Skip Navigation


JAC Advance Access originally published online on February 21, 2006
Journal of Antimicrobial Chemotherapy 2006 57(4):673-679; doi:10.1093/jac/dkl025
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
57/4/673    most recent
dkl025v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrow Search for citing articles in:
ISI Web of Science (2)
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Webber, M.
Right arrow Articles by Piddock, L. J. V.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Webber, M.
Right arrow Articles by Piddock, L. J. V.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© The Author 2006. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Overexpression of marA, soxS and acrB in veterinary isolates of Salmonella enterica rarely correlates with cyclohexane tolerance

Mark Webber1, Anthony M. Buckley1, Luke P. Randall2, Martin J. Woodward2 and Laura J. V. Piddock1,*

1 Antimicrobial Agents Research Group, Division of Immunity and Infection, University of Birmingham, Birmingham, B15 2TT, UK; 2 Veterinary Laboratories Agency (Weybridge), New Haw, Addlestone, Surrey, KT15 3NB, UK

* Corresponding author. Tel: +44-121-414-6966; Fax: +44-121-414-6815; E-mail: l.j.v.piddock{at}bham.ac.uk

Received 7 December 2005; returned 12 January 2006; revised 13 January 2006; accepted 19 January 2006


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Transparency Declarations
 References
 
Objectives: To determine the contribution of the AcrAB efflux system to cyclohexane tolerance in Salmonella enterica.

Methods: The expression of the efflux pump gene, acrB, and regulators marA and soxS from 46 isolates of S. enterica of 14 different serovars was determined by comparative RT–PCR and denaturing HPLC analysis.

Results: Twenty-one of the 46 isolates were cyclohexane tolerant, a phenotype associated with multiple antibiotic resistance (MAR) and overexpression of efflux pumps. Of the cyclohexane-tolerant isolates 81% were MAR, whereas only 44% of the cyclohexane-susceptible isolates were MAR, confirming the association between cyclohexane tolerance and MAR. However, there was no correlation between cyclohexane tolerance or MAR and overexpression of acrB, soxS or marA.

Conclusions: These data suggest that cyclohexane tolerance in S. enterica can be mediated by an acrB-independent mechanism.

Keywords: multiple antibiotic resistance , efflux , AcrAB , organic solvents


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Transparency Declarations
 References
 
There has been an increase in the number of bacteria being isolated that are resistant to multiple antibiotics. Two distinct mechanisms that contribute to this resistance have been identified. Multiple drug resistance (MDR) can be determined by acquisition of multiple specific antibiotic resistance genes carried on plasmids and integrons.1 Multiple antibiotic resistance (MAR) results from mutations that reduce accumulation of multiple antibiotics by the cell in a non-specific manner.1 One mechanism of resistance, which appears to be particularly important in MAR in Enterobacteriaceae, is the overexpression of efflux pumps. In Escherichia coli and Salmonella enterica serovar Typhimurium, overproduction of the efflux pump AcrAB has been shown to confer MAR and to also confer resistance to several non-antibiotic chemicals, such as dyes, detergents and biocides (disinfectants).2–4 In both E. coli and S. enterica the global regulators MarA and SoxS regulate the acrAB operon by binding to mar/sox boxes found downstream of the local repressor acrR.5–11 Both activators belong to the XylS/AraC family of transcriptional activators, and it has been shown that constitutive overexpression of either can confer clinically significant levels of antibiotic resistance in E. coli mediated by derepression of acrAB.9,11

