Alterations in the DNA topoisomerase IV grlA gene responsible for quinolone resistance in Staphylococcus aureus

Antimicrobial resistance in ocular infection: A review

Antimicrobial resistance (AMR) is a global public health threat with significant impact on treatment outcomes. The World Health Organization's Global Action Plan on AMR recommended strengthening the evidence base through surveillance programs and research. Comprehensive, timely data on AMR for organisms isolated from ocular infections are needed to guide treatment decisions and inform researchers and microbiologists of emerging trends. This article aims to provide an update on the development of AMR in ocular organisms, AMR in bacterial ocular infections and on AMR stewardship programs globally. The most common ocular pathogens are Pseudomonas aeruginosa, Staphylococcus spp., Streptococcus pneumoniae, and Haemophilus influenzae in ocular infections. A variety of studies and a few surveillance programs worldwide have reported on AMR in these infections over time. Fluoroquinolone resistance has increased particularly in Asia and North America. For conjunctivitis, the ARMOR cumulative study in the USA reported a slight decrease in resistance to ciprofloxacin. For keratitis, resistance to methicillin has remained stable for S. aureus and CoNS, while resistance to ciprofloxacin has decreased for MRSA globally. Methicillin-resistance and multidrug resistance are also emerging, requiring ongoing monitoring. Antimicrobial stewardship (AMS) programmes have a critical role in reducing the threat of AMR and improving treatment outcomes. To be successful AMS must be informed by up-to-date AMR surveillance data. As a profession it is timely for ophthalmology to act to prevent AMR leading to greater visual loss through supporting surveillance programmes and establishing AMS.

The Relationship between Ciprofloxacin Resistance and Genotypic Changes in S. aureus Ocular Isolates

Staphylococcus aureus (S. aureus) is a frequent cause of eye infections with some isolates exhibiting increased antimicrobial resistance to commonly prescribed antibiotics. The increasing resistance of ocular S. aureus to ciprofloxacin is a serious concern as it is a commonly used as a first line antibiotic to treat S. aureus keratitis. This study aimed to analyse genetic mutations in the genomes of 25 S. aureus isolates from infections or non-infectious ocular conditions from the USA and Australia and their relationship to ciprofloxacin resistance. Overall, 14/25 isolates were phenotypically resistant to ciprofloxacin. All isolates were analyzed for mutations in their quinolone resistance-determining regions (QRDRs) and efflux pump genes. Of the fourteen resistant isolates, 9/14 had ciprofloxacin resistance mutations within their QRDRs, at codons 80 or 84 within the parC subunit and codon 84 within the gyrA subunit of DNA gyrase. The highest resistance (MIC = 2560 μg/mL) was associated with two SNPs in both gyrA and parC. Other resistant isolates (3/14) had mutations within norB. Mutations in genes of other efflux pumps and their regulator (norA, norC, mepA, mdeA, sepA, sdrM, mepR, arlR, and arlS) or the DNA mismatch repair (MMR) system (mutL and mutS) were not associated with increased resistance to ciprofloxacin. The functional mutations associated with ciprofloxacin resistance in QRDRs (gyrA and parC) and norB suggests that these are the most common reasons for ciprofloxacin resistance in ocular isolates. Novel SNPs of gyrA Glu-88-Leu, Asn-860-Thr and Thr-845-Ala and IIe-855-Met, identified in this study, need further gene knock out/in studies to better understand their effect on ciprofloxacin resistance.

Molecular Characterization of Methicillin-Sensitive Staphylococcus aureus from the Intestinal Tracts of Adult Patients in China

Intestinal infections caused by methicillin-sensitive Staphylococcus aureus (MSSA) have posed a great challenge for clinical treatments. In recent years, the intestinal carriage rates of MSSA have risen steadily in hospital settings in China. However, the epidemiology and molecular characteristics of MSSA from the intestinal tracts of Chinese adult patients remain unknown. In the present study, a total of 80 S. aureus isolates, including 64 MSSA and 16 methicillin-resistant Staphylococcus aureus (MRSA), were recovered from 466 fecal swabs in adult patients between 2019 and 2021 in China. The MSSA isolates exhibited high resistance to penicillin (92.2%) and erythromycin (45.3%). In addition, a higher proportion of MSSA isolates (14.1%) were multidrug-resistant (MDR) strains than that of MRSA isolates (1.3%). Among the 64 MSSA isolates, we identified 17 MLST types, of which ST398 and ST15 were the most predominant types. The most frequently detected resistance genes were blaZ (87.5%) and erm(C) (21.9%). The hemolysin genes (hla, hld, hlgA, hlgB, hlgC) were detected in all the MSSA isolates, but the Panton–Valentine leucocidin (pvl) gene was identified in 1.7% of the MSSA isolates. Our findings indicated that the prevalence and antimicrobial resistance of intestinal MSSA was a serious concern among adult patients in China.

Paradox of complex diversity: Challenges in the diagnosis and management of bacterial keratitis

Bacterial keratitis continues to be one of the leading causes of corneal blindness in the developed as well as the developing world, despite swift progress since the dawn of the "anti-biotic era". Although, we are expeditiously developing our understanding about the different causative organisms and associated pathology leading to keratitis, extensive gaps in knowledge continue to dampen the efforts for early and accurate diagnosis, and management in these patients, resulting in poor clinical outcomes. The ability of the causative bacteria to subdue the therapeutic challenge stems from their large genome encoding complex regulatory networks, variety of unique virulence factors, and rapid secretion of tissue damaging proteases and toxins. In this review article, we have provided an overview of the established classical diagnostic techniques and therapeutics for keratitis caused by various bacteria. We have extensively reported our recent in-roads through novel tools for accurate diagnosis of mono- and poly-bacterial corneal infections. Furthermore, we outlined the recent progress by our group and others in understanding the sub-cellular genomic changes that lead to antibiotic resistance in these organisms. Finally, we discussed in detail, the novel therapies and drug delivery systems in development for the efficacious management of bacterial keratitis.

Genomic epidemiology of the opportunistic pathogen Staphylococcus coagulans from companion dogs

Introduction

Staphylococcus coagulans (formerly Staphylococcus schleiferi subsp. coagulans) is a common commensal and opportunistic pathogen of companion dogs. It carries a range of antimicrobial resistance genes and is an occasional zoonotic pathogen.

Hypothesis/Gap Statement

Despite the potential insight offered by genome sequencing into the biology of S. coagulans, few genomes are currently available for study.

Aim

To sequence and analyse S. coagulans genomes to improve understanding of this organism’s molecular epidemiology, antimicrobial resistance and bacterium–host interactions.

Methodology

Twenty-five genomes of clinical isolates collected at a veterinary referral hospital in Scotland, UK, were sequenced with Illumina technology. These genomes were analysed by a series of bioinformatics tools along with 16 previously sequenced genomes.

Results

Phylogenetic comparison of the 41 genomes shows that the current S. coagulans phylogeny is dominated by clades of closely related isolates, at least one of which has spread internationally. Ten of the 11 methicillin-resistant S. coagulans genomes in this collection of 41 encoded the mecA promoter and gene mutations that are predicted to render the isolates susceptible to penicillins in the presence of clavulanic acid, a feature only described to date in methicillin-resistant Staphylococcus aureus. Seven such isolates were from the current study and, in line with the genome-based prediction, all were susceptible to amoxicillin/clavulanic acid in vitro. S. coagulans shared very few highly conserved virulence-associated genes with Staphylococcus pseudintermedius, another common commensal and opportunistic canine pathogen.

