Salicylate induction of antibiotic resistance in Escherichia coli: activation of the mar operon and a mar-independent pathway

Effect of cyclo-oxygenase inhibitors on Helicobacter pylori susceptibility to metronidazole and clarithromycin

BACKGROUND

We previously reported that aspirin inhibited Helicobacter pylori growth and suppressed the mutagenic effect of metronidazole.

AIM

To determine the effects of a cyclo-oxygenase (COX)-2-specific inhibitor, SC-236, and a non-selective COX inhibitor, indometacin, on the growth, urease activity and antimicrobial susceptibility of H. pylori.

METHODS

Three H. pylori reference strains, and 18 clinical isolates were treated with SC-236 or indometacin for 24 and 48 h. Growth, urease activity and susceptibility to clarithromycin and metronidazole of the bacteria were assessed by viable colony counting, spectrophotometry and E-test respectively.

RESULTS

SC-236 and indometacin inhibited H. pylori growth in a dose-dependent manner with the lowest inhibitory concentrations of 0.03 and 0.1 mm, and the lethal concentrations of 0.09 and 0.3 mm, respectively. The numbers of CFU/mL in Brucella broth containing 0.09 mm SC-236 were 2 log lower at 24 h, and even 3 log lower at 48 h than that at 0 h (P = 0.035, compared with the vehicle control). Treatment of 0.3 mm indometacin reduced the number of CFU/mL by 1 log at 24 h compared with that at 0 h (P = 0.037 compared with the vehicle control). Helicobacter pylori urease activity began to decrease with 0.06 mm SC-236 at 24 h (P = 0.016), and 0.3 mm indometacin at 48 h (P = 0.025). MICs of metronidazole and clarithromycin against H. pylori were decreased significantly in the presence of 0.03 mm SC-236 or 0.1 mm indometacin (all P < 0.001).

CONCLUSION

Both SC-236 and indometacin suppressed the growth and urease activity of H. pylori in a dose-dependent manner, and increased its susceptibility to the antibiotics.

RamA is an alternate activator of the multidrug resistance cascade in Enterobacter aerogenes

Multidrug resistance (MDR) in Enterobacter aerogenes can be mediated by induction of MarA, which is triggered by certain antibiotics and phenolic compounds. In this study, we identified the gene encoding RamA, a 113-amino-acid regulatory protein belonging to the AraC-XylS transcriptional activator family, in the Enterobacter aerogenes ATCC 13048 type strain and in a clinical multiresistant isolate. Overexpression of RamA induced an MDR phenotype in drug-susceptible Escherichia coli JM109 and E. aerogenes ATCC 13048, as demonstrated by 2- to 16-fold-increased resistance to beta-lactams, tetracycline, chloramphenicol, and quinolones, a decrease in porin production, and increased production of AcrA, a component of the AcrAB-TolC drug efflux pump. We show that RamA enhances the transcription of the marRAB operon but is also able to induce an MDR phenotype in a mar-deleted strain. We demonstrate here that RamA is a transcriptional activator of the Mar regulon and is also a self-governing activator of the MDR cascade.

Proteolytic degradation of Escherichia coli transcription activators SoxS and MarA as the mechanism for reversing the induction of the superoxide (SoxRS) and multiple antibiotic resistance (Mar) regulons

In Escherichia coli, the SoxRS regulon confers resistance to redox-cycling compounds, and the Mar regulon provides a defence against multiple antibiotics. The response regulators, SoxS and MarA, are synthesized de novo in response to their inducing signals and directly activate transcription of a common set of target genes. Although the mechanisms of transcription activation by SoxS and MarA have been well studied, little is known about how the systems are shut-off once the inducing stress has subsided, except that de novo synthesis of the regulators is known to cease almost immediately. Here, we induced the SoxRS regulon and determined that, upon removal of the inducer, expression of the regulon's genes quickly returns to the preinduced level. This rapid shut-off indicates that the system is reset by an active process. We found that SoxS is unstable and infer that SoxS degradation is responsible for the rapid return of the system to the ground state upon removal of the inducing signal. We also found that MarA is unstable and that the instability of both proteins is intrinsic and unregulated. We used null mutations of protease genes to identify the proteases involved in the degradation of SoxS and MarA. Among single protease mutations, only lon mutations increased the half-life of SoxS and MarA. In addition, SoxS appeared to be nearly completely stable in a lon ftsH double mutant. Using hexahistidine tags placed at the respective ends of the activators, we found that access to the amino-terminus is essential for the proteolytic degradation.

Transcriptional and translational regulation of the marRAB multiple antibiotic resistance operon in Escherichia coli

The marRAB multiple antibiotic resistance operon of Escherichia coli is autorepressed by MarR. MarR binds to two palindromic sequences in vitro: site I lies between and overlaps the -35 and -10 hexamers for RNA polymerase binding; site II lies between the transcription start site and the GTG initiation codon of marR. To assess the importance of these sites in vivo, the effects of mutant sites on transcription were analysed using fusions to lacZ in the presence and absence of wild-type MarR. When both sites were wild type, transcription in the derepressed marR-deleted strain was 19-fold that of the wild-type strain; when only site I or site II was wild type, this ratio was reduced to 4.3- and 2.6-fold, respectively, showing that full repression requires both sites, but some repression can occur at one site independently of the other. Translational fusions of the wild-type promoter to lacZ demonstrated that marR translation proceeds at only 4.5% of the transcription rate. Analysis of translational fusions with mutant leader sequences demonstrated that the principal reason for inefficient translation is a weak Shine-Dalgarno (SD) sequence, AGG(G). Although the SD sequence is located within the potential stem-loop structure of site II, no evidence for occlusion of the SD sequence was found in the wild-type strain. However, a single basepair mutation that strengthens the stem-loop structure drastically reduced the translational efficiency. Substitution of ATG for GTG as the initiation codon increased translational efficiency by 50%. Increasing the 5 bp spacing between the SD sequence and the GTG codon by one to four bases reduced the translational efficiency by 50-75%. Inefficient translation of marR may help to sensitize the cell to environmental signals.

Novel pathway of salicylate degradation by Streptomyces sp. strain WA46

A novel salicylate-degrading Streptomyces sp., strain WA46, was identified by UV fluorescence on solid minimal medium containing salicylate; trace amounts of gentisate were detected by high-pressure liquid chromatography when strain WA46 was grown with salicylate. PCR amplification of WA46 DNA with degenerate primers for gentisate 1,2-dioxygenase (GDO) genes produced an amplicon of the expected size. Sequential PCR with nested GDO primers was then used to identify a salicylate degradation gene cluster in a plasmid library of WA46 chromosomal DNA. The nucleotide sequence of a 13.5-kb insert in recombinant plasmid pWD1 (which was sufficient for the complete degradation of salicylate) showed that nine putative open reading frames (ORFs) (sdgABCDEFGHR) were involved. Plasmid pWD1 derivatives disrupted in each putative gene were transformed into Streptomyces lividans TK64. Disruption of either sdgA or sdgC blocked salicylate degradation; constructs lacking sdgD accumulated gentisate. Cell extracts from Escherichia coli DH5 alpha transformants harboring pUC19 that expressed each of the sdg ORFs showed that conversions of salicylate to salicylyl-coenzyme A (CoA) and salicylyl-CoA to gentisyl-CoA required SdgA and SdgC, respectively. SdgA required CoA and ATP as cofactors, while NADH was required for SdgC activity; SdgC was identified as salicylyl-CoA 5-hydroxylase. Gentisyl-CoA underwent spontaneous cleavage to gentisate and CoA. SdgA behaved as a salicylyl-CoA ligase despite showing amino acid sequence similarity to an AMP-ligase. SdgD was identified as a GDO. These results suggest that Streptomyces sp. strain WA46 degrades salicylate by a novel pathway via a CoA derivative. Two-dimensional polyacrylamide gel electrophoresis and reverse transcriptase-PCR studies indicated that salicylate induced expression of the sdg cluster.