Organic solvents are toxic to bacteria and accumulate in the cytoplasmic membrane, where they disrupt vital membrane functions and also inhibit membrane protein functions.12 The intrinsic susceptibility to organic solvents differs between different bacterial species. The toxicity of different organic solvents varies and they can be ranked on the basis of the lowest logarithm of the partition coefficient (log Pow) of the given solvent in an equimolar mixture of n-octanol and water.13 Wild-type S. enterica and E. coli can grow in the presence of hexane (log Pow = 3.5) but not in the presence of cyclohexane (log Pow = 3.2); as a result hexane is considered the index solvent for both species.14 An association between MAR and increased organic solvent tolerance (i.e. tolerance to cyclohexane) has been observed for E. coli11 and S. enterica.15 In E. coli acrB overexpression mediated by overproduction of MarA, SoxS and Rob has been shown to result in cyclohexane tolerance.11 Therefore, it has been suggested that overexpression of efflux pumps can result in both MAR and organic solvent tolerance in serovars of S. enterica.15 The aim of the present study was to determine whether there was a correlation between MAR, organic solvent tolerance and expression of efflux-associated genes in S. enterica. Comparative (C) RT–PCR7 was used to determine the expression of efflux-associated genes in 21 cyclohexane-tolerant isolates and 25 cyclohexane-susceptible isolates of S. enterica.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Transparency Declarations
 References
 
Bacterial strains, growth and storage

Forty-six S. enterica were isolated from livestock species in Great Britain by the Veterinary Laboratory Agency (VLA). These isolates were representative of the 14 most commonly isolated serovars in England and Wales, which themselves were representative of Salmonellae isolated from animals during 1997–2000.15 The 46 isolates were chosen on the basis of serovar, ciprofloxacin MIC and mutation in the QRDR of gyrA, gyrB, parC and/or parE. Of the 28 ciprofloxacin-resistant isolates, all carried substitutions within GyrA and 9 also carried substitutions within ParC. Of the 18 isolates with an MIC of ciprofloxacin below 0.25 mg/L, 3 carried substitutions within GyrA, 9 carried substitutions within ParC and no isolates had substitutions within both GyrA and ParC. The most common substitutions identified were at codon 83 of GyrA where serine was replaced with phenylalanine in 14 isolates, tyrosine in 11 isolates and glycine in 1 isolate. Mutations were also identified at codon 87 of GyrA in six isolates (aspartic acid to asparagine or glycine). The only substitution identified within ParC was a threonine to serine change at codon 57. Isolates were serotyped by a microagglutination method and, where appropriate, were phage typed. All isolates were from geographically and temporally distinct samples. Isolates were also analysed by PFGE, which confirmed that no clones were examined. These 46 non-replicate isolates comprised 21 cyclohexane-tolerant isolates and 25 cyclohexane-susceptible isolates (Table 1). Fourteen different serovar wild-type, antibiotic-susceptible control NCTC/ATCC type strains were used throughout this study. In addition, two laboratory-selected cyclohexane-tolerant mutants (Salmonella Typhimurium L115 and L657) were isolated from veterinary isolate L113 and Salmonella Typhimurium SL1344 respectively, following the method of George and Levy.3 Both L113 and SL1344 are fully susceptible to antibiotics including ciprofloxacin. Two further laboratory mutants with the tolC or acrB genes disrupted were also used: L108 (tolC::aph) and L643 (acrB::aph).7 All strains were grown on Iso-Sensitest medium (Unipath) aerobically at 37°C for 18–24 h. All strains were stored on ProtectTM beads (Technical Service Consultants, Lancashire, UK) at –80°C.


View this table:
[in this window]
[in a new window]
  		Table 1.. Strains used in this study

 
Susceptibility of isolates to antibiotics, dyes, detergents and disinfectants

The MIC of each agent was determined using the standard British Society for Antimicrobial Chemotherapy (BSAC) doubling agar dilution.16 MIC breakpoint concentrations used were those recommended by the BSAC, except for ciprofloxacin, where a cut-off value of 0.25 mg/L was used to define resistance as suggested by other workers.17 Isolates that were resistant to three or more antibiotics of separate classes were considered to be MAR. MDR isolates were identified as those that carried three or more specific resistance determinants conferring resistance to antibiotics of different classes. The tolerance of all isolates to hexane and cyclohexane (Sigma) was determined according to the method of Aono et al.14

Presence of specific resistance genes

In order to determine the contribution of specific resistance determinants to the antibiotic resistance, the presence of the aadB, aphAI, aadA1, aadA2, bla(Carb(2)) or pse1, bla(Tem), catA1(AY123253 [GenBank] ), catA2(L06822 [GenBank] ), dfrA1, floR, strA, sul1, sul2, tet(A), tet(B) and tet(G) genes was identified previously.1

