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.
Center for Food Safety and Department of Food Science, University of Georgia, Griffin 30223, USA.
Zhinong
Z
Yan
Joshua B
JB
Gurtler
Jeffrey L
JL
Kornacki
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