Nalidixic acid-resistant mutations of the gyrB gene of Escherichia coli

Collective peroxide detoxification determines microbial mutation rate plasticity in E. coli

Mutagenesis is responsive to many environmental factors. Evolution therefore depends on the environment not only for selection but also in determining the variation available in a population. One such environmental dependency is the inverse relationship between mutation rates and population density in many microbial species. Here, we determine the mechanism responsible for this mutation rate plasticity. Using dynamical computational modelling and in culture mutation rate estimation, we show that the negative relationship between mutation rate and population density arises from the collective ability of microbial populations to control concentrations of hydrogen peroxide. We demonstrate a loss of this density-associated mutation rate plasticity (DAMP) when Escherichia coli populations are deficient in the degradation of hydrogen peroxide. We further show that the reduction in mutation rate in denser populations is restored in peroxide degradation-deficient cells by the presence of wild-type cells in a mixed population. Together, these model-guided experiments provide a mechanistic explanation for DAMP, applicable across all domains of life, and frames mutation rate as a dynamic trait shaped by microbial community composition.

Anaerobically Grown Escherichia coli Has an Enhanced Mutation Rate and Distinct Mutational Spectra

Oxidative stress is a major cause of mutation but little is known about how growth in the absence of oxygen impacts the rate and spectrum of mutations. We employed long-term mutation accumulation experiments to directly measure the rates and spectra of spontaneous mutation events in Escherichia coli populations propagated under aerobic and anaerobic conditions. To detect mutations, whole genome sequencing was coupled with methods of analysis sufficient to identify a broad range of mutational classes, including structural variants (SVs) generated by movement of repetitive elements. The anaerobically grown populations displayed a mutation rate nearly twice that of the aerobic populations, showed distinct asymmetric mutational strand biases, and greater insertion element activity. Consistent with mutation rate and spectra observations, genes for transposition and recombination repair associated with SVs were up-regulated during anaerobic growth. Together, these results define differences in mutational spectra affecting the evolution of facultative anaerobes.

Detection of a Novel gyrB Mutation Associated With Fluoroquinolone-Nonsusceptible Salmonella enterica serovar Typhimurium Isolated From a Bloodstream Infection in Ghana

A multidrug-resistant Salmonella enterica serovar Typhimurium with reduced susceptibility to ciprofloxacin was isolated from the blood of a hospitalized child in Ghana. DNA sequencing identified a novel gyrB mutation at codon 466 (Glu466Asp). An increase in fluoroquinolone susceptibility after the introduction of a wild-type gyrB(+) allele demonstrated that the gyrB466 mutation had a direct effect on fluoroquinolone susceptibility.

Markov Networks of Collateral Resistance: National Antimicrobial Resistance Monitoring System Surveillance Results from Escherichia coli Isolates, 2004-2012

Surveillance of antimicrobial resistance (AMR) is an important component of public health. Antimicrobial drug use generates selective pressure that may lead to resistance against to the administered drug, and may also select for collateral resistances to other drugs. Analysis of AMR surveillance data has focused on resistance to individual drugs but joint distributions of resistance in bacterial populations are infrequently analyzed and reported. New methods are needed to characterize and communicate joint resistance distributions. Markov networks are a class of graphical models that define connections, or edges, between pairs of variables with non-zero partial correlations and are used here to describe AMR resistance relationships. The graphical least absolute shrinkage and selection operator is used to estimate sparse Markov networks from AMR surveillance data. The method is demonstrated using a subset of Escherichia coli isolates collected by the National Antimicrobial Resistance Monitoring System between 2004 and 2012 which included AMR results for 16 drugs from 14418 isolates. Of the 119 possible unique edges, 33 unique edges were identified at least once during the study period and graphical density ranged from 16.2% to 24.8%. Two frequent dense subgraphs were noted, one containing the five β-lactam drugs and the other containing both sulfonamides, three aminoglycosides, and tetracycline. Density did not appear to change over time (p = 0.71). Unweighted modularity did not appear to change over time (p = 0.18), but a significant decreasing trend was noted in the modularity of the weighted networks (p < 0.005) indicating relationships between drugs of different classes tended to increase in strength and frequency over time compared to relationships between drugs of the same class. The current method provides a novel method to study the joint resistance distribution, but additional work is required to unite the underlying biological and genetic characteristics of the isolates with the current results derived from phenotypic data.

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.

Rate and molecular spectrum of spontaneous mutations in the bacterium Escherichia coli as determined by whole-genome sequencing

Knowledge of the rate and nature of spontaneous mutation is fundamental to understanding evolutionary and molecular processes. In this report, we analyze spontaneous mutations accumulated over thousands of generations by wild-type Escherichia coli and a derivative defective in mismatch repair (MMR), the primary pathway for correcting replication errors. The major conclusions are (i) the mutation rate of a wild-type E. coli strain is ~1 × 10(-3) per genome per generation; (ii) mutations in the wild-type strain have the expected mutational bias for G:C > A:T mutations, but the bias changes to A:T > G:C mutations in the absence of MMR; (iii) during replication, A:T > G:C transitions preferentially occur with A templating the lagging strand and T templating the leading strand, whereas G:C > A:T transitions preferentially occur with C templating the lagging strand and G templating the leading strand; (iv) there is a strong bias for transition mutations to occur at 5'ApC3'/3'TpG5' sites (where bases 5'A and 3'T are mutated) and, to a lesser extent, at 5'GpC3'/3'CpG5' sites (where bases 5'G and 3'C are mutated); (v) although the rate of small (≤4 nt) insertions and deletions is high at repeat sequences, these events occur at only 1/10th the genomic rate of base-pair substitutions. MMR activity is genetically regulated, and bacteria isolated from nature often lack MMR capacity, suggesting that modulation of MMR can be adaptive. Thus, comparing results from the wild-type and MMR-defective strains may lead to a deeper understanding of factors that determine mutation rates and spectra, how these factors may differ among organisms, and how they may be shaped by environmental conditions.

