Natural antimicrobials are of interest to replace traditional food decontamination methods: they are milder and maintain desirable sensory characteristics. However, efficacy can be affected by food structure/composition, thus structural effects in a co-culture pathogen/microflora system are investigated. Listeria was grown planktonically (liquid broth) or on a biphasic viscoelastic system, in monoculture with/without artificial nisin, or in co-culture with L. lactis (nisin/non-nisin producing). Microbial growth kinetics were monitored and advanced microscopy techniques were utilised to quantify cellular interactions and spatial organisation.

Microstructural effects are observed on the kinetics, with differences in monoculture/co- culture. Significant microscopic differences are observed in spatial organisation and colony size. We are the first to observe changing growth location for all species in monoculture/co- culture, with differences in colony size/organisation through stationary phase. This study provides insight into the environmental stress response/adaptation of Listeria grown on structured systems in response to L. lactis and natural antimicrobials.