Conclusion

The availability of a further 25 genome sequences from clinical S. coagulans isolates will aid in better understanding the epidemiology, bacterial–host interactions and antimicrobial resistance of this opportunistic pathogen.

Virulence factors and antimicrobial resistance in Staphylococcus aureus isolated from bovine mastitis in Brazil

This study aimed to evaluate virulence factors and genetic markers of antimicrobial resistance in 400 Staphylococcus aureus strains isolated from bovine mastitis in four Brazilian states, as well as to assess the association between these characteristics and field information. Virulence factors and drug resistance genes were identified by PCR screening. Biofilm-forming and hemolytic phenotype were detected using Congo red Tryptic Soy Broth and defibrinated sheep blood agar, respectively. Of all isolates, 83.5% were biofilm-forming and 98.5% strains exhibited biofilm gene icaAD, and a significant association between phenotype and genotype for biofilm was observed (P = 0.0005). Hemolysin genes were observed in 82.85% (hla⁺hlb⁺), 16.5% (hla⁺) and 0.75% (hlb⁺) isolates, whereas the hemolytic phenotype exhibited was complete and incomplete hemolysis in 64.25%, complete in 28.25%, incomplete in 4.75%, and negative in 2.75% of the strains. Virulence factors genes luk, seb, sec, sed, and tst were observed in 3.5%, 0.5%, 1%, 0.25%, and 0.74% isolates, respectively. The gene blaZ was detected in 82.03% of penicillin-resistant isolates, whereas tetK and aac(6')-Ie-aph(2')-Ia were observed in 33.87% and 45.15% of the tetracycline and aminoglycosides-resistant isolates, respectively. Fluoroquinolone resistance gene mepA was detected for the first time in S. aureus from bovine mastitis. Resistance genes tetM (3.22%), tetL (1.61%), ermA (14.29%), ermB (14.29%), ermC (33.3%), ermT (9.52%), ermY (4.76%), msrA (9.52%), and mphC (9.52%) were also detected among resistant isolates. No association between virulence factors or antimicrobial-resistant genes and year of isolation, geographic origin, or antimicrobial resistance profile was observed. Our results showed that S. aureus strains isolated from bovine mastitis in the four Brazilian states sampled are mainly biofilm-forming and hemolytic, whereas virulence genes associated with enterotoxins, luk and tst, were less frequently observed. Moreover, a wide variety of resistance genes that confer resistance to almost all classes of antimicrobial agents approved for use in animals and humans were found. Overall, the data point to a great pathogenic potential of S. aureus associated with bovine mastitis and to the non-negligible risks to public health of staphylococcal infections from animal origin.

MRSA CC398 recovered from wild boar harboring new SCCmec type IV J3 variant

A methicillin-resistant Staphylococcus aureus CC398 was recovered from a wild female boar (Sus scrofa) in the north of Portugal, in 2013 (Sousa et al. 2017). Whole genome sequencing (WGS) revealed this strain carries a new variant of a mecA-containing staphylococcal chromosomal gene cassette (SCCmec) type IV with an uncommon J3 region. WGS studies can facilitate surveillance and provide more detailed characterization of bacterial clones circulating in the wild, reinforcing the need for a one health perspective to better understand and control antimicrobial resistance.

Antimicrobial resistance and underlying mechanisms in Staphylococcus aureus isolates

OBJECTIVE

To investigate the antimicrobial susceptibility of 97 clinical Staphylococcus aureus (S. aureus) strains against 14 antimicrobials and corresponding resistance mechanisms.

METHODS

The antimicrobial susceptibility of the isolates was determined using a disk diffusion method and antimicrobial resistance genes were screened by polymerase chain reaction. Mutations responsible for ciprofloxacin and rifampicin resistance were investigated by polymerase chain reaction and DNA sequencing.

RESULTS

All isolates were found to be susceptible to vancomycin. Various rates of resistance to penicillin (83.5%), ampicillin (77.3%), erythromycin (63.9%), tetracycline (16.5%), amoxicillin/clavulanic acid (16.5%), ciprofloxacin (15.5%), trimethoprim/sulfamethoxazole (15.5%), oxacillin (13.4%), fusidic acid (12.4%), rifampin (6.2%), clindamycin (6.2%), gentamicin (6.2%) and mupirocin (5.2%) were determined. In addition, different combinations of resistance genes were identified among resistant isolates. Ciprofloxacin resistant isolates had mutations in codon 84 (Ser84Leu) and 106 (Gly106Asp) in the gyrA gene. Mutations in grlA were mostly related to Ser80Phe substitution. Leu466Ser mutation in the rpoB gene was detected in all rifampin resistant isolates. All methicillin resistant S. aureus isolates were SCCmec type V.

CONCLUSIONS

In conclusion, it was determined that the isolates were resistant to different classes of antimicrobials at varying rates and resistance was mediated by different genetic mechanisms. Therefore, continuous monitoring of resistance in S. aureus strains is necessary to control their resistance for clinically important antimicrobials.

The development of fluorescent protein tracing vectors for multicolor imaging of clinically isolated Staphylococcus aureus

Recent advances in fluorescent protein technology provide a wide variety of biological imaging applications; however current tools for bio-imaging in the Gram-positive bacterium Staphylococcus aureus has necessitated further developments for fluorescence intensity and for a multicolor palette of fluorescent proteins. To enhance the expression of multicolor fluorescent proteins in clinical S. aureus strains, we developed new fluorescent protein expression vectors, containing the blaZ/sodp promoter consisting of the β-lactamase gene (blaZ) promoter and the ribosome binding site (RBS) of superoxide dismutase gene (sod). We found S. aureus-adapted GFP (GFP_sa) driven by the blaZ/sodp promoter was highly expressed in the S. aureus laboratory strain RN4220, but not in the clinical strains, MW2 and N315, harboring the endogenous blaI gene, a repressor of the blaZ gene promoter. We therefore constructed a constitutively induced blaZ/sodp promoter (blaZ/sodp(Con)) by introducing substitution mutations into the BlaI binding motif, and this modification allowed enhanced expression of the multicolor GFP variants (GFP_sa, EGFP, mEmerald, Citrine, Cerulean, and BFP) as well as codon-optimized reef coral fluorescent proteins (mCherry and AmCyan) in the S. aureus clinical strains. These new fluorescent probes provide new tools to enhance expression of multicolor fluorescent proteins and facilitate clear visualization of clinical S. aureus strains.

Whole genome sequencing reveals high-resolution epidemiological links between clinical and environmental Klebsiella pneumoniae

Background

Klebsiella pneumoniae is a gram-negative bacterial species capable of occupying a broad range of environmental and clinical habitats. Known as an opportunistic pathogen, it has recently become a major causative agent of clinical infections worldwide. Despite growing knowledge about the highly diverse population of K. pneumoniae, the evolution and clinical significance of environmental K. pneumoniae, as well as the relationship between clinical and environmental K. pneumoniae, are poorly defined.

Methods

We isolated and sequenced K. pneumoniae from in-patients in a single hospital in Thailand, as well as hospital sewage, and surrounding canals and farms within a 20-km radius.