Quinolones: from antibiotics to autoinducers

Since quinine was first isolated, animals, plants and microorganisms producing a wide variety of quinolone compounds have been discovered, several of which possess medicinally interesting properties ranging from antiallergenic and anticancer to antimicrobial activities. Over the years, these have served in the development of many synthetic drugs, including the successful fluoroquinolone antibiotics. Pseudomonas aeruginosa and related bacteria produce a number of 2-alkyl-4(1H)-quinolones, some of which exhibit antimicrobial activity. However, quinolones such as the Pseudomonas quinolone signal and 2-heptyl-4-hydroxyquinoline act as quorum-sensing signal molecules, controlling the expression of many virulence genes as a function of cell population density. Here, we review selectively this extensive family of bicyclic compounds, from natural and synthetic antimicrobials to signalling molecules, with a special emphasis on the biology of P. aeruginosa. In particular, we review their nomenclature and biochemistry, their multiple properties as membrane-interacting compounds, inhibitors of the cytochrome bc₁ complex and iron chelators, as well as the regulation of their biosynthesis and their integration into the intricate quorum-sensing regulatory networks governing virulence and secondary metabolite gene expression.

Arabidopsis thaliana GYRB3 does not encode a DNA gyrase subunit

BACKGROUND

DNA topoisomerases are enzymes that control the topology of DNA in all cells. DNA gyrase is unique among the topoisomerases in that it is the only enzyme that can actively supercoil DNA using the free energy of ATP hydrolysis. Until recently gyrase was thought to be unique to bacteria, but has now been discovered in plants. The genome of the model plant, Arabidopsis thaliana, is predicted to encode four gyrase subunits: AtGyrA, AtGyrB1, AtGyrB2 and AtGyrB3.

METHODOLOGY/PRINCIPAL FINDINGS

We found, contrary to previous data, that AtGyrB3 is not essential to the survival of A. thaliana. Bioinformatic analysis suggests AtGyrB3 is considerably shorter than other gyrase B subunits, lacking part of the ATPase domain and other key motifs found in all type II topoisomerases; but it does contain a putative DNA-binding domain. Partially purified AtGyrB3 cannot bind E. coli GyrA or support supercoiling. AtGyrB3 cannot complement an E. coli gyrB temperature-sensitive strain, whereas AtGyrB2 can. Yeast two-hybrid analysis suggests that AtGyrB3 cannot bind to AtGyrA or form a dimer.

CONCLUSIONS/SIGNIFICANCE

These data strongly suggest that AtGyrB3 is not a gyrase subunit but has another unknown function. One possibility is that it is a nuclear protein with a role in meiosis in pollen.

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.

Use of gyrase resistance mutants to guide selection of 8-methoxy-quinazoline-2,4-diones

A series of 1-cyclopropyl-8-methoxy-quinazoline-2,4-diones was synthesized and evaluated for lowering the ratio of the antimicrobial MIC in gyrase resistance mutants to that in the gyr(+) (wild type) using isogenic strains of Escherichia coli. Dione features that lowered this ratio were a 3-amino group and C-7 ring structure (3-aminomethyl pyrrolidinyl < 3-aminopyrrolidinyl < diazobicyclo < 2-ethyl piperazinyl). The wild-type MIC was also lowered. With the most active derivative tested, many gyrA resistance mutant types were as susceptible as, or more susceptible than, wild-type cells. The most active 2,4-dione derivatives were also more active with two quinolone-resistant gyrB mutants than with wild-type cells. With respect to lethality, the most bacteriostatic 2,4-dione killed E. coli at a rate that was affected little by a gyrA resistance mutation, and it exhibited a rate of killing similar to its cognate fluoroquinolone at 10x the MIC. Population analysis with wild-type E. coli applied to agar showed that the mutant selection window for the most active 2,4-dione was narrower than that for the cognate fluoroquinolone or for ciprofloxacin. These data illustrate a new approach to guide early-stage antimicrobial selection. Use of antimutant activity (i.e., ratio of the antimicrobial MIC in a mutant strain to the antimicrobial MIC in a wild-type strain) as a structure-function selection criterion can be combined with traditional efforts aimed at lowering antimicrobial MICs against wild-type organisms to more effectively afford lead molecules with activity against both wild-type and mutant cells.

Real-time PCR detection of Pseudomonas aeruginosa in clinical and municipal wastewater and genotyping of the ciprofloxacin-resistant isolates

Real-time quantification of Pseudomonas aeruginosa was performed in various wastewater systems including clinical, municipal wastewaters and inflow from a wastewater treatment plant. The highest concentrations of P. aeruginosa-specific targets were detected in clinical wastewaters. Limitations of the detection system resulting from inhibition or cross-reaction were identified. Ciprofloxacin-resistant P. aeruginosa strains were isolated after specific enrichment from clinical and municipal wastewaters. In some cases they were also cultivated from effluent of a wastewater treatment plant, and from its downstream river water. A total of 119 isolates were phenotypically characterized as ciprofloxacin-resistant via antibiogram testing. Subsequently, the fluoroquinolone-resistance-mediating mutations in the genes gyrA codon positions 83 and 87, gyrB codon position 466 and parC codon positions 87 and 91 were determined by mini-sequencing. Ciprofloxacin resistance was mainly associated with mutations in gyrA codon position 83 and parC mutation in codon positions 87 or 91 of the bacterial gyrase and topoisomerase II genes. All ciprofloxacin-resistant P. aeruginosa strains were compared with genotypes from clinical data of fluoroquinolone-resistant P. aeruginosa infections. The results were in agreement with data from clinical analyses, with the exception that no gyrA 87 and no gyrB mutations were found in ciprofloxacin-resistant P. aeruginosa wastewater isolates.

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.