Efflux-mediated drug resistance in bacteria

Drug resistance in bacteria, and especially resistance to multiple antibacterials, has attracted much attention in recent years. In addition to the well known mechanisms, such as inactivation of drugs and alteration of targets, active efflux is now known to play a major role in the resistance of many species to antibacterials. Drug-specific efflux (e.g. that of tetracycline) has been recognised as the major mechanism of resistance to this drug in Gram-negative bacteria. In addition, we now recognise that multidrug efflux pumps are becoming increasingly important. Such pumps play major roles in the antiseptic resistance of Staphylococcus aureus, and fluoroquinolone resistance of S. aureus and Streptococcus pneumoniae. Multidrug pumps, often with very wide substrate specificity, are not only essential for the intrinsic resistance of many Gram-negative bacteria but also produce elevated levels of resistance when overexpressed. Paradoxically, 'advanced' agents for which resistance is unlikely to be caused by traditional mechanisms, such as fluoroquinolones and beta-lactams of the latest generations, are likely to select for overproduction mutants of these pumps and make the bacteria resistant in one step to practically all classes of antibacterial agents. Such overproduction mutants are also selected for by the use of antiseptics and biocides, increasingly incorporated into consumer products, and this is also of major concern. We can consider efflux pumps as potentially effective antibacterial targets. Inhibition of efflux pumps by an efflux pump inhibitor would restore the activity of an agent subject to efflux. An alternative approach is to develop antibacterials that would bypass the action of efflux pumps.

Bile-salt-mediated induction of antimicrobial and bile resistance in Salmonella typhimurium

By DNA microarray, the Salmonella typhimurium marRAB operon was identified as being bile-activated. Transcriptional assays confirm that marRAB is activated in the presence of bile and that this response is concentration-dependent. The bile salt deoxycholate is alone able to activate transcription, while there was no response in the presence of other bile salts tested or a non-ionic detergent. Deoxycholate is able to interact with MarR and interfere with its ability to bind to the mar operator. In addition, incubation of salmonellae in the presence of sublethal concentrations of bile is able to enhance resistance to chloramphenicol and bile, by means of both mar-dependent and mar-independent pathways. To further characterize putative marRAB-regulated genes that may be important for the resistance phenotype, acrAB, which encodes an efflux pump, was analysed. In S. typhimurium, acrAB is required for bile resistance, but while transcription of acrAB is activated by bile, this activation is independent of marRAB, as well as Rob, RpoS or PhoP–PhoQ. These data suggest that bile interacts with salmonellae to increase resistance to bile and other antimicrobials and that this can occur by marRAB- and acrAB-dependent pathways that function independently with respect to bile activation.

PepN is the major aminopeptidase in Escherichia coli: insights on substrate specificity and role during sodium-salicylate-induced stress

PepN and its homologues are involved in the ATP-independent steps (downstream processing) during cytosolic protein degradation. To obtain insights into the contribution of PepN to the peptidase activity in Escherichia coli, the hydrolysis of a selection of endopeptidase and exopeptidase substrates was studied in extracts of wild-type strains and two pepN mutants, 9218 and DH5alphaDeltapepN. Hydrolysis of three of the seven endopeptidase substrates tested was reduced in both pepN mutants. Similar studies revealed that hydrolysis of 10 of 14 exopeptidase substrates studied was greatly reduced in both pepN mutants. This decreased ability to cleave these substrates is pepN-specific as there is no reduction in the ability to hydrolyse exopeptidase substrates in E. coli mutants lacking other peptidases, pepA, pepB or pepE. PepN overexpression complemented the hydrolysis of the affected exopeptidase substrates. These results suggest that PepN is responsible for the majority of aminopeptidase activity in E. coli. Further in vitro studies with purified PepN revealed a preference to cleave basic and small amino acids as aminopeptidase substrates. Kinetic characterization revealed the aminopeptidase cleavage preference of E. coli PepN to be Arg>Ala>Lys>Gly. Finally, it was shown that PepN is a negative regulator of the sodium-salicylate-induced stress in E. coli, demonstrating a physiological role for this aminoendopeptidase under some stress conditions.

Cross-resistance to antibiotics of Escherichia coli adapted to benzalkonium chloride or exposed to stress-inducers

AIMS

To study the effects of adaptation and stress on the resistance to benzalkonium chloride (BC) and cross-resistance to antibiotics in Escherichia coli.

METHODS AND RESULTS

Precultivation of E. coli ATCC 11775 and E. coli DSM 682 in the presence of subinhibitory concentrations of BC or stress inducers (salicylate, chenodeoxycholate and methyl viologen) resulted in higher minimum inhibitory concentration (MIC) of BC and chloramphenicol (CHL). Adaptation to growth in sixfold of the initial MIC of BC resulted in stable BC resistance and enhanced tolerance to several antibiotics and ethidium bromide (EtBr). The MIC of CHL increased more than 10-fold for both strains. Enhanced efflux of EtBr in adapted E. coli ATCC 11775 indicated that the observed resistance was due to efflux. Changes in outer membrane protein profiles were detected in the BC-adapted cells. There were no indications of lower membrane permeability to BC.

CONCLUSIONS

Induction of stress response or gradual adaptation to BC or CHL results in acquired cross-tolerance between BC and antibiotics in E. coli. Enhanced efflux was one of the observed differences in adapted cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

Provided not taking due precautions, extensive use of disinfectants could lead to emergence of antibiotic-resistant isolates.

Salicylic acid attenuates virulence in endovascular infections by targeting global regulatory pathways in Staphylococcus aureus

Aspirin has been previously shown to reduce the in vivo virulence of Staphylococcus aureus in experimental endocarditis, through antiplatelet and antimicrobial mechanisms. In the present study, salicylic acid, the major in vivo metabolite of aspirin, mitigated two important virulence phenotypes in both clinical and laboratory S. aureus strains: alpha-hemolysin secretion and fibronectin binding in vitro. In addition, salicylic acid reduced the expression of the alpha-hemolysin gene promoter, hla, and the fibronectin gene promoter, fnbA. Transcriptional analysis, fluorometry, and flow cytometry revealed evidence of salicylic acid-mediated activation of the stress-response gene sigB. Expression of the sigB-repressible global regulon sarA and the global regulon agr were also mitigated by salicylic acid, corresponding to the reduced expression of the hla and fnbA genes in vitro. Studies in experimental endocarditis confirmed the key roles of both sarA and sigB in mediating the antistaphylococcal effects of salicylic acid in vivo. Therefore, aspirin has the potential to be an adjuvant therapeutic agent against endovascular infections that result from S. aureus, by downmodulating key staphylococcal global regulons and structural genes in vivo, thus abrogating relevant virulence phenotypes.

