Optimization of a screening strategy for simulated gastrointestinal survival of Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen known for its ability to survive the harsh conditions of the human gastrointestinal tract. Standardized methods simulating human digestion, such as the INFOGEST protocol, were primarily designed for studying food digestibility and have rarely been applied to bacterial survival studies. This study aimed to identify a practical and biologically relevant preliminary screening strategy for selecting L.monocytogenes isolates for subsequent evaluation under simulated gastrointestinal conditions. Sixty isolates from diverse ecological niches and food matrices were initially exposed to simplified mild gastric (pH 3.0), followed by sequential exposure to 0.3% bile salts using buffered peptone water (BPW). All isolates remained culturable with only limited reductions in culturable cell recovery, indicating limited discriminatory potential under these mild conditions. A subset of ten isolates was subsequently exposed to more stringent conditions: simulated gastric fluid (SGF), prepared according to the gastric phase described in the INFOGEST protocol and adjusted to pH 3.0, and BPW adjusted to pH 2.5. Across this subset, SGF produced the greatest overall reductions in culturable cells, whereas BPW pH 2.5 generated the widest numerical dispersion among isolate responses. SGF caused significantly greater reductions than BPW pH 2.5 for five isolates, indicating isolate-dependent responses. These results suggest that, within the tested subset, SGF is a promising, stringent, and physiologically relevant exploratory screening condition for the preliminary selection of L.monocytogenes isolates before full INFOGEST digestion assays, while also indicating that pH alone does not fully explain the observed responses under simulated gastric conditions. Further validation using a larger and more diverse isolate collection is required to confirm the broader applicability of this preliminary screening approach.