Results

Phylogenetic analysis of 77 K. pneumoniae (48 clinical and 29 non-clinical isolates) demonstrated that the two groups were intermixed throughout the tree and in some cases resided in the same clade, suggesting recent divergence from a common ancestor. Phylogenetic comparison of the 77 Thai genomes with 286 K. pneumoniae from a global collection showed that Thai isolates were closely related to the clinical sub-population of the global collection, indicating that Thai clinical isolates belonged to globally circulating lineages. Dating of four Thai K. pneumoniae clades indicated that they emerged between 50 and 150 years ago. Despite their phylogenetic relatedness, virulence factors and β-lactamase resistance genes were more numerous in clinical than in environmental isolates. Our results indicate that clinical and environmental K. pneumoniae are closely related, but that hospitals may select for isolates with a more resistant and virulent genotype.

Conclusions

These findings highlight the clinical relevance of environmental K. pneumoniae isolates.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-017-0397-1) contains supplementary material, which is available to authorized users.

Variety of Antimicrobial Resistances and Virulence Factors in Staphylococcus aureus Isolates from Meat Products Legally and Illegally Introduced to Germany

Food products of animal origin can serve as a vehicle for Staphylococcus (S.) aureus, a facultative pathogen involved in a variety of diseases. As a result, international trade and illegal transportation of foodstuffs can facilitate the distribution of S. aureus over long distances. In this study, we investigated S. aureus isolates recovered from meat products confiscated from passengers returning from non-EU countries at two German airports and from samples of legally imported meats from non-EU countries. The aim was to characterize isolates in regard to their genetic relatedness as well as their antimicrobial resistance profiles and major virulence factors in order to assess potential risks associated with these products. The isolates were characterized by spa typing, MLST, macrorestriction analysis, microarray analysis and antimicrobial susceptibility testing. MRSA isolates were further characterized by dru typing. The characteristics of the majority of the isolates indicated a human origin, rather than an association with livestock. The results further revealed a considerable heterogeneity among the MRSA isolates, despite their common origin. Overall, a plenitude of major virulence factors and antimicrobial resistances was detected among the isolates, highlighting the potential risks associated with contaminated meat products and the transportation of such products among different countries.

Single-molecule sequencing reveals the molecular basis of multidrug-resistance in ST772 methicillin-resistant Staphylococcus aureus

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of hospital-associated infection, but there is growing awareness of the emergence of multidrug-resistant lineages in community settings around the world. One such lineage is ST772-MRSA-V, which has disseminated globally and is increasingly prevalent in India. Here, we present the complete genome sequence of DAR4145, a strain of the ST772-MRSA-V lineage from India, and investigate its genomic characteristics in regards to antibiotic resistance and virulence factors.

RESULTS

Sequencing using single-molecule real-time technology resulted in the assembly of a single continuous chromosomal sequence, which was error-corrected, annotated and compared to nine draft genome assemblies of ST772-MRSA-V from Australia, Malaysia and India. We discovered numerous and redundant resistance genes associated with mobile genetic elements (MGEs) and known core genome mutations that explain the highly antibiotic resistant phenotype of DAR4145. Staphylococcal toxins and superantigens, including the leukotoxin Panton-Valentinin Leukocidin, were predominantly associated with genomic islands and the phage φ-IND772PVL. Some of these mobile resistance and virulence factors were variably present in other strains of the ST772-MRSA-V lineage.

CONCLUSIONS

The genomic characteristics presented here emphasize the contribution of MGEs to the emergence of multidrug-resistant and highly virulent strains of community-associated MRSA. Antibiotic resistance was further augmented by chromosomal mutations and redundancy of resistance genes. The complete genome of DAR4145 provides a valuable resource for future investigations into the global dissemination and phylogeography of ST772-MRSA-V.

DNA Topoisomerases

DNA topoisomerases are enzymes that control the topology of DNA in all cells. There are two types, I and II, classified according to whether they make transient single- or double-stranded breaks in DNA. Their reactions generally involve the passage of a single- or double-strand segment of DNA through this transient break, stabilized by DNA-protein covalent bonds. All topoisomerases can relax DNA, but DNA gyrase, present in all bacteria, can also introduce supercoils into DNA. Because of their essentiality in all cells and the fact that their reactions proceed via DNA breaks, topoisomerases have become important drug targets; the bacterial enzymes are key targets for antibacterial agents. This article discusses the structure and mechanism of topoisomerases and their roles in the bacterial cell. Targeting of the bacterial topoisomerases by inhibitors, including antibiotics in clinical use, is also discussed.

Shuttle vectors derived from pN315 for study of essential genes in Staphylococcus aureus

Using the par to rep region of the 24653 bp plasmid pN315, which is present in Staphylococcus aureus strain N315, we constructed three vectors that can be shuttled between Escherichia coli and S. aureus and maintained stably in S. aureus. Due to plasmid incompatibility, the resident plasmid in S. aureus cells can be replaced via transformation with an entering plasmid, which carries a different drug resistance gene. To evaluate the applicability of this plasmid-based approach for identifying genes essential for S. aureus cell growth, the chromosomal mraY gene, which is involved in peptidoglycan biosynthesis, was deleted in cells harboring a resident plasmid with an intact mraY gene. The resultant disruptant was then transformed with an empty vector. Cells with a chromosomal mraY deletion but lacking the plasmid supplying mraY could not be recovered, suggesting that mraY is indispensable for staphylococcal cell growth or viability. In contrast, other two genes were shown to be dispensable by this system. Thus, the pN315-based plasmids appear to be useful for studying genes essential for S. aureus cell growth.

Human serum mannose-binding lectin senses wall teichoic acid Glycopolymer of Staphylococcus aureus, which is restricted in infancy

Innate immunity is the first line of host defense against invading pathogens, and it is recognized by a variety of pattern recognition molecules, including mannose-binding lectin (MBL). MBL binds to mannose and N-acetylglucosamine residues present on the glycopolymers of microorganisms. Human serum MBL functions as an opsonin and activates the lectin complement pathway. However, which glycopolymer of microorganism is recognized by MBL is still uncertain. Here, we show that wall teichoic acid of Staphylococcus aureus, a bacterial cell surface glycopolymer containing N-acetylglucosamine residue, is a functional ligand of MBL. Whereas serum MBL in adults did not bind to wall teichoic acid because of an inhibitory effect of anti-wall teichoic acid antibodies, MBL in infants who had not yet fully developed their adaptive immunity could bind to S. aureus wall teichoic acid and then induced complement C4 deposition. Our data explain the molecular reasons of why MBL-deficient infants are susceptible to S. aureus infection.

Rapid exchange of bound ADP on the Staphylococcus aureus replication initiation protein DnaA

In Escherichia coli, regulatory inactivation of the replication initiator DnaA occurs after initiation as a result of hydrolysis of bound ATP to ADP, but it has been unknown how DnaA is controlled to coordinate cell growth and chromosomal replication in gram-positive bacteria such as Staphylococcus aureus. This study examined the roles of ATP binding and its hydrolysis in the regulation of the S. aureus DnaA activity. In vitro, S. aureus DnaA melted S. aureus oriC in the presence of ATP but not ADP by a mechanism independent of ATP hydrolysis. Unlike E. coli DnaA, binding of ADP to S. aureus DnaA was unstable. As a result, at physiological concentrations of ATP, ADP bound to S. aureus DnaA was rapidly exchanged for ATP, thereby regenerating the ability of DnaA to form the open complex in vitro. Therefore, we examined whether formation of ADP-DnaA participates in suppression of replication initiation in vivo. Induction of the R318H mutant of the AAA+ sensor 2 protein, which has decreased intrinsic ATPase activity, caused over-initiation of chromosome replication in S. aureus, suggesting that formation of ADP-DnaA suppresses the initiation step in S. aureus. Together with the biochemical features of S. aureus DnaA, the weak ability to convert ATP-DnaA into ADP-DnaA and the instability of ADP-DnaA, these results suggest that there may be unidentified system(s) for reducing the cellular ratio of ATP-DnaA to ADP-DnaA in S. aureus and thereby delaying the re-initiation of DNA replication.

