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1.  Cloning and Characterization of SmeT, a Repressor of the Stenotrophomonas maltophilia Multidrug Efflux Pump SmeDEF 
Antimicrobial Agents and Chemotherapy  2002;46(11):3386-3393.
We report on the cloning of the gene smeT, which encodes the transcriptional regulator of the Stenotrophomonas maltophilia efflux pump SmeDEF. SmeT belongs to the TetR and AcrR family of transcriptional regulators. The smeT gene is located upstream from the structural operon of the pump genes smeDEF and is divergently transcribed from those genes. Experiments with S. maltophilia and the heterologous host Escherichia coli have demonstrated that SmeT is a transcriptional repressor. S1 nuclease mapping has demonstrated that expression of smeT is driven by a single promoter lying close to the 5′ end of the gene and that expression of smeDEF is driven by an unique promoter that overlaps with promoter PsmeT. The level of expression of smeT is higher in smeDEF-overproducing S. maltophilia strain D457R, which suggests that SmeT represses its own expression. Band-shifting assays have shown that wild-type strain S. maltophilia D457 contains a cellular factor(s) capable of binding to the intergenic smeT-smeD region. That cellular factor(s) was absent from smeDEF-overproducing S. maltophilia strain D457R. The sequence of smeT from D457R showed a point mutation that led to a Leu166Gln change within the SmeT protein. This change allowed overexpression of both smeDEF and smeT in D457R. It was noteworthy that expression of wild-type SmeT did not fully complement the smeT mutation in D457R. This suggests that the wild-type protein is not dominant over the mutant SmeT.
PMCID: PMC128709  PMID: 12384340
2.  SmeDEF Multidrug Efflux Pump Contributes to Intrinsic Multidrug Resistance in Stenotrophomonas maltophilia 
Antimicrobial Agents and Chemotherapy  2001;45(12):3497-3503.
Stenotrophomonas maltophilia is an emerging nosocomial pathogen that displays high-level intrinsic resistance to a variety of structurally unrelated antimicrobial agents. Efflux mechanisms are known to contribute to acquired multidrug resistance in this organism, and indeed, one such multidrug efflux system, SmeDEF, was recently identified. Still, the importance of SmeDEF to intrinsic antibiotic resistance in S. maltophilia had not yet been determined. Reverse transcription-PCR confirmed expression of the smeDEF genes in wild-type S. maltophilia, and deletion of smeE or smeF in wild-type strains rendered the mutants hypersusceptible to several antimicrobials, suggesting that SmeDEF contributes to intrinsic antimicrobial resistance in this organism. Expression of smeDEF was also enhanced in an in vitro-selected multidrug-resistant mutant, although deletion of smeF but not of smeE in these mutants compromised antimicrobial resistance. Apparently, hyperexpressed SmeF is capable of functioning with additional multidrug efflux components to promote multidrug resistance in S. maltophilia.
PMCID: PMC90859  PMID: 11709330
3.  The Binding of Triclosan to SmeT, the Repressor of the Multidrug Efflux Pump SmeDEF, Induces Antibiotic Resistance in Stenotrophomonas maltophilia 
PLoS Pathogens  2011;7(6):e1002103.
The wide utilization of biocides poses a concern on the impact of these compounds on natural bacterial populations. Furthermore, it has been demonstrated that biocides can select, at least in laboratory experiments, antibiotic resistant bacteria. This situation has raised concerns, not just on scientists and clinicians, but also on regulatory agencies, which are demanding studies on the impact that the utilization of biocides may have on the development on resistance and consequently on the treatment of infectious diseases and on human health. In the present article, we explored the possibility that the widely used biocide triclosan might induce antibiotic resistance using as a model the opportunistic pathogen Stenotrophomonas maltophilia. Biochemical, functional and structural studies were performed, focusing on SmeDEF, the most relevant antibiotic- and triclosan-removing multidrug efflux pump of S. maltophilia. Expression of smeDEF is regulated by the repressor SmeT. Triclosan released SmeT from its operator and induces the expression of smeDEF, thus reducing the susceptibility of S. maltophilia to antibiotics in the presence of the biocide. The structure of SmeT bound to triclosan is described. Two molecules of triclosan were found to bind to one subunit of the SmeT homodimer. The binding of the biocide stabilizes the N terminal domain of both subunits in a conformation unable to bind DNA. To our knowledge this is the first crystal structure obtained for a transcriptional regulator bound to triclosan. This work provides the molecular basis for understanding the mechanisms allowing the induction of phenotypic resistance to antibiotics by triclosan.