Expression of 16S rRNA, gyrB, marA, soxS and acrB

RNA isolation, reverse transcription, multiplex PCR amplification of 16S rRNA, marA, soxS and acrB and PCR product quantification by denaturing HPLC analysis was exactly as described previously.7 16S rRNA and gyrB were included as control genes whose expression and copy number was not expected to differ significantly under the experimental conditions used in this study.18 A comparison of expression of both these genes from all serovar type strains indicated no significant difference in expression of either amongst all the serovar type strains and a very similar pattern of expression for each gene (see the Results section). Values obtained for gyrB were generally lower than those obtained for 16S rRNA and so were less sensitive measures of experimental fluctuations. As a result of these experiments 16S rRNA was considered a suitable candidate to use as a reference gene and was used for the rest of the expression analysis. Subsequent differences in amplification efficiencies amongst cDNA preparations were normalized by referring the peak area of each amplicon resulting from the 16S rRNA amplification to that of each amplicon resulting from the mean value of the control strain 16S rRNA amplifications. To gain an accurate mean value for 16S rRNA levels on which to base the normalization, five independent PCR amplifications from the cDNA of three separate RNA preparations from each control strain were used. The peak areas for marA, soxS and acrB were adjusted as necessary to compensate for variations between the mRNA levels in each RNA preparation.7 Data are presented as means ± standard deviations from at least three independent PCR amplifications. All comparisons between control strains and veterinary isolates were compared by Student's t-test. A P value of <0.05 was considered to indicate a significant change in gene expression. Data are expressed as fold changes in expression relative to the average expression of that gene by all wild-type serovars, i.e. a 1.5-fold increase is equivalent to a 50% increase in transcript level.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Transparency Declarations
 References
 
Susceptibility of isolates to cyclohexane, antibiotics, disinfectants, dyes and detergents

The MICs of nine antibiotics, two dyes and the biocide triclosan were determined for all isolates and control strains. Twenty-nine of the 46 (63%) isolates were resistant to ciprofloxacin. Thirty-one (67%) of the 46 isolates were resistant to tetracycline and 22 (48%) were resistant to chloramphenicol (Table 2). Fourteen isolates were resistant to ampicillin, whereas only six isolates were resistant to trimethoprim (data not shown). No resistance to kanamycin, paraquat or deoxycholate was detected. The MICs of acriflavine, ethidium bromide and triclosan for all 46 isolates were similar to those for the control strains (Table 2). Twenty-eight of the 46 isolates (61%) were resistant to three or more antibiotics (10 MDR, 18 MAR). All the isolates were tolerant to hexane and 21 (46%) of the isolates were tolerant to cyclohexane (Table 3). Of these 21 isolates, 13 (62%) were resistant to ciprofloxacin and 17 out of 21 (81%) were resistant to three or more antibiotics (13 MAR, 4 MDR). Of the 25 cyclohexane-susceptible isolates, 16 (64%) were resistant to ciprofloxacin and 11 (44%) isolates were resistant to three or more antibiotics (3 MAR, 8 MDR). The MICs of ciprofloxacin, chloramphenicol and tetracycline for the laboratory-selected cyclohexane-tolerant mutants (L115 and L657) were 2- to 4-fold higher than those for L113 or SL1344. Two laboratory mutants of SL1344 with disruption of either acrB or tolC were unable to grow in the presence of hexane, indicating a decrease in organic solvent tolerance in the absence of these genes.


View this table:
[in this window]
[in a new window]
  		Table 2.. Susceptibility of control strains and isolates to antibiotics, dyes and disinfectants

 

View this table:
[in this window]
[in a new window]
  		Table 3.. Summary of antibiotic-resistant and cyclohexane-tolerant isolates

 
Prevalence of resistance genes

Specific resistance genes were detected previously amongst both cyclohexane-tolerant and cyclohexane-susceptible isolates.1 Four of 21 (19%) cyclohexane-tolerant isolates carried resistance genes and 8 of 25 (32%) cyclohexane-susceptible isolates carried resistance genes. Of the 17 cyclohexane-tolerant isolates resistant to three or more antibiotics, 4 (24%) carried specific resistance genes, and 8 of 11 (73%) cyclohexane-susceptible isolates resistant to three or more antibiotics carried specific resistance genes. The most commonly detected genes were aadA2, floR and sul1, each found in six isolates. Many isolates carried a combination of resistance genes; the most common pattern of carriage was four genes seen in four isolates (the resistance genes varied between isolates).

