Inhibition of growth of Listeria strains by mesenterocin 5 and organic acids

Abstract

The influence of lactic and acetic acid concentration on specific growth rate (mu) of Listeria strains in tryptic soy broth (TSB) was investigated. For each strain and acid, minimal and maximal inhibitory concentrations (MIC(min) and MIC(max) corresponding respectively to mu = mu(max) and mu = 0) were determined. In the range of MIC(min) and MIC(max); and for all the Listeria strains tested, mu decreased with increasing lactic and acetic acid concentrations in the TSB medium, although it was little affected in buffered (0.15 mol/l phosphate) TSB. For acid concentrations exceeding the MIC(max) by 0.05% (w/v), viable counts of Listeria strains remained nearly constant in unbuffered TSB at almost-equal-to 10(7) cfu/ml for 24 h. Mesenterocin 5 produced during overnight culture of Leuconostoc mesenteroides UL5 in MRS medium at 30-degrees-C was used in an inhibition test. Unconcentrated ultrafiltered-diafiltered mesenterocin 5 supernatant (0.1 ml) was added to 9.9 ml of TSB and 0.4 ml of 6 separate Listeria suspensions. Mesenterocin 5 exhibited, for the first 2 h at pH 7.0 and 30-degrees-C, a bacteriostatic effect on one strain and a bactericidal effect on 5 strains, ie, initial viable counts of 10(7) cfu/ml decreased by 1.0 to 3.0 log. The total biomass measured by optical density (OD) then remained constant for a period of 3 to 16 h before it eventually increased. After this extended lag-phase, mesenterocin 5 concentration (1 x C to 8 x C) affected the specific growth rate of most Listeria strains to a much lesser extent in contrast to the influence of organic acid concentration.