Biological cost of single and multiple norfloxacin resistance mutations in Escherichia coli implicated in urinary tract infections

Resistance to fluoroquinolones in urinary tract infection (UTIs) caused by Escherichia coli is associated with multiple mutations, typically those that alter DNA gyrase and DNA topoisomerase IV and those that regulate AcrAB-TolC-mediated efflux. We asked whether a fitness cost is associated with the accumulation of these multiple mutations. Mutants of the susceptible E. coli UTI isolate Nu14 were selected through three to five successive steps with norfloxacin. Each selection was performed with the MIC of the selected strain. After each selection the MIC was measured; and the regions of gyrA, gyrB, parC, and parE, previously associated with resistance mutations, and all of marOR and acrR were sequenced. The first selection step yielded mutations in gyrA, gyrB, and marOR. Subsequent selection steps yielded mutations in gyrA, parE, and marOR but not in gyrB, parC, or acrR. Resistance-associated mutations were identified in almost all isolates after selection steps 1 and 2 but in less than 50% of isolates after subsequent selection steps. Selected strains were competed in vitro, in urine, and in a mouse UTI infection model against the starting strain, Nu14. First-step mutations were not associated with significant fitness costs. However, the accumulation of three or more resistance-associated mutations was usually associated with a large reduction in biological fitness, both in vitro and in vivo. Interestingly, in some lineages a partial restoration of fitness was associated with the accumulation of additional mutations in late selection steps. We suggest that the relative biological costs of multiple mutations may influence the evolution of E. coli strains that develop resistance to fluoroquinolones.

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.

Mutation rate and evolution of fluoroquinolone resistance in Escherichia coli isolates from patients with urinary tract infections

Escherichia coli strains from patients with uncomplicated urinary tract infections were examined by DNA sequencing for fluoroquinolone resistance-associated mutations in six genes: gyrA, gyrB, parC, parE, marOR, and acrR. The 54 strains analyzed had a susceptibility range distributed across 15 dilutions of the fluoroquinolone MICs. There was a correlation between the fluoroquinolone MIC and the number of resistance mutations that a strain carried, with resistant strains having mutations in two to five of these genes. Most resistant strains carried two mutations in gyrA and one mutation in parC. In addition, many resistant strains had mutations in parE, marOR, and/or acrR. No (resistance) mutation was found in gyrB. Thus, the evolution of fluoroquinolone resistance involves the accumulation of multiple mutations in several genes. The spontaneous mutation rate in these clinical strains varied by 2 orders of magnitude. A high mutation rate correlated strongly with a clinical resistance phenotype. This correlation suggests that an increased general mutation rate may play a significant role in the development of high-level resistance to fluoroquinolones by increasing the rate of accumulation of rare new mutations.

DNA gyrase and topoisomerase IV mutations associated with fluoroquinolone resistance in Proteus mirabilis

Mutations associated with fluoroquinolone resistance in clinical isolates of Proteus mirabilis were determined by genetic analysis of the quinolone resistance-determining region (QRDR) of gyrA, gyrB, parC, and parE. This study included the P. mirabilis type strain ATCC 29906 and 29 clinical isolates with reduced susceptibility (MIC, 0.5 to 2 microg/ml) or resistance (MIC, > or =4 microg/ml) to ciprofloxacin. Susceptibility profiles for ciprofloxacin, clinafloxacin, gatifloxacin, gemifloxacin, levofloxacin, moxifloxacin, and trovafloxacin were correlated with amino acid changes in the QRDRs. Decreased susceptibility and resistance were associated with double mutations involving both gyrA (S83R or -I) and parC (S80R or -I). Among these double mutants, MICs of ciprofloxacin varied from 1 to 16 microg/ml, indicating that additional factors, such as drug efflux or porin changes, also contribute to the level of resistance. For ParE, a single conservative change of V364I was detected in seven strains. An unexpected result was the association of gyrB mutations with high-level resistance to fluoroquinolones in 12 of 20 ciprofloxacin-resistant isolates. Changes in GyrB included S464Y (six isolates), S464F (three isolates), and E466D (two isolates). A three-nucleotide insertion, resulting in an additional lysine residue between K455 and A456, was detected in gyrB of one strain. Unlike any other bacterial species analyzed to date, mutation of gyrB appears to be a frequent event in the acquisition of fluoroquinolone resistance among clinical isolates of P. mirabilis.

Quinolone-binding pocket of DNA gyrase: role of GyrB

DNA gyrase is a prokaryotic type II topoisomerase and a major target of quinolone antibacterials. The majority of mutations conferring resistance to quinolones arise within the quinolone resistance-determining region of GyrA close to the active site (Tyr(122)) where DNA is bound and cleaved. However, some quinolone resistance mutations are known to exist in GyrB. Present structural data suggest that these residues lie a considerable distance from the quinolone resistance-determining region, and it is not obvious how they affect quinolone action. We have made and purified two such mutant proteins, GyrB(Asp(426)-->Asn) and GyrB(Lys(447)-->Glu), and characterized them in vitro. We found that the two proteins behave similarly to GyrA quinolone-resistant proteins. We showed that the mutations exert their effect by decreasing the amount of quinolone bound to a gyrase-DNA complex. We suggest that the GyrB residues form part of a quinolone-binding pocket that includes DNA and the quinolone resistance-determining region in GyrA and that large conformational changes during the catalytic cycle of the enzyme allow these regions to come into close proximity.

Type II topoisomerase quinolone resistance-determining regions of Aeromonas caviae, A. hydrophila, and A. sobria complexes and mutations associated with quinolone resistance

Most Aeromonas strains isolated from two European rivers were previously found to be resistant to nalidixic acid. In order to elucidate the mechanism of this resistance, 20 strains of Aeromonas caviae (n = 10), A. hydrophila (n = 5), and A. sobria (n = 5) complexes, including 3 reference strains and 17 environmental isolates, were investigated. Fragments of the gyrA, gyrB, parC, and parE genes encompassing the quinolone resistance-determining regions (QRDRs) were amplified by PCR and sequenced. Results obtained for the six sensitive strains showed that the GyrA, GyrB, ParC, and ParE QRDR fragments of Aeromonas spp. were highly conserved (> or =96.1% identity), despite some genetic polymorphism; they were most closely related to those of Vibrio spp., Pseudomonas spp., and members of the family Enterobacteriaceae (72.4 to 97.1% homology). All 14 environmental resistant strains carried a point mutation in the GyrA QRDR at codon 83, leading to the substitution Ser-83-->Ile (10 strains) or Ser-83-->Arg. In addition, seven strains harbored a mutation in the ParC QRDR either at position 80 (five strains), generating a Ser-80-->Ile (three strains) or Ser-80-->Arg change, or at position 84, yielding a Glu-84-->Lys modification. No amino acid alterations were discovered in the GyrB and ParE QRDRs. Double gyrA-parC missense mutations were associated with higher levels of quinolone resistance compared with the levels associated with single gyrA mutations. The most resistant strains probably had an additional mechanism(s) of resistance, such as decreased accumulation of the drugs. Our data suggest that, in mesophilic Aeromonas spp., as in other gram-negative bacteria, gyrase and topoisomerase IV are the primary and secondary targets for quinolones, respectively.

