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2 differential media
contains various nutrients that allow one to distinguish one bacterium from another by how they metabolize or change media with a waste product
mannitol salt agar
selective = contains a high salt concentration (only organism that are halophiles or halotolerant will grow)
differential = contains the sugar mannitol (looking for mannitol fermentation)
also contains pH indicator called phenol red
fermentation of mannitol
the medium will change from red to yellow due to the pH
pH color change for mannitol
pH >8.4 = pink
pH 6.9-8.4 = red
pH < 6.9 =
Microbiological Culture
method of multiplying microbial organisms by letting them reproduce in a predetermined culture media under controlled laboratory conditions
mixed culture
yellow, more than one type of organism
pure culture
pink, single type of organism, main idea is to dilute the original sample until the organism of interest is isolated and pure
ways to obtain pure culture
spread, pour, streak plate
details of methods of getting pure culture
they dilute or thin out a heavy population of bacteria across an agar surface. Once a pure culture is obtained it can be used to identify if the bacteria is sensitive or resistant to an antibiotic
spread plate
original culture is serially diluted then the final dilution is spread on the surface of the plate. surface colonies grow
pour plate
serial dilution than final dilution added to molten agar which is poured over an agar plate. surface and subsurface colonies grow
streak plate
original culture directly diluted across (agar) new plate with inoculating loop. 3 sections in a T pattern, in each section start with a line from the previous section
types of media
broth, agar
broth
liquid media; nutrients (used in motility experiment)
agar
jellylike substance derived from seaweed: thickening agent
why do we use agar
because microorganisms cannot digest agar. it’s a solid surface for microorganisms to grow and we can pick out individual colonies
all purpose/supportive media
contains nutrients that will support the growth of a large variety microorganisms
selective media
promote growth of some bacteria and/or limits growth of other bacteria
differential media
distinguish between different bacteria based on changes in colonies or changes in media
types of agar
tryptic soy agar, Eosin methylene blue agar
tryptic soy agar (TSA)
all-purpose/supportive medium used to grow most microorganisms
Eosin methylene blue agar (EMB)
selective media for gram negative organisms. inhibits the growth of gram-positive organisms due to the dye’s eosin Y and methylene Blue
lactose fermentation makes it a differential media. it causes precipitation of the dyes on the surface of the colonies resulting in different colors.
Lots of acid = green metallic sheen
small amount of acid = pink or blue center (fish eye)
no fermentation = colorless
aseptic transfer
conducting your work in a way that will not contaminate the culture itself, and also without contaminating your workspace to yourself with the specimen
keep the lid on the plate at all times, can come off to pick a colony and then immediately
how to achieve aseptic technique
disinfect work area
all tools that handle bacteria need to be sterile
loops/needles: flamed in the incinerator or Bunsen burner to sterilize
tubes, plates, etc: autoclaved
keep all cultures covered unless you are using it that second
if unaware if tools are sterile start over