BI 234; PCC JESSE; MICROBIOLOGY LAB EXAM 1 QUESTIONS
AND ANSWERS WITH COMPLETE SOLUTIONS
Culture medium
A nutrient material prepared for the growth of microorganisms in a laboratory.
Must provide an energy source, as well as carbon, nitrogen, sulfur, and phosphorus.
Selective medium
a culture medium with an ingredient that inhibits the growth of one type of bacterium,
while allowing another type to grow.
Facultative anaerobes
Aerobes that do not REQUIRE oxygen, but will use it when present.
Can continue to grow without oxygen via fermentation or anaerobic respiration.
E.g. E. coli and yeasts
differential medium
contains ingredients that react with, change in response to, or appear differently after
growth - due to some characterisitc of microbial growth or metabolism.
Culture
The microbes that grow in/on a culture medium.
which to use first, selective or differntial?
, If using a test that is both selective and differential, always observe first for the selective
test, and only observe differential if selective is positive.
Obligate aerobes
Require oxygen for energy
E.g. most fungi and protozoa, some bacteria
PEA (phenylethyl alcohol agar)
confirms gram stain, selects for gram+ bacteria as it inhibits the growth of gram-
bacteria.
2.5% phenylethyl alcohol is selective ingredient as it interferes with outer membrane in
gram-
Pure culture
A colony of millions of microbes that grew from only one (clones)
MSA (Mannitol Salt Agar)
both selective and differential; selects for halotolerant (salt), inhibits non-halotolerant,
and differentiates for ability to ferment mannitol (sugar).
If mannitol ferments, pH lowers and acid turns phenol red (pH indicator) to yellow.
conclusions: halotolerant or non-halotolerant, then mannitol fermenting or non-mannitol
fermenting
Obligate anaerobes
Cease growth and replication in the presence of oxygen and often die
AND ANSWERS WITH COMPLETE SOLUTIONS
Culture medium
A nutrient material prepared for the growth of microorganisms in a laboratory.
Must provide an energy source, as well as carbon, nitrogen, sulfur, and phosphorus.
Selective medium
a culture medium with an ingredient that inhibits the growth of one type of bacterium,
while allowing another type to grow.
Facultative anaerobes
Aerobes that do not REQUIRE oxygen, but will use it when present.
Can continue to grow without oxygen via fermentation or anaerobic respiration.
E.g. E. coli and yeasts
differential medium
contains ingredients that react with, change in response to, or appear differently after
growth - due to some characterisitc of microbial growth or metabolism.
Culture
The microbes that grow in/on a culture medium.
which to use first, selective or differntial?
, If using a test that is both selective and differential, always observe first for the selective
test, and only observe differential if selective is positive.
Obligate aerobes
Require oxygen for energy
E.g. most fungi and protozoa, some bacteria
PEA (phenylethyl alcohol agar)
confirms gram stain, selects for gram+ bacteria as it inhibits the growth of gram-
bacteria.
2.5% phenylethyl alcohol is selective ingredient as it interferes with outer membrane in
gram-
Pure culture
A colony of millions of microbes that grew from only one (clones)
MSA (Mannitol Salt Agar)
both selective and differential; selects for halotolerant (salt), inhibits non-halotolerant,
and differentiates for ability to ferment mannitol (sugar).
If mannitol ferments, pH lowers and acid turns phenol red (pH indicator) to yellow.
conclusions: halotolerant or non-halotolerant, then mannitol fermenting or non-mannitol
fermenting
Obligate anaerobes
Cease growth and replication in the presence of oxygen and often die
