Evaluation of thin agar layer method for recovery of acid-injured foodborne pathogens.

The thin agar layer (TAL) method of Kang and Fung was used to enumerate acid-injured foodborne pathogens. This method involves overlaying 14 ml of nonselective medium (tryptic soy agar [TSA]) onto a prepoured and solidified pathogen-specific, selective medium in a petri dish. After surface plating, injured cells resuscitated and grew on TSA during the first few hours of incubation; then, the selective agents from the selective medium diffused to the top layer, interacted with the recovered microorganisms, and started to produce typical reactions. Foodborne pathogens were exposed to 2% acetic acid for 1, 2, or 4 min, and the recovery rate with the TAL method was compared with the rate of TSA and pathogen-specific, selective media. No significant difference occurred between TSA and TAL (P > 0.05) for enumeration of acid-injured Escherichia coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus, and Yersinia enterocolitica, and both recovered significantly higher numbers than the selective medium for each respective pathogen (P 0.05) occurred among TSA. TAL, and selective media. However, fewer cells were recovered in the selective media. The TAL method is a one-step, convenient procedure for recovery of acid-injured cells. La methode de Kang et Fung couche mince de gelose a ete utilisee pour isoler et denombrer des bacteries pathogenes (Escherichia coli O157:H7, Yersinia enterocolitica, Staphylococcus aureus, Salmonella typhimurium) qui ont ete lesees par un traitement acide (acide acetique a 2% pendant 1, 2 ou 4 minutes). Un milieu selectif est etale en boite de Petri, puis recouvert par un milieu non selectif. Les bacteries lesees poussent sur le milieu non selectif, puis les substances actives diffusent depuis la couche inferieure vers la couche non selective et reagissent typiquement avec les microorganismes. Cette methode a ete comparee avec les methodes selectives ou non existant dans le commerce.