Effects of subinhibitory concentrations of antibiotics on SOS and DNA repair gene expression in Staphylococcus aureus

Abstract

Reporter clones of Staphylococcus aureus with different SOS response- and DNA repair-associated promoter-lux gene fusion constructs were constructed to study the effects of sub-MICs of antibiotics on the transcription of the SOS and methyl mismatch repair (MMR) genes. Fluoroquinolones (FQs) upmodulated both the SOS and the MMR genes. The patterns of antibiotic-induced transcriptional modulation were altered in FQ-resistant mutants.

FIG. 1.

(A) Promoter regions of SOS response genes. The SOS boxes (underlined) are conserved sequences upstream of many genes in the SOS regulon that repress transcription when they are bound by the LexA protein (5, 9). The putative −10 and −35 boxes (boldface), the direction of transcription (arrow), and intervening nucleotides not shown (parenthetical numbers) are indicated. (B). Expression of SOS genes. The luminescence in the presence of 0 μg/ml (diamonds), 0.2 μg/ml (squares), 0.5 μg/ml (triangles), and 1 μg/ml (circles) CIP in broth was measured with a Wallac 1420 Victor multilabel counter (Perkin-Elmer).

FIG. 2.

Effects of antibiotics on SOS and MMR gene expression in the indicated wild-type S. aureus strains (RN4220, Newman, and 458) (A) and Cip^r mutants (B). The first row in each set shows the inhibition zones in representative disk diffusion assays after 20 h; the other rows show the effects of the antibiotics on the indicated reporter strains, as seen with a luminograph LB980 photon camera (Berthold) and as converted to the color scale on the right. FQs and mitomycin C did not affect the promoterless-lux constructs. The antibiotic disks are as follows: CIP, CIP at 5 μg; ENR; enrofloxacin at 5 μg; FLO, florofloxacin at 5 μg; GAT, gatifloxacin at 5 μg; LVX, levofloxacin at 5 μg; MXF, moxifloxacin at 5 μg; MIT, mitomycin C at 5 μg; NAL, nalidixic acid at 30 μg; NOR, norfloxacin at 5 μg; PEF, pefloxacin at 5 μg; OFX, ofloxacin at 5 μg; H₂O, water; NOV, novobiocin at 5 μg; RIF, rifampin at 5 μg; TMP, trimethoprim at 5 μg; BLE, bleomycin A at 10 μg; DIS, distamycin at 10 μg; MTH, mithramycin A at 10 μg; NET, netropsin at 10 μg; PHL, phleomycin at 10 μg; STL, streptolydigin at 10 μg; GCV, ganciclovir at 10 μg; MET, methotrexate at 50 μg; ETBR, ethidium bromide at 10 μg.