Long-chain unsaturated free fatty acids reduce the host cell invasion of Listeria monocytogenes outbreak strains

The Gram-positive bacterium <i>Listeria monocytogenes</i> is a highly adaptable pathogen capable of causing severe foodborne infections, particularly in vulnerable populations. During infection, <i>L. monocytogenes</i> uses a variety of virulence factors to invade and multiply within host cells. The transcriptional regulator PrfA controls the expression of these virulence factors and is essential for the intracellular lifestyle of <i>L. monocytogenes</i>. Long-chain unsaturated free fatty acids (FFAs) have long been recognized for their antimicrobial activity and were recently shown to inhibit PrfA-dependent virulence gene expression in <i>L. monocytogenes</i>. To date, the antimicrobial and anti-virulent activities of FFAs have been primarily studied in laboratory strains. However, to fully evaluate their potential as anti-infective agents, it is essential to assess the effects of long-chain FFAs on clinically relevant isolates, including outbreak strains associated with high-fat food products. Here, we demonstrate that five different clinically relevant <i>L. monocytogenes</i> isolates are sensitive to the antimicrobial activity of long-chain unsaturated FFAs. Furthermore, at subinhibitory concentrations, these FFAs inhibit PrfA-regulated expression of virulence factors across all tested strains and reduce their invasive potential in non-phagocytic cells. These findings underscore the potential of long-chain unsaturated FFAs in developing new preventive strategies against <i>L. monocytogenes</i> strains associated with severe foodborne infections.