Bile salts and fatty acids induce the expression of Escherichia coli AcrAB multidrug efflux pump through their interaction with Rob regulatory protein

AcrAB of Escherichia coli, an archetype among bacterial multidrug efflux pumps, exports an extremely wide range of substrates including solvents, dyes, detergents and antimicrobial agents. Its expression is regulated by three XylS/AraC family regulators, MarA, SoxS and Rob. Although MarA and SoxS regulation works by the alteration of their own expression levels, it was not known how Rob, which is constitutively expressed, exerts its regulatory action. We show here that the induction of the AcrAB efflux pump by decanoate and the more lipophilic unconjugated bile salts is mediated by Rob, and that the low-molecular-weight inducers specifically bind to the C-terminal, non-DNA-binding domain of Rob. Induction of Rob is not needed for induction of AcrAB, and we suggest that the inducers act by producing conformational alterations in pre-existing Rob, as was suggested recently (Rosner, Dangi, Gronenborn and Martin, J Bacteriol 184: 1407-1416, 2002). Decanoate and unconjugated bile salts, which are present in the normal habitat of E. coli, were further shown to make the bacteria more resistant to lipophilic antibiotics, at least in part because of the induction of the AcrAB efflux pump. Thus, it is likely that E. coli is protecting itself by the Rob-mediated upregulation of AcrAB against the harmful effects of bile salts and fatty acids in the intestinal tract.

Potential impact of increased use of biocides in consumer products on prevalence of antibiotic resistance

There has recently been much controversy surrounding the increased use of antibacterial substances in a wide range of consumer products and the possibility that, as with antibiotics, indiscriminate use of biocides might contribute to the overall pattern of susceptibility in the general environment and in the clinic. Such speculation, based on the isolation of resistant mutants from in vitro monoculture experiments, is not reflected by an emergence of biocide-resistant strains in vivo. This review provides a broad coverage of the biocide and resistance literature and evaluates the potential risks, perceived from such laboratory monoculture experiments, against evidence gathered over 50 years of field studies. An explanation for the continued effectiveness of broad-spectrum biocidal agents against the decline in efficacy of therapeutic agents is provided based on the fitness costs of resistance and the ubiquity of naturally occurring substances that possess antibacterial effect. While we conclude from this review of the literature that the incorporation of antibacterial agents into a widening sphere of personal products has had little or no impact on the patterns of microbial susceptibility observed in the environment, the associated risks remain finite. The use of such products should therefore be associated with a clear demonstration of added value either to consumer health or to the product life. Hygienic products should therefore be targeted to applications for which the risks have been established.

Aspirin inhibits the growth of Helicobacter pylori and enhances its susceptibility to antimicrobial agents

BACKGROUND AND AIM

The role of Helicobacter pylori and aspirin in peptic ulcer formation and recurrence remains an important clinical topic. The interaction between aspirin and H pylori in vitro is also not clear. We investigated the effect of aspirin on the growth of H pylori and on the susceptibility of H pylori to antimicrobials.

METHODS

Time killing studies of H pylori were performed with different concentrations of aspirin and salicylate. Growth of bacteria was assessed spectrophotometrically and by viable colony count. The effects of aspirin on the efficiency of colony formation and on metronidazole induced mutation to rifampicin resistance in H pylori were determined. Minimal inhibitory concentrations (MICs) of aspirin and metronidazole were tested by the standard agar dilution method. MICs of amoxycillin and clarithromycin were determined by the E test method.

RESULTS

Aspirin and salicylate inhibited the growth of H pylori in a dose dependent manner and bactericidal activity was due to cell lysis. Aspirin 400 micro g/ml caused a 2 logs decrease in colony forming units/ml at 48 hours, and suppressed the normal ability of metronidazole to induce new mutations to rifampicin. The IC(90) of aspirin was 512 micro g/ml. Increased susceptibility of amoxycillin, clarithromycin, and metronidazole to H pylori was observed at 1 mM (180 micro g/ml) aspirin.

CONCLUSIONS

Aspirin inhibited the growth of H pylori, suppressed the mutagenic effect of metronidazole, and enhanced the susceptibility of H pylori to antimicrobial agents. This mechanism is important in future drug development for effective clearing and overcoming resistance.

The periplasmic protein MppA requires an additional mutated locus to repress marA expression in Escherichia coli

Escherichia coli strain TP985, which has an insertional mutation in the gene for the periplasmic murein tripeptide binding protein MppA, was previously reported to overproduce MarA and exhibit a multiple-antibiotic resistance (Mar) phenotype (H. Li and J. T. Park, J. Bacteriol. 181:4842-4847, 1999). We found that TP985 contained a previously unrecognized marR mutation which was responsible for the Mar phenotype. Transduction of the mppA mutation from TP985 to another wild-type strain did not affect antibiotic susceptibility. Overproduction of MppA repressed marA transcription in TP985 but not in other mppA or marR mutants. Therefore, TP985 contains an additional unknown mutation(s) that facilitates the repression of marA expression by MppA.

Phenotypic evidence for inducible multiple antimicrobial resistance in Salmonella choleraesuis

Multiple antimicrobial resistance (MAR) in Salmonella choleraesuis is becoming a major concern. It has been demonstrated that a MAR phenotype can be induced in Escherichia coli and other members of the Enterobacteriaceae by exposing the isolates to salicylates, various antimicrobials, or organic solvents used to combat and control bacterial infection. Therefore the purpose of the present study was to determine whether this marA-associated MAR-phenotype is inducible in S. choleraesuis. Isolates used in the present study were able to withstand toxic effects of the organic solvent cyclohexane naturally, or following exposure to the inducing compounds salicylate, tetracycline, or chloramphenicol. All isolates possessed fragments of marA with the predicted size of 408 bp when amplified using marA-specific primers by PCR. The resulting PCR products that were sequenced revealed that amplified S. choleraesuis marA was 99% and 85% homologous to the published Salmonella typhimurium and E. coli marA sequences respectively. Minimum inhibitory concentrations of tetracycline (P<0.08), chloramphenicol (P<0.001), rifampin (P<0.08), and nalidixic acid (P<0.001) against cyclohexane-tolerant mutants were significantly increased when compared with wild-type S. choleraesuis. Northern hybridization signals for both marA and acrB were increased in the induced isolates when compared to uninduced controls while soxS expression did not change between induced and uninduced cultures. The results suggest that marA is present in S. choleraesuis and a MAR-phenotype is inducible in S. choleraesuis presumably due to the overexpression of marA and acrB and not to the overexpression of soxS.

