Comparative recovery of uninjured and heat-injured Listeria monocytogenes cells from bovine milk

Abstract

The standard selective enrichment protocols of the Food and Drug Administration (FDA) and U.S. Department of Agriculture (USDA) were compared with an experimental nonselective broth enrichment (NSB) protocol and variations of the standard cold-enrichment (CE) protocol for the recovery of heat-injured Listeria monocytogenes. Bacterial cells (107/ml) were suspended in sterile milk and heated at 71.7°C in a slug-flow heat exchanger for holding times ranging from 1 to 30 s. Surviving cells were determined (50% endpoint) by the given protocols, and the following D values were obtained: NSB, D = 2.0 ± 0.5 s; FDA, D = 1.4 ± 0.3 s; USDA, D = 0.6 ± 0.2 s; CE, D ≤ 1.2 s. The respective direct-plating media used in these enrichments were also analyzed for recovery, and the following D values were calculated from the enumeration of surviving cells: NSB, D = 2.7 ± 0.8 s; FDA, D = 1.3 ± 0.4 s; USDA, D = 0.7 ± 0.2 s. The low levels of heat-injured L. monocytogenes cells which were detected at inactivation endpoints on the optimal nonselective media (25°C for 7 days) failed to recover and multiply during experimental CEs (4°C for 28 days). Initial inactivation experiments in which raw whole milk was used as the heating menstruum gave much lower recoveries with all protocols. The detectable limits for uninjured cells that were suspended inraw milk were similar (0.35 to 3.2 cells per ml) for the standard CE, FDA, and USDA protocols. Recovery by the NSB procedure (68 cells per ml) was compromised by background flora. The above data suggest that any cells surviving high-temperature, short-time pasteurization will be injured and unable to multiply either during cold storage of milk or in the FDA or USDA systems. Thus, L. monocytogenes cells recovered in finished pasteurized milk products by these detection methods probably represent uninjured environmental contaminants.