Mechanisms of quinolone resistance in a clinical isolate of Escherichia coli highly resistant to fluoroquinolones but susceptible to nalidixic acid

Transferable Mechanisms of Quinolone Resistance from 1998 Onward

While the description of resistance to quinolones is almost as old as these antimicrobial agents themselves, transferable mechanisms of quinolone resistance (TMQR) remained absent from the scenario for more than 36 years, appearing first as sporadic events and afterward as epidemics. In 1998, the first TMQR was soundly described, that is, QnrA. The presence of QnrA was almost anecdotal for years, but in the middle of the first decade of the 21st century, there was an explosion of TMQR descriptions, which definitively changed the epidemiology of quinolone resistance. Currently, 3 different clinically relevant mechanisms of quinolone resistance are encoded within mobile elements: (i) target protection, which is mediated by 7 different families of Qnr (QnrA, QnrB, QnrC, QnrD, QnrE, QnrS, and QnrVC), which overall account for more than 100 recognized alleles; (ii) antibiotic efflux, which is mediated by 2 main transferable efflux pumps (QepA and OqxAB), which together account for more than 30 alleles, and a series of other efflux pumps (e.g., QacBIII), which at present have been sporadically described; and (iii) antibiotic modification, which is mediated by the enzymes AAC(6')Ib-cr, from which different alleles have been claimed, as well as CrpP, a newly described phosphorylase.

The use of platensimycin and platencin to fight antibiotic resistance

Infectious diseases are known as one of the most life-threatening disabilities worldwide. Approximately 13 million deaths related to infectious diseases are reported each year. The only way to combat infectious diseases is by chemotherapy using antimicrobial agents and antibiotics. However, due to uncontrolled and unnecessary use of antibiotics in particular, surviving bacteria have evolved resistance against several antibiotics. Emergence of multidrug resistance in bacteria over the past several decades has resulted in one of the most important clinical health problems in modern medicine. For instance, approximately 440,000 new cases of multidrug-resistant tuberculosis are reported every year leading to the deaths of 150,000 people worldwide. Management of multidrug resistance requires understanding its molecular basis and the evolution and dissemination of resistance; development of new antibiotic compounds in place of traditional antibiotics; and innovative strategies for extending the life of antibiotic molecules. Researchers have begun to develop new antimicrobials for overcoming this important problem. Recently, platensimycin - isolated from extracts of Streptomyces platensis - and its analog platencin have been defined as promising agents for fighting multidrug resistance. In vitro and in vivo studies have shown that these new antimicrobials have great potential to inhibit methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and penicillin-resistant Streptococcus pneumoniae by targeting type II fatty acid synthesis in bacteria. Showing strong efficacy without any observed in vivo toxicity increases the significance of these antimicrobial agents for their use in humans. However, at the present time, clinical trials are insufficient and require more research. The strong antibacterial efficacies of platensimycin and platencin may be established in clinical trials and their use in humans for coping with multidrug resistance may be allowed in the foreseeable future.

Functional characterization of quinolone-resistant mechanisms in a lab-selected Salmonella enterica typhimurium mutant

Correlation has been widely accepted between quinolone resistance and topoisomerase point mutations in quinolone resistance determination regions (QRDRs). Acquirement of point mutations in QRDRs usually increases the microbial resistance to both nalidixic acid and fluoroquinolones. The quinolone-resistant mechanisms accumulated in a lab-selected mutant were characterized through the construction of isogenic mutants using phage λ Red recombinase system and phage P22. The function of a quinolone-resistant mechanism that increased resistance to fluoroquinolones, but decreased resistance to nalidixic acid was fully characterized. A previous reported point mutation in ParC (G78D) was identified in the lab-selected mutant LT2-128. Minimal inhibitory concentrations (MICs) of isogenic mutants showed that acquirement of this point mutation in the host with topoisomerase mutations in GyrA could increase 8- to 32-fold fluoroquinolones MICs, but decrease eight-fold nalidixic acid MICs. Multiple-resistant mechanisms, such as the overexpressed effluxes, were accumulated besides the point mutations in QRDRs in LT2-128 during the mutant selection process. Through biological costs comparison among isogenic mutants, we found the biological cost in LT2-128 was not from the mutations in QRDRs, instead it was from other mutations accumulated during the mutant selection process, such as the mechanisms related to constitutively overexpressed effluxes. Mutation in ParC (G78D) was responsible for the increased resistance to fluoroquinolones, but decreased resistance to nalidixic acid. The existence of this mechanism demonstrated mutations in ParC could play different roles in nalidixic acid and ciprofloxacin resistance.

The wzm gene located on the pRhico plasmid of Azospirillum brasilense Sp7 is involved in lipopolysaccharide synthesis

Several genes involved in the interaction between Azospirillum brasilense Sp7 and plants are located on the pRhico plasmid. Here we report the characterization of an Sp7 mutant strain with impairment of the pRhico-located gene wzm. This gene encodes an inner-membrane component of an ATP-binding cassette (ABC) transporter with similarity to transporters involved in surface polysaccharide export. Indeed, SDS-PAGE revealed that LPS synthesis is affected in the wzm mutant. No significant differences were observed between wild-type and mutant strains in exopolysaccharide (EPS) amount; however, several differences were observed between them in EPS monosaccharide composition, and only wild-type colonies stained positively with Congo red. Microscopy revealed that wzm mutant cells are longer and thinner, and exhibit several differences in their cell surface relative to the wild-type. The wzm mutant was more resistant to oxidative stress, starvation, desiccation, heat and osmotic shock than the wild-type. In contrast, the mutant was more susceptible than the wild-type to UV radiation and saline stress. The strains also differed in their susceptibility to different antibiotics. Differences between the strains were also observed in their outer-membrane protein composition. No differences were observed between strains in their ability to attach to sweet corn roots and seeds, and to promote growth under the tested conditions. As LPS plays an important role in cell envelope structural integrity, we propose that the pleiotropic phenotypic changes observed in the wzm mutant are due to its altered LPS relative to the wild-type.

