New methods for screening lactic acid bacteria that produce bacteriocin active against Listeria monocytogenes and for determining bacteriocin activity

New methods using a microtiter plate for screening lactic acid bacteria (LAB) that produce bacteriocin active against Listera monocytogenes, and determination of bacteriocin activity using Omnispec 4000 were developed. In Modified Oxford (MOX, Oxoid) medium, L. monocytogenes produces a black color around the growth. In this study inhibition of L. monocytogenes by bacteriocin was determined using the color change of the medium in a microtiter plate. Because each well of a microtiter plate limits the spread of microbial growth activity, this method can overcome the disadvantage of the agar overlay method in which a large clear zone can cover other microorganisms that are not producing bacteriocin. This new method is rapid, simple, and economical. Among the 96 strains of LAB isolated from meat 25 strains produced bacteriocin against L. monocytogenes. The Omnispec 4000 was used to automatically record color changes due to the growth of L. monocytogenes in MOX medium containing six different concentrations of Nisin (bacteriocin) in wells of a microtiter plate. Depending on bacteriocin activities (IU). the slopes of the color changes recorded were different. The bacteriocin activities were determined easily and accurately by using the slopes to construct a calibration curve (r(2)=0.96).