Listeria monocytogenes dry surface biofilms: a comparative study of mono- and coculture with Enterococcus faecium.

Dry surface biofilms (DSB) represent a challenge for microbial control in the food industry, since they show high desiccation resistance. This study evaluated the formation, viability, and composition of Listeria monocytogenes DSB in mono- and coculture with Enterococcus faecium. DSB were formed on polypropylene (PP) and stainless steel (SS) coupons using two hydrated-dry cycling protocols: P1 (48/48 h) and P2 (24/120 h), followed by storage (25 °C) for 42 days. At the forming endpoint (day 0), in DSB mono-species, Listeria monocytogenes was ca. 2 log CFU/cm2 higher on PP than on SS (p = 5.1 e-05). The pathogen remained viable for up to 42 days, but with reductions of ≥1.7 log CFU/cm2, compared to day 0. In dual-species DSB, Enterococcus faecium inhibited Listeria monocytogenes regardless of surface or protocol, maintaining pathogen levels at or below the detection limit (1.4 log CFU/cm2) throughout storage. Meanwhile, Enterococcus faecium achieved 5 log CFU/cm2 on day 0 and 3.4 log CFU/cm2 after 42 days. In addition, catalase treatment resuscitated VBNC Listeria monocytogenes cells on day 0 and within 14 days of storage, increasing culturability by up to 1.0 log CFU/cm2. In general, DSB matrices contained higher protein levels than carbohydrates and eDNA, and Listeria monocytogenes exhibited morphological alterations, including elongation and L-form-like cells. Overall, these findings highlight the need for strict hygiene measures to control DSB and suggest that Enterococcus faecium may be an effective strategy to inhibit the Listeria monocytogenes DSB formation.