Author Summary
The wide utilization of biocides for different purposes, including toothpastes, soaps, house-hold compounds surfaces' disinfectants and even their use as additives of different materials (from textiles to concrete used in germ-free buildings) to avoid their colonization by microorganisms, poses a concern on the impact of these compounds on natural bacterial populations. Furthermore, it has been demonstrated that such biocides can select, at least in laboratory experiments, bacteria resistant to antibiotics. This situation has raised concerns on the impact that the utilization of biocides may have on the development on resistance and consequently on the treatment of infectious diseases. In the present article we study whether biocides can induce phenotypic resistance to antibiotics, a process that would be barely detectable unless purposely searched out. In the article, we present functional, biochemical and structural data showing that the widely used biocide triclosan induces antibiotic resistance, mediated by the binding of the biocide to SmeT, the transcriptional regulator of the expression of the Stenotrophomonas maltophilia multidrug efflux pump SmeDEF, which can extrude an ample range of antibiotics. Our study provides an unambiguous link between the presence of this biocide and the increased efflux of antibiotics by the opportunistic pathogen S. maltophilia.
PMCID: PMC3128119  PMID: 21738470
4.  Expression of Multidrug Efflux Pump SmeDEF by Clinical Isolates of Stenotrophomonas maltophilia 
The presence of the multidrug efflux pump SmeDEF was assessed in a collection of clinical isolates of Stenotrophomonas maltophilia. All isolates encoded this pump, as demonstrated by PCR. Forty-seven percent of the strains overproduced a protein of the same size that was immunoreactive against an anti-SmeF antibody, and 33% overexpressed the gene semD when they were tested by reverse transcription-PCR. A correlation between smeDEF overexpression and antibiotic resistance was observed.
PMCID: PMC90562  PMID: 11353642
5.  Expression of Sme Efflux Pumps and Multilocus Sequence Typing in Clinical Isolates of Stenotrophomonas maltophilia 
Annals of Laboratory Medicine  2011;32(1):38-43.
Stenotrophomonas maltophilia has emerged as an important opportunistic pathogen, which causes infections that are often difficult to manage because of the inherent resistance of the pathogen to a variety of antimicrobial agents. In this study, we analyzed the expressions of smeABC and smeDEF and their correlation with antimicrobial susceptibility. We also evaluated the genetic relatedness and epidemiological links among 33 isolates of S. maltophilia.
In total, 33 S. maltophilia strains were isolated from patients in a tertiary hospital in Daejeon. Minimum inhibitory concentrations (MICs) of 11 antimicrobial agents were determined by using agar dilution method and E-test (BioMérieux, France). Real-time PCR analysis was performed to evaluate the expression of the Sme efflux systems in the S. maltophilia isolates. Additionally, an epidemiological investigation was performed using multilocus sequence typing (MLST) assays.
The findings of susceptibility testing showed that the majority of the S. maltophilia isolates were resistant to β-lactams and aminoglycosides. Twenty-one clinical isolates overexpressed smeABC and showed high resistance to ciprofloxacin. Moreover, a high degree of genetic diversity was observed among the S. maltophilia isolates; 3 sequence types (STs) and 23 allelic profiles were observed.
The smeABC efflux pump was associated with multidrug resistance in clinical isolates of S. maltophilia. In particular, smeABC efflux pumps appear to perform an important role in ciprofloxacin resistance of S. maltophilia. The MLST scheme for S. maltophilia represents a discriminatory typing method with stable markers and is appropriate for studying population structures.
PMCID: PMC3255492  PMID: 22259777
Stenotrophomonas maltophilia; smeABC; smeDEF; Efflux pump; MLST
6.  The Inactivation of Intrinsic Antibiotic Resistance Determinants Widens the Mutant Selection Window for Quinolones in Stenotrophomonas maltophilia 
Antimicrobial Agents and Chemotherapy  2012;56(12):6397-6399.
We have determined that the mutational inactivation of the SmeDEF efflux pump and the SmQnr quinolone resistance protein widens the mutant selection windows for ofloxacin and ciprofloxacin of Stenotrophomonas maltophilia by reducing their MICs. Resistant mutants arising from a strain lacking SmeDEF and SmQnr presented levels of susceptibility similar to those of the wild-type strain. This indicates that inactivation of intrinsic resistance determinants might increase the chances for selecting resistant mutants at low antibiotic concentrations.
PMCID: PMC3497180  PMID: 23006759
7.  Contribution of Resistance-Nodulation-Division Efflux Pump Operon smeU1-V-W-U2-X to Multidrug Resistance of Stenotrophomonas maltophilia ▿ 
Antimicrobial Agents and Chemotherapy  2011;55(12):5826-5833.