Type II topoisomerase mutations in fluoroquinolone-resistant clinical strains of Pseudomonas aeruginosa isolated in 1998 and 1999: role of target enzyme in mechanism of fluoroquinolone resistance

The major mechanism of resistance to fluoroquinolones for Pseudomonas aeruginosa is the modification of type II topoisomerases (DNA gyrase and topoisomerase IV). We examined the mutations in quinolone-resistance-determining regions (QRDR) of gyrA, gyrB, parC, and parE genes of recent clinical isolates. There were 150 isolates with reduced susceptibilities to levofloxacin and 127 with reduced susceptibilities to ciprofloxacin among 513 isolates collected during 1998 and 1999 in Japan. Sequencing results predicted replacement of an amino acid in the QRDR of DNA gyrase (GyrA or GyrB) for 124 of the 150 strains (82.7%); among these, 89 isolates possessed mutations in parC or parE which lead to amino acid changes. Substitutions of both Ile for Thr-83 in GyrA and Leu for Ser-87 in ParC were the principal changes, being detected in 48 strains. These replacements were obviously associated with reduced susceptibilities to levofloxacin, ciprofloxacin, and sparfloxacin; however, sitafloxacin showed high activity against isolates with these replacements. We purified GyrA (The-83 to Ile) and ParC (Ser-87 to Leu) by site-directed mutagenesis and compared the inhibitory activities of the fluoroquinolones. Sitafloxacin showed the most potent inhibitory activities against both altered topoisomerases among the fluoroquinolones tested. These results indicated that, compared with other available quinolones, sitafloxacin maintained higher activity against recent clinical isolates with multiple mutations in gyrA and parC, which can be explained by the high inhibitory activities of sitafloxacin against both mutated enzymes.

Effects of rice seed surface sterilization with hypochlorite on inoculated Burkholderia vietnamiensis

When a combination of hydrogen peroxide and hypochlorite was used to surface sterilize rice seeds, a 10(2)- to 10(4)-fold decrease in CFU was observed during the first 15 h after inoculation of the rice rhizosphere organism Burkholderia vietnamiensis TVV75. This artifact could not be eliminated simply by rinsing the seeds, even thoroughly, with sterile distilled water. When growth resumed, a significant increase in the frequency of rifampin- and nalidixic acid-resistant mutants in the population was observed compared to the control without seeds. This phenomenon was a specific effect of hypochlorite; it was not observed with hydrogen peroxide alone. It was also not observed when the effect of hypochlorite was counteracted by sodium thiosulfate. We hypothesized that the hypochlorite used for disinfection reacted with the rice seed surface, forming a chlorine cover which was not removed by rinsing and generated mutagenic chloramines. We studied a set of rifampin- and nalidixic acid-resistant mutants obtained after seed surface sterilization. The corresponding rpoB and gyrA genes were amplified and sequenced to characterize the induced mutations. The mutations in five of seven nalidixic acid-resistant mutants and all of the rifampin-resistant mutants studied were found to correspond to single amino acid substitutions. Hypochlorite surface sterilization can thus be a source of artifacts when the initial bacterial colonization of a plant is studied.

Genetic characterization of highly fluoroquinolone-resistant clinical Escherichia coli strains from China: role of acrR mutations

The genetic basis for fluoroquinolone resistance was examined in 30 high-level fluoroquinolone-resistant Escherichia coli clinical isolates from Beijing, China. Each strain also demonstrated resistance to a variety of other antibiotics. PCR sequence analysis of the quinolone resistance-determining region of the topoisomerase genes (gyrA/B, parC) revealed three to five mutations known to be associated with fluoroquinolone resistance. Western blot analysis failed to demonstrate overexpression of MarA, and Northern blot analysis did not detect overexpression of soxS RNA in any of the clinical strains. The AcrA protein of the AcrAB multidrug efflux pump was overexpressed in 19 of 30 strains of E. coli tested, and all 19 strains were tolerant to organic solvents. PCR amplification of the complete acrR (regulator/repressor) gene of eight isolates revealed amino acid changes in four isolates, a 9-bp deletion in another, and a 22-bp duplication in a sixth strain. Complementation with a plasmid-borne wild-type acrR gene reduced the level of AcrA in the mutants and partially restored antibiotic susceptibility 1.5- to 6-fold. This study shows that mutations in acrR are an additional genetic basis for fluoroquinolone resistance.

The antibiotic microcin B17 is a DNA gyrase poison: characterisation of the mode of inhibition

Microcin B17 is a 3.1-kDa bactericidal peptide; the putative target of this antibiotic is DNA gyrase. Microcin B17 has no detectable effect on gyrase-catalysed DNA supercoiling or relaxation activities in vitro and is unable to stabilise DNA cleavage in the absence of nucleotides. However, in the presence of ATP, or the non-hydrolysable analogue 5'-adenylyl beta,gamma-imidodiphosphate, microcin B17 stabilises a gyrase-dependent DNA cleavage complex in a manner reminiscent of quinolones, Ca(2+), or the bacterial toxin CcdB. The pattern of DNA cleavage produced by gyrase in the presence of microcin B17 is different from that produced by quinolones and more closely resembles Ca(2+)-mediated cleavage. Several gyrase mutants, including well-known quinolone-resistant mutants, are cross resistant to microcin-induced DNA cleavage. We suggest that microcin exerts its effects through a mechanism that has similarities to those of both the bacterial toxin CcdB and the quinolone antibacterial agents.

