Bacterial Growth Media: Defined & Complex

• Growing bacteria is a basic microbiology lab procedure
• A growth media, a liquid, solid, or semisolid substance, provides necessary nutrients for specimen growth
• Growth media come in hundreds of varieties
• Basic information is required about growth media and aseptic methods
• Methods used for species isolation and media used for differentiating bacterial species from one another are important
• Growth media have either a defined or complex ingredient

Defined Media

• Defined media have precisely known organic and inorganic ingredients

Complex Media

• Complex media contain less precisely defined organic nutrients
• Milk proteins, blood, and yeast extracts are examples of complex media
• Complex media are also called enriched media
• Most pathogens are fastidious, requiring complex nutrient sources, and will only grow on enriched media
• An example of complex media is nutrient broth, which contains peptone and beef extract
• Bacterial growth applications begin with solid or semisolid standard incubation temperatures
• Thickened media is poured into sterile petri plates to make agar plates, upright test tubes to make agar deeps, or tilted test tubes to make agar slants
• Plated media are used for isolating species, differential testing, and quantifying bacterial levels
• Agar slants are used for a variety of biochemical tests and maintaining stock cultures
• Agar deeps are used to determine specific bacterial metabolic properties
• All media requires aseptic technique

Liquid vs Solid Media

• Broth media is liquid
• Liquid media is optimal for quickly growing large quantities of bacteria
• High-volume batches can be readily prepared and easily inoculated in liquid media
• Agar, purified from seaweed, is added to liquid media to make solid media
• Agar has the same solidifying effect as gelatin, but is superior
• Glucose minimal salts media is an example of media that is accurately reproducible

Aseptic Technique

• Aseptic technique is required when transferring cultures to a new media to avoid environmental contamination and exposing oneself and surroundings to microbes
• There are several methods, all involving sterile media and sterile inoculating tools combined with precise sample handling methods
• One goal of aseptic methods uses sterile media
• Sterilizing media requires autoclaving, a chamber that steam heats a sample under pressure
• Anything that will directly contact the sample must be sterile including the interior of tubes/flasks and devices that hold media
• Media can be mixed and poured into tubes, then autoclaved or poured into presterilized petri plates and other plastic or glassware

Best Practices

• Limit the amount of time the sample is exposed to the environment
• Two things must be done to accomplish this: limit aerosol and decontaminate spills.
• Pay attention to environment and ensure workspace is clutter free
• Thoroughly label all media in advance with your name, date, type of medium, and the bacterium's name
• Keep test tubes in a test-tube rack
• Take time and avoid a frenzied pace

Tools

• Sterile inoculating tools transfer sample to the media
• A wire inoculating needle and wire inoculating loop are used to pick up culture amounts
• An inoculating loop has a handle that holds a thin tungsten, platinum, or nichrome wire twisted into a small loop at the end
• An inoculating needle is the same a a loop, but the wire is straight
• To sterilize the loop or needle, insert the wire filament into an incinerator or Bunsen burner flame until the filament glows red-hot
• Disposable inoculating loops or needles are used if presterilized
• Sterilization is skipped if disposable tools are used
• Other inoculating tools, like pipettes are sometimes used
• Appendices C and D cover the use of pipettes

Safety

• Aseptic methods help to avoid sample contamination
• Protecting yourself and others from exposure is importanti