Gene expression amongst serovar control strains

The expression of 16S rRNA and gyrB was determined for all serovar control strains. Figure 1 shows the mean standard deviation from the average expression for both genes. The pattern of expression of both 16S rRNA and gyrB was similar and there was little variation between both genes. For expression data from veterinary isolates 16S rRNA was used as a control to normalize variations in expression. The average level of expression of marA, soxS and acrB from all serovar controls was calculated and used as a reference for comparison of data from veterinary isolates (after normalization) in the absence of isogenic controls.


Figure 1
View larger version (21K):
[in this window]
[in a new window]
  		Figure 1.. Comparison of expression of 16S rRNA and gyrB amongst all serovar control strains. White bars, gyrB; black bars, 16S rRNA.

 
Expression of acrB, marA and soxS in veterinary isolates

The mean peak area from WaveTM DHPLC analysis for 16S rRNA for the 14 control serovars was 176185 ± 19380 (100 ± 11%). The mean peak areas of marA, soxS and acrB for all the control serovars was 129 737 ± 18 163 (100 ± 14%), 48 273 ± 5793 (100 ± 12%) and 226 241 ± 20 362 (100 ± 9%), respectively. There were no significant differences in the mean peak areas for marA, soxS or acrB for all the control strains except for Salmonella Livingstone, which had significantly higher levels of soxS. To determine whether this isolate of Salmonella Livingstone was typical for this serovar, expression in a second wild-type strain, Salmonella Livingstone NCTC 9125, was determined. This strain also showed significantly higher levels of soxS; thus the average of the mean peak areas from these two control strains was used for comparison with the isolates of Salmonella Livingstone.

Analysis of the cyclohexane-susceptible, ciprofloxacin-susceptible isolates (n = 9) revealed only one isolate (L649) that significantly overexpressed soxS (1.27-fold) and acrB (1.64-fold) (Figure 2). Two isolates expressed significantly less soxS transcript (0.51- to 0.66-fold) compared with the mean value for the control strains (Figure 2). Two isolates expressed significantly less marA transcript (0.59-fold) and four isolates expressed significantly less acrB (0.59- to 0.78-fold) compared with the mean values for the control strains. The remainder of the isolates showed no significant change in expression for soxS (0.8- to 1.09-fold), marA (0.69–1.08) and acrB (0.7–1.37) compared with the mean values for the control strains.


Figure 2
View larger version (15K):
[in this window]
[in a new window]
  		Figure 2.. Expression of acrB, marA and soxS by cyclohexane-susceptible isolates. Values are the area under the D-HPLC curve ± SD. White bars, acrB; grey bars, marA; black bars, soxS.

 
Three of 16 cyclohexane-susceptible, ciprofloxacin-resistant isolates had significantly increased marA expression (1.22- to 1.44-fold). Four isolates had significantly decreased expression (0.53- to 0.72-fold). The remainder of the isolates (n = 9) showed no significant change in the levels of marA transcript (0.58- to 1.02-fold) compared with the mean value for the control strains. Significant overexpression of soxS was seen in three isolates (1.45- to 1.76-fold). Five isolates showed significantly decreased expression (0.4–0.75) (Figure 2). The remainder of these isolates (n = 8) showed no change in soxS expression (0.89- to 1.26-fold) compared with the mean value for the control strains (Figure 2). acrB expression in four cyclohexane-susceptible, ciprofloxacin-resistant isolates was decreased (0.6- to 0.73-fold), but the remainder of the isolates (n = 12) showed no significant change in expression (0.58- to 0.95-fold).

