Control of listefia monocytogenes in a Biofilm by competitive-exclusion microorganisms

Biofilms from drains in food processing facilities with a recent history of no detectable Listeria monocytogenes in floor drains were cultured for microorganisms producing antilisterial metabolites. A total of 413 microbial isolates were obtained from 12 drain biofilm samples and were assayed at 15 and 37degreesC for activities that were bactericidal or inhibitory to L. monocytogenes, by two agar plate assays. Twenty-one of 257 bacterial isolates and 3 of 156 yeast isolates had antilisterial activity. All 24 isolates which produced metabolites inhibitory to L. monocytogenes were assayed for antilisterial activity in coinoculated broth cultures containing tryptic soy broth with yeast extract (TSB-YE). A five-strain mixture of 10(3) CFU of L. monocytogenes/ml and 10(5) CFU of the candidate competitive-exclusion microorganism/ml was combined in TSB-YE and incubated at 37degreesC for 24 h, 15degreesC for 14 days, 8degreesC for 21 days, and 4degreesC for 28 days. Substantial inhibition of L. monocytogenes growth (4 to 5 log CFU/ml) was observed for nine bacterial isolates at 37degreesC, two at 15 and 8degreesC, and three at 4degreesC. The inhibitory isolates were identified as Enterococcus durans (six isolates), Lactococcus lactis subsp. lactis (two isolates), and Lactobacillus plantarum (one isolate). The anti-L. monocytogenes activity of these isolates was evaluated in biotilms of L. monocytogenes on stainless steel coupons at 37, 15, 8, and 4degreesC. Results revealed that two isolates (E. durans strain 152 and L. lactis subsp. lactis strain C-1-92) were highly inhibitory to L. monocytogenes (growth inhibition of >5log(10) CFU of L.monocytogenes/cm(2)). These two bacterial isolates appear to be excellent competitive-exclusion candidates to control L. monocytogenes in biofilms at environmental temperatures of 4 to 37degreesC.