Inactivation of efflux pumps abolishes bacterial biofilm formation

Bacterial biofilms cause numerous problems in health care and industry; notably, biofilms are associated with a large number of infections. Biofilm-dwelling bacteria are particularly resistant to antibiotics, making it hard to eradicate biofilm-associated infections. Bacteria rely on efflux pumps to get rid of toxic substances. We discovered that efflux pumps are highly active in bacterial biofilms, thus making efflux pumps attractive targets for antibiofilm measures. A number of efflux pump inhibitors (EPIs) are known. EPIs were shown to reduce biofilm formation, and in combination they could abolish biofilm formation completely. Also, EPIs were able to block the antibiotic tolerance of biofilms. The results of this feasibility study might pave the way for new treatments for biofilm-related infections and may be exploited for prevention of biofilms in general.

Analysis of pleiotropic transcriptional profiles: a case study of DNA gyrase inhibition

Genetic and environmental perturbations often result in complex transcriptional responses involving multiple genes and regulons. In order to understand the nature of a response, one has to account for the contribution of the downstream effects to the formation of a response. Such analysis can be carried out within a statistical framework in which the individual effects are independently collected and then combined within a linear model. Here, we modeled the contribution of DNA replication, supercoiling, and repair to the transcriptional response of inhibition of the Escherichia coli gyrase. By representing the gyrase inhibition as a true pleiotropic phenomenon, we were able to demonstrate that: (1) DNA replication is required for the formation of spatial transcriptional domains; (2) the transcriptional response to the gyrase inhibition is coordinated between at least two modules involved in DNA maintenance, relaxation and damage response; (3) the genes whose transcriptional response to the gyrase inhibition does not depend on the main relaxation activity of the cell can be classified on the basis of a GC excess in their upstream and coding sequences; and (4) relaxation by topoisomerase I dominates the transcriptional response, followed by the effects of replication and RecA. We functionally tested the effect of the interaction between relaxation and repair activities, and found support for the model derived from the microarray data. We conclude that modeling compound transcriptional profiles as a combination of downstream transcriptional effects allows for a more realistic, accurate, and meaningful representation of the transcriptional activity of a genome.

Pleiotropism—a movement, or reaction, in multiple directions: although it was initially used specifically to describe the effect of a single genetic mutation on multiple characters in the offspring, the transcriptional responses of cells are often best described in terms of pleiotropy, when a single input affects multiple components inside the cell. This, in turn, presents a dilemma with the analysis and interpretation of the observed effects: which effects are directly due to the input itself and which are not? How are the effects related to each other and which are more important? And finally, can the overall transcriptional response be summarized as a combination of the effects? There is, however, a problem with recording the effects when they occur almost simultaneously in the same organism. The authors approached this by recording the effects independently, using mutants that could generate all of the effects of interest but one, and then estimating the effects and their interactions from a multivariate linear model. The authors applied this method to explain the transcriptional response of Escherichia coli to a quinolone antibacterial, a relative of Cipro (ciprofloxacin hydrochloride), and discovered unexpected interactions between DNA maintenance modules in the cell.

New quinolone resistance phenomenon in Salmonella enterica: nalidixic acid-susceptible isolates with reduced fluoroquinolone susceptibility

We describe the emergence of a new quinolone resistance pattern in Salmonella enterica isolates from Southeast Asia. These isolates are susceptible to nalidixic acid but exhibit reduced susceptibility to ciprofloxacin. The increase of such strains may threaten the value of the nalidixic acid disk test to screen for reduced fluoroquinolone susceptibility in salmonellas.

Mechanisms of quinolone resistance in Escherichia coli and Salmonella: Recent developments

Fluoroquinolones are broad-spectrum antimicrobials highly effective for treatment of a variety of clinical and veterinary infections. Their antibacterial activity is due to inhibition of DNA replication. Usually resistance arises spontaneously due to point mutations that result in amino acid substitutions within the topoisomerase subunits GyrA, GyrB, ParC or ParE, decreased expression of outer membrane porins, or overexpression of multidrug efflux pumps. In addition, the recent discovery of plasmid-mediated quinolone resistance could result in horizontal transfer of fluoroquinolone resistance between strains. Acquisition of high-level resistance appears to be a multifactorial process. Care needs to taken to avoid overuse of this important class of antimicrobial in both human and veterinary medicine to prevent an increase in the occurrence of resistant zoonotic and non-zoonotic bacterial pathogens that could subsequently cause human or animal infections.

