Impact of recA on levofloxacin exposure-related resistance development

Defects in DNA double-strand break repair resensitize antibiotic-resistant Escherichia coli to multiple bactericidal antibiotics

Antibiotic resistance is becoming increasingly prevalent amongst bacterial pathogens and there is an urgent need to develop new types of antibiotics with novel modes of action. One promising strategy is to develop resistance-breaker compounds, which inhibit resistance mechanisms and thus resensitize bacteria to existing antibiotics. In the current study, we identify bacterial DNA double-strand break repair as a promising target for the development of resistance-breaking co-therapies. We examined genetic variants of Escherichia coli that combined antibiotic-resistance determinants with DNA repair defects. We observed that defects in the double-strand break repair pathway led to significant resensitization toward five bactericidal antibiotics representing different functional classes. Effects ranged from partial to full resensitization. For ciprofloxacin and nitrofurantoin, sensitization manifested as a reduction in the minimum inhibitory concentration. For kanamycin and trimethoprim, sensitivity manifested through increased rates of killing at high antibiotic concentrations. For ampicillin, repair defects dramatically reduced antibiotic tolerance. Ciprofloxacin, nitrofurantoin, and trimethoprim induce the promutagenic SOS response. Disruption of double-strand break repair strongly dampened the induction of SOS by these antibiotics. Our findings suggest that if break-repair inhibitors can be developed they could resensitize antibiotic-resistant bacteria to multiple classes of existing antibiotics and may suppress the development of de novo antibiotic-resistance mutations.

Targeting the bacterial SOS response for new antimicrobial agents: drug targets, molecular mechanisms and inhibitors

Antimicrobial resistance is a pressing threat to global health, with multidrug-resistant pathogens becoming increasingly prevalent. The bacterial SOS pathway functions in response to DNA damage that occurs during infection, initiating several pro-survival and resistance mechanisms, such as DNA repair and hypermutation. This makes SOS pathway components potential targets that may combat drug-resistant pathogens and decrease resistance emergence. This review discusses the mechanism of the SOS pathway; the structure and function of potential targets AddAB, RecBCD, RecA and LexA; and efforts to develop selective small-molecule inhibitors of these proteins. These inhibitors may serve as valuable tools for target validation and provide the foundations for desperately needed novel antibacterial therapeutics.

Transcriptomic and proteomic profiling response of methicillin-resistant Staphylococcus aureus (MRSA) to a novel bacteriocin, plantaricin GZ1-27 and its inhibition of biofilm formation

Methicillin-resistant Staphylococcus aureus (MRSA) has become a worrisome superbug, due to its wide distribution and multidrug resistance. To characterize effects of a newly identified plantaricin GZ1-27 on MRSA, transcriptomic and proteomic profiling of MRSA strain ATCC43300 was performed in response to sub-MIC (16 μg/mL) plantaricin GZ1-27 stress. In total, 1090 differentially expressed genes (padj < 0.05) and 418 differentially expressed proteins (fold change > 1.2, p < 0.05) were identified. Centralized protein expression clusters were predicted in biological functions (biofilm formation, DNA replication and repair, and heat-shock) and metabolic pathways (purine metabolism, amino acid metabolism, and biosynthesis of secondary metabolites). Moreover, a capacity of inhibition MRSA biofilm formation and killing biofilm cells were verified using crystal violet staining, scanning electron microscopy, and confocal laser-scanning microscopy. These findings yielded comprehensive new data regarding responses induced by plantaricin and could inform evidence-based methods to mitigate MRSA biofilm formation.

ZeBRα a universal, multi-fragment DNA-assembly-system with minimal hands-on time requirement

The recently evolved field of synthetic biology has revolutionized the way we think of biology as an "engineerable" discipline. The newly sprouted branch is constantly in need of simple, cost-effective and automatable DNA-assembly methods. We have developed a reliable DNA-assembly system, ZeBRα (Zero-Background Redα), for cloning multiple DNA-fragments seamlessly with very high efficiency. The hallmarks of ZeBRα are the greatly reduced hands-on time and costs and yet excellent efficiency and flexibility. ZeBRα combines a "zero-background vector" with a highly efficient in vitro recombination method. The suicide-gene in the vector acts as placeholder, and is replaced by the fragments-of-interest, ensuring the exclusive survival of the successful recombinants. Thereby the background from uncut or re-ligated vector is absent and screening for recombinant colonies is unnecessary. Multiple fragments-of-interest can be assembled into the empty vector by a recombinogenic E. coli-lysate (SLiCE) with a total time requirement of less than 48 h. We have significantly simplified the preparation of the high recombination-competent E. coli-lysate compared to the original protocol. ZeBRα is the least labor intensive among comparable state-of-the-art assembly/cloning methods without a trade-off in efficiency.

