucose, antibiotics, serum, or blood before autoclaving.
Prevention: Filter-sterilize heat-labile additives and add them after the base medium has been autoclaved and cooled.
Cause: Convenience of using available human blood.
Prevention: Always use sheep, rabbit, horse, or ox blood — human blood may contain inhibitory antibiotics.
Cause: Time pressure skipping QC steps.
Prevention: Always test a representative sample of each new lot for growth performance and contamination.
Laboratory Tips from the Bench
Add blood to the medium only when it has cooled to just above the agar's solidifying point (~45–50°C) to avoid hemolysis of red blood cells.
Chocolate agar's brown colour comes from lysed blood cells — remember this is intentional and improves growth of fastidious organisms like Haemophilus and Neisseria.
Remember media types by function: "BEST" — Basal, Enriched, Selective, and differential/indicator Test media.
Important Notes
Certain highly selective media such as Wilson & Blair's medium and TCBS agar need not be sterilized, since their selective agents inhibit contaminating organisms.
Gelatin was used before agar as a solidifying agent but was problematic — it remained liquid at incubation temperatures (35–37°C) and was digested by certain bacteria.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer. Use arrow keys to navigate in single-card mode.
Clinical Case Study
Apply Your KnowledgeSuresh presents with dysuria, urgency, and lower abdominal discomfort. His physician orders a urine culture to investigate a suspected urinary tract infection.
Pink lactose-fermenting colonies on the selective/differential MacConkey agar, combined with significant colony count and pyuria, strongly suggest E. coli as the causative agent of a urinary tract infection.
- →MacConkey agar is both selective (bile salts) and differential (lactose + neutral red).
- →>10⁵ CFU/mL is the standard threshold for significant bacteriuria.
- →Selecting the right medium accelerates presumptive identification.
Frequently Asked Questions
Agar melts at 95°C and solidifies at 42°C, staying solid at incubation temperatures (35–37°C), unlike gelatin which liquefies at these temperatures and can be digested by some bacteria.
Selective media are agar-based and inhibit unwanted bacteria while allowing target organisms to grow as visible colonies. Enrichment media are liquid and similarly inhibit commensals, but are used to increase the numbers of a target pathogen before subculturing onto solid media.
Glucose can caramelize or break down under high autoclave temperatures, altering the medium's properties. It is filter-sterilized and added separately after the base medium has cooled.
Quick Revision
10-Minute ReviewKey Takeaways
- Culture media are essential for diagnosis, morphology study, antigen production, and viable counting.
- Media are classified by consistency (liquid, semi-solid, solid), nutrition (simple, complex, synthetic), and function (basal, enriched, selective, differential).
- Agar is the preferred solidifying agent due to its stability at incubation temperatures.
- Selective media inhibit commensals; differential media reveal biochemical activity via colour change.
- Proper preparation includes pH adjustment, correct sterilization, and quality control testing of each lot.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling (NIOS). Microbiology — Lesson 9: Bacterial Culture Media.
- Collee JG et al. Mackie & McCartney Practical Medical Microbiology.
- Cheesbrough M. District Laboratory Practice in Tropical Countries.