β-Lactams Interfering with PBP1 Induce Panton-Valentine Leukocidin Expression by TriggeringsarAandrotGlobal Regulators of Staphylococcus aureus

Abstract

ABSTRACTPrevious articles reported that beta-lactam antibiotics increase the expression ofStaphylococcus aureusPanton-Valentine leukocidin (PVL) by activating its transcription. We investigated the mechanisms underlying the inductor effect of beta-lactams on PVL expression by determining targets and regulatory pathways possibly implicated in this process. We measured PVL production in the presence of oxacillin (nonselective), imipenem (penicillin-binding protein 1 [PBP1] selective), cefotaxime (PBP2 selective), cefaclore (PBP3 selective), and cefoxitin (PBP4 selective).In vitro, we observed increased PVL production consistent withluk-PV mRNA levels that were 20 to 25 times higher for community-acquired methicillin-resistantS. aureus(CA-MRSA) cultures treated with PBP1-binding oxacillin and imipenem than for cultures treated with other beta-lactams or no antibiotic at all. This effect was also observedin vivo, with increased PVL mRNA levels in lung tissues from CA-MRSA-infected mice treated with imipenem but not cefoxitin. To confirm the involvement of PBP1 inhibition in this pathway, PBP1 depletion by use of an induciblepbp1antisense RNA showed a dose-dependent relationship between the level ofpbp1antisense RNA and theluk-PV mRNA level. Upon imipenem treatment of exponential-phase cultures, we observed an increasedsarAmRNA level after 30 min of incubation followed by a decreasedrotmRNA level after 1 to 4 h of incubation. Unlike theagrandsaeRSpositive regulators, which were nonessential for PVL induction by beta-lactams, thesarA(positive) androt(negative) PVL regulators were necessary for PVL induction by imipenem. Our results suggest that antibiotics binding to PBP1 increase PVL expression by modulatingsarAandrot, which are essential mediators of the inductor effect of beta-lactams on PVL expression.