Development and validation of a diagnostic DNA microarray to detect quinolone-resistant Escherichia coli among clinical isolates

The incidence of resistance against fluoroquinolones among pathogenic bacteria has been increasing in accordance with the worldwide use of this drug. Escherichia coli is one of the most relevant species for quinolone resistance. In this study, a diagnostic microarray for single-base-mutation detection was developed, which can readily identify the most prevalent E. coli genotypes leading to quinolone resistance. Based on genomic sequence analysis using public databases and our own DNA sequencing results, two amino acid positions (83 and 87) on the A subunit of the DNA gyrase, encoded by the gyrA gene, have been identified as mutation hot spots and were selected for DNA microarray detection. Oligonucleotide probes directed against these two positions were designed so that they could cover the most important resistance-causing and silent mutations. The performance of the array was validated with 30 clinical isolates of E. coli from four different hospitals in Germany. The microarray results were confirmed by standard DNA sequencing and were in full agreement with phenotypic antimicrobial susceptibility testing.

Laboratory detection of Haemophilus influenzae with decreased susceptibility to nalidixic acid, ciprofloxacin, levofloxacin, and moxifloxacin due to GyrA and ParC mutations

The detection of clinical isolates with decreased fluoroquinolone susceptibilities and a resistance mechanism is of epidemiological and clinical interest. We studied the susceptibilities of 62 clinical isolates and 2 American Type Culture Collection reference strains of Haemophilus influenzae to ciprofloxacin, levofloxacin, moxifloxacin, and nalidixic acid by the microdilution and disk diffusion methods. The ciprofloxacin MICs for 34 of the isolates were >/=0.12 micro g/ml (range, 0.12 to 32 micro g/ml), and the ciprofloxacin MICs for 28 matched control isolates were /=0.5 micro g/ml and the vast majority of those for which nalidixic acid MICs were >/=32 micro g/ml exhibited amino acid changes in GyrA and ParC. Nalidixic acid and the other three fluoroquinolones studied could be used to screen H. influenzae isolates for the detection of decreased susceptibilities to quinolones due to the acquisition of two amino acid changes in the QRDRs of GyrA and ParC (sensitivity, >95%; specificity, >80%).

Campylobacter and fluoroquinolones: a bias data set?

There is no universally accepted standard method for the isolation of Campylobacter spp. and it is considered that currently available isolation media are not yet optimal for the recovery of Campylobacter spp. from a range of sample types. Almost all methods incorporate antibiotics into the isolation media to inhibit growth of other bacteria within the sample. It is established that the incorporation of such antibiotics into isolation media will inhibit the growth of some Campylobacter spp. as well as other bacteria. The results of the use of such suboptimal isolation methods are that the isolates which 'survive' the isolation procedure will be those which: (i) are able to 'out compete' the rest of the bacteria in the sample, i.e. they are able to grow faster; (ii) are resistant to the antibiotics used in the isolation media; and (iii) are randomly selected by the laboratory technician as being a 'typical'Campylobacter spp. It is clear that such a procedure is intrinsically biased and will mean that species resistant to the antibiotics used in the media will be isolated. This introduces real doubt that the bacteria isolated are truly representative of those initially found on the sample. It is also becoming clear that Campylobacter spp. are rather difficult to isolate as pure cultures and many are in fact mixtures of more than one strain. Again this introduces great uncertainty as to the prevalence and distribution of respective species from the different sample types. This is especially true when considering isolation of Campylobacter spp. causing disease in man as there is no certainty that the selected isolate is that which was responsible for disease. The incorporation of antibiotics into the isolation media not only introduces the issue of species bias but perhaps more importantly exposes the Campylobacter spp. to a cocktail of antibiotics thereby providing the potential for them to 'switch on' antibiotic resistance mechanisms. It might be argued that this has always been the case for isolation of Campylobacter spp., however, we know that the antibiotic cocktails used in media over the last 10 years have changed and indeed there was a time when the filtration protocol which didn't use antibiotics was more widely used. As most reports in the literature do not state what methods were used to isolate Campylobacter spp. it is not possible to quantify any relationship between antibiotics used in the isolation media and susceptibility data. An approved method for Campylobacter susceptibility testing was not available until May 2002, all data generated prior to this date will have been generated using non-standard methods. As tremendous variability in the reproducibility data for Campylobacter spp. was observed during the development of the standard agar dilution susceptibility method, data generated with disk diffusion and broth microdilution methods must be considered with caution. It has been shown that, compared with the conventional agar dilution method, the E-test tends to give rise to lower minimal inhibitory concentrations (MICs) for sensitive strains and higher MICs for resistant strains. There are no recommended antibiotic breakpoint concentrations for Campylobacter spp. A breakpoint is used to separate sensitive from resistant strains of bacteria and is thus crucial to any discussion of antibiotic resistance. This discussion is further complicated by introduction of the terms microbiological and clinical breakpoints. While a microbiological breakpoint can be a useful parameter with regard to identifying resistance factors it cannot on its own be used to predict whether that bacteria will respond to treatment from an appropriate antibiotic. Predicting clinical response is a function of the clinical breakpoint which considers the pharmacokinetic profile of the antimicrobial compound, i.e. the concentration of the antimicrobial compound in the body and the MIC. The National Committee for Clinical Laboratory Standards (NCCLS) uses microbiological, pharmacokinetic and clinical data to establish breakpoints, without c and clinical data to establish breakpoints, without such considerations it is not possible to consider what is truly clinically sensitive and resistant. There are no reported studies that have systematically determined appropriate breakpoints for Campylobacter, there are data however, which relate MICs to clinical outcome. It is without dispute that microbiological resistance in Campylobacter spp. occurs as a result of mutation in the gyrA gene with single point mutations most frequently causing a four- to eightfold shift in the MIC. What is also clear is that if a high enough concentration of antimicrobial relative to MIC of the infecting organism can be achieved not only will the parent organism be killed but also the 'resistant' mutant. Considering the above and the concentrations of ciprofloxacin achieved in the gastro-intestinal tract it is not surprising that clinical cure can be demonstrated for organisms with an MIC of 32 microg ml(-1).

