Make Campylobacter easier to count.

Imagine trying to count the raindrops on your car's windshield after a light rain. That's what it can be like for scientists to count colonies of Campylobacter that are growing in round petri dishes.

The agar medium that's used to grow Campylobacter contains blood components or charcoal, giving the agar a dark color. Unfortunately, Campylobacter colonies are clear, often appearing like water droplets on the agar. USDA-ARS technologists have found a way to make the task of counting them easier. Their new technique, available for licensing, can be used in laboratories to conduct diagnostic testing.

Scientists typically use the direct plating technique to isolate and count microscopic organisms. The direct plating approach can be used to grow and count Campylobacter from a variety of samples. But it is difficult to distinguish Campylobacter from non-Campylobacter contaminants that often grow on many existing agars.

The researchers found that exposing Campylobacter to low levels of triphenyltetrazolium chloride does not harm its growth, yet stains the colonies deep red to magenta. New agars used for Campylobacter growth are translucent, resulting in a contrast of dark colonies on the translucent background. This greatly facilitates Campylobacter isolation and makes it possible to count the number of colonies by placing the medium on light boxes or on other electronic sources.

But contamination by other organisms can still occur. In this instance, even if contaminated colonies show up as red, most of them are easily distinguished from Campylobacter based on differences in shape and structure.

FYI: Campylobacter is thought to be the leading cause of bacterial food poisoning in humans and is likely the perpetrator of more than 400 million cases of diarrhea every year. More than 10,000 cases of human campylobacteriosis are reported to the U.S. Centers for Disease Control and Prevention each year, though many more cases go undiagnosed or unreported. This illness is characterized by diarrhea, cramping, abdominal pain and fever.

Further information. Eric Line, USDA-ARS Poultry Microbiological Safety Research Unit, Richard B. Russell Agricultural Research Center, 950 College Station Rd., Athens, GA 32604; phone:706-546-3522; fax: 706-546-3772; email: eric.line@ars.usda.gov.

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