Rapid phenotypic change and diversification of a soil bacterium during 1000 generations of experimental evolution

Evolutionary pathways open to even relatively simple organisms, such as bacteria, may lead to complex and unpredictable phenotypic changes, both adaptive and non-adaptive. The evolutionary pathways taken by 18 populations of Ralstonia strain TFD41 while they evolved in defined environments for 1000 generations were examined. Twelve populations evolved in liquid media, while six others evolved on agar surfaces. Phenotypic analyses of these derived populations identified some changes that were consistent across all populations and others that differed among them. The evolved populations all exhibited morphological changes in their cell envelopes, including reductions of the capsule in each population and reduced prostheca-like surface structures in most populations. Mean cell length increased in most populations (in one case by more than fourfold), although a few populations evolved shorter cells. Carbon utilization profiles were variable among the evolved populations, but two distinct patterns were correlated with genetic markers introduced at the outset of the experiment. Fatty acid methyl ester composition was less variable across populations, but distinct patterns were correlated with the two physical environments. All 18 populations evolved greatly increased sensitivity to bile salts, and all but one had increased adhesion to sand; both patterns consistent with changes in the outer envelope. This phenotypic diversity contrasts with the fairly uniform increases in competitive fitness observed in all populations. This diversity may represent a set of equally probable adaptive solutions to the selective environment; it may also arise from the chance fixation of non-adaptive mutations that hitchhiked with a more limited set of beneficial mutations.

The complex of DNA gyrase and quinolone drugs on DNA forms a barrier to the T7 DNA polymerase replication complex

Quinolone drugs can inhibit bacterial DNA replication, via interaction with the type II topoisomerase DNA gyrase. Using a DNA template containing a preferred site for quinolone-induced gyrase cleavage, we have demonstrated that the passage of the bacteriophage T7 replication complex is blocked in vitro by the formation of a gyrase-drug-DNA complex. The majority of the polymerase is arrested approximately 10 bp upstream of this preferred site, although other minor sites of blocking have been observed. The ability of mutant gyrase proteins to arrest DNA replication in vitro has been investigated. Gyrase containing mutations in the A subunit at either the active-site tyrosine (Tyr122) or Ser83 (a residue known to be involved in quinolone interaction) failed to halt the progress of the polymerase. A low-level, quinolone-resistant mutation in the B subunit of gyrase showed reduced blocking compared to wild-type. We have demonstrated that DNA cleavage and replication blocking occur on similar time-scales and we conclude that formation of the cleavable complex is a prerequisite for polymerase blocking. Additionally, we have shown that collision of the replication proteins with the gyrase-drug-DNA complex is not sufficient to render this complex irreversible and that further factors must be involved in processing this stalled complex.

Cellular resistance to the antitumor DNA topoisomerase II inhibitor S16020-2: importance of the N-[2(Dimethylamino)ethyl]carbamoyl side chain

The new olivacine derivative S16020-2 (NSC-659687) is a DNA topoisomerase II inhibitor endowed with a remarkable antitumor activity against various experimental tumors. In vitro physicochemical properties of this compound, in particular its interaction with DNA and DNA topoisomerase II, were very similar to those of ellipticine derivatives, except for a strictly ATP-dependent mechanism of cleavable complex induction. From the Chinese hamster lung fibroblast cell line DC-3F, a subline resistant to S16020-2, named DC-3F/S16, was selected by adding stepwise increasing concentrations of the drug to the cell growth medium. Whereas DC-3F/9-OH-E cells, a DC-3F subline resistant to 9-hydroxy-ellipticine, are cross-resistant to S16020-2, DC-3F/S16 cells are only very weakly cross-resistant to ellipticine derivatives, indicating that, despite their structural similarity, these compounds may differ in their mechanisms of action. Uptake and efflux rates of S16020-2 were identical in the resistant and the sensitive cells. Topoisomerase IIalpha was expressed at the same level in both sensitive and resistant cells, whereas expression of the beta-enzyme was approximately 50% lower in the resistant cells. Sequencing of both alpha- and beta-isoform cDNAs revealed a point mutation that converts Arg(486) to a Gly in the alpha cDNA, whereas the beta cDNA was not modified. This amino acid substitution in a highly conserved sequence of the enzyme appears to be responsible for the resistance to S16020-2. Comparative analysis of the properties of the ellipticine and S16020-2-resistant cells suggests that S16020-2, which is a DNA intercalator, might also interact with this enzyme amino acid sequence through its side chain.

The interaction of drugs with DNA gyrase: a model for the molecular basis of quinolone action

DNA gyrase supercoils DNA in bacteria. The fact that it is essential in all bacteria and absent from eukaryotes makes it an ideal drug target. We discuss the action of coumarin and quinolone drugs on gyrase. In the case of coumarins, the drugs are known to be competitive inhibitors of the gyrase ATPase reaction. From a combination of structural and biochemical studies, the molecular details of the gyrase-coumarin complex are well established. In the case of quinolones, the drugs are thought to act by stabilising a cleavage complex between gyrase and DNA that arrests polymerases in vivo. The exact nature of the gyrase-quinolone-DNA complex is not known; we propose a model for this complex based on structural and biochemical data.

Analysis of the mutations involved in fluoroquinolone resistance of in vivo and in vitro mutants of Escherichia coli

We searched for the mutations involved in high-level fluoroquinolone resistance (ciprofloxacin MIC > or = 8 microg/ml) of 11 clinical isolates of Escherichia coli. trans-Complementation tests with the wild-type gyrA and parC genes were positive for all strains whereas negative results were observed with the wild-type gyrB and parE genes. By PCR and sequencing, two mutations in gyrA, leading to Ser-83 --> Leu and Asp-87 --> Asn (7) or Gly (2) or Tyr (1) changes, were found in 10 strains, the eleventh presenting only the Ser-83 --> Leu change. In addition, all strains carried one change in ParC: Ser-80 --> Ile (8) or Arg (2); Glu-84 --> Lys (1). We described a novel and simple method permitting detection of the mutations in parC at codon 80, PCR-RFLP with HaeII. In vitro mutants, selected with ciprofloxacin in three successive steps were also studied. The first-step mutants were complemented by pJSW101 (gyrA+) but not by pEN260 (parC+), whereas the second-step and third-step mutants were complemented by both plasmids. Mutations occurred in the following order: (i) gyrA at codon 83 (Ser to Leu change), (ii) parC at codon 80 (Ser to Ile change), and (iii) gyrA at codon 87 (Asp to Asn change). Thus, these sequential mutations appear to be frequently involved in high-level fluoroquinolone resistance of E. coli.