Ciprofloxacin-Mediated Mutagenesis Is Suppressed by Subinhibitory Concentrations of Amikacin in Pseudomonas aeruginosa

Resistance to antibiotics is a global health problem. Activation of the SOS response, and the subsequent elevation in mutagenesis, contributes to the appearance of resistance mutations. Among currently used drugs, quinolones are the most potent inducers of the SOS response. In the present study, we show that amikacin inhibits ciprofloxacin-mediated SOS induction and mutagenesis in Pseudomonas aeruginosa.

Identification and Characterization of Novel Compounds Blocking Shiga Toxin Expression in Escherichia coli O157:H7

Infections caused by Shiga toxin (Stx)-producing E. coli strains constitute a health problem, as they are problematic to treat. Stx production is a key virulence factor associated with the pathogenicity of enterohaemorrhagic E. coli (EHEC) and can result in the development of haemolytic uremic syndrome in infected patients. The genes encoding Stx are located on temperate lysogenic phages integrated into the bacterial chromosome and expression of the toxin is generally coupled to phage induction through the SOS response. We aimed to find new compounds capable of blocking expression of Stx type 2 (Stx2) as this subtype of Stx is more strongly associated with human disease. High-throughput screening of a small-molecule library identified a lead compound that reduced Stx2 expression in a dose-dependent manner. We show that the optimized compound interferes with the SOS response by directly affecting the activity and oligomerization of RecA, thus limiting phage activation and Stx2 expression. Our work suggests that RecA is highly susceptible to inhibition and that targeting this protein is a viable approach to limiting production of Stx2 by EHEC. This type of approach has the potential to limit production and transfer of other phage induced and transduced determinants.

The Regulatory Networks That Control Clostridium difficile Toxin Synthesis

The pathogenic clostridia cause many human and animal diseases, which typically arise as a consequence of the production of potent exotoxins. Among the enterotoxic clostridia, Clostridium difficile is the main causative agent of nosocomial intestinal infections in adults with a compromised gut microbiota caused by antibiotic treatment. The symptoms of C. difficile infection are essentially caused by the production of two exotoxins: TcdA and TcdB. Moreover, for severe forms of disease, the spectrum of diseases caused by C. difficile has also been correlated to the levels of toxins that are produced during host infection. This observation strengthened the idea that the regulation of toxin synthesis is an important part of C. difficile pathogenesis. This review summarizes our current knowledge about the regulators and sigma factors that have been reported to control toxin gene expression in response to several environmental signals and stresses, including the availability of certain carbon sources and amino acids, or to signaling molecules, such as the autoinducing peptides of quorum sensing systems. The overlapping regulation of key metabolic pathways and toxin synthesis strongly suggests that toxin production is a complex response that is triggered by bacteria in response to particular states of nutrient availability during infection.

SOS response and its regulation on the fluoroquinolone resistance

Bacteria can survive fluoroquinolone antibiotics (FQs) treatment by becoming resistant through a genetic change-mutation or gene acquisition. The SOS response is widespread among bacteria and exhibits considerable variation in its composition and regulation, which is repressed by LexA protein and derepressed by RecA protein. Here, we take a comprehensive review of the SOS gene network and its regulation on the fluoroquinolone resistance. As a unique survival mechanism, SOS may be an important factor influencing the outcome of antibiotic therapy in vivo.

The SOS Response Master Regulator LexA Is Associated with Sporulation, Motility and Biofilm Formation in Clostridium difficile

The LexA regulated SOS network is a bacterial response to DNA damage of metabolic or environmental origin. In Clostridium difficile, a nosocomial pathogen causing a range of intestinal diseases, the in-silico deduced LexA network included the core SOS genes involved in the DNA repair and genes involved in various other biological functions that vary among different ribotypes. Here we describe the construction and characterization of a lexA ClosTron mutant in C. difficile R20291 strain. The mutation of lexA caused inhibition of cell division resulting in a filamentous phenotype. The lexA mutant also showed decreased sporulation, a reduction in swimming motility, greater sensitivity to metronidazole, and increased biofilm formation. Changes in the regulation of toxin A, but not toxin B, were observed in the lexA mutant in the presence of sub-inhibitory concentrations of levofloxacin. C. difficile LexA is, therefore, not only a regulator of DNA damage but also controls many biological functions associated with virulence.

