Thin agar layer method for recovery of heat-injured Listeria monocytogenes

A thin agar layer (TAL) method was developed to recover heat-injured Listeria monocytogenes. Modified Oxford medium (MOX), a selective plating medium, inhibits heat-injured L. monocytogenes from growing, whereas tryptic soy agar (TSA), a nonselective medium, does not. In order to facilitate recovery of heat-injured L. monocytogenes cells while providing selectivity of isolation of L. monocytogenes from other bacteria in the sample, a unique TAL procedure was developed by overlaying 5 mi of nonselective medium (TSA) onto prepoured and solidified MOX medium in an 8.5-cm-diameter petri dish. The injured L. monocytogenes repaired and started to grow in the TSA during the first few hours after incubation of the plate. During the resuscitation of injured cells, the selective agents from MOX diffused to the TSA top layer to inhibit other microorganisms. L. monocytogenes showed a typical reaction (black colonies) on TAL after 24 h of incubation at 37 degrees C. The recovery rate for heat-injured L. monocytogenes with the TAL method was compared with those rates associated with TSA, MOX, and the traditional overlay method (OV; pouring selective agar on top of resuscitated cells on TSA agar after 3 h incubation). Milk and 0.1% peptone water that were inoculated with L. monocytogenes (4 to 5 log CFU/ml) were heated for 15 min at 55 degrees C. L. monocytogenes was enumerated on TSA, MOX, QV, and TAL media and procedures. No significant difference occurred among TSA, OV, and TAL (P > 0.05) in terms of enumeration of heat-injured L. monocytogenes, but these media recovered significantly higher numbers than did MOX agar (P < 0.05)-in both samples. The TAL method involves only one step, whereas OV is a more cumbersome two-step procedure.