Two of eight cyclohexane-tolerant, ciprofloxacin-susceptible isolates significantly overexpressed (1.52- to 2.96-fold) marA. The remainder of the isolates (n = 6) showed no change in marA expression (0.64- to 1.29-fold) compared with the mean value for the control strains. soxS expression was significantly increased (1.67-fold) for one isolate (L394). Two isolates expressed significantly less (0.57- to 0.59-fold) soxS transcript (Figure 3). The remainder of the isolates showed no significant change in soxS expression (0.78- to 1.19-fold) compared with the mean value for the control strains (Figure 3). One isolate significantly overexpressed acrB (1.5-fold). The remainder of these isolates showed no significant change in acrB expression (0.76–1.05) compared with the mean value for the control strains.


Figure 3
View larger version (17K):
[in this window]
[in a new window]
  		Figure 3.. Expression of acrB, marA and soxS by cyclohexane-tolerant isolates. Values are the area under the D-HPLC curve ± SD. White bars, acrB; grey bars, marA; black bars, soxS.

 
Analysis of the 13 cyclohexane-tolerant, ciprofloxacin-resistant isolates showed that one isolate significantly overexpressed marA (1.29-fold). The remainder of these isolates showed no significant change in expression (0.77- to 1.17-fold) compared with the mean value for the control strains. soxS expression of these isolates showed that four isolates had significantly increased expression (1.4- to 2.35-fold). One isolate had significantly decreased soxS transcript (0.72-fold), but the remainder (n = 8) of the isolates showed no change in expression (0.72- to 1.34-fold) compared with the mean value for the control strains (Figure 3). No significant acrB overexpression was detected with these isolates; however, three isolates significantly underexpressed acrB (0.57- to 0.75-fold). The remainder showed no change in expression (0.72- to 1.23-fold) compared with the mean value for the control strains.

Two cyclohexane-tolerant Salmonella Typhimurium mutants (L115 and L657) were selected in the laboratory. Expression of soxS and acrB in L115 were significantly decreased (0.77- and 0.85-fold, respectively; P = 0.027/0.047) compared with expression by the parent (L113). However, the level of marA transcript was significantly increased by 1.17-fold (P = 0.025) compared with L113. Expression analysis of the second laboratory-selected cyclohexane-tolerant mutant (L657) revealed a similar pattern of gene expression to L113.


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Transparency Declarations
 References
 
Cyclohexane tolerance has been proposed to be a cheap and quick marker for MAR mediated by overexpression of efflux genes.14,15 Of the 46 strains tested 28 were resistant to three or more antibiotics, the most common phenotype being resistance to a fluoroquinolone, tetracycline and chloramphenicol. Cyclohexane tolerance was seen in 21 out of 46 isolates (46%), of which 17 were resistant to three or more antibiotics (81%; 15 MAR, 2 MDR) and 13 (62%) were ciprofloxacin resistant. All the cyclohexane-tolerant, ciprofloxacin-resistant isolates carried substitutions at codon 83 of GyrA and three also carried a substitution at codon 57 of ParC. These data confirm the association between cyclohexane tolerance and resistance to multiple antibiotics. However, 19% of cyclohexane-tolerant mutants were not MAR or MDR, indicating a significant level of false-positive results from this test. Of the 25 cyclohexane-susceptible isolates 11 (44%) were resistant to three or more antibiotics, in contrast to the cyclohexane-tolerant isolates. The majority of these isolates were MDR (seven) rather than MAR (five), indicating that the presence of specific resistance genes accounted for most of the isolates that were cyclohexane susceptible and resistant to three or more antibiotics.

Artificial overexpression of marA, soxS and rob on a plasmid has been shown to lead to increased organic solvent tolerance in E. coli as has overexpression of marA in Salmonella Typhimurium.11,15 In this study there was no clear association between cyclohexane tolerance and increased expression of marA, soxS or acrB. No overexpression of acrB was seen in any cyclohexane-tolerant isolate or laboratory mutant, which is counterintuitive as acrB has been shown to mediate intrinsic tolerance to organic solvents in E. coli, and the Salmonella Typhimurium strains lacking acrB or tolC were less tolerant to organic solvents than wild-type strains. However, amongst the cyclohexane-tolerant, ciprofloxacin-resistant isolates, 8 of 13 strains overexpressed either marA or soxS, although none of these strains overexpressed acrB. It is unclear whether overexpression of marA or soxS in these strains is contributing to their MAR phenotype in an acrB-independent manner.