Novel ciprofloxacin-resistant, nalidixic acid-susceptible mutant of Staphylococcus aureus

A ciprofloxacin-resistant, nalidixic acid-susceptible mutant of Staphylococcus aureus (F145) contained no mutations within gyrA, gyrB, grlA, and grlB or within norA or its promoter region. MICs and accumulation studies suggest the role of a novel multidrug efflux pump.

Topoisomerase II and IV quinolone resistance-determining regions in Stenotrophomonas maltophilia clinical isolates with different levels of quinolone susceptibility

The quinolone resistance-determining regions (QRDRs) of topoisomerase II and IV genes from Stenotrophomonas maltophilia ATCC 13637 were sequenced and compared with the corresponding regions of 32 unrelated S. maltophilia clinical strains for which ciprofloxacin MICs ranged from 0.1 to 64 microg/ml. GyrA (Leu-55 to Gln-155, Escherichia coli numbering), GyrB (Met-391 to Phe-513), ParC (Ile-34 to Arg-124), and ParE (Leu-396 to Leu-567) fragments from strain ATCC 13637 showed high degrees of identity to the corresponding regions from the phytopathogen Xylella fastidiosa, with the degrees of identity ranging from 85.0 to 93.5%. Lower degrees of identity to the corresponding regions from Pseudomonas aeruginosa (70.9 to 88.6%) and E. coli (73.0 to 88.6%) were observed. Amino acid changes were present in GyrA fragments from 9 of the 32 strains at positions 70, 85, 90, 103, 112, 113, 119, and 124; but there was no consistent relation to higher ciprofloxacin MICs. The absence of changes at positions 83 and 87, commonly involved in quinolone resistance in gram-negative bacteria, was unexpected. The GyrB sequences were identical in all strains, and only one strain (ciprofloxacin MIC, 16 microg/ml) showed a ParC amino acid change (Ser-80-->Arg). In contrast, a high frequency (16 of 32 strains) of amino acid replacements was present in ParE. The frequencies of alterations at positions 437, 465, 477, and 485 were higher (P < 0.05) in strains from cystic fibrosis patients, but these changes were not linked with high ciprofloxacin MICs. An efflux phenotype, screened by the detection of decreases of at least twofold doubling dilutions of the ciprofloxacin MIC in the presence of carbonyl cyanide m-chlorophenylhydrazone (0.5 microg/ml) or reserpine (10 microg/ml), was suspected in seven strains. These results suggest that topoisomerases II and IV may not be the primary targets involved in quinolone resistance in S. maltophilia.

Resolution of high-molecular-weight components in lipopolysaccharides of Escherichia coli, Morganella morganii, Citrobacter freundii and Citrobacter diversus strains with sodium dodecyl sulfate polyacrylamide gels

The use of 0.5% sodium dodecyl sulfate in polyacrylamide separation gels allowed the resolution in several bands of high-molecular-mass components in smooth lipopolysaccharide of bacterial outer membrane from Escherichia coli, Morganella morganii, Citrobacter freundii and Citrobacter diversus. With or without 0.1% SDS, however, such a result was not possible.

Mechanisms involved in the development of resistance to fluoroquinolones in Escherichia coli isolates

Eighteen quinolone-resistant isolates of Escherichia coli were selected by exposing ten clinical isolates to increasing concentrations of norfloxacin and lomefloxacin. The mutant isolates showed a multiple-antibiotic-resistance phenotype. All of them contained single mutations in gyrA consisting of the substitution of Ser-83-->Leu (n = 14), Val (n = 1) or Ala (n = 1) and the substitution of Asp-87-->Asn (n = 2). Only one concomitant mutation in parC (Ser-80-->Arg) was detected. Four parent isolates exhibited a single mutation in gyrA which required < or = 12 mg/L of norfloxacin to be inhibited. Fluoroquinolone resistance, in the 18 quinolone-resistant mutants, was a result of mutations affecting DNA gyrase plus decreased fluoroquinolone uptake. This latter mechanism of resistance was a combined effect of an absence of OmpF and an increase in active efflux in eight isolates, or an increased active efflux alone in the remaining ten selected mutants.

gyrA mutations associated with fluoroquinolone resistance in eight species of Enterobacteriaceae

