A NOVEL STRICTLY ANAEROBIC RECOVERY AND ENRICHMENT SYSTEM INCORPORATING LITHIUM FOR DETECTION OF HEAT-INJURED LISTERIA-MONOCYTOGENES IN PASTEURIZED MILK CONTAINING BACKGROUND MICROFLORA

Heat-injured cells of Listeria monocytogenes were recovered from heate d raw milk containing noninjured Enterococcus faecium by combining a s imple method for obtaining strict anaerobiosis with a novel enrichment broth, Penn State University broth (PSU broth). Strictly anaerobic co nditions were rapidly achieved by adding 0.5 g of filter-sterilized cy steine per liter to PSU broth and then purging the preparation with N- 2 gas. Little resuscitation or growth occurred in strictly anaerobic P SU broth without lithium chloride because of overgrowth by E. faecium. The growth of E. faecium decreased dramatically with increasing LiCl concentration; LiCl concentrations of 8 and 10 g/liter were completely bacteriostatic. The mechanism of inhibition by LiCl appeared to invol ve competition with the divalent cations Ca2+ and Mg2+. Heat-injured L . monocytogenes consistently recovered and grew rapidly in strictly an aerobic PSU broth containing 4, 6, or 7 g of LiCl per liter. The use o f strictly anaerobic PSU broth containing 7 g of LiCl per liter permit ted detection of severely heat-injured L. monocytogenes in one simple recovery-enrichment step by eliminating oxygen toxicity and inhibiting the growth of background microflora, without preventing the resuscita tion and subsequent growth of heat-injured L. monocytogenes. L. monocy togenes heated in raw milk at 62.8 degrees C for 10, 15, and 20 min co uld be consistently recovered from strictly anaerobic PSU broth enrich ment cultures at 30 degrees C after 48, 96, and 144 h, respectively, a nd hence, use of PSU broth may result in better recovery of both injur ed and noninjured cells from foods than currently used U.S. Department of Agriculture and Food and Drug Administration preenrichment procedu res.