Weak organic acids: a panoply of effects on bacteria

Weak organic acids have been used for centuries to preserve foods, but only recently has the possible mechanism for bacterial growth inhibition been investigated. Although the lowering of internal pH was favored as the cause of growth inhibition, the emphasis has shifted to the anion and its specificity. There are a number of applications of weak organic acids to foods and in the food industry be they pre- or postharvest, However, there is concern that the ability of foodborne pathogens to adapt to these acids may allow longer survival in these commodities and also to better survive transit through the gastric acid barrier of the stomach. Genomic and proteomic approaches have been applied to the identification of genes and proteins that may allow prokaryotes to cope with organic acid stress. These technologies in combination with genetic approaches may provide better identification of genes essential for survival to organic acids. These acids may have other roles: they can induce phenotypic antibiotic resistance, and the high concentrations of these acids in the colon may signal a relationship to diet, colonic microflora, and human health.

Loss of a 29-kilodalton outer membrane protein in Acinetobacter baumannii is associated with imipenem resistance

We analyzed the possible causes of imipenem (IPM) resistance in multidrug-resistant isolates of Acinetobacter baumannii. Comparison of the outer membrane protein (OMP) profiles of two genomically related strains (Ab288 [IPM sensitive] and Ab242 [IPM resistant]) indicated the conspicuous loss of a 29-kDa polypeptide in the Ab242 strain. No carbapenemase activity was detected in any of these strains. The treatment of Ab288 with sodium salicylate resulted in IPM resistance and the loss of the 29-kDa OMP. In addition, IPM-resistant clones of Ab288 which were selected by repetitive culturing in increasing concentrations of this antibiotic also showed the absence of this 29-kDa OMP.

A comprehensive alanine scanning mutagenesis of the Escherichia coli transcriptional activator SoxS: identifying amino acids important for DNA binding and transcription activation

SoxS is the direct transcriptional activator of the superoxide regulon. SoxS recognizes a highly degenerate "soxbox" DNA sequence, and activates transcription from class I and class II promoters. SoxS is the smallest member of the AraC/XylS family of transcription regulators whose hallmark is dual helix-turn-helix (HTH) DNA-binding motifs. Evidence suggests that the N-terminal HTH motif of SoxS interacts with a highly conserved region of the soxbox termed recognition element 1 (RE1), while the C-terminal HTH motif interacts with the less conserved recognition element 2 (RE2). In the work described here, we prepared a complete library of 101 SoxS mutants containing single alanine substitutions of SoxS, and we characterized the mutant proteins in vivo and in vitro. With SoxS being closely related to MarA, we analyzed the effects of the SoxS mutations in the context of the MarA-mar crystal structure and with respect to the NMR study of MarA-DNA complexes in solution. From the properties of the alanine substitutions, we conclude the following. (1) Surface-exposed residues of helix 3 and helix 6, the recognition helices of the dual HTH motifs, are important to DNA binding and transcription activation; however, substitutions of residues predicted from the MarA-mar crystal structure to make contact with the sugar-phosphate backbone are more detrimental to DNA binding than mutations predicted to make base-specific contacts. (2) Substitution of several residues within the recognition helix predicted to make base-specific contacts with RE2 have relatively little effect on DNA-binding, suggesting the possibility of alternative protein-DNA interactions than those inferred from the MarA-mar crystal structure. (3) DNA binding and transcription activation were reduced by substitution of conserved amino acid residues comprising the hydrophobic core, presumably because they disrupt the structural integrity of SoxS. (4) Mutant K30A appears to be a positive control mutant defective in a protein-protein interaction with RNA polymerase that is required for transcription activation at all SoxS-dependent promoters because it binds and bends DNA normally but fails to activate transcription from both classes of promoters. Alanine substitutions of surface-exposed residues H3, K5, D9, S31, and V45 confer a similar phenotype. Since these residues are near K30 on the surface of the protein, the surface formed by the six residues may be used to make protein-protein interactions with RNA polymerase that are required for transcription activation at both class I and class II SoxS-dependent promoters. (5) Mutants F74A, D75A, M78A, D79A and Q85A appear to define a surface required for protein-protein interaction with RNA polymerase specifically at class II promoters because these positive control mutants bind and bend DNA normally but are defective in activation of class II promoters but not class I promoters. These SoxS mutants that bind and bend DNA normally but are defective in transcription activation represent the first positive control mutants with putative defects in protein-protein interactions with RNA polymerase among the SoxS/MarA/Rob subset of the AraC/XylS family of transcription regulators.

Salicylate reduces susceptibility of Mycobacterium tuberculosis to multiple antituberculosis drugs

Salicylate induces multiple antibiotic resistance in various bacterial species. Here we investigated the effect of salicylate on the susceptibility of Mycobacterium tuberculosis to a range of antituberculosis (anti-TB) drugs. In the presence of salicylate, the killing effects of isoniazid (INH), rifampin (RMP), ethambutol (EMB), streptomycin (STR), and p-aminosalicylate (PAS) were reduced, as shown with a tetrazolium redox dye viability assay and a bacterial survival assay. Salicylate-induced resistance was more pronounced for PAS, STR, and EMB but was not apparent for INH and RMP when salicylate and the anti-TB agents were incorporated into 7H11 plates. The significance of these findings for TB treatment needs to be further evaluated in vivo.

The baeSR two-component regulatory system activates transcription of the yegMNOB (mdtABCD) transporter gene cluster in Escherichia coli and increases its resistance to novobiocin and deoxycholate

Screening of random fragments of Escherichia coli genomic DNA for their ability to increase the novobiocin resistance of a hypersusceptible (Delta)acrAB mutant resulted in the isolation of a plasmid containing baeR, which codes for the response regulator of the two-component regulatory system BaeSR. When induced for expression, baeR cloned in multicopy plasmid pTrc99A significantly increased the resistance of the (Delta)acrAB host strain to novobiocin (16-fold) and to deoxycholate (8-fold). Incubation of cells with novobiocin followed by a chromatographic assay for intracellular drug showed that overproduced BaeR decreased drastically the drug accumulation, presumably via increased active efflux. The genes baeSR are part of a putative operon, yegMNOB baeSR. Direct binding of BaeR to the yegM promoter was demonstrated in vitro by gel retardation assay. The gene yegB, which codes for a major facilitator superfamily transporter, was not necessary for increased resistance, but deletion of yegO or an in-frame deletion of yegN, both of which code for resistance-nodulation-cell division-type multidrug transporters, abolished the BaeR-induced increase in resistance. It is likely that both YegN and YegO produce a complex(es) with the membrane fusion protein family member YegM and pump out novobiocin and deoxycholate. We accordingly propose to rename yegMNOB as mdtABCD (mdt for multidrug transporter). Finally, the expression of two other genes, yicO and ygcL, was shown to be regulated by BaeR, but it is not known if they play any roles in resistance.

Toxic waste disposal in Escherichia coli

About 10% of the nalidixic acid-resistant (Nal(r)) mutants in a transposition-induced library exhibited a growth factor requirement as the result of cysH, icdA, metE, or purB mutation. Resistance in all of these mutants required a functional AcrAB-TolC efflux pump, but the EmrAB-TolC pump played no obvious role. Transcription of acrAB was increased in each type of Nal(r) mutant. In the icdA and purB mutants, each of the known signaling pathways appeared to be used in activating the AcrAB-TolC pump. The metabolites that accumulate upstream of the blocks caused by the mutations are hypothesized to increase the levels of the AcrAB-TolC pump, thereby removing nalidixic acid from the organism.