KJ09C, a multidrug-resistant mutant of Stenotrophomonas maltophilia KJ, was generated by in vitro selection with chloramphenicol. The multidrug-resistant phenotype of KJ09C was attributed to overexpression of a resistance nodulation division (RND)-type efflux system encoded by an operon consisting of five genes: smeU1, smeV, smeW, smeU2, and smeX. Proteins encoded by smeV, smeW, and smeX were similar to the membrane fusion protein, RND transporter, and outer membrane protein, respectively, of known RND-type systems. The proteins encoded by smeU1 and smeU2 were found to belong to the family of short-chain dehydrogenases/reductases. Mutant KJ09C exhibited increased resistance to chloramphenicol, quinolones, and tetracyclines and susceptibility to aminoglycosides; susceptibility to β-lactams and erythromycin was not affected. The expression of the smeU1-V-W-U2-X operon was regulated by the divergently transcribed LysR-type regulator gene smeRv. Overexpression of the SmeVWX pump contributed to the acquired resistance to chloramphenicol, quinolones, and tetracyclines. Inactivation of smeV and smeW completely abolished the activity of the SmeVWX pump, whereas inactivation of smeX alone decreased the activity of the SmeVWX pump. The enhanced aminoglycoside susceptibility observed in KJ09C resulted from SmeX overexpression.
PMCID: PMC3232770  PMID: 21930878
8.  The Biocide Triclosan Selects Stenotrophomonas maltophilia Mutants That Overproduce the SmeDEF Multidrug Efflux Pump 
The possibility that triclosan selects Stenotrophomonas maltophilia mutants overexpressing the multidrug resistance pump SmeDEF is analyzed. Five out of 12 triclosan-selected mutants were less susceptible to antibiotics than the wild-type strain and overproduced SmeDEF. Results are discussed in relation to current debates on the potential selection of antibiotic-resistant bacteria by household biocides.
PMCID: PMC547238  PMID: 15673767
9.  SmeC, an Outer Membrane Multidrug Efflux Protein of Stenotrophomonas maltophilia 
A homologue of the mexAB-oprM multidrug efflux operon of Pseudomonas aeruginosa, smeABC, was cloned from Stenotrophomonas maltophilia by using, as a probe, a PCR product amplified from this organism with primers based on the mexB sequence. The smeABC genes were hyperexpressed in a mutant strain displaying resistance to several antimicrobials, including aminoglycosides, β-lactams, and fluoroquinolones. Deletions in smeC but not smeB compromised this resistance, suggesting that SmeC contributed to the multidrug resistance of the mutant as part of another, as-yet-unidentified multidrug efflux system. Consistent with SmeC functioning independently of SmeAB, a promoter activity was identified upstream of smeC. Upstream of the smeABC genes, a putative two-gene operon, smeSR, encoding homologues of bacterial two-component regulatory systems was identified. The cloned smeR gene activated expression of a smeA-lacZ fusion, indicating that SmeR positively regulates expression of the smeABC genes. Consistent with this, the multidrug resistance of the SmeABC-hyperexpressing mutant was compromised by deletion of smeR. Intriguingly, SmeC expression in S. maltophilia paralleled a β-lactamase activity provided by a C-terminally truncated L2 enzyme, which was apparently responsible for the β-lactam resistance of the SmeABC-hyperexpressing mutant. This represents the first report of coregulation of an efflux resistance determinant and a β-lactamase.
PMCID: PMC127032  PMID: 11796339
10.  Multiple Antibiotic Resistance in Stenotrophomonas maltophilia: Involvement of a Multidrug Efflux System 
Clinical strains of Stenotrophomonas maltophilia are often highly resistant to multiple antibiotics, although the mechanisms of resistance are generally poorly understood. Multidrug resistant (MDR) strains were readily selected by plating a sensitive reference strain of the organism individually onto a variety of antibiotics, including tetracycline, chloramphenicol, ciprofloxacin, and norfloxacin. Tetracycline-selected MDR strains typically showed cross-resistance to erythromycin and fluoroquinolones and, in some instances, aminoglycosides. MDR mutants selected with the other agents generally displayed resistance to chloramphenicol and fluoroquinolones only, although two MDR strains (e.g., K1385) were also resistant to erythromycin and hypersusceptible to aminoglycosides. Many of the MDR strains expressed either moderate or high levels of a novel outer membrane protein (OMP) of ca. 50 kDa molecular mass, a phenotype typical of MDR strains of Pseudomonas aeruginosa hyperexpressing drug efflux systems. Indeed, the 50-kDa OMP of these S. maltophilia MDR strains reacted with antibody to OprM, the outer membrane component of the MexAB-OprM MDR efflux system of P. aeruginosa. Similarly, a ca. 110-kDa cytoplasmic membrane protein of these MDR strains also reacted with antibody to the MexB component of the P. aeruginosa pump. The outer and cytoplasmic membranes of several clinical S. maltophilia strains also reacted with the anti-OprM and anti-MexB antibodies. N-terminal amino acid sequencing of a cyanogen bromide-generated peptide of the 50-kDa OMP of MDR strain K1385, dubbed SmeM (Stenotrophomonas multidrug efflux), revealed it to be very similar to a number of outer membrane multidrug efflux components of P. aeruginosa and Pseudomonas putida. Deletion of the L1 and L2 β-lactamase genes confirmed that these enzymes were responsible for the bulk of the β-lactam resistance of K1385 and its parent. Still, overexpression of the MDR efflux mechanism in an L1- and L2-deficient derivative of K1385 did yield a modest increase in resistance to a few β-lactams. These data are consistent with the MDR efflux mechanism(s) playing a role in the multidrug resistance of S. maltophilia.
