Topic 3: Microbes in the Environment
- Microbes are everywhere
- found in water, air, and the earth
- live in and on our bodies
- occupy ecological niches on all forms of life and in most environments
- Most microbes are harmless
- When a medium is selected for culturing bacteria, macronutrients, an energy source, and any necessary growth factors must be provided.
- Chemically defined medium: a medium whose exact chemical composition is known.
- Most chemoheterotrophic bacteria are grown on complex media.
- Complex media: media for which the exact chemical composition varies slightly from batch to batch.
- Organic carbon, energy, and nitrogen sources are usually supplied by protein in the form of meat extracts and partially digested proteins called peptones.
- Nutrient broth: commonly used liquid complex medium.
- Nutrient agar: when agar is added to a nutrient broth medium and becomes a solid medium.
- Agar is an extract from marine red algae.
- Few microbes can degrade agar so it remains solid during microbial growth.
- Liquefies in 100 °C and remains in a liquid state until cooled to 40 °C.
- Once solidified, it can be incubated at temperatures of up to 100 °C and remain solid.
- Media must be sterilized after preparation.
- Most common method is steam sterilization or autoclaving → using steam under pressure.
- Material to be sterilized is placed in the autoclave and heated to 121 °C at 15 lbs of pressure (15 psi) for 15 minutes
- Petri plates, containing a solid media, provide a large surface area for examination of colonies
- Inoculated: intentionally introduced
- Bacteria that is inoculated on media will increase in number during an incubation period.
- Liquid media becomes turbid, cloudy, due to bacterial growth.
- On solid media, colonies will be visible
- Colony: population of cells that arises from a single bacterial cell,
- Colony-forming unit: a colony that may arise from a group of the same microbes attached to one another