Biofilms of Listeria monocytogenes Produced at 12 ° C either in Pure Culture or in Co-Culture with Pseudomonas aeruginosa Showed Reduced Susceptibility to Sanitizers

The biofilm-forming ability of 21Listeria monocytogenesisolates, previously pulsotyped and corresponding to 16 strains, from different origins was evaluated using the Calgary Biofilm Device^®, at 37 °C. Biofilms of 4 selected strains were also produced either on pure cultures or on co-cultures withPseudomonas aeruginosa(PAO1), at 12 °C and at 37 °C. For these biofilms, the minimum biofilm eradication concentrations (MBECs) of 4 commercial dairy sanitizers (1 alkyl amine acetate based-T99, 2 chlorine based-T66 and DD, and 1 phosphoric acid based-BP) were determined.Listeria monocytogenesbiofilms grown, either at 37 °C or 12 °C, were able to achieve similar cell densities by using different incubation periods (24 h and 7 d, respectively). In co-culture biofilms,P. aeruginosawas the dominant species, either at 37 °C or at 12 °C, representing 99% of a total biofilm population of 6 to 7 log CFU/peg. Co-culture biofilms were generally less susceptible thanL. monocytogenespure cultures. More interestingly, the biofilms produced at 12 °C were usually less susceptible to the sanitizers than when produced at 37 °C. Single or co-culture biofilms ofL. monocytogenesand PAO1, particularly produced at 12 °C, retrieved MBEC values for agents T99 and BP that were, at times, above the maximum in-use recommended concentrations for these agents. The results presented here reinforce the importance of the temperature used for biofilm formation, when susceptibility to sanitizers is being assessed.