DNA Topoisomerases

DNA topoisomerases are enzymes that control the topological state of DNA in all cells; they have central roles in DNA replication and transcription. They are classified into two types, I and II, depending on whether they catalyze reactions involving the breakage of one or both strands of DNA. Structural and mechanistic distinctions have led to further classifications: IA, IB, IC, IIA, and IIB. The essence of the topoisomerase reaction is the ability of the enzymes to stabilize transient breaks in DNA, via the formation of tyrosyl-phosphate covalent intermediates. The essential nature of topoisomerases and their ability to stabilize DNA breaks has led to them being key targets for antibacterial and anticancer agents. This chapter reviews the basic features of topoisomerases focussing mainly on the prokaryotic enzymes. We highlight recent structural advances that have given new insight into topoisomerase mechanisms and into the molecular basis of the action of topoisomerase-specific drugs.

Pleiotropic roles of polyglycerolphosphate synthase of lipoteichoic acid in growth of Staphylococcus aureus cells

Lipoteichoic acid (LTA) is one of two anionic polymers on the surface of the gram-positive bacterium Staphylococcus aureus. LTA is critical for the bacterium-host cell interaction and has recently been shown to be required for cell growth and division. To determine additional biological roles of LTA, we found it necessary to identify permissive conditions for the growth of an LTA-deficient mutant. We found that an LTA-deficient S. aureus Delta ltaS mutant could grow at 30 degrees C but not at 37 degrees C. Even at the permissive temperature, Delta ltaS mutant cells had aberrant cell division and separation, decreased autolysis, and reduced levels of peptidoglycan hydrolases. Upshift of Delta ltaS mutant cells to a nonpermissive temperature caused an inability to exclude Sytox green dye. A high-osmolarity growth medium remarkably rescued the colony-forming ability of the Delta ltaS mutant at 37 degrees C, indicating that LTA synthesis is required for growth under low-osmolarity conditions. In addition, the Delta ltaS mutation was found to be synthetically lethal with the Delta tagO mutation, which disrupts the synthesis of the other anionic polymer, wall teichoic acid (WTA), at 30 degrees C, suggesting that LTA and WTA compensate for one another in an essential function.

DnaB and DnaI temperature-sensitive mutants of Staphylococcus aureus: evidence for involvement of DnaB and DnaI in synchrony regulation of chromosome replication

DnaB and DnaI proteins conserved in low-GC content Gram-positive bacteria are apparently involved in helicase loading at the replication initiation site and during the restarting of stalled replication forks. In this study, we found five novel dnaB mutants and three novel dnaI mutants by screening 750 temperature-sensitive Gram-positive Staphylococcus aureus mutants. All of the mutants had a single amino acid substitution in either DnaB or DnaI that controlled temperature-sensitive growth, as confirmed by transduction experiments using phage 80alpha. DNA synthesis as measured by [(3)H]thymine incorporation, origin-to-terminus ratios and flow cytometric analysis revealed that the dnaB and dnaI mutants were unable to initiate DNA replication at restrictive temperatures, which is similar to previous findings in Bacillus subtilis. Furthermore, some of the mutants were found to exhibit asynchrony in the initiation of DNA replication. Also, a fraction of the dnaI mutant cells showed arrested replication, and the dnaI mutant tested was sensitive to mitomycin C, which causes DNA lesions. These results suggest that DnaB and DnaI are required not only for replication initiation and but also for regulation of its synchrony, and they provide support for the involvement of DnaI activity in the restart of arrested replication forks in vivo.

Antimicrobial susceptibility and mechanism of resistance to fluoroquinolones in Staphylococcus intermedius and Staphylococcus schleiferi

The objective of the present study was to determine the antimicrobial susceptibility of 136 canine isolates of Staphylococcus intermedius and 10 canine isolates of S. schleiferi subspecies coagulans to 16 fluoroquinolones (FQs), and to investigate the mechanisms of resistance in the nonsusceptible isolates. Of the 136 of S. intermedius tested 98.5% were susceptible to all 16 FQs whereas only 40% of the 10 isolates of S. schleiferi subspecies coagulans were susceptible. Two isolates of S. intermedius and six isolates of S. schleiferi, were found to be resistant to 13 out of 16 FQs, while they retained their susceptibility to fourth generation FQs such as gatifloxacin, moxifloxacin and trovafloxacin. Sequencing of the quinolone-resistance determining regions of gyrA and grlA genes showed that in S. intermedius, dichotomous resistance to FQs was associated with the occurrence of one alteration in GyrA-84 and one in GrlA-80, while in S. schleiferi the same pattern of resistance was observed in isolates showing these changes only in gyrA. This study is the first to screen FQs of the second, third and fourth generation for antimicrobial resistance in clinical isolates of S. intermedius and S. schleiferi of canine origin, and to describe mutations in gyrA and grlA associated with FQ resistance in these bacterial species.

Dual targeting of DNA gyrase and topoisomerase IV: target interactions of heteroaryl isothiazolones in Staphylococcus aureus

Heteroaryl isothiazolones (HITZs) are antibacterial agents that display excellent in vitro activity against Staphylococcus aureus. We recently identified a series of these compounds that show potent bactericidal activities against methicillin-resistant Staphylococcus aureus (MRSA). We report here the results of in vitro resistance studies that reveal potential underlying mechanisms of action. HITZs selected gyrA mutations exclusively in first-step mutants of wild-type S. aureus, indicating that in contrast to the case with most quinolones, DNA gyrase is the primary target. The compounds displayed low mutation frequencies (10(-9) to 10(-10)) at concentrations close to the MICs and maintained low MICs (< or =0.016 microg/ml) against mutants with single mutations in either gyrA or grlA (parC). These data suggested that HITZs possess significant inhibitory activities against target enzymes, DNA gyrase and topoisomerase IV. This dual-target inhibition was supported by low 50% inhibitory concentrations against topoisomerase IV as measured in a decatenation activity assay and against DNA gyrase as measured in a supercoiling activity assay. Good antibacterial activities (< or =1 microg/ml) against staphylococcal gyrA grlA double mutants, as well as low frequencies (10(-9) to 10(-10)) of selection of still higher-level mutants, also suggested that HITZs remained active against mutant enzymes. We further demonstrated that HITZs exhibit good inhibition of both S. aureus mutant enzymes and thus continue to possess a novel dual-targeting mode of action against these mutant strains. In stepwise acquisition of mutations, HITZs selected quinolone resistance determining region mutations gyrA(Ser84Leu), grlA(Ser80Phe), grlA(Ala116Val), and gyrA(Glu88Lys) sequentially, suggesting that the corresponding amino acids are key amino acids involved in the binding of HITZs to topoisomerases. The overall profile of these compounds suggests the potential utility of HITZs in combating infections caused by S. aureus, including multidrug-resistant MRSA.

Molecular characterization of 8-methoxyfluoroquinolone resistance in a clinical isolate of methicillin-resistant Staphylococcus aureus

BACKGROUND

Activity of gatifloxacin against clinical isolates of fluoroquinolone-resistant Staphylococcus aureus is more potent than that of other fluoroquinolones such as norfloxacin and levofloxacin. To date, few reports have described high-level resistance to gatifloxacin in clinical isolates of S. aureus, although in vitro studies have shown that mutations in both DNA gyrase and topoisomerase IV were required for gatifloxacin resistance in S. aureus.

