Modified methicillin-resistant Staphylococcus aureus detected in neonatal intensive care patients

Abstract

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.

Figure 1.

Genetic variants uniquely identified in MODSA strains. (a) Epidemiological timeline for MSSA/MRSA surveillance of infants 1 and 2. Stays in different units are indicated by horizontal bars shaded in grey. Results from periodic surveillance are indicated by symbols, with the two distinct S. aureus strain types identified in each infant shown as triangles and circles, respectively. The two isolates with conflicting clinical (MRSA) and research (MSSA) assay results are highlighted in red. Arrows connecting sequenced isolates from infant 2 denote the number of core-genome SNV differences. (b) Variants seen in MODSA strains in infant 1 that were not present in MSSA strains from the same patient. (c) Same as (b) but for infant 2. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.