Effect of relative inoculum concentration on Listeria monocytogenes growth in co-culture

Growth suppression of multi-species bacterial populations in batch cultures by a single ‘dominant’ strain has been referred to in the literature as the ‘Jameson Effect’. The effect is often attributed to production of specific inhibitors of growth by one species against another. To explore its basis, we hypothesised that the Jameson Effect is often a non-specific interaction and that growth inhibition of species in co-culture can be controlled by manipulation of inoculum concentration and growth rate so as to enable a selected species to achieve stationary phase first. The hypothesis was assessed by co-culturing pairs of bacterial species under conditions selected to ensure that both strains grew at a similar rate and manipulating the initial concentration of each species. Specifically, the effect of inoculum concentration on the growth kinetics of Listeria monocytogenes when co-cultured in complex laboratory media with Escherichia coli, Pseudomonas fluorescens or a commercial strain of the lactic acid bacterium, Lactobacillus plantarum, was studied. Starting numbers of the second strain were either higher than (∼ 106 cfu mL− 1), equal to (∼ 104 cfu mL− 1) or lower than (∼ 103 cfu mL− 1) L. monocytogenes. In most trials, the initial inoculum concentration governed which species became dominant and suppressed the growth of the other strain. L. monocytogenes was suppressed by all other strains when its inoculum level was lower. Conversely, when L. monocytogenes was initially present at higher concentration than either P. fluorescens or L. plantarum, their growth was suppressed. E. coli, however, was not suppressed by L. monocytogenes even when the E. coli density was initially lower. While simple competition for nutrients could explain most of the observations, in some co-culture experiments pH reduction also seemed to play a role in inhibition of growth of some species. In other cases, available growth substrates were apparently not utilised by both co-cultured species, and the Jameson Effect did not occur. Thus, while it appears that under many conditions the Jameson Effect may be largely due to non-specific inhibition, more complex interactions between co-cultured strains involving species-specific pH limits for growth and differential utilisation of growth substrates may also occur and confound the simple Jameson Effect.