Fluoroquinolone resistance (FQ-R) in clinical isolates of Enterobacteriaceae species has been reported with increasing frequency in recent years. Two mechanisms of FQ-R have been identified in gram-negative organisms: mutations in DNA gyrase and reduced intracellular drug accumulation. A single point mutation in gyrA has been shown to reduce susceptibility to fluoroquinolones. To determine the extent of gyrA mutations associated with FQ-R in enteric bacteria, one set of oligonucleotide primers was selected from conserved sequences in the flanking regions of the quinolone resistance-determining regions (QRDR) of Escherichia coli and Klebsiella pneumoniae. This set of primers was used to amplify and sequence the QRDRs from 8 Enterobacteriaceae type strains and 60 fluoroquinolone-resistant clinical isolates of Citrobacter freundii, Enterobacter aerogenes, Enterobacter cloacae, E. coli, K. pneumoniae, Klebsiella oxytoca, Providencia stuartii, and Serratia marcescens. Although similarity of the nucleotide sequences of seven species ranged from 80.8 to 93.3%, when compared with that of E. coli, the amino acid sequences of the gyrA QRDR were highly conserved. Conservative amino acid substitutions were detected in the QRDRs of the susceptible type strains of C. freundii, E. aerogenes, K. oxytoca (Ser-83 to Thr), and P. stuartii (Asp-87 to Glu). Strains with ciprofloxacin MICs of >2 microg/ml expressed amino acid substitutions primarily at the Gly-81, Ser-83, or Asp-87 position. Fluoroquinolone MICs varied significantly for strains exhibiting identical gyrA mutations, indicating that alterations outside gyrA contribute to resistance. The type and position of amino acid alterations also differed among these six genera. High-level FQ-R frequently was associated with single gyrA mutations in all species of Enterobacteriaceae in this study except E. coli.

Mechanisms of quinolone resistance in clinical isolates of Enterobacter cloacae

OBJECTIVE: To study and evaluate changes in the gyrA gene and the outer-membrane protein patterns in relation to evolution of resistance against the quinolones in Enterobacter cloacae. METHODS: Strains expressing gyrA-mediated quinolone resistance become susceptible to quinolones upon insertion of the plasmid pNJR3-2. This plasmid (containing wild-type Escherichia coli quinolone-susceptible DNA gyrase A subunits) and pLA2917 (the vector) were introduced into 10 resistant or moderately susceptible clinical isolates of Enterobacter cloacae by conjugation. The transconjugants, the original isolates, the plasmid and the vector control were screened for susceptibility to ofloxacin, ciprofloxacin and sparfloxacin. Additionally, examinations of the outer-membrane proteins were performed. RESULTS: A reduction of MICs by a factor of 8--32 was found for the transconjugants of five Enterobacter cloacae isolates in the presence of the gene probe, suggesting that these isolates harbored mutations in gyrA. No discernible difference in the patterns of outer-membrane proteins of sensitive and resistant strains could be detected. CONCLUSIONS: It seems that changes in the target site such as alterations in gyrA are important factors leading to a change in the susceptibility of bacteria to the quinolones, whereas there were no evident changes in the outer-membrane proteins to account for evolution of resistance.

A novel, double mutation in DNA gyrase A of Escherichia coli conferring resistance to quinolone antibiotics

A spontaneous Escherichia coli mutant, named Q3, resistant to nalidixic acid was obtained from a previously described clinical isolate of E. coli, Q2, resistant to fluoroquinolones but susceptible to nalidixic acid (E. Cambau, F. Bordon, E. Collatz, and L. Gutmann, Antimicrob. Agents Chemother. 37:1247-1252, 1993). Q3 harbored the mutation Asp82Gly in addition to the Gly81Asp mutation of Q2. The different mutations leading to Gly81Asp, Asp82Gly, and Gly81AspAsp82Gly were introduced into the gyrA gene harbored on plasmid pJSW102, and the resulting plasmids were introduced into E. coli KNK453 (gyrAts) by transformation. The presence of Asp82Gly or Gly81Asp alone led to a low-level resistance to fluoroquinolones but not to nalidixic acid resistance. When both mutations were present, resistance to both nalidixic acid and fluoroquinolones was expressed. Purified gyrases of the different mutants showed similar rates of supercoiling. Dominance of the various gyrA mutant alleles harbored on plasmids was examined. The susceptibility to quinolones associated with wild-type gyrA was always dominant. The susceptibility to nalidixic acid expressed by the Gly81Asp mutant was dominant, while that expressed by the Asp82Gly mutant was recessive. From these results, we hypothesize that some amino acids within the quinolone resistance-determining region of gyrase A are more important for the association of subunits rather than for the activity of the holoenzyme.

Characterization of clinical isolates of Escherichia coli showing high levels of fluoroquinolone resistance

During the years 1992 to 1994, an increase in fluoroquinolone-resistant Escherichia coli was observed at the Medical Center of the Technical University in Munich, Germany. Nineteen strains were collected and were thus available for further analysis. Pulsed-field gel electrophoresis showed clonal diversity in all but two strains. The majority of the patients from whom the strains were isolated had been previously treated with fluoroquinolones. Quinolone resistance was associated with mutations of the quinolone resistance-determining region of the gyrA gene in all cases. Direct sequencing of gyrA PCR amplification products revealed a mutation in codon 83 of the gyrA gene. In some instances the Ser-83-->Leu mutation was accompanied by an Asp-87-->Asn or Asp-87-->Gly mutation. Furthermore, the strains exhibited two different genotypes: in almost half of the fluoroquinolone-resistant E. coli strains as well as in the fluoroquinolone-susceptible E. coli reference strains ATCC 25922 and 35218, silent mutations were detected at bases 255, 273, 300, and 333. Although fluoroquinolones solved major problems in antimicrobial chemotherapy, in certain departments of our hospital the number of resistant E. coli isolates has become so high that susceptibility to fluoroquinolones can no longer be taken for granted.

