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). For recovery of acid-injured Listeria monocytogenes, no difference (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.
Wu
V C
VC
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-1600, USA.
Fung
D Y
DY
Kang
D H
DH
Thompson
L K
LK
D000143:Acids; D000362:Agar; D001419:Bacteria / Q000302:isolation & purification*; D001431:Bacteriological Techniques; D003470:Culture Media; D019453:Escherichia coli O157 / Q000302:isolation & purification; D005516:Food Microbiology; D008089:Listeria monocytogenes / Q000302:isolation & purification; D012486:Salmonella typhimurium / Q000302:isolation & purification; D013211:Staphylococcus aureus / Q000302:isolation & purification; D015008:Yersinia enterocolitica / Q000302:isolation & purification
D000143:Acids; D003470:Culture Media; D000362:Agar