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United States Department of Agriculture

Agricultural Research Service

Title: Development of a Selective Differential Agar for Isolation and Enumeration of Campylobacter Spp.

Author
item Line, John

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 28, 2001
Publication Date: N/A

Interpretive Summary: Direct plating is a relatively easy, proven method for determining how many bacteria may be present in a variety of sample types: however, distinguishing certain human foodborne pathogens, such as campylobacter, from non-campylobacter contaminants which frequently grow on many existing agars is difficult. More selective agars were developed to make detecting and counting Campylobacter easier by combining selective antibiotics and triphenyltetrazolium chloride (TTC). Growth of many bacteria turn the initially colorless TTC to compounds which give a red color to the growing colonies. We found that exposing Campylobacter to low levels of TTC did not inhibit its growth, yet was sufficient to give a deep red/magenta color to the colonies. The new agars (Campy-Line agar [CLA] and Campy-Line Blood Agar [CLBA]) are translucent. The contrast of deep red colonies on a translucent background makes Campylobacter isolation easier and makes counting on light boxes or by electronic means possible and will benefit those in the Food Safety or Medical fields. In comparison trials, approximately the same number of Campylobacter were recovered on each of the agars (P<0.05); however, isolation was much easier on the CLA or CLBA. Fewer than 3 non-campylobacter colonies were observed on the CLA or CLBA, whereas the Cefex agar supported the growth of between 0 and 72 contamination colonies per ml of sample.

Technical Abstract: Direct plating is an effective technique for isolation and enumeration of campylobacters from a variety of sample types: however, distinguishing campylobacters from non-campylobacter contaminants which frequently grow on many existing agars is difficult. More selective and differential agars were developed to facilitate Campylobacter enumeration by combining selective antibiotics and triphenyltetrazolium chloride (TTC). Growth of many microorganisms reduce the initially colorless tetrazolium salts to insoluble formazan compounds which impart a red color to the growing colonies. We found that exposing Campylobacter to low levels of TTC (200mg/L) was not inhibitory to growth, yet was sufficient to give a deep red/magenta color to the colonies. The new agars (Campy-Line agar [CLA] and Campy-Line Blood Agar [CLBA]) are translucent. The contrast of deep red colonies on a translucent background greatly facilitates Campylobacter isolation and makes enumeration on light boxes or by electronic means possible. Direct plating of broiler carcass rinse samples was compared on Campy-Cefex agar and CLA (n=76) or CLBA (n=61). Recovery of Campylobacter populations was not significantly different between the agars (P<0.05); however, isolation was much easier on the CLA or CLBA. Fewer than 3 non-campylobacter colonies were observed on the CLA or CLBA, whereas the Cefex agar supported the growth of between 0 and 72 contaminating cfu/ml.

Last Modified: 12/22/2014
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