Association between double mutation in gyrA gene of ciprofloxacin-resistant clinical isolates of Escherichia coli and MICs

The mutations in the quinolone resistance-determining region of the gyrA and gyrB genes from 27 clinical isolates of Escherichia coli with a range of MICs of ciprofloxacin from 0.007 to 128 micrograms/ml and of nalidixic acid from 2 to > 2,000 micrograms/ml were determined by DNA sequencing. All 15 isolates with ciprofloxacin MICs of > or = 1 micrograms/ml showed a change in Ser-83 to Leu of GyrA protein, whereas in clinical isolates with a MIC of > or = 8 micrograms/ml (11 strains), a double change in Ser-83 and Asp-87 was found. All isolates with a MIC of nalidixic acid of > or = 128 micrograms/ml showed a mutation at amino acid codon Ser-83. Only 1 of the 27 clinical isolates of E. coli analyzed showed a change in Lys-447 of the B subunit of DNA gyrase. A change in Ser-83 is sufficient to generate a high level of resistance to nalidixic acid, whereas a second mutation at Asp-87 in the A subunit of DNA gyrase may play a complementary role in developing the strain's high levels of ciprofloxacin resistance.

Characterization of fluoroquinolone-resistant mutants of escherichia coli selected in vitro

Wild-type mutants highly resistant to fluoroquinolones were selected in vitro from a quinolone-susceptible Escherichia coli isolate by stepwise exposure to increasing concentrations of nalidixic acid and ciprofloxacin (CIP) either in liquid medium or on solid medium. Mutant R17 was selected by serial passage in liquid medium; the MIC of CIP for mutant R17 was 256 micrograms/ml. On solid medium, consecutive mutants MI, MII, MIII, MIVa, and MIVb were selected in four steps. The frequencies of mutations were between 10(-9) and 10(-11), and the MICs of CIP ranged from 0.5 microgram/ml (for mutant MI) to 256 micrograms/ml (for mutant MIVb). From the results of a dominance test with the gyrB+ plasmid (pBP547), no gyrB mutations were detectable. In the first step, mutant MI, a mutation from a Ser to a Leu residue at position 83 (a Ser-83-->Leu mutation), was detected in the quinolone resistance-determining region of the gyrA gene. In addition, the second-step mutation was associated with a reduced uptake of CIP and an altered outer membrane protein profile. The third mutation was identified as an Asp-87-->Gly mutation in the quinolone resistance-determining region of the gyrA gene. Concomitantly, a slight increase in the doubling time was detected. For two different four-step mutants, mutants MIVa and MIVb, the MICs of only some quinolones, including CIP, increased. The accumulation of CIP in the mutants was comparable to that in their parent MIII. The doubling time of mutant MIVa was similar to that of mutant MIII, but differed by a factor of 3 from that of the very slow growing mutant MIVb. In contrast, a clinical isolate of E.coli (isolate 205096) described previously (P. Heisig, H. Schedletzky, and H. Falkenstein-Paul, Antimicrob. Agents Chemother. 37:696-701, 1993) which has the same double mutation in gyrA had a doubling time comparable to that of the wild-type isolate.

Emergence of fluoroquinolone-resistant Escherichia coli at a cancer center

Prophylactic treatment with fluoroquinolones of patients with profound neutropenia has been found to be useful for preventing gram-negative bacteremia and has become a standard preventive-therapy strategy in many cancer centers, but the development of bacterial resistance is a cause of concern. During the past few years, we have observed an increasing number of patients with leukemia from whom fluoroquinolone-resistant strains of Escherichia coli were isolated. The increase was significant in this patient population, and among patients with other underlying diseases, the rates of isolation of such strains per number of discharges were significantly lower and did not increase. Most of the leukemia case patients (16 of 19) had been pretreated with an oral quinolone (ofloxacin), with cumulative doses until the first isolation of a resistant E. coli strain ranging from 0 to 97.8 g (median, 14.4 g). Repeated isolation of such strains was seen in 8 of 17 patients during a follow-up period of > or = 4 weeks and in 1 of 6 patients during a follow-up period of > or = 16 weeks. Ten patients developed bacteremia (mortality, 1 of 10). On the basis of the number of patients with leukemia admitted to the hematology-oncology service, the incidence of bacteremia caused by fluoroquinolone-resistant E. coli increased from < 0.5% in 1988-1989 and 0.8% in 1990-1991 to 4.5% in 1992-1993 (P < 0.01). MICs for nine isolates obtained from cultures of blood from different patients ranged between 8 and 16 microgram/ml (ciprofloxacin and PD 131628), 8 and 32 microgram/ml (ofloxacin and BAY Y 3118), and 16 and 32 microgram/ml (sparfloxacin) and indicated resistance to trimethoprim-sulfamethoxazole, ampicillin, doxycycline, and chloramphenicol. Of nine isolates obtained from cultures of blood from different patients and that were subjected to genomic DNA typing by pulsed-field gel electrophoresis of XbaI digests, seven were typeable. Among these, four different genotypes were identified, suggesting both the independent development and the horizontal spread of resistant clones of E. coli.

