(A) waaL15 provides a strain-independent increase in vancomycin resistance. Isogenic strains, differing by a point mutation in waaL, were plated by serial dilution on LB agar containing indicated amounts of vancomycin. (B) waaL15 does not improve resistance against other antibiotics. Antibiotic discs containing either 15 μg erythromycin or 5 μg rifampin were placed on LB agar overlays inoculated with the indicated strains. Diametric zones of growth inhibition were measured across the disc. The disc diameter was 6 mm and this value represents growth at the disc.