Phage P100 resistance in clinical and foodborne Listeria monocytogenes isolates is associated with adsorption-inhibiting mutations and fitness trade-offs

• Most mutations occurred in genes associated with WTA glycosylation. • All mutants exhibited impaired P100 phage binding. • Gene mutation patterns appeared serovar-dependent. • Serovar 4b-derived mutants showed greater fitness loss than 2a or 2b. • Phage-resistance induction led to increased antibiotics sensitivity. Listeria monocytogenes is a common bacterial pathogen causing listeriosis. Phage-based products are increasingly used in food safety, but their use raises concerns about the emergence of phage-resistant bacteria. The broad-host-range phage P100 is applied in foods, however, little is known about the genetic basis of resistance and associated fitness trade-offs in L. monocytogenes . In this study, recently (2014-2020) circulating, clinical and food-associated L. monocytogenes strains (n = 14) were exposed to a commercial P100 phage preparation in soft agar at 20 °C and 10 °C. Eight mutants were isolated and confirmed as phage-resistant by spot assays and flow cytometry. Whole-genome sequencing revealed that most mutants derived from serovar 4b strains carried mutations in genes for polysaccharide synthesis indirectly involved in cell wall teichoic acid glycosylation, whereas mutants from serovar 2a or 2b strains had mutations in a gene directly involved in WTA glycosylation. Confocal microscopy and flow cytometry confirmed reduced phage adsorption in all mutants. The resistant mutants displayed increased antibiotic sensitivity, with two showing pronounced sensitivity to β-lactam antibiotics. These two mutants also showed impaired growth at 37°C. Together with four additional mutants, growth was also impaired at 10°C, both without and with sodium chloride and sodium nitrite. Overall, serovar 4b-derived mutants exhibited stronger fitness trade-offs than those of serovar 2a or 2b. The findings of this study contribute to the understanding of the P100 phage-resistance mechanisms in different serovar strains of L. monocytogenes and reveal associated fitness trade-offs that may be relevant for future antimicrobial and biocontrol strategies.