Quinolone resistance mutations in the gyrA gene of clinical isolates of Salmonella

S. typhimurium AlhR, S. enteritidis OulR, and S. hadar GueR resistant to fluoroquinolones (QR), ciprofloxacin MICs, 0.25 to 1 microgram/ml; norfloxacin MICs, 0.5 to 4 micrograms/ml; nalidixic acid MIC, 256 micrograms/ml were isolated from urinary tract infections (AlhR and OulR) during FQ therapy in immunocompromised patients infected by the parent FQ-susceptible strains (AlhS and OulS) (ciprofloxacin MICs, 0.032-0.063; norfloxacin MICs, 0.125-0.25; nalidixic acid MICs, 4-8) or from intestinal infection (GueR). Transformation of AlhR, OulR, and GueR by plasmid pJSW101 carrying the wild-type gyrA gene of Escherichia coli resulted in complementation (nalidixic acid MICs, 4 to 8), proving that these strains had a gyrA mutation. A 800-bp fragment of gyrA from the five strains was amplified by PCR. Direct DNA sequencing of 252-bp region of this fragment identified a single point mutation leading to a substitution Ser-83 to Tyr in AlhR and to a substitution Ser-83 to Phe in OulR and in GueR. These results emphasize the potential risk of selection of FQ-resistant Salmonella during FQ therapy in immunocompromised patients and suggest that these strains differ from the parent strains at least by one mutation in the gyrA gene. They also confirm the role of substitutions in position 83 of gyrA in FQ-resistant clinical isolates of Salmonella.

Analysis of the mechanisms of quinolone resistance in clinical isolates of Citrobacter freundii

The presence of gyrA, gyrB and/or parC mutations, quinolone uptake, outer membrane protein profiles and epidemiological relationship were studied in 12 clinical isolates of Citrobacter freundii. No alterations were observed in the gyrB gene of any of the strains, or gyrA or parC of the four quinolone-susceptible strains (nalidixic acid MIC of 2-4 mg/L, and a ciprofloxacin MIC of 0.006-0.06 mg/L). The quinolone-resistant strains were classified into two groups: one group (group A) composed of strains resistant to nalidixic acid but not to ciprofloxacin and another (group B) including those resistant to both antibiotics with a mutation at codon 83 of the gyrA gene (Thr-->Ile), but no alteration in either parC or gyrB genes. In group B, three of the four resistant isolates, with a nalidixic acid MIC > 1024 mg/L and ciprofloxacin MIC of 8-32 mg/L, showed concomitant mutations at codons 83 and 87 of the gyrA gene (Thr-->Ile and Asp-->Tyr, respectively) as well as a single mutation in codon 80 of the parC gene (Ser-->Ile). The fourth isolate did not possess the mutation at codon 87 of gyrA. Two strains belong to the same clone and, although they had the same type of mutations in the gyrA and parC genes, showed different MICs of ciprofloxacin. This difference was related to an efflux pump mechanism. Mutations in the gyrA and parC genes play the main role in quinolone resistance development in Citrobacter freundii, although other factors such as overexpression of efflux pumps can play a complementary role and thus modulate the final quinolone MIC.

Activities of fluoroquinolones against Streptococcus pneumoniae type II topoisomerases purified as recombinant proteins

Streptococcus pneumoniae topoisomerase IV and DNA gyrase have been purified from a fluoroquinolone-susceptible Streptococcus pneumoniae strain, from first-step mutants showing low-level resistance to ciprofloxacin, sparfloxacin, levofloxacin, and ofloxacin, and from two clinical isolates showing intermediate- and high-level fluoroquinolone resistance by a gene cloning method that produces recombinant proteins from Escherichia coli. The concentrations of ciprofloxacin, sparfloxacin, levofloxacin, or ofloxacin required to inhibit wild-type topoisomerase IV were 8 to 16 times lower than those required to inhibit wild-type DNA gyrase. Furthermore, low-level resistance to these fluoroquinolones was entirely due to the reduced inhibitory activity of fluoroquinolones against topoisomerase IV. For all the laboratory strains, the 50% inhibitory concentration for topoisomerase IV directly correlated with the MIC. We therefore propose that with S. pneumoniae, ciprofloxacin, sparfloxacin, levofloxacin, and ofloxacin target topoisomerase IV in preference to DNA gyrase. Sitafloxacin, on the other hand, was found to be equipotent against either enzyme. This characteristic is unique for a fluoroquinolone. A reduction in the sensitivities of both topoisomerase IV and DNA gyrase are required, however, to achieve intermediate- or high-level fluoroquinolone resistance in S. pneumoniae.

Isoleucine 10 is essential for DNA gyrase B function in Escherichia coli

DNA gyrase is an essential enzyme that regulates the DNA topology in bacteria. It belongs to the type II DNA topoisomerase family and is responsible for the introduction of negative supercoils into DNA at the expense of hydrolysis of ATP molecules. The aim of the present work was to study the contribution of I10, one of the most important residues responsible for the stabilization of GyrB dimer and involved in the ATP-binding step, in the ATP-hydrolysis reaction and in the DNA supercoiling mechanism. We constructed MBP-tagged GyrB mutants I10G and Delta4-14. Our results demonstrate that both mutations severely affect the DNA-dependent ATPase activity and DNA supercoiling. Mutation of Y5 residue involved in the formation of ATPase catalytic site (Y5G mutant) had only little effect on the DNA-dependent ATPase activity and DNA supercoiling. Interestingly, the DNA-relaxation activity of MBP-GyrB mutants and wild type was completely inhibited by ATP. Binding of ADPNP to MBP-tagged mutants was significantly decreased. ADPNP had no effect on DNA-relaxation activity of MBP-tagged mutants but was able to inhibit MBP-tagged wild type enzyme. Our results demonstrate that GyrB N-terminal arm, and specially I10 residue is essential for ATP binding/hydrolysis efficiency and DNA transfer through DNA gyrase.

