MadSci Network: Microbiology

Re: I don't know how to count the bacteria.

Date: Tue Apr 20 21:00:33 1999
Posted By: Glynis Kolling, Grad student, Food Science, Rutgers Univeristy
Area of science: Microbiology
ID: 924217149.Mi

Kim - 
   This is a very good question, and is of importance when you need an idea 
of how many bacteria are actually present.  As you may have already seen, 
temperature will have marked effects of bacterial growth.  
   Microbiologists base their counting on the assumption that one single 
bacteria can give rise to one colony on solid media.  Each colony is 
referred to as a Colony Forming Unit or CFU for short.  If you are 
measuring bacteria in water, a known amount would be plated out (i.e. 1 ml 
of water/ agar plate).  Once colonies are visible, you can count these (say 
you count 100 colonies) and then express your final value as 100 CFU/ml of 
water.  Another word on counting.  If you have so many bacteria that you 
begin to loose count, divide the plate into quarters, halves, etc. and 
count one section.  To get the total number, multiply back by how many 
divisions the plate was split into (i.e. 100 CFU in one quarter of the 
plate x 4 = 400 CFU total).  

   If this is not feasible for your experiments, you could just express 
them as CFU.  You could count the different types of colonies based on 
their morphology (shape) and color and graph these CFU. If you have access 
to different agar media, you could also compare how your bacteria grow on 
these.  Some bacteria require many special nutrients for growth 
(fastidious) while others can grow with limited nutrients (non-fastidious).

   For graphing purposes, CFU are usually expressed on a log scale.  So 400 
CFU would be equal to 2.60 CFU (on a log scale).  The reason that we use a 
log scale is because bacteria multiply by fission and therefore undergo 
exponential growth.  Numbers of bacteria can be as high as 10,000,000,000 
CFU/ ml, which is easily expressed as 10 CFU/ml on a log scale.

   As far as your studies go, you may classify your bacteria into three 
classes based on their temperature requirement.  The three general classes 
are psychrophiles (<7 C - 20 C), mesophiles (22 C - 45 C), and thermophiles 
(45 C - 90 C!).  These ranges are pretty rough, but from these you may 
conclude that you may find psychrophilic bacteria in your refrigerator 
while you may find mesophilic bacteria in your mouth.  Hope that this 
information has helped you!  Good luck!

    Glynis Kolling
    Graduate Student
    Food Microbiology



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