METHODS

Minimum inhibitory concentrations were determined for fluoroquinolones and other antimicrobials in a methicillin-resistant S. aureus isolate that was cultured from blood of a patient with septicemia. Fluoroquinolone resistance was characterized by DNA sequencing and microbiologic assay.

RESULTS

The isolate showed high-level resistance to fluoroquinolones including an 8-methoxyfluoroquinolone, gatifloxacin (minimum inhibitory concentration 64 microg/ml). Amino acid mutations of Ser80Tyr and Glu84Lys in GrlA and Ser84Leu and Ser85Pro in GyrA were possibly related to this resistance in methicillin-resistant S. aureus HU2000-062, although efflux may play a minor role in resistance as well.

CONCLUSION

GyrA and GrlA mutations mainly conferred to 8-methoxyfluoroquinolone resistance in this isolate.

Antibacterial Effects of Moxifloxacin and Levofloxacin Simulating Epithelial Lining Fluid Concentrations against Community-Acquired Methicillin-Resistant Staphylococcus aureus

BACKGROUND AND OBJECTIVE

Current North American guidelines advocate the use of respiratory fluoroquinolones for the empirical management of community-acquired pneumonia (CAP). While community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) has emerged as a pathogen frequently encountered in skin and skin structure infections, it has also now been recognised as a causative pathogen in CAP. Since fluoroquinolones may be used empirically to treat unsuspected CA-MRSA pneumonia, the objective of this study was to evaluate the antibacterial properties of levofloxacin and moxifloxacin using human simulated drug exposures in epithelial lining fluid (ELF).

METHODS

An in vitro model was used to simulate the ELF concentrations, previously determined in older adults receiving multiple doses, of levofloxacin 500 mg once daily and moxifloxacin 400mg once daily. Four CA-MRSA isolates were studied at a starting inoculum of 10(6) colony-forming units (CFU)/mL; selected isolates were also studied at 10(8) CFU/mL. Bacterial density and resistance were quantitatively assessed over 48 hours. Drug exposure (area under the concentration-time curve [AUC]) was confirmed using validated drug assays.

RESULTS

At a standard 10(6) starting inoculum, sustained bacterial kill (3.6-4.5 log) with both fluoroquinolones was noted for CA-MRSA isolates 27 and 44 (AUC/minimum inhibitory concentration [MIC] = 383-3923). Despite an MIC of 8 microg/mL (AUC/MIC = 25) for isolate 3, levofloxacin displayed a 2.8 log kill, while moxifloxacin (MIC 1 microg/mL) sustained a 4.5 log kill (AUC/MIC = 207) over 48 hours. Against isolate 59, levofloxacin displayed no antibacterial effect (AUC/MIC = 3), while moxifloxacin with an MIC of 8 microg/mL (AUC/MIC = 31) killed 4.6 log. At a high inoculum (10(8)), both fluoroquinolones showed 5.2-5.6 log kill for the susceptible isolate (44), while moxifloxacin showed no antibacterial activity against isolate 59. Drug exposure (AUC/MIC) appeared to correlate well (r(2) = 0.99) with the change in log CFU/mL. Maximal activity was observed for both drugs at an AUC/MIC of approximately 30.

CONCLUSION

When evaluated at human simulated ELF concentrations, both levofloxacin and moxifloxacin appeared to demonstrate sustained antibacterial activity for CA-MRSA isolates with MICs <or=8 microg/mL at a starting inoculum of 10(6). Use of a high inoculum (10(8)) appeared to compromise the antimicrobial activity of the fluoroquinolones when the MIC was 8 microg/mL, but did not mitigate antibacterial kill for susceptible isolates.

Topoisomerase mutations and efflux are associated with fluoroquinolone resistance in Enterococcus faecalis

To understand better the mechanisms of fluoroquinolone resistance in Enterococcus faecalis, fluoroquinolone-resistant mutants isolated from Ent. faecalis ATCC 29212 by stepwise selection with sparfloxacin (SPX) and norfloxacin (NOR) were analysed. The results showed the following. (i) In general, fluoroquinolone-resistance mechanisms in Ent. faecalis are similar to those in other Gram-positive bacteria, such as Staphylococcus aureus and Streptococcus pneumoniae, namely, mutants with amino acid changes in both GyrA and ParC exhibited high fluoroquinolone resistance, and single GyrA mutants and a single ParC mutant were more resistant to SPX and NOR, respectively, than the parent strain, indicating that the primary targets of SPX and NOR in Ent. faecalis are DNA gyrase and topoisomerase IV, respectively. (ii) Alterations in GyrB (ΔKGA, residues 395–397) and ParE (Glu-459 to Lys) were associated with fluoroquinolone resistance in some mutants. Moreover, the facts that the NOR MIC, but not the SPX MIC, decreased in the presence of multidrug efflux pump inhibitors, that NOR accumulation decreased in the cells, and that the EmeA mRNA expression level did not change, strongly suggested that a NorA-like efflux pump, rather than EmeA, was involved in resistance to NOR.

The anti-methicillin-resistant Staphylococcus aureus quinolone WCK 771 has potent activity against sequentially selected mutants, has a narrow mutant selection window against quinolone-resistant Staphylococcus aureus, and preferentially targets DNA gyrase

WCK 771 is a broad-spectrum fluoroquinolone with enhanced activity against quinolone-resistant staphylococci. To understand the impact of the target-level interactions of WCK 771 on its antistaphylococcal pharmacodynamic properties, we determined the MICs for genetically defined mutants and studied the mutant prevention concentrations (MPCs), the frequency of mutation, and the cidality against the wild type and double mutants. There was a twofold increase in the MICs of WCK 771 for single gyrA mutants, indicating that DNA gyrase is its primary target. All first- and second-step mutants selected by WCK 771 revealed gyrA and grlA mutations, respectively. The MICs of WCK 771 and clinafloxacin were found to be superior to those of other quinolones against strains with double and triple mutations. WCK 771 was also cidal for high-density double mutants at low concentrations. WCK 771 and clinafloxacin showed narrow mutant selection windows compared to those of the other quinolones. Against a panel of 50 high-level quinolone-resistant clinical isolates of staphylococci (ciprofloxacin MIC > or = 16 microg/ml), the WCK 771 MPCs were < or =2 microg/ml for 68% of the strains and < or =4 microg/ml for 28% of the strains. Our results demonstrate that gyrA is the primary target of WCK 771 and that it has pharmacodynamic properties remarkably different from those of quinolones with dual targets (garenoxacin and moxifloxacin) and topoisomerase IV-specific quinolones (trovafloxacin). WCK 771 displayed an activity profile comparable to that of clinafloxacin, a dual-acting quinolone with a high affinity to DNA gyrase. Overall, the findings signify the key role of DNA gyrase in determining the optimal antistaphylococcal features of quinolones.

