A novel strictly anaerobic recovery and enrichment system incorporating lithium for detection of heat-injured Listeria monocytogenes in pasteurized milk containing background microflora

Abstract

Heat-injured cells of Listeria monocytogenes were recovered from heated raw milk containing noninjured Enterococcusfaecium by combining a simple method for obtaining strict anaerobiosis with a novel enrichment broth, Penn State University broth (PSU broth). Strictly anaerobic conditions were rapidly achieved by adding 0.5 g of filter-sterilized cysteine per liter to PSU broth and then purging the preparation with N2 gas. Little resuscitation or growth occurred in strictly anaerobic PSU broth without lithium chloride because of overgrowth by E. faecium. The growth ofE.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 involve competition with the divalent cations Ca2+ and Mg2+. Heat-injured L. monocytogenes consistently recovered and grew rapidly in strictly anaerobic PSU broth containing 4, 6, or 7 g of LiCl per liter. The use of strictly anaerobic PSU broth containing 7 g of LiCl per liter permitted 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 resuscitation and subsequent growth of heat-injured L. monocytogenes. L. monocytogenes heated in raw milk at 62.8°C for 10, 15, and 20 min could be consistently recovered from strictly anaerobic PSU broth enrichment cultures at 30°C after 48, 96, and 144 h, respectively, and hence, use of PSU broth may result in better recovery of both injured and noninjured cells from foods than currently used U.S. Department of Agriculture and Food and Drug Administration preenrichment procedures.

Inorganic salts and compatible solutes help mesophilic bacteria inhabit the high temperature waters of a Trans-Himalayan sulfur-borax spring

“…Among the four kosmotropic solutes tested, the alkali metal lithium was found to be the most thermoprotective at both 50°C and 70°C. This finding is consistent with previous studies where Listeria monocytogenes heated in raw milk at 62.8°C for 10, 15, and 20 min was recovered after 48, 96 and 144 h 775 respectively, by adding 7 g L -1 LiCl to the revival culture incubated at 30°C in rich heterotrophic medium (Mendonca and Knabel, 1994). Thermoprotective effect of lithium could be linked, but not necessarily restricted, to its activities at the periphery of cells.…”

Recovery of heat-injured Listeria innocua

“…The presence of LiCl, even in low concentrations, inhibits bacterial growth by competing with essential divalent cations, such as calcium and magnesium. It is possible that critical metalloenzymes are inactivated when the monovalent cation Li + replaces divalent cations on these enzymes (Mendonca and Knabel, 1994).…”

Impact of monocytic cells on recovery of uncultivable bacteria from atherosclerotic lesions

“…The pelleted THP‐1 cells were re‐suspended in 2 mL of PBS, washed with this volume of medium two times by centrifugation at 800 g for 7 min, re‐suspended in 1 mL of water to lyse the THP‐1 cells, and after concentration via high‐speed centrifugation, the entire lysate was plated. Mendonca [12] reported that noncultivable Listeria could be recovered only under strictly anaerobic conditions. Accordingly, we plated the lysate on FAA plates and incubated them under anaerobic conditions at 37 °C for 14 days.…”