A solid agar overlay method for recovery of heat-injured Listeria monocytogenes.
A solid agar overlay method was developed for recovery of heat-injured Listeria monocytogenes. Presolidified nonselective tryptic soy agar with 0.6% yeast extract (TSAYE, 2% agar) was overlaid on top of solidified modified Oxford agar (MOX). Heat injury of L. monocytogenes was conducted at 58 degrees C for 6 min in a jacketed flask filled with tryptic soy broth. Both noninjured and heat-treated L. monocytogenes cells were plated onto TSAYE, MOX, and TSAYE-MOX plates. No significant differences (P > 0.05) in recovery were found among the three media for noninjured bacterial cells. Recovery of heat-injured L. monocytogenes cells on TSAYE-MOX overlay plates was equivalent to that on the nonselective TSAYE medium, whereas recovery on the selective MOX medium was significantly lower (P < 0.05) compared with both TSAYE and the overlay plates. There were no significant differences (P > 0.05) among the overlay plates prepared 0, 2, 4, 6, 8, 16, and 24 h prior to plating heat-injured bacterial cells. The TSAYE-MOX overlay also allowed differentiation of L. monocytogenes from a mixture of four other types of foodborne pathogens. This solid agar overlay method for recovery of heat-injured L. monocytogenes cells is less time-consuming and less complicated than the conventional overlay-underlay technique and the double overlay modification of the thin agar layer method and may allow for greater laboratory plating efficiencies.
Yan
Zhinong
Z
Center for Food Safety and Department of Food Science, University of Georgia, Griffin 30223, USA.
Gurtler
Joshua B
JB
Kornacki
Jeffrey L
JL
D000362:Agar / Q000737:chemistry*; D001431:Bacteriological Techniques; D015169:Colony Count, Microbial / Q000379:methods*; D003470:Culture Media; D005516:Food Microbiology; D006358:Hot Temperature; D008089:Listeria monocytogenes / Q000302:isolation & purification*; D012680:Sensitivity and Specificity; D013696:Temperature; D013997:Time Factors
D003470:Culture Media; D000362:Agar