Trends in the rates of resistance of Ureaplasma urealyticum to antibiotics and identification of the mutation site in the quinolone resistance-determining region in Chinese patients

The resistance of Ureaplasma urealyticum to antibacterials, isolated from 804 patients from the outpatient clinic of the Sir Run Run Shaw Hospital Hangzhou, China, from March to June over six consecutive years (1999-2004) was reviewed. The quinolone resistance-determining region of six strains of U. urealyticum was analyzed. The level of resistance to doxycycline, josamycin, tetracycline, azithromycin, clarithromycin and pristinamycin was below 20% and did not change during the study period. The rate of resistance to fluoroquinolones (ofloxacin, ciprofloxacin) was much greater than these (>50%) and has increased since 1999. The rate of resistance to Erythromycin decreased from 63.9% in 1999 to about 20% from 2000 onwards. The widespread use of fluoroquinolones had led to high resistance rates in U. urealyticum and the emergence of quinolone resistance. Analysis of the gene sequence of topoisomerase IV and DNA gyrase suggested a role for the topoisomerase IV ParE subunit in fluoroquinolone-resistant U. urealyticum.

Emerging multiple mutations and high-level fluoroquinolone resistance in methicillin-resistant Staphylococcus aureus isolated from ocular infections

We investigated the prevalence of mutations in the gyrA, gyrB, grlA, and grlB genes in 21 clinical isolates of methicillin-resistant Staphylococcus aureus recovered from ocular infections. These isolates were classified into 16 groups on the basis of pulsed-field gel electrophoresis (PFGE) patterns. Among these nonidentical strains, 9 different combinations of mutations were observed. We observed an unusually high frequency of mutations: 10 (62.5%) of the isolates had four or five mutations, 5 (31.3%) of the isolates had double mutations, and 1 isolate had a single mutation. Different mutations were detected in strains of the same PFGE type, and this is consistent with the independent acquisition of mutations. The minimun inhibitory concentrations values ranged from 0.5 to 512 microg/mL for levofloxacin and 0.125 to 128 microg/mL for gatifloxacin. Only 1 isolate was found to be susceptible to both agents. These results suggest that application of high concentration fluoroquinolone ophthalmic solutions selects for isolates presenting such mutations that may result in a high level of resistance to fluoroquinolones.

Combination of known and unknown mechanisms confers high-level resistance to fluoroquinolones in Enterococcus faecium

In order to elucidate the mechanisms of fluoroquinolone resistance in Enterococcus faecium, spontaneous mutants isolated from Ent. faecium ATCC 19434 by stepwise selection with sparfloxacin (SPX) or norfloxacin (NOR) and 13 clinical isolates of Ent. faecium were characterized by analysing quinolone-resistance-determining regions (QRDRs) of the gyrA, gyrB, parC and parE genes and examining changes in MICs of SPX and NOR in the presence of efflux pump inhibitors. The SPX-selected first-step mutant had a point mutation only in gyrA, and the mutants QR7-18 and QR7-39, and clinical isolates that had point mutations in parC, showed NOR resistance. These results indicate that the primary targets of SPX and NOR are DNA gyrase and topoisomerase IV, respectively, and therefore that the primary target of fluoroquinolones in Ent. faecium differs depending on the structure of the compound used. The characterization of the spontaneous mutants and the clinical isolates demonstrates that in addition to the previously reported alterations in GyrA and ParC, an alteration in GyrB, a NorA-like pump, an unknown efflux pump, which excretes both SPX and NOR from bacterial cells, and probably other unknown mechanism(s) all contribute to fluoroquinolone resistance in Ent. faecium.

Anucleate cell blue assay: a useful tool for identifying novel type II topoisomerase inhibitors

About 95,000 compounds were screened by the anucleate cell blue assay. Fifty-one of the hit compounds had various structures and showed inhibitory activity against DNA gyrase and/or topoisomerase IV. Moreover, the compounds exhibited antibacterial activity against a fluoroquinolone- and novobiocin-resistant strain of Staphylococcus aureus. The anucleate cell blue assay is therefore a useful tool for finding novel type II topoisomerase inhibitors.

Susceptibility and resistance genes to fluoroquinolones in methicillin-resistant Staphylococcus aureus isolated in 2002

The activity of six fluoroquinolones (FQs) was determined against 100 methicillin-resistant Staphylococcus aureus (MRSA) isolated in 2002 along with mutations in the grlA and gyrA genes and in the norA promoter of these isolates. Of the isolates tested, 97% had mutations in grlA and gyrA. A single mutation in grlA and gyrA resulted in a decrease of susceptibility to old generation FQs (norfloxacin, enoxacin, ciprofloxacin, fleroxacin, sparfloxacin and levofloxacin) but not to new generation FQs (gatifloxacin and moxifloxacin). Double mutations of both grlA and gyrA resulted in high-level resistance to all FQs tested. All norA mutants (15%) contained double mutations in grlA and gyrA and showed no decrease of MIC in the presence of reserpine, which is known to inhibit the drug-efflux pump. Our results showed that double mutations in grlA and gyrA were necessary for the expression of high-level resistance to new generation FQs. As different FQ-resistant mutants occur in the same PFGE type, FQ-resistant MRSA may well develop individually.

Accumulation of mutations in both gyrB and parE genes is associated with high-level resistance to novobiocin in Staphylococcus aureus

Coumarin-resistant mutants of Staphylococcus aureus were isolated by three-step selection with novobiocin at different concentrations. Sequencing analysis of the gyrB and parE genes of the first-, second-, and third-step mutants revealed that successive point mutations first occurred specifically in the gyrB gene, followed by a point mutation in the parE gene and then an additional point mutation in the gyrB gene. These findings demonstrate that DNA gyrase is the primary target and that topoisomerase IV is the secondary target for novobiocin and that the accumulation of point mutations in both the gyrB and the parE genes is associated with high-level resistance to novobiocin in S. aureus. Moreover, our results show that the amino acid substitutions (Asp-89 to Gly and Ser-128 to Leu) found in GyrB are associated with resistance to novobiocin but not to coumermycin A1, suggesting that the interactions of novobiocin and coumermycin A1 with GyrB differ at the molecular level.

An improved expression plasmid for affinity purification of Staphylococcus aureus gyrase A subunit

Of the bacterial topoisomerases, the gyrase A subunit (GyrA) of Staphylococcus aureus is particularly difficult to purify because of its tendency to form inclusion bodies. Previous attempts at purification yielded low concentrations of protein with reduced specific activity. To overcome this problem, we modified the commercially available plasmid expression vector, pBAD/Thio-TOPO, via the addition of DNA sequences encoding a hexahistidine tag upstream and a cleavage site for tobacco etch virus protease downstream of the gene encoding thioredoxin. The resulting expression system consisting of the modified plasmid, pSAGA7, and the recommended host strain, Escherichia coli TOP 10, facilitated high level expression of soluble GyrA and its affinity purification to over 95% homogeneity. Purified GyrA had high biological activity as evidenced by a specific activity of 4.3x10(5)U/mg. The pSAGA7/TOP10 expression system also facilitated the expression and purification of a subunit of S. aureus topoisomerase IV, ParE, and a recently discovered protein unrelated to topoisomerases, QnrB, two "hard to purify" proteins. We conclude that pSAGA7 might be useful for high-level soluble expression and purification of diverse microbial proteins.

Increased antibacterial activity of DW286, a novel fluoronaphthyridone antibiotic, against Staphylococcus aureus strains with defined mutations in DNA gyrase and topoisomerase IV

To investigate the activity of DW286, a new fluoronaphthyridone, the quinolone resistance determining regions (QRDRs) of gyrA, gyrB, grlA and grlB genes in 64 Staphylococcus aureus clinical isolates were analyzed and the MICs of DW286 and comparator quinolones determined. Double and triple mutants in gyrA and grlA were resistant to ciprofloxacin, sparfloxacin, trovafloxacin and gemifloxacin but susceptible to DW286 (MIC 0.25-0.5 mg/l). The fourth alteration, Ser85Pro of GyrA was required to make a strain resistant to DW286 (MIC 4-32 mg/l). For a strain with the mutations at GyrA Ser84Leu and GrlA Ser80Phe, the MBC of DW286 was two-fold higher than its corresponding MIC, in contrast to ciprofloxacin which was not bactericidal.

