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Background As part of active surveillance in our NICU for methicillin-resistant Staphylococcus aureus (MRSA), two isolates representing modified S. aureus (MODSA), which are Methicillin resistant but lacking mecA or C were identified. Our current microbiology laboratory workflow for screening for MRSA involves plating isolates on chromoID agar (bioMérieux, Marcy-l’Etoile, France) as well as on sheep blood agar (SBA). Β hemolytic colonies on SBA that are catalase and coagulase positive are set up for confirmation and antimicrobial susceptibility testing on the Vitek 2 (bioMérieux, Marcy-l’Etoile, France). Methods These 2 isolates (from Baby 1 and Baby 2) tested positive for green colonies on the chromoagar plates. The Vitek 2 subsequently identified both these isolates as MRSA. However, for research purposes, all positive NICU MRSA isolates are tested via whole-genome sequencing (WGS). Both isolates were identified by WGS as methicillin-susceptible Staphylococcus aureus (MSSA). We subsequently went back and performed additional workup on these isolates. Isolates were plated on SBA and chromagar again and incubated for 24 hours. 2 colonies of different morphologies from the chromagar plates and 3 from the SBA were randomly selected and subcultured to chromagar and SBA for a total of 5 subcultures. Each of the subcultures was tested using staphaurex, mannitol salt agar and the Cepheid Xpert MRSA assay and all were confirmed to be Staphylococcus aureus. Results All 10 isolates tested negative by the Cepheid Xpert MRSA assay for MRSA. Phenotypic testing was set up again for all ten isolates using the vitek GP panel, as well as cefoxitin disk, and oxacillin E test using Mueller–Hinton agar supplemented with 2% NaCl as per CLSI methods. See table attached for results. Conclusion In conclusion, these two cases highlight the difficulty in identifying non-MecA, non-MecC-mediated MRSA isolates in the clinical microbiology laboratory. This is particularly important as more laboratories rely on testing for MecA by PCR for surveillance testing. These 2 cases were further complicated by heterogeneous sub-populations of Staphylococcus aureus. Failure to recognize these variant forms of MRSA can lead to difficulties in implementing appropriate therapy and infection control measures. Improved methodologies are needed. Disclosures All authors: No reported disclosures.

2129. When Is Methicillin-resistant Staphylococcus aureus not Methicillin-resistant Staphylococcus aureus?