Viable Cell Counts
By Daniel Y. C. Fung, Professor of Food Science, Kansas State University
From Bioscience World, Summer 2009
THE WELL-KNOWN viable cell
count has been serving the field
of applied microbiology effectively
formore than a century. Simply
stated, on or in the appropriate agar
medium, one living cell or a few living
cells grouped together can grow
to a mass visible to the human eye,
usually as a round colony. The number
of organisms in one visible colony
is actually about one billion cells (1 x
109 or log 9 cells). Since a round visible
colony could have been developed
from one or more individual
cells, it is referred to as a Colony Forming Unit or CFU.
Just knowing the number of CFU’s
on or in an agar medium has very little
meaning unless the number is
related to per volume of liquid (ml),
per weight of matters (g), per area of a
surface (cm square), or per volume of
air (cubic meter). After more than 30
years of working in applied and food
microbiology, I have developed the
Fung Scales for CFU for general and
food microbiology.
These scales are for general and
normal environmental microbial populations. No frank pathogen, such as
Escherichia coli O157:H7, Salmonella,
Listeria monocytogenes, etc., is allowed
in any ready-to-eat foods or drinks for
consumption or for food preparation
environments. On the other hand, in
fermented foods and drinks, high
numbers of desirable microorganisms,
such as, Lactobacillus acidophilus,
Streptococcus thermophilus, Saccharomyces
cerevisiae, etc. are encouraged.
To obtain microbial counts from
the air, an impactor air sampler, such
as the SAS Microbial Air
Sampler, is used to pull a
known volume of air and
impact the particles which
may contain live
microbes on an
agar medium
and later incubate
the agar
plate at the
appropriate temperature
for about 24 to 48 hours
to let the organisms
grow to visible colonies
and then convert the
CFU into cubic meters
by knowing the volume
of air impacted over the
agar plate.
Viable cell counts of normal microbial flora and of specific target pathogens in/on air, food, water, surfaces and the general environment are important information for the protection of humans, animals and plants.
Long Live Viable Cell Count! We can Count on You.
(An expanded version of this article,
which covers standards for surfaces,
may be requested from Bioscience
International.)
| Fung Scales for CFU | |
| For Air | |
| 0-100 CFU/cubic meter | Acceptable level, no concern |
| 100-300 CFU/cubic meter | Intermediate level, slight concern |
| >300 CFU/cubic meter | Serious concern, need corrective actions |
The above scale is for general food processing environments. |
|
| For solid, liquid food, or food surfaces | |
| Log 0 to Log 2 CFU/g, ml, 10² | Low count, no concern |
| Log 3 to Log 4 CFU/g, ml, 10² | Intermediate count, slight concern |
| Log 5 to Log 6 CFU/g, ml, 10² | High count, serious concern |
| Log 7 CFU/g, ml, 10² | Index of Spoilage, food will soon spoil |
| Log 8 CFU/g, ml, 10² | Odor development, not acceptable |
| Log 9 CFU/g, ml, 10² | Slime development, highly unacceptable |
| Log 10 CFU/g, ml, 10² | Discard immediately |
Daniel Y.C. Fung, Ph. D. |
A world-renowned applied microbiologist in
rapid methods and automation in microbiology,
Dr. Fung has published
nearly 600 publications For reprints on research generated from
the SAS Air Sampler or information on |
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