Comparison of the nucleotide sequence and expression of norA genes and microbial susceptibility in 21 strains of Staphylococcus aureus

The norA gene, which is located on the chromosome of Staphylococcus aureus, encodes the multidrug efflux protein with 12 transmembrane segments (TMS), and the overexpression of norA by the mutation of the transcriptional control region confers resistance to fluoroquinolones and antiseptics. In this study, the nucleotide sequence and transcriptional expression of norA genes were studied in six standard laboratory strains and 15 clinical isolates, which included 5 antiseptic-susceptible strains and 10 resistant strains. Seven mutations leading to up-regulation of norA transcription were identified. In addition, a novel allele of norA (norAII) was identified whose 3'-terminal region sequence and profile of antimicrobial susceptibility differs from those of original norA.

Evaluation of a high-density oligonucleotide array for characterization of grlA, grlB, gyrA and gyrB mutations in fluoroquinolone resistant Staphylococcus aureus isolates

Using high-density oligonucleotide array technology, 30 Staphylococcus aureus strains were studied for the presence of mutations in genes involved in fluoroquinolone resistance: grlA, gyrA, grlB and gyrB. For the two most important genes, gyrA and grlA, correlation with sequencing reached 95.1%. If all genes were considered, correlation was 88.8%.

Evolution of ciprofloxacin-resistant Staphylococcus aureus in in vitro pharmacokinetic environments

The development of novel antibacterial agents is decreasing despite increasing resistance to presently available agents among common pathogens. Insights into relationships between pharmacodynamics and resistance may provide ways to optimize the use of existing agents. The evolution of resistance was examined in two ciprofloxacin-susceptible Staphylococcus aureus strains exposed to in vitro-simulated clinical and experimental ciprofloxacin pharmacokinetic profiles for 96 h. As the average steady-state concentration (C(avg ss)) increased, the rate of killing approached a maximum, and the rate of regrowth decreased. The enrichment of subpopulations with mutations in grlA and low-level ciprofloxacin resistance also varied depending on the pharmacokinetic environment. A regimen producing values for C(avg ss) slightly above the MIC selected resistant variants with grlA mutations that did not evolve to higher levels of resistance. Clinical regimens which provided values for C(avg ss) intermediate to the MIC and mutant prevention concentration (MPC) resulted in the emergence of subpopulations with gyrA mutations and higher levels of resistance. A regimen producing values for C(avg ss) close to the MPC selected grlA mutants, but the appearance of subpopulations with higher levels of resistance was diminished. A regimen designed to maintain ciprofloxacin concentrations entirely above the MPC appeared to eradicate low-level resistant variants in the inoculum and prevent the emergence of higher levels of resistance. There was no relationship between the time that ciprofloxacin concentrations remained between the MIC and the MPC and the degree of resistance or the presence or type of ciprofloxacin-resistance mutations that appeared in grlA or gyrA. Regimens designed to eradicate low-level resistant variants in S. aureus populations may prevent the emergence of higher levels of fluoroquinolone resistance.

Role of topoisomerase mutations and efflux in fluoroquinolone resistance of Bacteroides fragilis clinical isolates and laboratory mutants

Twelve laboratory mutants and 32 ciprofloxacin-resistant isolates of Bacteroides fragilis were examined for the mechanism(s) of fluoroquinolone resistance. Five mutants had mutations in gyrA. One mutant and two clinical isolates contained a mutation in gyrB. Eight mutants and five clinical isolates accumulated significantly less ciprofloxacin than did wild-type isolates; the mutants and clinical isolates were restored to wild-type characteristics when carbonyl cyanide m-chlorophenylhydrazone was used.

Quinolone resistance due to reduced target enzyme expression

We report for the first time low-level quinolone resistance mediated by decreased expression of topoisomerase IV in Staphylococcus aureus. A single-step mutant of wild-type S. aureus strain ISP794, P18 selected by using twice the MIC of premafloxacin, had four- and four- to eightfold greater MICs of premafloxacin and ciprofloxacin, respectively, than the wild type. Sequencing of parEC and gyrBA with their promoter regions revealed a point mutation (G-->A) 13 bp upstream of the start codon of parE. Genetic linkage studies showed that there was a high level of correlation between the mutation and the resistance phenotype, and allelic exchange confirmed the contribution of the mutation to resistance. Decreased expression of ParE and decreased steady-state levels of parEC transcripts in P18 and in resistant allelic exchange mutants were observed. The steady-state levels of gyrBA and topB transcripts were increased in P18 but not in two resistant allelic exchange mutants, and sequencing upstream of either gene did not reveal a difference between ISP794 and P18. The steady-state levels of topA transcripts were similar in the various strains. Growth competition experiments performed at 30, 37, and 41 degrees C with a susceptible allelic exchange strain and a resistant allelic exchange strain suggested that loss of fitness was associated with reduced levels of ParE at 41 degrees C. However, P18 had a growth advantage over ISP794 at all temperatures, suggesting that a compensatory mechanism was associated with the increased levels of gyrBA and topB transcripts. Thus, reduced levels of ParE appear to be compatible with cell survival, although there may be a fitness cost during rapid cell multiplication, which might be overcome by compensatory mechanisms without reversion of the resistance phenotype.

Characteristic expression of three genes, msr(A), mph(C) and erm(Y), that confer resistance to macrolide antibiotics on Staphylococcus aureus

We have reported that the gene mph(C) (formally referred to as 'mphBM') is located on plasmid pMS97 342 bp downstream of the msr(A) gene. msr(A) specifies resistance to macrolides by ABC-transporter-mediated efflux, and mph(C) has 49% identity to the amino acid sequence of MPH(2')II, which encodes a phosphotransferase that inactivates some macrolide antibiotics. A strain of Staphylococcus aureus NCTC8325 containing plasmid pMS97 inactivated unlabeled and (14)C-labeled erythromycin when tested by bioautographic and radioautographic techniques. In addition to erythromycin, other 14-membered ring macrolides (except for ketolides), 15-membered ring macrolides and 16-membered ring macrolides, mycinamicin, rosamicin and YM133, were inactivated by the strain. Erythromycin inactivation products produced by the strain carrying pMS97 were completely different from those produced by Escherichia coli BM694 bearing plasmid pAT63, which contains the ereA gene encoding an esterase that hydrolyzes macrolide lactones. Constructs formed with the msr(A) and mph(C) genes, and with the msr(A), mph(C) and erm(Y) genes, showed erythromycin-inactivating activity, but another construct built with the mph(C) gene alone failed to show such activity. This result suggests that any region of the msr(A) gene is needed for the expression of mph(C).

