Surface growth diversity between Listeria monocytogenes strains resulting from oxygen and pH interactions.

The food industry's focus on sustainability has driven efforts to replace multilayer packaging with recyclable monolayers. Monolayers face challenges in providing adequate gas barriers in modified atmosphere packaging (MAP), making oxygen's impact on food safety a critical concern. This study investigated the effects of oxygen on the surface growth of Listeria monocytogenes under refrigerated conditions. Both the absence and presence of mild acid stress were considered, with particular attention to strain-specific differences. Growth experiments were conducted using three L. monocytogenes strains (LMG 23905, LMG 23194 and ADQP 105) at 7 °C on brain heart infusion (BHI) agar under controlled oxygen (0 % and 21 %) and pH conditions (6.2 and 7.6). Significant differences in growth rates were observed for strain LMG 23905 under mildly acidic conditions (pH 6.2) at 0 % oxygen versus 21 % oxygen (p = 1.99e-13). Under neutral conditions (pH 7.6) however, no significant differences were observed (p = 0.09). Strain ADQP 105 exhibited no oxygen sensitivity under both mildly acidified (p = 0.1), and neutral pH conditions (p = 0.52). The same holds for strain LMG 23194, having identical growth rates under air and 100 % nitrogen at neutral (p = 0.28) and mildly reduced pH (p = 0.38). These findings highlight the strain-dependent effects of oxygen on the surface growth of L. monocytogenes. This also demonstrates that there is interaction between mild acid stress and oxygen availability, offering important insights for improving shelf-life predictions and better reflecting real-world conditions for bacterial contamination in solid foods.