Overexpression of Salmonella enterica serovar Typhi recA gene confers fluoroquinolone resistance in Escherichia coli DH5α

A spontaneous fluoroquinolone-resistant mutant (STM1) was isolated from its parent Salmonella enterica serovar Typhi (S. Typhi) clinical isolate. Unlike its parent isolate, this mutant has selective resistance to fluoroquinolones without any change in its sensitivity to various other antibiotics. DNA gyrase assays revealed that the fluoroquinolone resistance phenotype of the STM1 mutant did not result from alteration of the fluoroquinolone sensitivity of the DNA gyrase isolated from it. To study the mechanism of fluoroquinolone resistance, a genomic library from the STM1 mutant was constructed in Escherichia coli DH5α and two recombinant plasmids were obtained. Only one of these plasmids (STM1-A) conferred the selective fluoroquinolone resistance phenotype to E. coli DH5α. The chromosomal insert from STM1-A, digested with EcoRI and HindIII restriction endonucleases, produced two DNA fragments and these were cloned separately into pUC19 thereby generating two new plasmids, STM1-A1 and STM1-A2. Only STM1-A1 conferred the selective fluoroquinolone resistance phenotype to E. coli DH5α. Sequence and subcloning analyses of STM1-A1 showed the presence of an intact RecA open reading frame. Unlike that of the wild-type E. coli DH5α, protein analysis of a crude STM1-A1 extract showed overexpression of a 40 kDa protein. Western blotting confirmed the 40 kDa protein band to be RecA. When a RecA PCR product was cloned into pGEM-T and introduced into E. coli DH5α, the STM1-A11 subclone retained fluoroquinolone resistance. These results suggest that overexpression of RecA causes selective fluoroquinolone resistance in E. coli DH5α.

'Supermutators' found amongst highly levofloxacin-resistant E. coli isolates: a rapid protocol for the detection of mutation sites

Fluoroquinolone resistance is gradually acquired through several mechanisms. In particular, chromosomal mutations in the genes encoding topoisomerases II and IV and increased expression of the multidrug efflux pump AcrAB-TolC are the most common mechanisms. In this study, multiplex polymerase chain reaction (PCR) protocols were designed for high-throughput sequencing of the quinolone resistance determining regions of topoisomerases genes (gyrA, parC and parE) and/or the expression regulation systems of multidrug efflux pump AcrAB (acrRAB, marRAB and soxSR). These protocols were applied to sequence samples from five subpopulations of 103 clinical Escherichia coli isolates. These subpopulations were classified according to their levofloxacin susceptibility pattern as follows: highly resistant (HR), resistant (R), intermediate (I), reduced susceptibility (RS) and susceptible (S). All HR isolates had mutations in the six genes surveyed, with two ‘supermutator' isolates harboring 13 mutations in these six genes. Strong associations were observed between mutations in acrR and HR isolates, parE and R/HR isolates and parC and I/R/HR isolates, whereas surprisingly, gyrA mutations were common in RS/I/R/HR isolates. Further investigation revealed that strong associations were limited to the triple mutations gyrA-S83L/D87N/R237H and HR isolates and the double mutations S83L/D87N and I/R/HR isolates, whereas the single mutation S83L was common in RS/I/R/HR isolates. Interestingly, two novel mutations (gyrA-R237H and acrR-V29G) were located and found to strongly associate with HR isolates. To the best of our knowledge, the gyrA-R237H and acrR-V29G mutations have never been reported and require further investigation to determine their exact role in resistance or ‘fitness' as defined by their ability to compensate for the organismal cost of gaining resistance.

Suramin is a potent and selective inhibitor of Mycobacterium tuberculosis RecA protein and the SOS response: RecA as a potential target for antibacterial drug discovery

OBJECTIVES

In eubacteria, RecA is essential for recombinational DNA repair and for stalled replication forks to resume DNA synthesis. Recent work has implicated a role for RecA in the development of antibiotic resistance in pathogenic bacteria. Consequently, our goal is to identify and characterize small-molecule inhibitors that target RecA both in vitro and in vivo.

METHODS

We employed ATPase, DNA strand exchange and LexA cleavage assays to elucidate the inhibitory effects of suramin on Mycobacterium tuberculosis RecA. To gain insights into the mechanism of suramin action, we directly visualized the structure of RecA nucleoprotein filaments by atomic force microscopy. To determine the specificity of suramin action in vivo, we investigated its effect on the SOS response by pull-down and western blot assays as well as for its antibacterial activity.

RESULTS

We show that suramin is a potent inhibitor of DNA strand exchange and ATPase activities of bacterial RecA proteins with IC(50) values in the low micromolar range. Additional evidence shows that suramin inhibits RecA-catalysed proteolytic cleavage of the LexA repressor. The mechanism underlying such inhibitory actions of suramin involves its ability to disassemble RecA-single-stranded DNA filaments. Notably, suramin abolished ciprofloxacin-induced recA gene expression and the SOS response and augmented the bactericidal action of ciprofloxacin.

