Overview
This lecture covers the classification and use of culture media for microbial growth, including types based on physical state, composition, and purpose, and discusses cultivation techniques for bacteria and viruses.
Sterile Collection & Media Basics
- Sterile collection prevents contamination of samples by unwanted microorganisms on instruments.
- Medium (plural: media) is any substance that supports microbial growth.
Physical State of Media
- Liquid media allows cultivation of large amounts of microorganisms and can include indicators to detect metabolic changes (e.g., pH shifts).
- Solid media (typically agar-based) provides a surface for microbial colony characterization and isolation.
- Agar, a solidifying agent, is derived from algae; gelatin can also be used but may be degraded by some bacteria.
- Semi-solid media is used mainly to study microorganism motility and growth patterns, not for bulk cultivation.
Applications & Examples of Media
- Semi-solid media helps determine bacterial motility by observing spread from the inoculation point.
- Triple Sugar Iron Agar (TSI) is a differential semi-solid medium used to identify bacterial species based on sugar fermentation, gas, and hydrogen sulfide production.
Classification by Composition
- Complex media is made from undefined mixtures like peptone, yeast extract, or brain/heart infusions; used for cultivating diverse microorganisms.
- Synthetic (defined) media has a known, precise chemical composition, useful for controlled studies.
Classification by Purpose
- General purpose media supports growth of many microbes (example: tryptic soy agar).
- Enriched media contains additional nutrients for fastidious organisms (example: blood or chocolate agar).
- Selective media inhibits some microbes while allowing others to grow (example: mannitol salt agar for Staphylococci, EMB for inhibiting Gram-positives).
- Differential media distinguishes organisms based on appearance or metabolic activity (example: blood agar for hemolysis, MacConkey agar for lactose fermentation).
- Some media are both selective and differential (example: MacConkey agar).
Types of Cultures
- Pure culture contains a single microbial species.
- Mixed culture contains multiple known species.
- Contaminated culture contains unwanted microbes.
Virus Cultivation
- Viruses are obligate intracellular parasites and require living cells for growth.
- Early virus cultivation used animals; now, viruses are grown using cell culture techniques (cells in flasks with growth medium), greatly advancing virology.
Key Terms & Definitions
- Medium (media) — Substance used to grow microorganisms.
- Agar — Gel-like substance used to solidify culture media, derived from algae.
- Complex media — Media with an undefined composition, often from animal or yeast extracts.
- Synthetic (defined) media — Media with precisely known chemical components.
- Selective media — Media that suppresses unwanted microbes and favors desired ones.
- Differential media — Media enabling distinction between microbial species based on their biochemical properties.
- Fastidious organism — Microbe requiring specific nutrients/growth factors not found in general media.
- Cell culture — Growth of cells in a controlled artificial environment, used for virus cultivation.
Action Items / Next Steps
- Review lab video on triple sugar iron agar and hydrogen sulfide production (Lab #10, "Entry Tube").
- Prepare for lab #4: practice streak plating to produce pure cultures.
- Read about selective and differential media in your textbook.