Revised manuscript AAC00346-09 VanA-type vancomycin-resistant Staphylococcus aureus

2 Staphylococcus aureus is one of the most common causes of hospital-and community-acquired infections and treatment of staphylococcal infections is complicated by the ability of this bacterial species to become resistant to antibiotics. Vancomycin is the drug of choice for therapy of infections due to methicillin-resistant S. aureus (MRSA) but increase in vancomycin use has led to the emergence of two types of glycopeptide-resistant S. aureus. The first one, designated vancomycin intermediate-resistant S. aureus (VISA), is associated with a thickened and poorly cross-linked cell wall resulting in accumulation of acyl-D-alanyl-D-alanine (X-D-Ala-D-Ala) targets in the periphery that sequester glycopeptides (8). The second type, vancomycin-resistant S. aureus (VRSA), is due to acquisition from Enterococcus spp. of the vanA operon carried by transposon Tn1546 resulting in high-level resistance (4, 5). This review is devoted to the genetics of acquisition and to the phenotypic expression of the vanA operon in S. aureus. VanA-type glycopeptide resistance. Glycopeptides inhibit cell wall synthesis in gram-positive bacteria by binding to the C-terminal D-Ala-D-Ala of the pentapeptide precursors of peptidoglycan thus blocking the transglycosylation and transpeptidation reactions (28). High-level resistance to glycopeptides was first reported in enterococci in 1988 (19, 39) approximately 30 years after the introduction of this antibiotic into clinical practice. Since then, glycopeptide-resistant enterococci (GRE) have disseminated throughout the world. (i) synthesis of a new target (peptidoglycan precursors ending in D-Ala-D-lactate (D-Ala-D-Lac) in VanA, B, and D-type or D-Ala-D-serine (D-Ala-D-Ser) in VanC, E, G, and L-type) having a reduced affinity for glycopeptides and (ii) elimination of the normal D-Ala-D-Ala-terminating precursors (26). VanA-type resistance, which was the first to be elucidated and is the more common, is characterized by high levels of resistance to glycopeptides, vancomycin and teicoplanin, and is mediated by transposon Tn1546, or closely-related elements, that are chromosomally-or plasmid-located. This 11-kb mobile genetic element, which belongs to the Tn3 family of transposons, codes for nine polypeptides responsible for transposition (ORF1 and ORF2), regulation of expression of resistance (VanR and VanS), synthesis of modified peptidoglycan precursors ending in D-Lac (VanH and VanA), hydrolysis of normal precursors (VanX and VanY), and for an unknown function (VanZ) (Fig. 1A). ORF1 and ORF2 encode, respectively, a transposase and a resolvase, responsible for the movements of 3 the transposon. VanH is a dehydrogenase that reduces pyruvate to D-Lac and VanA a ligase allowing the formation of the D-Ala-D-Lac depsipeptide that replaces the D-Ala-D-Ala dipeptide in peptidoglycan synthesis. This substitution dramatically …