The Secondary Colonizer Hypothesis: an Ecological View of Listeria monocytogenes Persistence in Food Processing Environments

Listeria monocytogenes demonstrates remarkable persistence in food processing environments, particularly in areas with established biofilm communities. This communication synthesizes emerging evidence for a novel ecological strategy whereby L. monocytogenes functions as a secondary colonizer, exploiting central voids created during Gram-negative bacterial biofilm dispersal. We propose the "secondary colonizer hypothesis" - that L. monocytogenes utilizes a sophisticated temporal succession strategy, colonizing biofilm voids characterized by steep oxygen gradients and phosphatidylethanolamine-rich debris from dispersed primary colonizers. This strategy employs specialized metabolic pathways including cobalamin-dependent catabolism of ethanolamine, propylene glycol, and glycerol while simultaneously utilizing ferric iron reduction for respiration. The metabolic coupling allows each bacterial cell to accumulate iron via ferritin-like protein storage, creating competitive advantages through localized limitation. Recent evidence identifies Veillonella as a key bridging species that may facilitate this ecological succession through metabolic complementarity. Notably, while L. monocytogenes co-occurs with Gram-positive bacteria, its metabolic specialization for ethanolamine and iron scavenging suggests a preferential adaptation to the nutrient-dense voids of dispersed Gram-negative biofilms. The implications for food safety protocols, particularly cleaning chemical formulations and ecological monitoring strategies, warrant immediate attention.