Characterizing the culturable bacteria isolated from imported, ready-to-eat (RTE) foods for their ability to control Listeria monocytogenes

Listeria monocytogenes, an important foodborne pathogen, remains a significant threat to public health. This thesis investigated the culturable microbiota of select imported, RTE foods to see whether the existing bacterial microflora could inactivate, inhibit the growth and/or cause a reduction in the virulence of L. monocytogenes. Among all the foods tested (dried apple slices, cumin seeds, date fruits, fennel seeds, pistachios, pollen, raisins and seaweed), the date fruit microbiota displayed the most promise for harbouring antagonistic properties against L. monocytogenes. Of the 191 isolates recovered from five different date fruits, 36 (19%) produced zones of inhibition against L. monocytogenes that ranged from 0.3 to 5.8 mm. The inhibitory strains were all identified as Bacillus spp. Among those Bacillus spp. that were tested for their ability to inhibit PrfA, all caused a significant reduction in the activation of the PrfA protein (p-value < 0.05). In addition, the anti-Listeria compound(s) produced by B. altitudinis DS11 were found to be proteinaceous in nature, acid and alkali-tolerant and resistant to temperature treatments up to 100oC.