Preparing Laboratory Media; Defined, Complex, Selective and Differential Media

selective and differential

macconkey agar, above, selects for gram-negative bacteria and simultaneously detects lactose fermentation. mannitol-salt agar, above, selects for staphylococcus species but distinguishes staphylococcus aureus from all other staph because it ferments the sugar mannitol and the colonies turn "bright yellow" as a result of pH change)

differential

media contains some substance that allows the observer to visualize some distinct biochem characteristic of the organisms (macconkey agar contains lactose and a pH indicator - if a gram-negative organisms on the plate is a lactose-fermenter like e. coli, then it will ferment the sugar and produce lost of acid resulting in a bright pink colony, vs. a lactose-nonfermenter which would remain colorless

selective medium

medium formulated to permit the growth of certain bacteria but not others

differential medium

medium permitting certain organisms to be distinguished from others by the appearance of their colonies

fastidious

organisms that require many complex nutritional supplements

Non-selective / enriched

will grow virtually anything (tryptic soy agar, brain heart infusion, etc.)

growth factor

a compound such as a vitamin or amino acid that is required by an organism for growth

glucose mineral medium

a chemically defined medium containing glucose as a carbon source and such salts... for a nitrogen and phosphate source

rich or enriched medium

a medium, usually complex, containing many growth factors. sometimes blood or additional vitamins are added to permit the growth of organisms with special requirements

what is agar?

a polysaccharide from seaweed that when in solid form will not melt until heated to nearly boiling, and when a liquid, will not solidify until cooled to below 37 degrees celsius. this means that it will remain a solid throughout the temp range that most bacterial will grow

chemically defined medium

a synthetic medium composed of inorganic salts and usually a carbon source such as glucose

undefined medium

also termed a complex medium. a medium in which the exact amounts of chemical components and their composition are unknown because it is made of extracts or enzymatic digests of meat, plants, or yeast

complex medium

also termed an undefined medium. a medium in which the exact amounts of chemical components and their composition are unknown because it is made of extracts or enzymatic digests of meat, plants, or yeast

enteric

associated with the intestine

selective

contains some substance that inhibits the growth of some organisms while permitting others to grow. macConkey agar, eosin-methylene blue agar inibit gram-positives while allowing gram-negatives to grow; phenylethylalcohol agar inhibits gram-negatives while allowing gram-positives to grow; mannitol-salt agar allows only staphlococcus species to grow because of high salt content; etc.