CONCLUSIONS

Our findings suggest a strategy to chemically disrupt the vital processes controlled by RecA and hence the promise of small molecules for use against drug-susceptible as well as drug-resistant strains of M. tuberculosis for better infection control and the development of new therapies.

In vitro pharmacodynamics of AZD5206 against Staphylococcus aureus

AZD5206 is a novel antimicrobial agent with potent in vitro activity against Staphylococcus aureus. We evaluated the in vitro pharmacodynamics of AZD5206 against a standard wild-type methicillin-susceptible strain (ATCC 29213) and a clinical strain of methicillin-resistant S. aureus (SA62). Overall, bacterial killing against a low baseline inoculum was more remarkable. Low dosing exposures and/or high baseline inoculum resulted in early reduction in bacterial burden, followed by regrowth and selective amplification of the resistant population.

High-throughput screening for RecA inhibitors using a transcreener adenosine 5'-O-diphosphate assay

The activities of the bacterial RecA protein are involved in the de novo development and transmission of antibiotic resistance genes, thus allowing bacteria to overcome the metabolic stress induced by antibacterial agents. RecA is ubiquitous and highly conserved among bacteria, but has only distant homologs in human cells. Together, this evidence points to RecA as a novel and attractive antibacterial drug target. All known RecA functions require the formation of a complex formed by multiple adenosine 5'-O-triphosphate (ATP)-bound RecA monomers on single-stranded DNA. In this complex, RecA hydrolyzes ATP. Although several methods for assessing RecA's ATPase activity have been reported, these assay conditions included relatively high concentrations of enzyme and ATP and thereby restricted the RecA conformational state. Herein, we describe the validation of commercial reagents (Transcreener(®) adenosine 5'-O-diphosphate [ADP](2) fluorescence polarization assay) for the high-throughput measurement of RecA's ATPase activity with lower concentrations of ATP and RecA. Under optimized conditions, ADP detection by the Transcreener reagent provided robust and reproducible activity data (Z'=0.92). Using the Transcreener assay, we screened 113,477 small molecules against purified RecA protein. In total, 177 small molecules were identified as confirmed hits, of which 79 were characterized by IC(50) values ≤ 10 μM and 35 were active in bioassays with live bacteria. This set of compounds comprises previously unidentified scaffolds for RecA inhibition and represents tractable hit structures for efforts aimed at tuning RecA inhibitory activity in both biochemical and bacteriological assays.

Optimizing dosage to prevent emergence of resistance - lessons from in vitro models

The widespread emergence of resistance to antimicrobial agents has taken mammoth dimension and warrants immediate steps to minimize it. Pharmacokinetics/pharmacodynamics of antimicrobial agents, differences among bacterial species, and time-dependent changes in the bacterial population are important factors involved in the development of drug resistance. The key to minimizing resistance lies in understanding how these factors affect resistance emergence and incorporating them in dosing regimen design. In vitro models have proven to be a valuable tool to study these factors and their contribution in resistance emergence. This review summarizes the key factors implicated in resistance development and the lessons learnt from in vitro studies optimizing antimicrobial dosing to prevent resistance emergence.

Topoisomerase I function during Escherichia coli response to antibiotics and stress enhances cell killing from stabilization of its cleavage complex

OBJECTIVES

To explore the role of topoisomerase I in gene activation and increased RecA levels during the bacterial SOS response, as well as the effect of antibiotic treatment and stress challenge on cell killing initiated by trapped topoisomerase I cleavage complex.

METHODS

A mutant Escherichia coli strain with a ΔtopA mutation was used to investigate the role of topoisomerase I function in the SOS response to trimethoprim and mitomycin C. Induction of the recA and dinD1 promoters was measured using luciferase reporters of these promoters fused to luxCDABE. An increase in the RecA level following trimethoprim treatment was quantified directly by western blotting. The effect of stress challenge from trimethoprim and acidified nitrite treatments on cell killing by topoisomerase I cleavage complex accumulation was measured by the decrease in viability following induction of recombinant mutant topoisomerase I that forms a stabilized cleavage complex.

RESULTS

Topoisomerase I function was found to be required for efficient transcriptional activation of the recA and dinD1 promoters during the E. coli SOS response to trimethoprim and mitomycin C. The role of topoisomerase I in the SOS response was confirmed with quantitative western blot analysis of RecA following trimethoprim treatment. The bactericidal effect from topoisomerase I cleavage complex accumulation was shown to be enhanced by stress challenge from trimethoprim and acidified nitrite.

CONCLUSIONS

Bacterial topoisomerase I function is actively involved in the SOS response to antibiotics and stress challenge. Cell killing initiated by the topoisomerase I cleavage complex would be enhanced by antibiotics and the host response. These findings provide further support for bacterial topoisomerase I as a therapeutic target.