The multiple antibiotic resistance (mar) locus and its significance

Chromosomally encoded systems involved in low level resistance of bacteria to different classes of antibiotics (mainly beta-lactams, chloramphenicol, quinolones and tetracycline), disinfectants and in resistance to organic solvents have been the focus of considerable interest in recent years. The multiple antibiotic resistance (mar) locus of Escherichia coli and Salmonella is perhaps the best described system involved in this type of resistance which is induced by MarA, the activator protein encoded by the marRAB locus. The mar -locus is reported to mediate resistance primarily by up-regulating efflux of some antibiotics, disinfectants and organic solvents via the AcrAB-TolC efflux pump and down regulating influx through Outer Membrane Protein F (OmpF). Whilst the level of antibiotic resistance conferred by marRAB is only low level, there are increasing data to suggest that marRAB and related systems are important in clinical antibiotic resistance, possibly as a 'stepping stone' to higher levels of resistance. Other related systems include up-regulation of RobA, SoxS and AcrAB which give rise to a similar resistance phenotype to that conferred by up-regulation of MarA. The aim of this paper is to review the function and significance of the mar -locus and related systems with a particular focus on its implications in veterinary medicine.

Fluoroquinolone resistance in Salmonella serovars isolated from humans and food animals

Quinolone-resistant Salmonella enterica usually contain a mutation in gyrA within the region encoding the quinolone resistance determining region of the A subunit of DNA gyrase. These mutations confer substitutions analogous to Escherichia coli Ser83-->Phe and Asp87-->Gly or Tyr, or a novel mutation resulting in Ala119-->Glu or Val. Mutations in gyrB are rare, and no mutations in parC or parE have been described. Quinolone-resistant Salmonella can also be cross-resistant to other agents including chloramphenicol and tetracycline. Increased efflux has been demonstrated and for some strains this has been associated with increased expression of acrB. Mutation in soxR has also been shown for one isolate. Detection of low level resistance (minimum inhibitory concentrations <0.5 microg ml(-1)) to fluoroquinolones is proving an increasing problem in the treatment of invasive Salmonella infections.

Posttranscriptional activation of the transcriptional activator Rob by dipyridyl in Escherichia coli

The transcriptional activator Rob consists of an N-terminal domain (NTD) of 120 amino acids responsible for DNA binding and promoter activation and a C-terminal domain (CTD) of 169 amino acids of unknown function. Although several thousand molecules of Rob are normally present per Escherichia coli cell, they activate promoters of the rob regulon poorly. We report here that in cells treated with either 2,2"- or 4,4"-dipyridyl (the latter is not a metal chelator), Rob-mediated transcription of various rob regulon promoters was increased substantially. A small, growth-phase-dependent effect of dipyridyl on the rob promoter was observed. However, dipyridyl enhanced Rob's activity even when rob was regulated by a heterologous (lac) promoter showing that the action of dipyridyl is mainly posttranscriptional. Mutants lacking from 30 to 166 of the C-terminal amino acids of Rob had basal levels of activity similar to that of wild-type cells, but dipyridyl treatment did not enhance this activity. Thus, the CTD is not an inhibitor of Rob but is required for activation of Rob by dipyridyl. In contrast to its relatively low activity in vivo, Rob binding to cognate DNA and activation of transcription in vitro is similar to that of MarA, which has a homologous NTD but no CTD. In vitro nuclear magnetic resonance studies demonstrated that 2,2"-dipyridyl binds to Rob but not to the CTD-truncated Rob or to MarA, suggesting that the effect of dipyridyl on Rob is direct. Thus, it appears that Rob can be converted from a low activity state to a high-activity state by a CTD-mediated mechanism in vivo or by purification in vitro.

Molecular cloning and characterization of the Salmonella enterica Serovar Paratyphi B rma Gene, which confers multiple drug resistance in Escherichia coli

A genomic library from a strain of Salmonella enterica serovar Paratyphi B that exhibits multiple drug resistance (MDR) was constructed in Escherichia coli. Two of the recombinant plasmids, pNOR5 and pNOR5, conferred resistance only to fluoroquinolones in E. coli, whereas the third, pNCTR4, conferred the MDR phenotype. Sequence and subcloning analysis showed that it is the presence of RecA on the first two plasmids which confers resistance to fluoroquinolones in E. coli. A similar analysis established that the MDR phenotype conferred by pNCTR4 is due to a gene, rma (resistance to multiple antibiotics), which encodes a 13.5-kDa polypeptide. The derived sequence for Rma exhibits a high degree of similarity to those of a group of MarA-like activators that confer MDR in E. coli. A MalE-Rma fusion protein was purified to near homogeneity and was shown to interact with a DNA fragment carrying a MarA operator sequence. Furthermore, overexpression of rma in E. coli caused changes in the outer membrane protein profile that were similar to those reported for MarA. These results suggest that Rma might act as a transcriptional activator of the marA regulon.

Complex formation between activator and RNA polymerase as the basis for transcriptional activation by MarA and SoxS in Escherichia coli

Transcriptional activation in Escherichia coli is generally considered to proceed via the formation of an activator-DNA-RNA polymerase (RNP) ternary complex. Although the order of assembly of the three elements is thermodynamically irrelevant, a prevalent idea is that the activator-DNA complex is formed first, and recruitment of RNP to the binary complex occurs subsequently. We show here that the closely related activators, MarA, SoxS and Rob, which activate the same family of genes, are capable of forming complexes with RNP core or holoenzyme in the absence of DNA. In addition, we find that the ternary MarA-DNA-RNP and SoxS-DNA-RNP complexes are more stable than the corresponding Rob-DNA-RNP complex, although the binary Rob-DNA complex is often more stable than the corresponding MarA- or SoxS-DNA complexes. These results may help to explain certain puzzling aspects of the MarA/SoxS/Rob system. We suggest that activator-RNP complexes scan the chromosome and bind promoters of the regulon more efficiently than either RNP or the activators alone.

MicF: an antisense RNA gene involved in response of Escherichia coli to global stress factors

The micF gene is a stress response gene found in Escherichia coli and related bacteria that post-transcriptionally controls expression of the outer membrane porin gene ompF. The micF gene encodes a non-translated 93 nt antisense RNA that binds its target ompF mRNA and regulates ompF expression by inhibiting translation and inducing degradation of the message. In addition, other factors, such as the RNA chaperone protein StpA also play a role in this regulatory system. Expression of micF is controlled by both environmental and internal stress factors. Four transcriptional regulators are known to bind the micF promoter region and activate micF expression. The crystal structure of one these transcriptional activators, Rob, complexed with the micF promoter has been reported. Here, we review new developments in the micF regulatory network.

