In-vitro evaluation of Pseudomonas fluorescens antibacterial activity against Listeria spp. isolated from New Zealand horticultural environments

Beneficial bacteria with antibacterial properties are an attractive alternative to chemical-based antibacterial or bactericidal agents. The aim of our study was to source such bacteria from horticultural produce and environments and to explore the mechanisms of their antimicrobial properties. Four strains of Pseudomonas fluorescens were isolated that possessed antibacterial activity against the pathogen Listeria monocytogenes. These strains (PFR46H06, PFR46H07, PFR46H08 and PFR46H09) were tested against L. monocytogenes (n=31), L. seeligeri (n=1) and L. innocua (n=1) isolated from seafood and horticultural sources, and two L. monocytogenes from clinical cases (Scott A and ATCC 49594, n=2). All Listeria strains were inhibited by the three strains PFR46H07, PFR46H08 and PFR46H09 with average zones of inhibition of 14.8, 15.1 and 18.2 mm, respectively, with PFR46H09 having significantly more inhibition than the other two (p<0.05). PFR46H06 showed minimal inhibition towards the L. seeligeri and L. innocua isolates and no inhibition towards most of the L. monocytogenes isolates. In order to investigate the functional protein differences between these strains, all four strains were subjected to whole cell lysate proteomics; data are available via ProteomeXchange with identifier PXD019965. We found significant differences in the peptide profiles and protein summaries between these isolates. The total number of proteins identified was 1781 in PFR46H06, 2030 in PFR46H07, 2228 in PFR46H08 and 1994 in PFR46H09. We paid special attention to secretion system proteins as these play a main role in the defense mechanisms of bacteria, particularly those of the type VI secretion system (T6SS) and found that they varied significantly (p<0.001) among the four isolates. PFR46H06 and PFR46H07 possessed the fewest secretion system structural proteins (12 and 11 respectively) while PFR46H08 and PFR46H09 each had 18. PFR46H09, which showed the greatest antimicrobial effect, had nine T6SS proteins compared to just four in the other three strains.