PMCID: PMC89673  PMID: 10639352
11.  Involvement of the SmeAB Multidrug Efflux Pump in Resistance to Plant Antimicrobials and Contribution to Nodulation Competitiveness in Sinorhizobium meliloti▿† 
The contributions of multicomponent-type multidrug efflux pumps to antimicrobial resistance and nodulation ability in Sinorhizobium meliloti were comprehensively analyzed. Computational searches identified genes in the S. meliloti strain 1021 genome encoding 1 pump from the ATP-binding cassette family, 3 pumps from the major facilitator superfamily, and 10 pumps from the resistance-nodulation-cell division family, and subsequently, these genes were deleted either individually or simultaneously. Antimicrobial susceptibility tests demonstrated that deletion of the smeAB pump genes resulted in increased susceptibility to a range of antibiotics, dyes, detergents, and plant-derived compounds and, further, that specific deletion of the smeCD or smeEF genes in a ΔsmeAB background caused a further increase in susceptibility to certain antibiotics. Competitive nodulation experiments revealed that the smeAB mutant was defective in competing with the wild-type strain for nodulation. The introduction of a plasmid carrying smeAB into the smeAB mutant restored antimicrobial resistance and nodulation competitiveness. These findings suggest that the SmeAB pump, which is a major multidrug efflux system of S. meliloti, plays an important role in nodulation competitiveness by mediating resistance toward antimicrobial compounds produced by the host plant.
PMCID: PMC3126421  PMID: 21398477
12.  nalD Encodes a Second Repressor of the mexAB-oprM Multidrug Efflux Operon of Pseudomonas aeruginosa▿  
Journal of Bacteriology  2006;188(24):8649-8654.
The Pseudomonas aeruginosa nalD gene encodes a TetR family repressor with homology to the SmeT and TtgR repressors of the smeDEF and ttgABC multidrug efflux systems of Stenotrophomonas maltophilia and Pseudomonas putida, respectively. A sequence upstream of mexAB-oprM and overlapping a second promoter for this efflux system was very similar to the SmeT and TtgR operator sequences, and NalD binding to this region was, in fact, demonstrated. Moreover, increased expression from this promoter was seen in a nalD mutant, consistent with NalD directly controlling mexAB-oprM expression from a second promoter.
PMCID: PMC1698243  PMID: 17028276
13.  Regulatory Regions of smeDEF in Stenotrophomonas maltophilia Strains Expressing Different Amounts of the Multidrug Efflux Pump SmeDEF 
The smeT-smeDEF region and the smeT gene, which encodes the smeDEF repressor, are highly polymorphic. Few changes in smeT might be associated with smeDEF overexpression. The results obtained with cellular extracts suggest that mutant SmeT proteins cannot bind to the operator and that other transcription factors besides SmeT are involved in the regulation of smeDEF expression.
PMCID: PMC415575  PMID: 15155232
14.  Coordinate Hyperproduction of SmeZ and SmeJK Efflux Pumps Extends Drug Resistance in Stenotrophomonas maltophilia 
A Stenotrophomonas maltophilia mutant that coordinately hyper-expresses three resistance nodulation division-type efflux pump genes, smeZ, smeJ, and smeK, has been identified. SmeZ is responsible for elevating aminoglycoside MICs; SmeJ and SmeK are jointly responsible for elevating tetracycline, minocycline, and ciprofloxacin MICs and conferring levofloxacin resistance. One clinical isolate with this same phenotype was identified from a sample of six, and the isolate also coordinately hyper-expresses smeZ and smeJK, confirming the clinical relevance of our findings.
