Modified Oxacillin Resistant Staphylococcus aureus detected in neonatal intensive care patients

Objectives As part of an active MRSA surveillance programme in our neonatal ICU, we identified nares surveillance cultures from two infants that displayed heterogeneity in methicillin resistance between isolated subclones that lacked mecA and mecC. Methods The underlying mechanism for the modified Staphylococcus aureus (MODSA) methicillin-resistance phenotype was investigated by WGS. Results Comparison of finished-quality genomes of four MODSA and four MSSA subclones demonstrated that the resistance changes were associated with unique truncating mutations in the gene encoding the cyclic diadenosine monophosphate phosphodiesterase enzyme GdpP or a non-synonymous substitution in the gene encoding PBP2. Conclusions These two cases highlight the difficulty in identifying non-mecA, non-mecC-mediated MRSA isolates in the clinical microbiology laboratory, which leads to difficulties in implementing appropriate therapy and infection control measures.