Activation of multiple antibiotic resistance in uropathogenic Escherichia coli strains by aryloxoalcanoic acid compounds

Clofibric and ethacrynic acids are prototypical pharmacological agents administered in the treatment of hypertrigliceridemia and as a diuretic agent, respectively. They share with 2,4-dichlorophenoxyacetic acid (the widely used herbicide known as 2,4-D) a chlorinated phenoxy structural moiety. These aryloxoalcanoic agents (AOAs) are mainly excreted by the renal route as unaltered or conjugated active compounds. The relatedness of these agents at the structural level and their potential effect on therapeutically treated or occupationally exposed individuals who are simultaneously undergoing a bacterial urinary tract infection led us to analyze their action on uropathogenic, clinically isolated Escherichia coli strains. We found that exposure to these compounds increases the bacterial resistance to an ample variety of antibiotics in clinical isolates of both uropathogenic and nonpathogenic E. coli strains. We demonstrate that the AOAs induce an alteration of the bacterial outer membrane permeability properties by the repression of the major porin OmpF in a micF-dependent process. Furthermore, we establish that the antibiotic resistance phenotype is primarily due to the induction of the MarRAB regulatory system by the AOAs, while other regulatory pathways that also converge into micF modulation (OmpR/EnvZ, SoxRS, and Lrp) remained unaltered. The fact that AOAs give rise to uropathogenic strains with a diminished susceptibility to antimicrobials highlights the impact of frequently underestimated or ignored collateral effects of chemical agents.

Low-level antibacterial resistance: a gateway to clinical resistance

The huge amount of antibiotic substances released in the human environment has probably resulted in an acceleration in the rate of bacterial evolution. It is to note that most interactions between chemotherapeutic agents and microbial populations occur at very low antibiotic concentrations. Thus, natural selection is expected to act on very small increases in the bacterial ability to resist to antibiotic inhibitory effects. On the other hand, there is a wealth of mechanisms to resist to these low antibiotic concentrations. The progressive enrichment in low-level resistant populations favours secondary selections for more specific and effective mechanisms of resistance, particularly in treated patients. These adaptations may have a biological cost in the absence of antibiotics, but frequently compensatory mutations occur, minimizing such genetic burden. In this way, a phenomenon of directional selection takes place, with low possibilities of return to susceptibility. Moreover, low antibiotic concentrations are not only able to select low-level antibiotic resistant variants, but may produce a substantial stress in bacterial populations, that eventually influences the rate of genetic variation and the diversity of adaptive responses. More attention should be devoted to the mechanisms of low-level resistance in microorganisms, as they can serve as stepping stones to develop high level, clinically relevant resistance. These mechanisms should be identified early in the development of drugs in order to adapt the therapeutic strategies (for instance dosage) to minimize the selection of low-level resistant variants, as frequently they emerge by means of concentration-specific selection. At the same time, conventional susceptibility testing should probably be able to detect low-level resistance, and not only clinically-relevant resistance. We should be vigilant of the evolutionary trends of microorganisms; for that a purpose, knowledge of the biology and epidemiology of low-level resistance is becoming a real need.

Multiple antibiotic resistance (mar) locus in Salmonella enterica serovar typhimurium DT104

In order to understand the role of the mar locus in Salmonella with regard to multiple antibiotic resistance, cyclohexane resistance, and outer membrane protein F (OmpF) regulation, a marA::gfp reporter mutant was constructed in an antibiotic-sensitive Salmonella enterica serovar Typhimurium DT104 background. Salicylate induced marA, whereas a number of antibiotics, disinfectants, and various growth conditions did not. Increased antibiotic resistance was observed upon salicylate induction, although this was shown to be by both mar-dependent and mar-independent pathways. Cyclohexane resistance, however, was induced by salicylate by a mar-dependent pathway. Complementation studies with a plasmid that constitutively expressed marA confirmed the involvement of mar in Salmonella with low-level antibiotic resistance and cyclohexane resistance, although the involvement of mar in down regulation of OmpF was unclear. However, marA overexpression did increase the expression of a ca. 50-kDa protein, but its identity remains to be elucidated. Passage of the marA::gfp reporter mutant with increasing levels of tetracycline, a method reported to select for mar mutants in Escherichia coli, led to both multiple-antibiotic and cyclohexane resistance. Collectively, these data indicate that low-level antibiotic resistance, cyclohexane resistance, and modulation of OMPs in Salmonella, as in E. coli, can occur in both a mar-dependent and mar-independent manner.

Role of SspA in the density-dependent expression of the transcriptional activator AarP in Providencia stuartii

The AarP protein in Providencia stuartii encodes a small transcriptional activator which activates the chromosomal aminoglycoside acetyltransferase aac(2')-Ia gene. In addition, AarP activates genes involved in a multiple antibiotic resistance (Mar) phenotype. Expression of an aarP-lacZ fusion increased in a density-dependent manner and reached peak levels at stationary phase. The expression of an aarP-lacZ fusion could be prematurely activated in cells at early to mid-exponential phase by the addition of spent culture supernatants from stationary phase cultures or by ethyl acetate extracts of these supernatants. Nutrient starvation had a negligible effect on aarP expression. In a search for mutations that block aarP activation at stationary phase, a mini-Tn5Cm insertion has been identified within a gene whose product was 77% identical to SspA, a regulatory protein involved in stationary phase gene expression and virulence. An unmarked sspA null allele (sspA2) was created by allelic replacement to further examine the role of sspA in P. stuartii. The sspA2 allele resulted in substantial decrease in aarP mRNA accumulation at various phases of growth. Furthermore, in an sspA mutant background, the aarP-lacZ fusion was no longer activated by an extracellular signal.

Expression in Escherichia coli of native and chimeric phenolic acid decarboxylases with modified enzymatic activities and method for screening recombinant E. coli strains expressing these enzymes

Four bacterial phenolic acid decarboxylases (PAD) from Lactobacillus plantarum, Pediococcus pentosaceus, Bacillus subtilis, and Bacillus pumilus were expressed in Escherichia coli, and their activities on p-coumaric, ferulic, and caffeic acids were compared. Although these four enzymes displayed 61% amino acid sequence identity, they exhibit different activities for ferulic and caffeic acid metabolism. To elucidate the domain(s) that determines these differences, chimeric PAD proteins were constructed and expressed in E. coli by exchanging their individual carboxy-terminal portions. Analysis of the chimeric enzyme activities suggests that the C-terminal region may be involved in determining PAD substrate specificity and catalytic capacity. In order to test phenolic acid toxicity, the levels of growth of recombinant E. coli displaying and not displaying PAD activity were compared on medium supplemented with different concentrations of phenolic acids and with differing pHs. Though these acids already have a slight inhibitory effect on E. coli, vinyl phenol derivatives, created during decarboxylation of phenolic acids, were much more inhibitory to the E. coli control strain. To take advantage of this property, a solid medium with the appropriate pH and phenolic acid concentration was developed; in this medium the recombinant E. coli strains expressing PAD activity form colonies approximately five times smaller than those formed by strains devoid of PAD activity.