PMCID: PMC3535947  PMID: 23147729
15.  Expression of Pseudomonas aeruginosa Multidrug Efflux Pumps MexA-MexB-OprM and MexC-MexD-OprJ in a Multidrug-Sensitive Escherichia coli Strain 
The mexCD-oprJ and mexAB-oprM operons encode components of two distinct multidrug efflux pumps in Pseudomonas aeruginosa. To assess the contribution of individual components to antibiotic resistance and substrate specificity, these operons and their component genes were cloned and expressed in Escherichia coli. Western immunoblotting confirmed expression of the P. aeruginosa efflux pump components in E. coli strains expressing and deficient in the endogenous multidrug efflux system (AcrAB), although only the ΔacrAB strain, KZM120, demonstrated increased resistance to antibiotics in the presence of the P. aeruginosa efflux genes. E. coli KZM120 expressing MexAB-OprM showed increased resistance to quinolones, chloramphenicol, erythromycin, azithromycin, sodium dodecyl sulfate (SDS), crystal violet, novobiocin, and, significantly, several β-lactams, which is reminiscent of the operation of this pump in P. aeruginosa. This confirmed previous suggestions that MexAB-OprM provides a direct contribution to β-lactam resistance via the efflux of this group of antibiotics. An increase in antibiotic resistance, however, was not observed when MexAB or OprM alone was expressed in KZM120. Thus, despite the fact that β-lactams act within the periplasm, OprM alone is insufficient to provide resistance to these agents. E. coli KZM120 expressing MexCD-OprJ also showed increased resistance to quinolones, chloramphenicol, macrolides, SDS, and crystal violet, though not to most β-lactams or novobiocin, again somewhat reminiscent of the antibiotic resistance profile of MexCD-OprJ-expressing strains of P. aeruginosa. Surprisingly, E. coli KZM120 expressing MexCD alone also showed an increase in resistance to these agents, while an OprJ-expressing KZM120 failed to demonstrate any increase in antibiotic resistance. MexCD-mediated resistance, however, was absent in a tolC mutant of KZM120, indicating that MexCD functions in KZM120 in conjunction with TolC, the previously identified outer membrane component of the AcrAB-TolC efflux system. These data confirm that a tripartite efflux pump is necessary for the efflux of all substrate antibiotics and that the P. aeruginosa multidrug efflux pumps are functional and retain their substrate specificity in E. coli.
PMCID: PMC105457  PMID: 9449262
16.  A MATE Family Multidrug Efflux Transporter Pumps out Fluoroquinolones in Bacteroides thetaiotaomicron 
Antimicrobial Agents and Chemotherapy  2001;45(12):3341-3346.
We cloned a gene, bexA, that codes for a multidrug efflux transporter from the chromosomal DNA of Bacteroides thetaiotaomicron ATCC 29741 by using an Escherichia coli ΔacrAB ΔacrEF mutant as a host. Although the initial recombinant construct contained other open reading frames, the presence of bexA alone was sufficient to confer to the E. coli host elevated levels of resistance to norfloxacin, ciprofloxacin, and ethidium bromide. Disruption of bexA in B. thetaiotaomicron made the strain more susceptible to norfloxacin, ciprofloxacin, and ethidium bromide, showing that this gene is expressed in this organism and functions as a multidrug efflux pump. The deduced BexA protein sequence was homologous to the protein sequence of Vibrio parahaemolyticus NorM, a multidrug efflux transporter, and thus, BexA belongs to the multidrug and toxic compound extrusion (MATE) family.
PMCID: PMC90835  PMID: 11709306
17.  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 
Journal of Bacteriology  2002;184(15):4168-4176.
Screening of random fragments of Escherichia coli genomic DNA for their ability to increase the novobiocin resistance of a hypersusceptible ΔacrAB 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 ΔacrAB 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.
PMCID: PMC135214  PMID: 12107134
18.  Comparison of the postantibiotic and postantibiotic sub-MIC effects of levofloxacin and ciprofloxacin on Staphylococcus aureus and Streptococcus pneumoniae. 
