Innovative Sampling of Injured Microbial Cells

By Daniel Y. C. Fung, Professor of Food Science, Kansas State University

From Bioscience World

WHEN MICROBIAL CELLS ARE TREATED with heat, cold, chemicals, dehydration, radiation, or pressure, three populations of cells usually exist. One survives; another dies; a third survives but is injured. These injured cells can repair under favorable conditions, and later grow and metabolize as healthy cells. If we do not use the proper method to resuscitate injured cells, we may underestimate the existence of potentially pathogenic organisms in environmental samples.

At Kansas State University, I led a group of researchers to work on a one-step Thin Agar Layer (TAL) method to recover injured cells. Alayer of selective agar is first placed at the bottom of the Petri dish to solidify. Then, a layer of non-selective agar is poured over the selective agar. After solidification, the population of healthy and injured cells is spread above the thin agar layer. In three hours, the injured cells on top will repair while the inhibitory compounds from the selective agar at the bottom migrate upward. When the compounds reach the top of the non-selective agar, injured target cells are repaired and resist toxic effects of the selective agar.

Dr. Crozier-Dodson modified the TAL method for use in the SAS Super 180 Microbial Air Sampler and collected data on the recovery of microorganisms in the air of beef, poultry, and swine confinement units. Typically, microbes in the air are injured due to lack of nutrients and dehydration. It is difficult to recover Gram-negative organisms directly from air due to the injury. Gram-positive organisms are more resistant to the air environment and can be recovered more easily. So, the recovery of airborne bacteria mostly consists of Gram-positive bacteria. In fact, many Gram-negative bacteria may be present and not recovered due to lack of a proper technique. Dr. Crozier-Dodson’s research found that the use of selective agar alone greatly underestimated the amount of Gram-negative organisms. The TAL, when used with the SAS Air Sampler, can
improve the study of aeromicrobiology.

This TAL method has been tested on a variety of food pathogens such as Salmonella, Listeria monocytogenes, Staphylococcus aureus, Yersinia enterocolitica, E. coli O157:H7, after these cells have been injured by heat, cold, organic acids, salt, radiation, and high pressure.

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



For More Information Contact:

Bioscience International
11333 Woodglen Drive
Rockville MD 20852
Tel: 301-231-7400
FAX: 301-231-7277