Bacteremia due to fluoroquinolone-resistant Escherichia coli in two immunocompromised patients

Two patients with acute leukemia developed Escherichia coli bacteremia while receiving oral ofloxacin for antibacterial prophylaxis during profound neutropenia. The isolates were resistant to ofloxacin (MIC 25 and 12.5 micrograms/ml respectively), other fluoroquinolones and several unrelated agents. Whole cell drug accumulation studies with four different fluoroquinolones suggested major differences between drugs but only minor alterations in individual drug permeability in the two resistant isolates compared with a susceptible control strain. In a supercoiling assay using the purified DNA gyrase and plasmid pBR322, high concentrations (> or = 250 micrograms/ml) of both ofloxacin and ciprofloxacin were needed for visible inhibition of enzyme activity suggesting mutations in the DNA gyrase gene as the significant mechanism of resistance in both strains.

Reduced susceptibilities of Shigella sonnei strains isolated from patients with dysentery to fluoroquinolones

Seven clinical isolates of Shigella sonnei with reduced susceptibilities to fluoroquinolones (sparfloxacin, ciprofloxacin, and ofloxacin) were obtained. The MICs of fluoroquinolones against these S. sonnei strains were 16 to 32 times higher than those obtained against typical strains that are highly susceptible to these agents. The kinetics of [14C]ofloxacin accumulation in these clinical strains were not different from those in the fully susceptible strains. However, DNA synthesis was much less inhibited by ofloxacin in the strains with reduced susceptibility. Analysis of the in vitro activity of the partially purified DNA gyrase from these isolates showed that the decreased quinolone susceptibility of the S. sonnei strains was likely due to mutation of the DNA gyrase subunit A gene.

Novel gyrA point mutation in a strain of Escherichia coli resistant to fluoroquinolones but not to nalidixic acid

We have previously described a clinical isolate of Escherichia coli (Q2) that is highly resistant to fluoroquinolones (MIC of ciprofloxacin, 16 micrograms/ml) but susceptible to nalidixic acid (MIC of nalidixic acid, 4 micrograms/ml) (N. Moniot-Ville, J. Guibert, N. Moreau, J.F. Acar, E. Collatz, and L. Gutmann, Antimicrob. Agents Chemother. 35:519-523, 1991). Transformation of strain Q2 with a plasmid carrying the wild-type gyrA gene from E. coli K-12(pAFF801) resulted in a 32-fold decrease in the MIC of ciprofloxacin, suggesting that at least one mutation in gyrA was involved in the resistance of Q2. Intragenic gyrA fragments of 668 and 2,500 bp from strain Q2 were amplified by the polymerase chain reaction. We sequenced the 668-bp fragment and identified a single novel point mutation (transition from G to A at position 242), leading to an amino acid substitution (Gly-81 to Asp) in the gyrase A subunit. We constructed hybrid plasmids by substituting either the 668-bp fragment or the 2,500-bp fragment from Q2 DNA, both of which contained the gyrA point mutation, for the corresponding fragments in wild-type gyrA (2,625 bp) of E. coli K-12. When introduced into E. coli KNK453 (gyrA temperature sensitive), both plasmids conferred an eightfold increase in the MIC of ciprofloxacin, but only a twofold increase in the MIC of nalidixic acid. When introduced into E. coli Q2, neither plasmid conferred any change in the MICs of ciprofloxacin or nalidixic acid, suggesting that only the point mutation found in gyrA was involved in the resistance that we observed.

Mutations in the gyrA gene of a highly fluoroquinolone-resistant clinical isolate of Escherichia coli

We have determined the DNA sequence of the gyrA gene of the fluoroquinolone-resistant Escherichia coli isolate 205096 (MIC of ciprofloxacin, 128 micrograms/ml), which was recently demonstrated to be a gyrA mutant (P. Heisig and B. Wiedemann, Antimicrob. Agents Chemother. 35:2031-2036, 1991). Compared with the gyrA+ gene of E. coli K-12, 55 nucleotide changes were found. Three of these resulted in amino acid exchanges: Ser-83-->Leu, Asp-87-->Gly, and Asp-678-->Glu. A 0.7-kb DNA fragment containing two of these mutations (Ser-83-->Leu and Asp-87-->Gly) was isolated and fused in frame to the residual 3' coding region of gyrA+ in a plasmid to yield a chimeric gyrA gene (gyrA#). After introduction into E. coli 205096, this gyrA# gene does not increase the fluoroquinolone susceptibility of the resulting heterodiploid strain in a dominance test, while the gyrA+ gene does. The ciprofloxacin concentration necessary to inhibit by 90% (IC90) the supercoiling activity of gyrase isolated from E. coli 205096 is above 2,000 micrograms/ml. An identical result was found for gyrase reconstituted in vitro from the gyrB+ gene product and the chimeric gyrA# gene product. This is more than a 4,000-fold increase compared with the IC90 determined for gyrase from E. coli K-12 (gyrA+) (IC90, 0.5 microgram of ciprofloxacin per ml). No indications for the involvement of the gyrB gene or for alterations in quinolone permeation were found.