The postantibiotic subminimum inhibitory concentration effect (PA SME) may simulate in vivo drug exposure more accurately than the postantibiotic effect (PAE) since subinhibitory concentrations of drug persist between antibiotic dosings. In this study, we compared the PAEs and PA SMEs of levofloxacin and ciprofloxacin for clinical isolates of fluoroquinolone-susceptible Staphylococcus aureus and Streptococcus pneumoniae. At two times the MIC, PAEs of levofloxacin were an average of 0.6 h longer than the PAEs obtained for ciprofloxacin for methicillin-susceptible and methicillin-resistant S. aureus strains. The PAEs of levofloxacin and ciprofloxacin ranged from 1.8 to 3.1 and 1.1 to 2.4 h, respectively. Continued exposure of the methicillin-resistant strain to 1/16, 1/8, and 1/4 the MIC resulted in PA SMEs of 6.5, 15.3, and >22.3 h, respectively, for levofloxacin and 3.8, 8.0, and 12.3 h, respectively, for ciprofloxacin. For isolates of S. pneumoniae, at two times the MIC of both fluoroquinolones, the average PAEs of levofloxacin and ciprofloxacin were equivalent: 1.3 h for the penicillin-susceptible isolate and 0.6 h for the penicillin-resistant isolate. Continued exposure of the penicillin-susceptible S. pneumoniae strain to 1/16, 1/8, and 1/4 the MIC resulted in average PA SMEs of 1.0, 1.4, and 2.8 h, respectively, for levofloxacin and 1.8, 2.0, and 2.5 h, respectively, for ciprofloxacin. Continued exposure of penicillin-resistant S. pneumoniae to 1/16, 1/8, and 1/4 the MIC of the same fluoroquinolones resulted in average PA SMEs of 0.6, 1.1, and 2.9 h, respectively, for levofloxacin and 0.6, 1.1, and 1.5 h, respectively, for ciprofloxacin. The PA SMEs observed demonstrate the superior activity of levofloxacin against methicillin-susceptible or methicillin-resistant S. aureus. Although PAEs were similar for the penicillin-susceptible and penicillin-resistant S. pneumoniae strains, the PA SME of levofloxacin at one-fourth the MIC was longer for penicillin-resistant S. pneumoniae.
PMCID: PMC163831  PMID: 9145850
19.  Role of Histone-Like Protein H-NS in Multidrug Resistance of Escherichia coli 
Journal of Bacteriology  2004;186(5):1423-1429.
The histone-like protein H-NS is a major component of the bacterial nucleoid and plays a crucial role in global gene regulation of enteric bacteria. It is known that the expression of a variety of genes is repressed by H-NS, and mutations in hns result in various phenotypes, but the role of H-NS in the drug resistance of Escherichia coli has not been known. Here we present data showing that H-NS contributes to multidrug resistance by regulating the expression of multidrug exporter genes. Deletion of the hns gene from the ΔacrAB mutant increased levels of resistance against antibiotics, antiseptics, dyes, and detergents. Decreased accumulation of ethidium bromide and rhodamine 6G in the hns mutant compared to that in the parental strain was observed, suggesting the increased expression of some drug exporter(s) in this mutant. The increased drug resistance and decreased drug accumulation caused by the hns deletion were completely suppressed by deletion of the multifunctional outer membrane channel gene tolC. At least eight drug exporter systems require TolC for their functions. Among these, increased expression of acrEF, mdtEF, and emrKY was observed in the Δhns strain by quantitative real-time reverse transcription-PCR analysis. The Δhns-mediated multidrug resistance pattern is quite similar to that caused by overproduction of the AcrEF exporter. Deletion of the acrEF gene greatly suppressed the level of Δhns-mediated multidrug resistance. However, this strain still retained resistance to some compounds. The remainder of the multidrug resistance pattern was similar to that conferred by overproduction of the MdtEF exporter. Double deletion of the mdtEF and acrEF genes completely suppressed Δhns-mediated multidrug resistance, indicating that Δhns-mediated multidrug resistance is due to derepression of the acrEF and mdtEF drug exporter genes.
PMCID: PMC344412  PMID: 14973023
20.  Postantibiotic effect of sanfetrinem compared with those of six other agents against 12 penicillin-susceptible and -resistant pneumococci. 
Antimicrobial Agents and Chemotherapy  1997;41(10):2173-2176.
