Studies on the biocontrol mechanisms of Pseudomonas graminis strain CPA-7 against food-borne pathogens in vitro and on fresh-cut melon

•CPA-7 inhibits Listeria monocytogenes growth in cv 'Galia' melon.•CPA-7 has not shown proteolytic or hemolytic activity on agar medium.•CPA-7 does not produce alginate or form biofilms on inert plastic surfaces.•No detectable antibiotics were produced by CPA-7 in cv 'Galia' melon or synthetic media.•CPA-7 has no antagonistic effect against food-borne pathogens in melon juice.

The present study was aimed at gaining insight into the mode of action of the antagonistic bacteria Pseudomonas graminis CPA-7, which has been previously identified as an effective biocontrol agent against Listeria monocytogenes, Salmonella enterica and Escherichia coli O157:H7 on fresh-cut fruit. In vitro experiments did not reveal any antimicrobial or proteolytic activity on solid media or any biosurfactant activity on hydrophobic surfaces. Metabolites produced by CPA-7 in two different culture media and on 'Galia' melon were unable to inhibit L. monocytogenes populations on 'Galia' melon plugs at 25 °C or 5 °C. In contrast, at 25 °C the population of this pathogen on 'Galia' plugs was reduced by 2.1 and 3.3 log-units when co-inoculated with the antagonist in water, after 24 and 48 h, respectively. CPA-7 did not form biofilms after 72 h at 25 °C (OD = 0.03) or at 30 °C (OD = 0.01) on polystyrene plates and the production of alginate was close to the negative control. Studies of nutritional profiles showed high overlap (NOI > 0.9) between CPA-7 and E. coli O157:H7 regarding the use of carboxylic acids. This functional group could also contain putative targets for competiveness between CPA-7 and S. enterica, although overlapping was not restrictive enough (NOI = 0.83).