Biofilm formation by acid-adapted and nonadapted Listeria monocytogenes in fresh beef decontamination washings and its subsequent inactivation with sanitizers

Abstract

The antimicrobial effects of sodium hypochlorite (SH, 200 ppm, at an adjusted pH of 6.80 ± 0.20 and at an unadjusted pH of 10.35 ± 0.25), quaternary ammonium compound (pH 10.20 ± 0.12, 200 ppm), and peroxyacetic acid (PAA, pH 3.45 ± 0.20, 150 ppm) on previously acid-adapted or nonadapted Listeria monocytogenes inoculated (105 CFU/ml) into beef decontamination water washings were evaluated. The effects of the sanitizers on suspended cells (planktonic or deattached) and on cells attached to stainless steel coupons obtained from inoculated washings stored at 15°C for up to 14 days were studied. Cells were exposed to sanitizers on days 2, 7, and 14. The pathogen had formed a biofilm of 5.3 log CFU/cm2 by day 2 of storage (which was reduced to 4.6 log CFU/cm2 by day 14), while the total microbial populations showed more extensive attachment (6.1 to 6.6 log CFU/cm2). The sanitizers were more effective in reducing populations of cells in suspension than in reducing populations of attached cells. Overall, there were no differences between previously acid-adapted and nonadapted L. monocytogenes with regard to sensitivity to sanitizers. The total microbial biofilms were the most sensitive to all of the sanitizers on day 2, but their resistance increased during storage, and they were at their most resistant on day 14. Listeria monocytogenes displayed stronger resistance to the effects of the sanitizers on day 7 than on day 2 but had become sensitized to all sanitizers by day 14. SH at the adjusted pH (6.80) (ASH) was generally more effective in reducing bacterial populations than was SH at the unadjusted pH. PAA generally killed attached cells faster at 30 to 300 s of exposure than did the other sanitizers, except for ASH on day 2. PAA was more effective in killing attached cells than in killing cells treated in suspension, in contrast to the other sanitizers.