It would appear that cyclohexane tolerance in Salmonella Typhimurium can be mediated by a mechanism other than overexpression of acrB. There are numerous other efflux pumps in Salmonella, including AcrD and AcrF, that are highly homologous to the AcrAB system. Kobayashi et al.19 showed overexpression of acrE and acrF were able to restore organic solvent tolerance in an acrB-inactivated E. coli mutant. However, Eaves et al.7 failed to detect an association between expression of acrF or acrD and MAR or cyclohexane tolerance in S. enterica. Alternatively, the MarA homologue RamA (RmaA) has been shown to be able to mediate a MAR phenotype in a range of Enterobacteriaceae, including S. enterica.20 It may be that this gene or another regulator is involved in mediating cyclohexane tolerance in an acrB-independent manner.

In conclusion, whilst several publications on E. coli show that overexpression of marA, soxS, rob and/or the efflux pump AcrAB-TolC system are associated with organic solvent tolerance, this study shows that the same is not true for isolates of various serovars of S. enterica. The effector(s) of cyclohexane tolerance in S. enterica remain unknown; further experiments are in progress to identify the alternative effectors of cyclohexane tolerance in S. enterica.


   Transparency Declarations
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Transparency Declarations
References
 
None to declare.


   Acknowledgements
 
This work was supported in part by a Bristol-Myers Squibb Unrestricted Grant in Infectious Diseases to L.J.V.P.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Transparency Declarations
 References
 
1. Randall LP, Cooles SW, Osborn MK et al. Antibiotic resistance genes, integrons and multiple antibiotic resistance in thirty-five serotypes of Salmonella enterica isolated from humans and animals in the UK. J Antimicrob Chemother 2004; 53: 208–16.[Abstract/Free Full Text]

2. Baucheron S, Tyler S, Boyd D et al. AcrAB-TolC directs efflux-mediated multidrug resistance in Salmonella enterica serovar Typhimurium phage type DT104. Antimicrob Agents Chemother 2004; 48: 3729–35.[Abstract/Free Full Text]

3. George A, Levy SB. Amplifiable resistance to tetracycline, chloramphenicol and other antibiotics in Escherichia coli: involvement of a non-plasmid-determined efflux of tetracycline. J Bacteriol 1983; 155: 531–40.[Abstract/Free Full Text]

4. Lacriox F, Cloeckaert A, Grepinet O et al. Salmonella typhimurium acrB-like gene: identification and role in resistance to biliary salts and detergents and in murine infection. FEMS Microbiol Lett 1996; 135: 161–7.[CrossRef][ISI][Medline]

5. Asako H, Nakajima H, Kobayashi K et al. Organic solvent tolerance and antibiotic resistance increased by overexpression of marA in Escherichia coli. Appl Environ Microbiol 1997; 63: 1428–33.[Abstract]

6. Barbosa T, Levy SB. Differential expression of over 60 chromosomal genes in Escherichia coli by using transposon mutagenesis. J Bacteriol 2000; 182: 3467–74.[Abstract/Free Full Text]

7. Eaves DJ, Ricci V, Piddock LJ. Expression of acrB, acrF, acrD, marA, and soxS in Salmonella enterica serovar Typhimurium: role in multiple antibiotic resistance. Antimicrob Agents Chemother 2004; 48: 1145–50.[Abstract/Free Full Text]

8. Miller P, Gambino L, Sulavik M et al. Genetic relationship between soxRS and marA loci in promoting multiple antibiotic resistance in Escherichia coli. Antimicrob Agents Chemother 1994; 38: 1773–9.[Abstract/Free Full Text]

9. Oethinger M, Podgaljen I, Kern W et al. Overexpression of the marA or soxS regulatory gene in clinical topoisomerase mutants of Escherichia coli. Antimicrob Agents Chemother 1998; 42: 2089–94.[Abstract/Free Full Text]

10. Sulavick M, Dazer M, Miller P. The Salmonella typhimurium mar locus: molecular and genetic analyses and assessment of its role in virulence. J Bacteriol 1997; 179: 1857–66.[Abstract/Free Full Text]

