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 in raw 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.

EFFECT OF CONCURRENT HIGH HYDROSTATIC PRESSURE, ACIDITY AND HEAT ON THE INJURY AND DESTRUCTION OF LISTERIA MONOCYTOGENES

“…Control and pressure-treated cell suspensions (202 MPa, 45C, pH 5.2 and 6.0, 10 min) were plated on each medium. The criterion for selection of the optimal salt level was to define the salt concentration that suppressed the nonpressurized cells minimally while suppressing the pressurized cells maximally, relative to the nonsupplemented TPA (Crawford et al 1989). TPAN achieved differentiation via both reduced water activity (%=0.973) and intolerance of injured cells to salt (Busch and DOMelly 1992).…”

A study of the occurrence of Listeria species in raw sheep milk

“…Also they confirm that whole sheep, cow and goat milks containing high levels of L. monocytogenes (1 × 10 6 cfu/mL) could not survive the current high temperature short-time (HTST) plate pasteurizer protocol. Crawford et al (1989) suggest that two theoretical possibilities exist when L. monocytogenes cells are detected in an HTST-pasteurized milk product: either the prepasteurized raw milk was contaminated with a high enough level of the pathogen to allow some organisms to survive the pasteurization process; or the postpasteurized product was contaminated from the surrounding environment. The data from their report suggest, however, that any cells surviving HTST minimal processing (at inoculums of 1 × 10 7 /mL exposure to 71.7°C for ≤ 15 s in the absence of competitors) will be injured and unable to multiply during cold storage of milk, and it can be reasoned from their observations that L. monocytogenes cells detected in finished pasteurized milk products probably represent uninjured environment contaminants.…”

ENUMERATING LISTERIA MONOCYTOGENES IN FOOD: A PROBLEM OF LOW NUMBERS

“…Direct plates counts are quite simple and rapid, but are characterized by poor performances in terms of sensitivity, variability of results, recovery of stressed cells, and sometimes selectivity (Golden el al. 1988;Crawford et al 1989;Warburton et al 1992;Scotter et al 2001). The European and International Standard method for enumeration of L. monocytogenes EN IS0 11290-2 (Anon.…”