Effectiveness of chlorine wash on Listeria monocytogenes biofilm on onions

An investigation into the effectiveness of chlorine wash on Listeria monocytogenes biofilm on onions was carried out. Whole white onion bulbs were purchased from a retail shop at Nottingham, UK, and a strain of L. monocytogenes (NCTC 11944) was obtained from Food Microbiology Laboratory, University of Nottingham, UK. Chlorine treatment on L. monocytogenes biofilm was done according to standard method. Using different chlorine concentrations (20, 50, 100, 150 and 200 mg/l) for different time durations (0.5, 1, 2, 5, 10, 15 and 20 minutes) at pH 6.5, inoculated onion pieces were treated. Bacterial enumeration was done using serial dilution and spread plate method on chromogenic Listeria selective plates. Reduction of L. monocytogenes biofilm ranged from 1.5 – 2.4 log10 with the maximum reduction observed with 200 mg/l chlorine at 20 minutes treatment time. Similar treatment of L. monocytogenes in suspension (planktonic cells) gave up to 5 log reduction when 200 mg/l chlorine was applied for 20 minutes. The effectiveness of chlorine was more on L. monocytogenes in suspension than as a biofilm on onion.Keywords: Chlorine, biofilm, Listeria monocytogenes, onion, inoculation