Each group will receive two bacterial cultures, Escherichia
coli
and Streptococcus
mutans. You will have 1 ml of
each, but the number of
bacteria in that culture is unknown. Your mission is to determine the
number of
colony forming units (c. f. u.) in your 1 ml cultures.
You will perform a serial dilution, or in other
words,
dilute your bacterial culture more and more until the number of
colonies is
easy to count.
Label all plates and tubes before you
start! You will do two dilution series for each culture (these are
called replicates, and should be labeled A and B). Tubes should be
labeled with the culture type, replicate, and dilution. Plates should
be labeled with your group name, dilution, replicate, and culture type.
Put 900 µl of LB broth into each tube
for each dilution.
Take 100 µl of the original inoculum
and pipet into the first dilution tube (1:10). Mix the contents of the
tube well.
Take 100 µl of the previous dilution
and pipet into the next dilution and mix well. For example, for the
1:100 dilution, take 100 µl
of the 1:10 dilution and mix with 900ul of LB broth in the 1:100
dilution tube.
Repeat until all dilutions are mixed.
Spread 100 µl of each dilution onto its
corresponding plate.
Incubate plates overnight at 37° C, and
we will count colonies that grow tomorrow!
Materials necessary
Overnight
liquid culture of bacteria (E. coli and S.
mutans)