Purification and inhibition by quinolones of DNA gyrases from Mycobacterium avium, Mycobacterium smegmatis and Mycobacterium fortuitum bv. peregrinum

The DNA gyrases from Mycobacterium avium, Mycobacterium smegmatis and Mycobacterium fortuitum bv. peregrinum, which are species naturally resistant, moderately susceptible and susceptible to fluoroquinolones, respectively, were purified by affinity chromatography on novobiocin-Sepharose columns. The DNA gyrase inhibiting activities (IC50 values) of classical quinolones and fluoroquinolones were determined from the purified enzymes and were compared to the corresponding antibacterial activities (MICs). Regarding M. fortuitum bv. peregrinum, which is nearly as susceptible as Escherichia coli, the corresponding MIC and IC50 values of quinolones were significantly lower than those found for M. avium and M. smegmatis (e.g. for ofloxacin, MICs of 0.25 versus 32 and 1 microg ml(-1), and IC50 values of 1 versus 8 and 6 microg ml(-1), respectively). Such a result could be related to the presence of Ser-83 in the quinolone-resistance-determining region of the gyrase A subunit of M. fortuitum bv. peregrinum, as found in wild-type E. coli, instead of Ala-83 in M. avium and M. smegmatis, as found in fluoroquinolone-resistant E. coli mutants. The IC50 values of quinolones against the M. avium and M. smegmatis DNA gyrases were similar, while the corresponding MICs were 32-fold higher for M. avium when compared to M. smegmatis, suggesting that an additional mechanism, such as a low cell wall permeability or a drug efflux, could contribute to the low antibacterial potency of quinolones against M. avium.

Cloning and characterization of the gene encoding Aspergillus nidulans DNA topoisomerase II

We have determined the complete nucleotide sequence of a 5544bp genomic DNA fragment from Aspergillus nidulans that encodes DNA topoisomerase II (topo II). It contains a single open reading frame of 4740bp that codes for 1579 amino acid residues with a molecular weight of 178kDa; when expressed in Escherichia coli and Saccharomyces cerevisiae the molecular weight was 180kDa. The gene (TOP2) is divided into three exons. Two introns, 54bp and 60bp in length, are located at nucleotide positions 187 and 3214 respectively. Comparison of the deduced amino acid sequence with other eukaryotic topo II sequences showed a higher degree of identity with other fungal enzymes than the human topo IIalpha. One of monoclonal antibodies raised against human topo II, 6H8, can cross-react with Aspergillus topo II.

Sequence analysis of the gyrA and parC homologues of a wild-type strain of Vibrio parahaemolyticus and its fluoroquinolone-resistant mutants

Vibrio parahaemolyticus causes seafood-borne gastroenteritis in humans. It is particularly important in Japan, where raw seafood is frequently consumed. Fluoroquinolone is one of the current drugs of choice for treating patients infected by V. parahaemolyticus because resistant strains are rarely found. To study a possible fluoroquinolone resistance mechanism in this organism, nucleotide sequences that are homologous to known gyrA and parC genes have been cloned from V. parahaemolyticus AQ3815 and sequenced by amplification with degenerate primers of the quinolone resistance-determining region (QRDR), followed by cassette ligation-mediated PCR. Open reading frames encoding polypeptides of 878 and 761 amino acid residues were detected in the gyrA and parC homologues, respectively. The V. parahaemolyticus GyrA and ParC sequences were most closely related to Erwinia carotovora GyrA (76% identity) and Escherichia coli ParC (69% identity) sequences, respectively. Ciprofloxacin-resistant mutants of AQ3815 were obtained on an agar medium by multistep selection with increasing levels of the quinolone. One point mutation only in the gyrA QRDR was detected among mutants with low- to intermediate-level resistance, while point mutations in both the gyrA and parC QRDRs were detected only in strains with high-level resistance. These results strongly suggest that, as in other gram-negative bacteria, GyrA and ParC are the primary and secondary targets, respectively, of ciprofloxacin in V. parahaemolyticus.

Cloning, expression, and enzymatic characterization of Pseudomonas aeruginosa topoisomerase IV

The topoisomerase IV subunit A gene, parC homolog, has been cloned and sequenced from Pseudomonas aeruginosa PAO1, with cDNA encoding the N-terminal region of Escherichia coli parC used as a probe. The homolog and its upstream gene were presumed to be parC and parE through sequence homology with the parC and parE genes of other organisms. The deduced amino acid sequence of ParC and ParE showed 33 and 32% identity with that of the P. aeruginosa DNA gyrase subunits, GyrA and GyrB, respectively, and 69 and 75% identity with that of E. coli ParC and ParE, respectively. The putative ParC and ParE proteins were overexpressed and separately purified by use of a fusion system with a maltose-binding protein, and their enzymatic properties were examined. The reconstituted enzyme had ATP-dependent decatenation activity, which is the main catalytic activity of bacterial topoisomerase IV, and relaxing activities but had no supercoiling activity. So, the cloned genes were identified as P. aeruginosa topoisomerase IV genes. The inhibitory effects of quinolones on the activities of topoisomerase IV and DNA gyrase were compared. The 50% inhibitory concentrations of quinolones for the decatenation activity of topoisomerase IV were from five to eight times higher than those for the supercoiling activities of P. aeruginosa DNA gyrase. These results confirmed that topoisomerase IV is less sensitive to fluoroquinolones than is DNA gyrase and may be a secondary target of new quinolones in wild-type P. aeruginosa.

