The CFS of all of the LAB strains assayed showed antilisteria activity on LM1. By comparison, some strains showed a lower antagonistic effect while others showed none when LAB supernatants were neutralized. These results agree with those recently reported by Aguilar and Klotz (2010). They reported antimicrobial activity of CFS from Lactobacillus species and found a higher inhibitory effect with untreated supernatants, as compared to neutralized supernatants. Similar findings were obtained by Sica et al. (2010), who stated that all CFS from LAB showed antimicrobial activity on L. monocytogenes, and only some supernatants maintained their inhibitory activity after being neutralized with 5 N
sodium hydroxide. The authors explained that the loss of antimicrobial activity after neutralizing the supernatant was due to the fact that the strainswere only capable of producing organic acids. In contrast, Botina et al. (2008) found that the supernatants of 11 LAB did not show antimicrobial activity on L. monocytogenes when tested by the well diffusion method. Also, Mezaini et al. (2009) demonstrated that only 2 LAB supernatants inhibited the growth of the pathogen. In a recent study with lactic bacteria from fish, Marguet et al. (2011) showed that 7 of 74 CFS (9.45%) presented
antilisterial activity. While the percentage found by these authors is very low, our results are in agreement because CFS of some strains maintained their activity on Listeria spp. after being neutralized. In conclusion, the isolation of lactic acid bacteria with antilisterial activity from brewer's grains is promising based on their
capacity to produce antimicrobial compounds and not only organic acids. The production of antimicrobial metabolites by LAB in the substrate would generate an unfavorable environment for the growth of the pathogenic bacterium under study. Further studies
are needed in order to determine the potential efficacy of these bacteria as antilisterial agents.