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BackgroundMethicillin-resistant Staphylococcus aureus (MRSA) causes threatening infection-related mortality worldwide. Currently, spread of multi-drug resistance (MDR) MRSA limits therapeutic options and requires new approaches to “druggable” target discovery, as well as development of novel MRSA-active antibiotics. RNA polymerase primary σ70 (encoded by gene rpoD) is a highly conserved prokaryotic factor essential for transcription initiation in exponentially growing cells of diverse S. aureus, implying potential for antisense inhibition.Methodology/Principal FindingsBy synthesizing a serial of cell penetrating peptide conjugated peptide nucleic acids (PPNAs) based on software predicted parameters and further design optimization, we identified a target sequence (234 to 243 nt) within rpoD mRNA conserved region 3.0 being more sensitive to antisense inhibition. A (KFF)3K peptide conjugated 10-mer complementary PNA (PPNA2332) was developed for potent micromolar-range growth inhibitory effects against four pathogenic S. aureus strains with different resistance phenotypes, including clinical vancomycin-intermediate resistance S. aureus and MDR-MRSA isolates. PPNA2332 showed bacteriocidal antisense effect at 3.2 fold of MIC value against MRSA/VISA Mu50, and its sequence specificity was demonstrated in that PPNA with scrambled PNA sequence (Scr PPNA2332) exhibited no growth inhibitory effect at higher concentrations. Also, PPNA2332 specifically interferes with rpoD mRNA, inhibiting translation of its protein product σ70 in a concentration-dependent manner. Full decay of mRNA and suppressed expression of σ70 were observed for 40 µM or 12.5 µM PPNA2332 treatment, respectively, but not for 40 µM Scr PPNA2332 treatment in pure culture of MRSA/VISA Mu50 strain. PPNA2332 (≥1 µM) essentially cleared lethal MRSA/VISA Mu50 infection in epithelial cell cultures, and eliminated viable bacterial cells in a time- and concentration- dependent manner, without showing any apparent toxicity at 10 µM.ConclusionsThe present result suggested that RNAP primary σ70 is a very promising candidate target for developing novel antisense antibiotic to treat severe MRSA infections.

Abstract. The treatment of septicemia caused by antibiotic-resistant bacteria is a great challenge in the clinic. Because traditional antibiotics inevitably induce bacterial resistance, which is responsible for many treatment failures, there is an urgent need to develop novel antibiotic drugs. Amino-terminated Poly (amidoamine) dendrimers (PAMAM-NH 2 ) are reported to have antibacterial activities. However, previous studies focused on high generations of PAMAM-NH 2 , which have been found to exhibit high toxicities. The present study aimed to clarify whether low generations of PAMAM-NH 2 could be used as novel antibacterial agents. We found that generation 2 (G2.0) PAMAM-NH 2 showed significant antibacterial effects against antibiotic-sensitive and antibiotic-resistant strains but exhibited little toxicity to human gastric epithelial cells and did not induce antibiotic resistance in bacteria. Scanning and transmission electron microscopy analyses suggested that G2.0 PAMAM-NH 2 might inhibit the growth of bacteria by destroying their cell membranes. The administration of G2.0 PAMAM-NH 2 dosedependently improved the animal survival rate of mice infected with extended-spectrum beta lactamase-producing Escherichia coli (ESBL-EC) and of animals infected with a combination of ESBL-EC and methicillin-resistant Staphylococcus aureus. A treatment regimen of 10 mg/kg of G2.0 PAMAM-NH 2 starting 12 h before inoculation followed by 10 mg/kg at 0.5 h after inoculation rescued 100% of singly infected mice and 60% of multiply infected mice. The protective effects were associated with the reduction of the bacterial titers in the blood and with the morphological amelioration of infected tissues. These findings demonstrate that the G2.0 PAMAM-NH 2 is a potential broad-spectrum and nonresistance-inducing antibiotic agent with relatively low toxicity.