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 for more 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 hospitals and clean room operations, more stringent requirements are needed.
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 and received many awards, including the International Award from The Institute of Food Technologists. In addition to teaching, research and university services, he has initiated and directed the internationally renowned Workshop in Rapid Methods and Automation in Microbiology from 1981 to the present. For reprints on research generated from the SAS Air Sampler or information on the workshop, contact Dr. Fung at DFUNG@OZ.OZNET.KSU.EDU.

 

   

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