Molecular properties of bacterial multidrug transporters

One of the mechanisms that bacteria utilize to evade the toxic effects of antibiotics is the active extrusion of structurally unrelated drugs from the cell. Both intrinsic and acquired multidrug transporters play an important role in antibiotic resistance of several pathogens, including Neisseria gonorrhoeae, Mycobacterium tuberculosis, Staphylococcus aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Vibrio cholerae. Detailed knowledge of the molecular basis of drug recognition and transport by multidrug transport systems is required for the development of new antibiotics that are not extruded or of inhibitors which block the multidrug transporter and allow traditional antibiotics to be effective. This review gives an extensive overview of the currently known multidrug transporters in bacteria. Based on energetics and structural characteristics, the bacterial multidrug transporters can be classified into five distinct families. Functional reconstitution in liposomes of purified multidrug transport proteins from four families revealed that these proteins are capable of mediating the export of structurally unrelated drugs independent of accessory proteins or cytoplasmic components. On the basis of (i) mutations that affect the activity or the substrate specificity of multidrug transporters and (ii) the three-dimensional structure of the drug-binding domain of the regulatory protein BmrR, the substrate-binding site for cationic drugs is predicted to consist of a hydrophobic pocket with a buried negatively charged residue that interacts electrostatically with the positively charged substrate. The aromatic and hydrophobic amino acid residues which form the drug-binding pocket impose restrictions on the shape and size of the substrates. Kinetic analysis of drug transport by multidrug transporters provided evidence that these proteins may contain multiple substrate-binding sites.

Cloning and characterization of SmeDEF, a novel multidrug efflux pump from Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is a nosocomial bacterial pathogen intrinsically resistant to several antibiotics. The mechanisms involved in this intrinsic multiresistance phenotype are poorly understood. A library of chromosomal DNA from a spontaneous multidrug-resistant S. maltophilia D457R mutant (A. Alonso and J. L. Martinez, Antimicrob. Agents Chemother. 41:1140-1142, 1997) was screened for complementation of erythromycin susceptibility on an antibiotic-hypersusceptible Escherichia coli DeltaacrAB strain. Cloning and further analysis revealed that a 6-kbp region constituting a transcriptional unit was capable of complementing the antibiotic-susceptible phenotype of an E. coli DeltaacrAB strain. We identified three open reading frames, smeD, smeE and smeF, which code for members of the membrane fusion protein, resistance nodulation division, and outer membrane factor families, respectively. Drug susceptibility assays indicated that the SmeDEF system cloned in E. coli mediates resistance to a wide range of antibiotics. Ethidium bromide and norfloxacin accumulation experiments in the presence and in the absence of carbonyl cyanide m-chlorophenylhydrazone showed that this system constitutes a drug efflux pump dependent on the membrane proton motive force. The presence of high levels of smeDEF mRNA in the multiresistant D457R mutant was consistent with the high levels of SmeF (formerly Omp54) observed in the same strain. In contrast, transcription levels of smeDEF in the D457 strain were tiny, which correlates with the low levels of SmeF observed for this strain. Also, for both the D457 and D457R strains, we observed growth phase-dependent regulation in which the highest level of transcription corresponded to early exponential phase, with transcription decreasing throughout the growth curve to undetectable levels at 24 h.

The effects of salicylate on bacteria

Salicylate and related compounds, such as aspirin, have a variety of effects in eucaryotic systems and are well known for their medicinal properties. Salicylate also has numerous effects on bacteria, yet only a handful of individuals within the scientific community appreciate these findings. From a bacterial viewpoint, growth in the presence of salicylate can be both beneficial and detrimental. On one hand, growth of certain bacteria in the presence of salicylate can induce an intrinsic multiple antibiotic resistance phenotype. On the other hand, growth in the presence of salicylate can reduce the resistance to some antibiotics and affect virulence factor production in some bacteria. This review provides an overview of the effects salicylate has on various bacterial species.

Prevalence of the multiple antibiotic resistance operon (marRAB) in the genus Salmonella

The multiple antibiotic resistance operon (marRAB) is a member of the multidrug resistance (mdr) systems. Similar to other mdr systems, this operon when induced encodes resistance to structurally and functionally unrelated antibiotics. marRAB has been shown to be conserved in the family Enterobacteriaceae, but within the genus Salmonella certain species appeared to be lacking this operon. To investigate how conserved the marRAB operon was in Salmonella, 30 veterinary isolates were examined by PCR, Southern blot, and dot blot analysis. Using DNA primers based on the marRAB operon of S. typhimurium, a predicted 2.3-kb amplicon resulted after PCR in 16 of the 30 organisms. The 2.3-kb DNA band from S. enteritidis was cloned and sequenced and shown to possess 99% sequence homology to marRAB from S. typhimurium. Using a labeled marRAB gene probe from S. enteritidis, Southern blot and dot blot analysis confirmed the presence of the operon in all 30 Salmonella species examined. Furthermore, when these isolates were induced with low levels of either tetracycline or chloramphenicol, increased antimicrobial resistance was observed to structurally and functionally unrelated antibiotics. Thus, the widespread occurrence of the marRAB locus in this genus prescribes judicious use of antimicrobials to avoid induction of a mdr phenotype.

An evaluation of the potential of the multiple antibiotic resistance operon (mar) and the multidrug efflux pump acrAB to moderate resistance towards ciprofloxacin in Escherichia coli biofilms

The chromosomal multiple antibiotic resistance operon, mar, is widely represented amongst Gram-negative bacteria and has been implicated in resistance towards oxidative stress agents, organic solvents and a large number of structurally unrelated antimicrobial agents. The major mechanism associated with such increased resistance is an upregulation of the efflux pump acrAB. Growth as a biofilm is often associated with similar generalized reductions in susceptibility to inimical agents. Escherichia coli K12 (AG100), an isogenic mutant of AG100 constitutive for mar expression (AG102) and an isolate deleted of the mar locus (MCH164) were grown as biofilms in cellulose-fibre depth filters and perfused with a simple salts, minimal medium (CDM) over 120 h. Biofilms were exposed to various concentrations of ciprofloxacin (0.004, 0.015 and 0.1 mg/L) for 42 h. The numbers of viable cells within the perfusate and within the biofilm were estimated throughout. Whereas no differences were seen between the wild-type and mar-deleted isolates, that constitutive for mar displayed reduced susceptibility to ciprofloxacin at concentrations of 0.004 mg/L (MIC for AG100 was 0.0052 mg/L). Similar antibiotic perfusion experiments were conducted using isolates in which the efflux pump acrAB was either deleted (AG100-A) or constitutively expressed (AG100-B). Exposure of AG100-A biofilms to ciprofloxacin at 0.004 and 0.1 mg/L showed similar susceptibilities to those seen in the wild-type (AG100) and mar-deleted (MCH164) isolates and suggested that acrAB was not induced within the attached population. On the other hand, constitutive expression of acrAB (AG100-B) protected biofilms against the lower concentration of ciprofloxacin used (0.004 mg/L). This protection was again lost at concentrations of 0.1 mg/L. Overall, these results show that ciprofloxacin resistance in biofilms is not mediated by the upregulation of the mar or acrAB operons.

Differential expression of over 60 chromosomal genes in Escherichia coli by constitutive expression of MarA

In Escherichia coli, the MarA protein controls expression of multiple chromosomal genes affecting resistance to antibiotics and other environmental hazards. For a more-complete characterization of the mar regulon, duplicate macroarrays containing 4,290 open reading frames of the E. coli genome were hybridized to radiolabeled cDNA populations derived from mar-deleted and mar-expressing E. coli. Strains constitutively expressing MarA showed altered expression of more than 60 chromosomal genes: 76% showed increased expression and 24% showed decreased expression. Although some of the genes were already known to be MarA regulated, the majority were newly determined and belonged to a variety of functional groups. Some of the genes identified have been associated with iron transport and metabolism; other genes were previously known to be part of the soxRS regulon. Northern blot analysis of selected genes confirmed the results obtained with the macroarrays. The findings reveal that the mar locus mediates a global stress response involving one of the largest networks of genes described.