The postantibiotic effect (PAE) and postantibiotic sub-MIC effect (PAE-SME) of sanfetrinem were compared to those of penicillin G, amoxicillin, cefpodoxime, ceftriaxone, imipenem, and clarithromycin against four penicillin-susceptible, four intermediately susceptible, and four resistant pneumococci. The MICs of imipenem were the lowest against all of the strains (0.03 to 0.5 microg/ml), followed by those of sanfetrinem (0.016 to 1.0 microg/ml), amoxicillin and ceftriaxone (0.016 to 2.0 microg/ml), and cefpodoxime (0.03 to 8.0 microg/ml). High-level resistance to clarithromycin (MIC, >64.0 microg/ml) was seen in three selected strains. The PAEs of all of the oral beta-lactams tested were similar for all of the strains, ranging from 1 to 6.5 h. The PAEs of ceftriaxone and imipenem ranged from 1 to 8 h, and those of clarithromycin ranged from 1 to 7 h. The mean PAEs of all of the beta-lactams and clarithromycin were 2.8 to 4.3 and 2.5 h, respectively. PAE-SMEs could not be determined for all of the strains due to complete killing, especially at high subinhibitory concentrations. However, the overall pattern with all of the compounds tested was that PAE-SMEs were longer than PAEs. Measurable PAE-SMEs of sanfetrinem at the three subinhibitory concentrations (0.125, 0.25, and 0.5 times the MIC) were 2 to 7, 2 to 7, and 3 to 6 h, while those of amoxicillin and cefpodoxime were 1 to 7.5, 2 to 4, and 4 to 9 and 2 to 7, 4 to 7, and 4 to 6 h, respectively. Measurable PAE-SMEs of ceftriaxone and imipenem were 1 to 6.5, 2 to 9, and 2 to 9 and 1.5 to 6, 2 to 5.8, and 4 to 7.7 h, respectively. Measurable clarithromycin PAE-SMEs were 1 to 5, 1 to 5, and 1 to 6 h at the three concentrations.
PMCID: PMC164088  PMID: 9333043
21.  SME-Type Carbapenem-Hydrolyzing Class A β-Lactamases from Geographically Diverse Serratia marcescens Strains 
Antimicrobial Agents and Chemotherapy  2000;44(11):3035-3039.
Three sets of carbapenem-resistant Serratia marcescens isolates have been identified in the United States: 1 isolate in Minnesota in 1985 (before approval of carbapenems for clinical use), 5 isolates in Los Angeles (University of California at Los Angeles [UCLA]) in 1992, and 19 isolates in Boston from 1994 to 1999. All isolates tested produced two β-lactamases, an AmpC-type enzyme with pI values of 8.6 to 9.0 and one with a pI value of approximately 9.5. The enzyme with the higher pI in each strain hydrolyzed carbapenems and was not inhibited by EDTA, similar to the chromosomal class A SME-1 β-lactamase isolated from the 1982 London strain S. marcescens S6. The genes encoding the carbapenem-hydrolyzing enzymes were cloned in Escherichia coli and sequenced. The enzyme from the Minnesota isolate had an amino acid sequence identical to that of SME-1. The isolates from Boston and UCLA produced SME-2, an enzyme with a single amino acid change relative to SME-1, a substitution from valine to glutamine at position 207. Purified SME enzymes from the U.S. isolates had β-lactam hydrolysis profiles similar to that of the London SME-1 enzyme. Pulsed-field gel electrophoresis analysis revealed that the isolates showed some similarity but differed by at least three genetic events. In conclusion, a family of rare class A carbapenem-hydrolyzing β-lactamases first described in London has now been identified in S. marcescens isolates across the United States.
PMCID: PMC101599  PMID: 11036019
22.  In Vitro Evaluation of CBR-2092, a Novel Rifamycin-Quinolone Hybrid Antibiotic: Microbiology Profiling Studies with Staphylococci and Streptococci ▿  
We present data from antimicrobial assays performed in vitro that pertain to the potential clinical utility of a novel rifamycin-quinolone hybrid antibiotic, CBR-2092, for the treatment of infections mediated by gram-positive cocci. The MIC90s for CBR-2092 against 300 clinical isolates of staphylococci and streptococci ranged from 0.008 to 0.5 μg/ml. Against Staphylococcus aureus, CBR-2092 exhibited prolonged postantibiotic effects (PAEs) and sub-MIC effects (SMEs), with values of 3.2, 6.5, and >8.5 h determined for the PAE (3× MIC), SME (0.12× MIC), and PAE-SME (3× MIC/0.12× MIC) periods, respectively. Studies of genetically defined mutants of S. aureus indicate that CBR-2092 is not a substrate for the NorA or MepA efflux pumps. In minimal bactericidal concentration and time-kill studies, CBR-2092 exhibited bactericidal activity against staphylococci that was retained against rifampin- or intermediate quinolone-resistant strains, with apparent paradoxical cidal characteristics against rifampin-resistant strains. In spontaneous resistance studies, CBR-2092 exhibited activity consistent with balanced contributions from its composite pharmacophores, with a mutant prevention concentration of 0.12 μg/ml and a resistance frequency of <10−12 determined at 1 μg/ml in agar for S. aureus. Similarly, CBR-2092 suppressed the emergence of preexisting rifamycin resistance in time-kill studies undertaken at a high cell density. In studies of the intracellular killing of S. aureus, CBR-2092 exhibited prolonged bactericidal activity that was superior to the activities of moxifloxacin, rifampin, and a cocktail of moxifloxacin and rifampin. Overall, CBR-2092 exhibited promising activity in a range of antimicrobial assays performed in vitro that pertain to properties relevant to the effective treatment of serious infections mediated by gram-positive cocci.
