Laboratory-evolved, vancomycin-resistant, clinical isolates of Staphylococcus aureus
Staphylococcus aureus has remained susceptible to vancomycin despite ~4 decades of this drug being the primary treatment for MRSA infections. Vancomycin resistance is commonly seen in Enterococcus (VRE) and at a low level in MRSA (VISA), but high-level resistance in SA (VRSA) has remained remarkably rare. Many have speculated the rarity of VRSA is due to the fitness cost imposed by the van operon which imparts resistance. To determine if this cost can be overcome, we propagated four clinical VRSA isolates on solid media in the presence and absence of vancomycin for 50 propagation cycles of 72 hours growth. Growth parameter measurements (colony expansion rate, growth rate, lag time) reveal a fitness tradeoff which occurs during adaptation. However, this tradeoff can be ameliorated by propagating the evolved lineages in the absence of vancomycin for some period. A few high-fitness lineages which retain resistance in the absence of selective pressure were identified, suggesting the durability of vancomycin in the treatment of MRSA may not last. The cycle 50 isolates were sequenced (short read Illumina) to identify vancomycin-specific mutations which may be responsible for the fitness tradeoff and compensation.