Type II topoisomerase mutations in ciprofloxacin-resistant strains of Pseudomonas aeruginosa

We determined the sequences of the quinolone resistance-determining regions of gyrA, gyrB, and parC genes for 30 clinical strains of Pseudomonas aeruginosa resistant to ciprofloxacin that were previously complemented by wild-type gyrA and gyrB plasmid-borne alleles and studied for their coresistance to imipenem (E. Cambau, E. Perani, C. Dib, C. Petinon, J. Trias, and V. Jarlier, Antimicrob. Agents Chemother. 39:2248-2252, 1995). In the present study, we found mutations in type II topoisomerase genes for all strains. Twenty-eight strains had a missense mutation in gyrA (codon 83 or 87). Ten of them had an additional mutation in parC (codon 80 or 84), including a novel mutation of Ser-80 to Trp, but all were fully complemented by a plasmid-borne wild-type gyrA allele. The remaining two strains harbored the first gyrB mutation described in P. aeruginosa, leading to the substitution of phenylalanine for serine 464. The strains which had two mutations in type II topoisomerase genes (i.e., gyrA and parC) were significantly more resistant to fluoroquinolones than those with a single mutation in gyrA or gyrB (geometric mean MICs of ciprofloxacin, 39.4 versus 10.9 microg/ml, P < 0.01; geometric mean MICs of sparfloxacin, 64.0 versus 22.6, P < 0. 01). No mutant with a parC mutation alone was observed, which favors DNA gyrase being the primary target for fluoroquinolones. These results demonstrate that gyrA mutations are the major mechanism of resistance to fluoroquinolones for clinical strains of P. aeruginosa and that additional mutations in parC lead to a higher level of quinolone resistance.

DNA gyrase and topoisomerase IV: biochemical activities, physiological roles during chromosome replication, and drug sensitivities

DNA gyrase and topoisomerase IV are the two type II topoisomerases present in bacteria. Though clearly related, based on amino acid sequence similarity, they each play crucial, but distinct, roles in the cell. Gyrase is involved primarily in supporting nascent chain elongation during replication of the chromosome, whereas topoisomerase IV separates the topologically linked daughter chromosomes during the terminal stage of DNA replication. These different roles can be attributed to differences in the biochemical properties of the two enzymes. The biochemical activities, physiological roles, and drug sensitivities of the enzymes are reviewed.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Conformational changes in DNA gyrase revealed by limited proteolysis

We have used limited proteolysis to identify conformational changes in DNA gyrase. Gyrase exhibits a proteolytic fingerprint dominated by two fragments, one of approximately 62 kDa, deriving from the A protein, and another of approximately 25 kDa from the B protein. Quinolone binding to the enzyme-DNA complex induces a conformational change which is reflected in the protection of the C-terminal 47-kDa domain of the B protein. An active site mutant (Tyr122 to Ser in the A protein) that binds quinolones but cannot cleave DNA still gives the quinolone proteolytic pattern, while stabilization of a cleaved-DNA intermediate by calcium ions does not reveal any protection, suggesting that the quinolone-induced conformational change is different from an "open-gate" state of the enzyme. A quinolone-resistant mutant of gyrase fails to give the characteristic quinolone-associated proteolytic signature. The ATP-induced dimerization of the B subunits is a key step of the gyrase mechanism. The proteolytic fingerprint of this conformation (stabilized by the non-hydrolyzable ATP analog 5'-adenylyl-beta, gamma-imidodiphosphate (ADPNP) shows a protection of the 43-kDa N-terminal domain of the B subunit. The presence of quinolones does not prevent dimerization since incubation of the enzyme-DNA complex with both ADPNP and quinolones gives rise to a complex whose proteolytic pattern retains the characteristic signature of dimerization but has lost the quinolone-induced protection. As a result, the quinolone-gyrase complex can still hydrolyze ATP, albeit with different kinetic characteristics. We interpret the proteolytic signatures observed in terms of four complexes of gyrase, each representing a particular conformational state.

Escherichia coli HU protein suppresses DNA-gyrase-mediated illegitimate recombination and SOS induction

BACKGROUND

The HU protein is an abundant DNA binding protein of bacteria and is a major constituent of the bacterial nucleoid. HU protein is known to be involved in several fundamental biological functions, including DNA supercoiling, DNA replication, site-specific DNA inversion, and transposition. It is generally thought that a functional relationship exists between HU protein and DNA gyrase.

RESULTS

We found that an hupA hupB double mutant displays enhanced spontaneous illegitimate recombination during the formation of lambdabio transducing phage in Escherichia coli. Nucleotide sequence analysis of the resulting transducing phages showed that the E. coli bio and lambda recombination sites did not have any homologous sequence. This mutation also enhanced the spontaneous expression of SOS functions. Furthermore, either overproduced GyrA protein or a temperature-sensitive gyrB mutation suppressed the illegitimate recombination enhanced by the defect of HU protein.

CONCLUSION

These results show that the defect of HU induces illegitimate recombination and SOS response, which are probably mediated by DNA gyrase, implying that HU protein plays roles in suppression of illegitimate recombination and SOS response through interaction with DNA gyrase.

Bacterial live vaccines with graded level of attenuation achieved by antibiotic resistance mutations: transduction experiments on the functional unit of resistance, attenuation and further accompanying markers

At least 10% of spontaneous chromosomal antibiotic resistant mutants of bacteria express a strain-dependent graded reduction of virulence; this correlates linearly with a prolonged generation time. Occasionally, these mutants are temperature sensitive or/and auxotrophe. The work described in this paper provides evidence that in such strains the resistance and the accompanying markers exist only as a functional genetic unit. In a series of transduction experiments with a pathogenic strain of Salmonella typhimurium, it was found that without exception, the resistance and the additional markers were 100% simultaneoulsy transferred. Furthermore, antibiotic-resistant Escherichia coli mutants with prolonged generation time, were isolated from faecal samples; it is thus indicated that, such innocuous mutants occur at any time in the intestine. It is concluded that concerns connecting such mutants to the possibility of resistance dissemination are unfounded; furthermore, even if transfer of resistance occurred, only attenuated strains would be disseminated.