# In Vivo Emergence of Oxacillin Resistance in mecA‐Negative MSSA Endocarditis With Metastatic Infection

## TL;DR

A patient with methicillin-sensitive Staphylococcus aureus infection developed oxacillin resistance during treatment, highlighting the need for careful monitoring and multidisciplinary care.

## Contribution

This case reports the rare in vivo emergence of oxacillin resistance in a mecA-negative MSSA infection and emphasizes the role of MMAE in enabling cardiac surgery after intracranial bleeding.

## Key findings

- Phenotypic oxacillin resistance emerged in a mecA-negative MSSA isolate during treatment.
- Middle meningeal artery embolization enabled time-sensitive cardiac surgery after intracranial hemorrhage.
- Serial susceptibility testing is crucial for detecting emerging beta-lactam resistance in high-risk SAB.

## Abstract

Staphylococcus aureus
bacteremia (SAB) is particularly challenging when complicated by infective endocarditis (IE), metastatic spread, or evolving antimicrobial resistance. We describe a 76‐year‐old woman with poorly‐controlled diabetes who presented with bilateral knee septic arthritis and persistent methicillin‐sensitive
S. aureus
(MSSA) bacteremia complicated by in vivo emergence of oxacillin resistance despite a mecA‐negative genotype. Her clinical course was marked by an aortomitral intervalvular fibrosa abscess, vertebral osteomyelitis with epidural extension, a psoas abscess, and septic renal infarctions. She experienced breakthrough bacteremia despite initial clearance on oxacillin. Surgery was delayed due to intracranial hemorrhage but was ultimately enabled by middle meningeal artery embolization (MMAE). This case highlights the rare emergence of phenotypic oxacillin resistance in mecA‐negative MSSA during therapy and illustrates the role of MMAE in facilitating time‐sensitive cardiac surgery in the setting of recent intracranial bleeding. It underscores the importance of dynamic diagnostic reassessment, serial susceptibility testing, and multidisciplinary coordination in complicated SAB.

In
Staphylococcus aureus
bacteremia with high‐risk features, clinicians should pursue early transesophageal echocardiography, maintain vigilance for metastatic foci, and consider serial susceptibility testing to detect emerging beta‐lactam resistance—even in mecA‐negative isolates.