11. White D, Goldman J, Demple B et al. Role of the acrAB locus in organic solvent tolerance mediated by expression of marA, soxS, or robA in Escherichia coli. J Bacteriol 1997; 179: 6122–6.[Abstract/Free Full Text]

12. Sikkema J, de Bont J, Poolman, B. Mechanisms of membrane toxicity of hydrocarbons. Microbiol Rev 1995; 59: 201–22.[Abstract/Free Full Text]

13. Sardessai Y, Bhosle S. Tolerance of bacteria to organic solvents. Res Microbiol 2002; 153: 263–8.[Medline]

14. Aono R, Kobayashi M, Nakajima H et al. A close correlation between improvement of organic-solvent tolerance levels and alteration of resistance towards low-levels of multiple antibiotics in Escherichia coli. Biosci Biotechnol Biochem 1995; 59: 213–8.[Medline]

15. Randall LP, Cooles SW, Sayers A et al. Association between cyclohexane resistance in Salmonella of different serovars and increased resistance to multiple antibiotics, disinfectants and dyes. J Med Microbiol 2001; 50: 919–24.[Abstract/Free Full Text]

16. Andrews J. Determination of minimum inhibitory concentrations. J Antimicrob Chemother 2001; 48 Suppl 1: 5–16.[Abstract]

17. Aarestrup F, Mølbak K, Threlfall J. Is it time to change fluoroquinolone breakpoints for Salmonella spp.? Antimicrob Agents Chemother 2003; 47: 827–9[Free Full Text]

18. Klappenbach JA, Saxman PR, Cole JR et al. rrndb: the Ribosomal RNA Operon Copy Number Database. Nucleic Acids Res 2001; 29: 181–4.[Abstract/Free Full Text]

19. Kobayashi, K, Tsukagoshi N, Aono R. Suppression of hypersensitivity of Escherichia coli acrB mutant to organic solvents by integrational activation of the acrEF operon with the IS1 or IS2 element. J Bacteriol 2001; 183: 2646–53.[Abstract/Free Full Text]

20. Van der Straaten T, Janssen R, Mevius DJ et al. Salmonella gene rma (ramA) and multiple-drug-resistant Salmonella enterica serovar Typhimurium. Antimicrob Agents Chemother 2004; 48: 2292–4.[Abstract/Free Full Text]


Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 Antimicrob. Agents Chemother.Home page
Y. M. Abouzeed, S. Baucheron, and A. Cloeckaert
ramR Mutations Involved in Efflux-Mediated Multidrug Resistance in Salmonella enterica Serovar Typhimurium
Antimicrob. Agents Chemother., July 1, 2008; 52(7): 2428 - 2434.
[Abstract] [Full Text] [PDF]

Home page
 J Antimicrob ChemotherHome page
L. P. Randall, S. W. Cooles, N. G. Coldham, E. G. Penuela, A. C. Mott, M. J. Woodward, L. J. V. Piddock, and M. A. Webber
Commonly used farm disinfectants can select for mutant Salmonella enterica serovar Typhimurium with decreased susceptibility to biocides and antibiotics without compromising virulence
J. Antimicrob. Chemother., December 1, 2007; 60(6): 1273 - 1280.
[Abstract] [Full Text] [PDF]

Home page
 J Antimicrob ChemotherHome page
C. Kehrenberg, A. de Jong, S. Friederichs, A. Cloeckaert, and S. Schwarz
Molecular mechanisms of decreased susceptibility to fluoroquinolones in avian Salmonella serovars and their mutants selected during the determination of mutant prevention concentrations
J. Antimicrob. Chemother., May 1, 2007; 59(5): 886 - 892.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow All Versions of this Article:
57/4/673    most recent
dkl025v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrow Search for citing articles in:
ISI Web of Science (2)
Right arrowRequest Permissions
Right arrow Disclaimer
Google Scholar
Right arrow Articles by Webber, M.
Right arrow Articles by Piddock, L. J. V.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Webber, M.
Right arrow Articles by Piddock, L. J. V.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?