Contribution of the 8-methoxy group to the activity of gatifloxacin against type II topoisomerases of Streptococcus pneumoniae

The inhibitory activities (50% inhibitory concentrations [IC(50)s]) of gatifloxacin and other quinolones against both DNA gyrase and topoisomerase IV of the wild-type Streptococcus pneumoniae IID553 were determined. The IC(50)s of 10 compounds ranged from 4.28 to 582 microg/ml against DNA gyrase and from 1.90 to 35.2 microg/ml against topoisomerase IV. The inhibitory activity against DNA gyrase was more varied than that against topoisomerase IV among fluoroquinolones. The IC(50)s for DNA gyrase of the 8-methoxy quinolones gatifloxacin and AM-1147 were approximately seven times lower than those of their 8-H counterparts AM-1121 and ciprofloxacin, whereas the IC(50)s for topoisomerase IV were 1.5 times lower. Moreover, the IC(50) ratios (IC(50) for DNA gyrase/IC(50) for topoisomerase IV) of gatifloxacin, AM-1147, and moxifloxacin, which possess 8-methoxy groups, were almost the same. The 8-methoxy quinolones showed higher antibacterial activity and less mutant selectivity against IID553 than their 8-H counterparts. These results suggest that the 8-methoxy group enhances both target inhibition, especially for DNA gyrase, leading to potent antipneumococcal activity and dual inhibition against both DNA gyrase and topoisomerase IV in the bacterial cell.

Topoisomerase targeting with and resistance to gemifloxacin in Staphylococcus aureus

Gemifloxacin, a novel quinolone with potent activity against Staphylococcus aureus, was 8- to 16-fold more active against wild-type S. aureus than ciprofloxacin. The two- to fourfold increase in the MIC of gemifloxacin in genetically defined grlBA mutants and the twofold increase in a single gyrA mutant, supported by the low frequency of selection of resistant mutants at twice the MIC (7.4 x 10(-11) to 1.1 x 10(-10)), suggested similar targeting of the two enzymes by gemifloxacin. Dual mutations in both gyrase and topoisomerase IV caused a 64- to 128-fold increase in the MIC of gemifloxacin, similar to that seen with ciprofloxacin. Gemifloxacin also had similar activity in vitro against topoisomerase IV and gyrase purified from S. aureus (50% inhibitory concentrations of 0.25 and 0.31 micro g/ml, respectively). This activity was 10- to 20-fold higher than that of ciprofloxacin for topoisomerase IV and 33-fold higher than that for gyrase. In contrast to the in vitro findings, only topoisomerase IV mutants were selected in first-step mutants. Overexpression of the NorA efflux pump had a minimal effect on resistance to gemifloxacin, and a mutation in the promoter region of the gene for NorA was selected only in the sixth step of serial selection of mutants. Our data show that although gemifloxacin targets purified topoisomerase IV and gyrase similarly in vitro, topoisomerase IV is the preferred target in the bacteria. Selection of novel resistance mutations in grlA requires further expansion of quinolone-resistance-determining regions, and their study may provide increased insight into enzyme-quinolone interactions.

Dual targeting of DNA gyrase and topoisomerase IV: target interactions of garenoxacin (BMS-284756, T-3811ME), a new desfluoroquinolone

We determined the target enzyme interactions of garenoxacin (BMS-284756, T-3811ME), a novel desfluoroquinolone, in Staphylococcus aureus by genetic and biochemical studies. We found garenoxacin to be four- to eightfold more active than ciprofloxacin against wild-type S. aureus. A single topoisomerase IV or gyrase mutation caused only a 2- to 4-fold increase in the MIC of garenoxacin, whereas a combination of mutations in both loci caused a substantial increase (128-fold). Overexpression of the NorA efflux pump had minimal effect on resistance to garenoxacin. With garenoxacin at twice the MIC, selection of resistant mutants (<7.4 x 10(-12) to 4.0 x 10(-11)) was 5 to 6 log units less than that with ciprofloxacin. Mutations inside or outside the quinolone resistance-determining regions (QRDR) of either topoisomerase IV, or gyrase, or both were selected in single-step mutants, suggesting dual targeting of topoisomerase IV and gyrase. Three of the novel mutations were shown by genetic experiments to be responsible for resistance. Studies with purified topoisomerase IV and gyrase from S. aureus also showed that garenoxacin had similar activity against topoisomerase IV and gyrase (50% inhibitory concentration, 1.25 to 2.5 and 1.25 micro g/ml, respectively), and although its activity against topoisomerase IV was 2-fold greater than that of ciprofloxacin, its activity against gyrase was 10-fold greater. This study provides the first genetic and biochemical data supporting the dual targeting of topoisomerase IV and gyrase in S. aureus by a quinolone as well as providing genetic proof for the expansion of the QRDRs to include the 5' terminus of grlB and the 3' terminus of gyrA.

Small-colony mutants of Staphylococcus aureus allow selection of gyrase-mediated resistance to dual-target fluoroquinolones

Fluoroquinolones acting equally through DNA gyrase and topoisomerase IV in vivo are considered desirable in requiring two target mutations for emergence of resistant bacteria. To investigate this idea, we have studied the response of Staphylococcus aureus RN4220 to stepwise challenge with sparfloxacin, a known dual-target agent, and with NSFQ-105, a more potent sulfanilyl fluoroquinolone that behaves similarly. First-step mutants were obtained with both drugs but only at the MIC. These mutants exhibited distinctive small-colony phenotypes and two- to fourfold increases in MICs of NSFQ-105, sparfloxacin, and ciprofloxacin. No changes were detected in the quinolone resistance-determining regions of the gyrA, gyrB, grlA, or grlB gene. Quinolone-induced small-colony mutants shared the delayed coagulase response but not the requirement for menadione, hemin, or thymidine characteristic of small-colony variants, a subpopulation of S. aureus that is often defective in electron transport. Second-step mutants selected with NSFQ-105 had gyrA(S84L) alterations; those obtained with sparfloxacin carried a gyrA(D83A) mutation or a novel gyrB deletion (DeltaRKSAL, residues 405 to 409) affecting a trypsin-sensitive region linking functional domains of S. aureus GyrB. Each mutation was associated with four- to eightfold increases in MICs of NSFQ-105 and sparfloxacin, but not of ciprofloxacin, which we confirm targets topoisomerase IV. The presence of wild-type grlB-grlA gene sequences in second-step mutants excluded involvement of topoisomerase IV in the small-colony phenotype. Growth revertants retaining mutant gyrA or gyrB alleles were quinolone susceptible, indicating that resistance to NSFQ-105 and sparfloxacin was contingent on the small-colony mutation. We propose that small-colony mutations unbalance target sensitivities, perhaps through altered ATP or topoisomerase levels, such that gyrase becomes the primary drug target. Breaking of target parity by genetic or physiological means eliminates the need for two target mutations and provides a novel mechanism for stepwise selection of quinolone resistance.

Antimicrobial susceptibility and molecular characterization of community-acquired methicillin-resistant Staphylococcus aureus

Community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infections are increasing. Since most published data are on nosocomial MRSA, our goal was to identify the antimicrobial susceptibility profile and resistance mechanisms of pretreatment MRSA isolates obtained from adult subjects participating in recent clinical treatment trials of community respiratory infections. Out of 465 S. aureus isolates, 43 were identified as MRSA. Antimicrobial susceptibility testing indicated susceptibility rates to: vancomycin (100%), gentamicin (86%), clindamycin (39%), quinolones (49%), and erythromycin (12%). Among our MRSA isolates, the MLS constitutive phenotype and ermA were more prevalent than the MLS inducible phenotype and ermC. No isolates had ermB or msrA. All ciprofloxacin resistant isolates had an amino acid change in GyrA and GrlA. The relatedness of our MRSA strains was assessed by ribotyping. Our results indicate that MRSA from adult subjects with community respiratory infections have similar antimicrobial susceptibility profiles and resistance mechanisms as nosocomial MRSA, and represent a genetically diverse group.