Transcriptome analysis of Listeria monocytogenes exposed to beef fat reveals antimicrobial and pathogenicity attenuation mechanisms.

Listeria monocytogenes is a deadly intracellular pathogen mostly associated with consumption of ready-to-eat foods. This study investigated the effectiveness of total beef fat (BF-T) from flaxseed fed cattle, and its fractions enriched with monounsaturated fatty acids (BF-MUFA) and polyunsaturated fatty acids (BF-PUFA) along with commercially available long chain fatty acids (LC-FA) as natural antimicrobials against L. monocytogenes BF-T was ineffective at concentrations up to 6 mg/ml, while L. monocytogenes was susceptible to BF-MUFA and BF-PUFA with the minimum inhibitory concentrations (MICs) at pH 7 being 0.33 ± 0.21 mg/ml and 0.06 ± 0.03 mg/ml, respectively. The MIC of C14:0 was significantly lower than those of C16:0 and C18:0 (P < 0.05). c9-C16:1, C18:2n-6 and C18:3n-3, showed stronger inhibitory activity than c9-C18:1 and conjugated C18:2, with MICs < 1 mg/ml. Furthermore, global transcriptional analysis by RNA-seq was performed to characterize the response of L. monocytogenes to selected FA. Function analysis indicated that antimicrobial LC-UFA repressed the expression of genes associated with nutrient transmembrane transport, energy generation, and oxidative stress resistance. On the other hand, upregulation of ribosome assembly and translation process is possibly associated with adaptive and repair mechanisms in response to LC-UFA. Virulence genes and genes involved in bile, acid and osmotic stress were largely downregulated, more so for c9-C16:1, C18:2n-6 and C18:3n-3, likely through interaction the master virulence regulator PrfA and the alternative Sigma factor σB Importance Listeria monocytogenes is a bacterial pathogen known for its ability to survive and thrive under adverse environments and as such its control poses a significant challenge, especially with the trend of minimally processed and ready-to-eat foods. This work investigated the effectiveness of fatty acids from various sources as natural antimicrobials against L. monocytogenes, and evaluated their potential role in L. monocytogenes pathogenicity modulation, using the strain ATCC 19111. The findings show LC-UFA including unsaturated beef fat fractions from flaxseed fed cattle could have the potential to be used as effective antimicrobials for L. monocytogenes, through controlling growth as well as virulence attenuation. This not only advances our understanding on the mode of action of LC-UFA against L. monocytogenes, but also suggests the potential use of beef fat or its fractions as natural antimicrobials for controlling foodborne pathogens.