Non-target gene mutations in the development of fluoroquinolone resistance in Escherichia coli

Mutations in loci other than genes for the target topoisomerases of fluoroquinolones, gyrA and parC, may play a role in the development of fluoroquinolone resistance in Escherichia coli. A series of mutants with increasing resistance to ofloxacin was obtained from an E. coli K-12 strain and five clinical isolates. First-step mutants acquired a gyrA mutation. Second-step mutants reproducibly acquired a phenotype of multiple antibiotic resistance (Mar) and organic solvent tolerance and showed enhanced fluoroquinolone efflux. None of the second-step mutants showed additional topoisomerase mutations. All second-step mutants showed constitutive expression of marA and/or overexpressed soxS. In some third-step mutants, fluoroquinolone efflux was further enhanced compared to that for second-step mutants, even when the mutant had acquired additional topoisomerase mutations. Attempts to circumvent the second-step Mar mutation by induction of the mar locus with sodium salicylate and thus to select for pure topoisomerase mutants at the second step were not successful. At least in vitro, non-target gene mutations accumulate in second- and third-step mutants upon exposure to a fluoroquinolone and typically include, but do not appear to be limited to, mutations in the mar or sox regulons with consequent increased drug efflux.

Mutational analysis of MarR, the negative regulator of marRAB expression in Escherichia coli, suggests the presence of two regions required for DNA binding

MarR, the negative regulator of the Escherichia coli multiple antibiotic resistance (marRAB) operon, is a member of a newly recognized family of regulatory proteins. The amino acid sequences of these proteins do not display any apparent homologies to the DNA binding domains of prokaryotic transcription regulators and a DNA binding motif for any one of the MarR homologues is currently unknown. In order to define regions of MarR required for DNA binding, mutant repressors, selected based on their ability to interfere with (negatively complement) the activity of wild-type MarR, were isolated. As determined using gel mobility shift assays, 13 out of 14 negative complementing mutants tested were unable to bind DNA in vitro. Three negative complementing alleles presumably specify truncated repressors and one of these proteins, a 120 residue MarR, can bind DNA in vitro. Most of the negative complementing mutations were clustered within two areas of MarR with features related to a helix-turn-helix DNA binding motif. These regions are presumed to be required for the DNA binding activity of the repressor.

Expression of the multiple antibiotic resistance operon (mar) during growth of Escherichia coli as a biofilm

The multiple antibiotic resistance (mar) operon is a global regulator controlling the expression of various genes in Escherichia coli which constitutes the mar regulon. Upregulation of mar leads to a multi-drug resistant phenotype, which includes resistance towards structurally unrelated antibiotics, organic solvents and the disinfectant pine oil. Biofilms also display similar decreases in susceptibility to antimicrobial agents. A marOII-lacZ fusion strain (SPC105) of E. coli was used to monitor mar expression under various growth conditions including batch, continuous and biofilm culture. In chemically-defined media (CDM), mar expression was maximal in mid-log and declined in the stationary phase. Conversely, in rich media (Luria-Bertani broth), minimal expression in mid-log was followed by an increase in the stationary phase. In continuous culture, expression was inversely related to specific growth rate (mu = 0.05-0.4 h-1). LacZ expression by the marOII-lacZ fusion was generally low within the total biofilm population and equivalent to that of stationary phase cultures grown in batch culture. When the expression of mar in CDM batch culture was compared with that in biofilm populations, beta-galactosidase activity was generally higher throughout batch culture than in the attached population. Overall, these results suggest that while mar expression will be greatest within the depths of a biofilm where growth rates are suppressed, its probable induction within biofilms cannot explain the elevated levels of antibiotic resistance observed.

Promoter discrimination by the related transcriptional activators MarA and SoxS: differential regulation by differential binding

MarA and SoxS are closely related proteins ( approximately 45% identical) that transcriptionally activate a common set of unlinked genes, resulting in multiple antibiotic and superoxide resistance in Escherichia coli. Both proteins bind as monomers to a 20 bp degenerate asymmetric recognition sequence, the 'marbox', located upstream of the promoter. However, the proteins differ widely in the extents to which they activate particular promoters, with the consequence that overexpression of SoxS leads to greater superoxide resistance than does overexpression of MarA. This 'discrimination' between activators by promoters was demonstrated in vivo, using promoters fused to lacZ, and in vitro, using purified RNA polymerase, promoter DNA and MarA or SoxS. The marbox was found to be a critical element in discrimination by in vivo and in vitro assays of hybrid promoters containing the marbox from one gene and the core promoter from another. Furthermore, by sequential mutation of its marbox, a promoter that discriminated 35-fold in favour of SoxS was converted into one that did not discriminate. The relative activation of a promoter by MarA or SoxS was paralleled by the relative binding of the two activators to the promoter's marbox as assayed by band shift experiments. Thus, differential recognition of closely related marbox sequences by the closely related activators is the primary basis for promoter discrimination. Discrimination enables the cell to customize its response to the stresses that trigger synthesis of the activators.

Mutational analysis of a role for salicylic acid in iron metabolism of Mycobacterium smegmatis

The role of salicylic acid in iron metabolism was examined in two wild-type strains (mc(2)155 and NCIMB 8548) and three mutant strains (mc(2)1292 [lacking exochelin], SM3 [lacking iron-dependent repressor protein IdeR] and S99 [a salicylate-requiring auxotroph derived in this study]) of Mycobacterium smegmatis. Synthesis of salicylate in SM3 was derepressed even in the presence of iron, as was synthesis of the siderophores exochelin, mycobactin, and carboxymycobactin. S99 was dependent on salicylate for growth and failed to grow with the three ferrisiderophores, suggesting that salicylate fulfills an additional function(s) other than being a precursor of mycobactin and carboxymycobactin. Salicylic acid at 100 microgram/ml repressed the formation of a 29-kDa cell envelope protein (putative exochelin receptor protein) in S99 grown both iron deficiently and iron sufficiently. In contrast, synthesis of this protein was affected only under iron-limited conditions in the parent strain, mc(2)155, and remained unaltered in SM3, suggesting an interaction between the IdeR protein and salicylate. Thus, salicylate may also function as a signal molecule for recognition of cellular iron status. Growth of all strains and mutants with p-aminosalicylate (PAS) at 100 microgram/ml increased salicylate accumulation between three- and eightfold under both iron-limited and iron-sufficient growth conditions and decreased mycobactin accumulation by 40 to 80% but increased carboxymycobactin accumulation by 50 to 55%. Thus, although PAS inhibited salicylate conversion to mycobactin, presumptively by blocking salicylate AMP kinase, PAS also interferes with the additional functions of salicylate, as its effect was heightened in S99 when the salicylate concentration was minimal.