Risk of false-negative diagnostics of Listeria monocytogenes due to weak phospholipase activity

We investigated an atypical Listeria monocytogenes strain isolated from the food processing environment, which may evade widely used diagnostic approaches. After species confirmation using PCR and MALDI-TOF, enzyme analysis revealed reduced phospholipase activity, indicating a link between its activity and the white appearance of atypical strains on chromogenic agar. While no mutation was detected in the phospholipase C gene, comparative genomics revealed a distinct seven base pair insertion in the prfA gene, resulting in gene truncation. These results highlight the risk of false negatives in current diagnostics and a need for improved detection methods to ensure food safety.