PMCID: PMC2443883  PMID: 18443106
23.  Entry into and Release of Solvents by Escherichia coli in an Organic-Aqueous Two-Liquid-Phase System and Substrate Specificity of the AcrAB-TolC Solvent-Extruding Pump 
Journal of Bacteriology  2000;182(17):4803-4810.
Growth of Escherichia coli is inhibited upon exposure to a large volume of a harmful solvent, and there is an inverse correlation between the degree of inhibition and the log POW of the solvent, where POW is the partition coefficient measured for the partition equilibrium established between the n-octanol and water phases. The AcrAB-TolC efflux pump system is involved in maintaining intrinsic solvent resistance. We inspected the solvent resistance of ΔacrAB and/or ΔtolC mutants in the presence of a large volume of solvent. Both mutants were hypersensitive to weakly harmful solvents, such as nonane (log POW = 5.5). The ΔtolC mutant was more sensitive to nonane than the ΔacrAB mutant. The solvent entered the E. coli cells rapidly. Entry of solvents with a log POW higher than 4.4 was retarded in the parent cells, and the intracellular levels of these solvents were maintained at low levels. The ΔtolC mutant accumulated n-nonane or decane (log POW = 6.0) more abundantly than the parent or the ΔacrAB mutant. The AcrAB-TolC complex likely extrudes solvents with a log POW in the range of 3.4 to 6.0 through a first-order reaction. The most favorable substrates for the efflux system were considered to be octane, heptane, and n-hexane.
PMCID: PMC111357  PMID: 10940021
24.  Postantibiotic sub-MIC effects of vancomycin, roxithromycin, sparfloxacin, and amikacin. 
The sub-MIC effects (SMEs) and the postantibiotic sub-MIC effects (PA SMEs) of vancomycin, roxithromycin, and sparfloxacin for Streptococcus pyogenes and Streptococcus pneumoniae and of amikacin for Escherichia coli and Pseudomonas aeruginosa were investigated. A postantibiotic effect was induced by exposing strains to 10x the MIC of the antibiotic for 2 h in vitro. After the induction, the exposed cultures were washed to eliminate the antibiotics. Unexposed controls were treated similarly. Thereafter, the exposed cultures (PA SME) and the controls (SME) were exposed to different subinhibitory concentrations (0.1, 0.2, and 0.3x the MIC) of the same drug and growth curves for a period of 24 h were compared. In general, the PA SMEs were much more pronounced than the SMEs. However, for amikacin and E. coli the SME of 0.2 and 0.3x the MIC also had an initial bactericidal effect. The longest PA SMEs were demonstrated for the combinations with the most pronounced killing during the induction and for the combinations which exhibited the longest PAEs.
PMCID: PMC192199  PMID: 1329631
25.  Postantibiotic and Sub-MIC Effects of Azithromycin and Isepamicin against Staphylococcus aureus and Escherichia coli 
Investigations of pharmacodynamic parameters (postantibiotic effect [PAE], sub-MIC effects [SMEs], etc.) have been progressively employed for the design of dosing schedules of antimicrobial agents. However, there are fewer in vivo than in vitro data, probably because of the simplicity of the in vitro procedures. In this study, we have investigated the in vitro PAE, SME, and previously treated (postantibiotic [PA]) SME (1/2 MIC, 1/4 MIC and 1/8 MIC) of azithromycin and isepamicin against standard strains of Staphylococcus aureus and Escherichia coli by using centrifugation to remove the antibiotics. In addition, the in vivo PAE and SME have been studied with the thigh infection model in neutropenic mice. Finally, in vivo killing curves with two dosing schedules were determined to examine whether the PAE can cover the time that antimicrobial agents are below the MIC. The two antimicrobial agents induced moderate-to-high in vitro PAEs, SMEs, and PA SMEs against S. aureus (>8 h) and E. coli (3.38 to >7.64 h). The in vivo PAEs were also high (from 3.0 to 3.6 h), despite the fact that isepamicin had lower times above the MIC in serum. Only azithromycin showed a high in vivo SME against the two strains (1.22 and 1.75 h), which indicated that the in vivo PAEs were possibly overestimated. In the killing kinetics, no great differences (<0.5 log10) were observed between the schedule that took the PAE into account and the continuous administration of doses. These results are comparable with those of other authors and suggest that these antimicrobial agents could be administered at longer intervals without losing effectiveness.
PMCID: PMC105424  PMID: 9527796

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