Mechanisms of resistance to quinolones

Mechanisms of resistance to the quinolones have been described for several bacterial species, but mainly for Escherichia coli and Staphylococcus aureus. Two principal mechanisms have been described: 1) alteration of the DNA gyrase, which is the target site of the quinolones; and 2) diminished accumulation in the cell as a result of either decreased uptake or increased efflux. Alteration of DNA gyrase is usually the result of a mutation in the gyrA, or more rarely, the gyrB gene. All substitutions in subunit A of the gyrase are located in the 67 to 106 amino-acid domain and are clustered around Ser-83 in E. coli and Ser-84 in S. aureus. A decrease in uptake has been described for Gram-negative bacteria such as Enterobacteriaceae and Pseudomonas aeruginosa. It has almost always been correlated with a modified electrophoretic profile of outer membrane proteins of the quinolone-resistant mutants. In E. coli, a decrease in OmpF seemed to be linked to the activation of the micF operon in most of the mutants described. These mutants were cross-resistant to unrelated antibiotics, such as trimethoprim, chloramphenicol, tetracycline, and some beta-lactams. In all these mutants the normal or enhanced efflux of quinolones increased the level of resistance. Enhanced efflux has been described as the second mechanism of resistance in S. aureus. Acquired resistance to the quinolones was thought, until recently, to result from chromosomal mutation. Plasmid-mediated resistance associated with an enhanced efflux has been described in S. aureus, but this needs to be confirmed. When a high level of resistance is observed, 2 or 3 mechanisms may be involved.(ABSTRACT TRUNCATED AT 250 WORDS)

Quinolone accumulation in Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus

The accumulation of quinolones by Escherichia coli JF568, Pseudomonas aeruginosa PAO1, and Staphylococcus aureus ATCC 29213 was measured by a modified fluorometric assay (J. S. Chapman and N. H. Georgopapadakou, Antimicrob. Agents Chemother. 33:27-29, 1989). The quinolones examined were fleroxacin, pefloxacin, norfloxacin, difloxacin, A56620, ciprofloxacin, ofloxacin, and Ro 09-1168. In all three organisms, uptake was complete in less than 5 min and was proportional to extracellular quinolone concentrations between 2 and 50 micrograms/ml, which is consistent with simple diffusion. Washing cells with quinolone-free buffer decreased accumulation by up to 70% in E. coli and P. aeruginosa but not in S. aureus. Similarly, incubation with the uncouplers 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone increased accumulation up to fourfold in E. coli and P. aeruginosa, though not in S. aureus, suggesting endogenous, energy-dependent efflux. High quinolone hydrophobicity was generally associated with decreased accumulation in E. coli and P. aeruginosa (except in the case of pefloxacin) but was associated with increased accumulation in S. aureus (except in the case of difloxacin). Ciprofloxacin had the highest accumulation in E. coli and P. aeruginosa, while pefloxacin had the highest accumulation in S. aureus.

Do we need an intravenous fluoroquinolone?

Intravenous ciprofloxacin, the first parenteral fluoroquinolone available in this country, represents another class of antimicrobial agents from which physicians must choose when treating nosocomial infections. Fluoroquinolones are bactericidal antimicrobial agents that act by inhibiting DNA gyrase. They are active in vitro against most gram-negative bacteria and methicillin-susceptible staphylococci. Activity against anaerobic bacteria and streptococci is poor. The rapid development of bacterial resistance in centers with high quinolone usage is of great concern. Resistance develops most commonly in Pseudomonas aeruginosa and staphylococci. Resistance emerges most often when quinolones are used to treat chronic infections or in patients with poorly drained abscesses, necrotic tissue, or indwelling catheters. Clinical trials have shown ciprofloxacin to be as effective as ceftazidime in the treatment of infections caused by gram-negative bacteria. Although the overall frequency of side effects to fluoroquinolones is low, seizures and allergic reactions have been attributed to their use. Ciprofloxacin inhibits the metabolism of theophylline, and morbidity and death have been reported in patients taking the two drugs concomitantly. Parenteral fluoroquinolones should be reserved for the treatment of gram-negative bacterial infections in patients in whom standard agents cannot be used.

Mechanism of action of sparfloxacin against and mechanism of resistance in gram-negative and gram-positive bacteria

The inhibition of DNA synthesis by sparfloxacin; accumulation of sparfloxacin into members of the family Enterobacteriaceae, Pseudomonas aeruginosa, and staphylococci; induction of recA in Escherichia coli; and the optimum bactericidal concentration (OBC) were measured, and killing kinetics at the OBC were estimated. The OBC and maximum recA-inducing concentration in E. coli were both 1 microgram of sparfloxacin per ml. Accumulation was rapid; two- to threefold more sparfloxacin than ciprofloxacin accumulated in staphylococci and more sparfloxacin accumulated in staphylococci than in gram-negative bacteria. Laboratory mutants with decreased susceptibilities to quinolones alone or multiply resistant were selected from the Enterobacteriaceae and Staphylococcus aureus by using sparfloxacin.

Ofloxacin-resistant Pseudomonas aeruginosa mutants with elevated drug extrusion across the inner membrane

We selected the quinolone-resistant mutants from the protein F deficient Pseudomonas aeruginosa. The mutants showed cross-resistance to tetracycline, minocycline and chloramphenicol, but not to the beta-lactam antibiotics. The MIC of ofloxacin (OFLX) against the mutants, but not in the parent, became 2 to 4 times lower as the medium pH was raised from 6.5 to 8.5. The mutants accumulated about half as much OFLX as the parent. The OFLX accumulation in the mutants increased 6.5- to 8-fold in the presence of 100 mM carbonyl cyanide m-chlorophenylhydrazone, while that in the parent was 4-fold. The OFLX sensitivity of the mutant DNA-gyrase was comparable with that of the parent's enzyme. These results suggest that the resistance of these mutants to OFLX is associated with the membrane potential dependent drug efflux.