Biofilm formation and cell-to-cell signalling in Gram-negative bacteria isolated from a food processing environment

To investigate the biofilm-forming capacity and the production of quorum signals in Gram-negative bacteria isolated from a food production environment, and the possible correlation between both phenotypes.

Sixty-eight Gram-negative bacteria were isolated from equipment and working surfaces in a raw vegetable processing line, and tested for biofilm-forming capacity using an in vitro microplate assay. All isolates showed significantly higher biofilm-forming capacity than Escherichia coli laboratory strain DH5alpha, which was included as a negative control, and differed up to 56-fold in relative biofilm-forming capacity. Various assays based on reporter bacteria were used to detect quorum signals produced by the isolates. Twenty-six isolates produced autoinducer-2, five isolates produced N-acyl-homoserine lactones (AHLs), and none produced the Pseudomonas quinolone signal.

No correlation was found between in vitro biofilm-forming capacity and production of quorum signalling molecules among the 68 strains isolated from the raw vegetable processing line.

Several recent studies have shown a role of AHL-based quorum sensing in biofilm formation of specific Gram-negative bacterial strains. The current work shows that production of AHL and other quorum signals is not widespread in Gram-negative isolates from a raw vegetable processing line, and is not a general requirement for biofilm formation, at least in vitro.