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Antimicrobial Agents and Chemotherapy, May 2006, p. 1615-1622, Vol. 50, No. 5
0066-4804/06/$08.00+0     doi:10.1128/AAC.50.5.1615-1622.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Polyamines Induce Resistance to Cationic Peptide, Aminoglycoside, and Quinolone Antibiotics in Pseudomonas aeruginosa PAO1

Dong H. Kwon and Chung-Dar Lu^*

Department of Biology, Georgia State University, Atlanta, Georgia 30303

Received 14 June 2005/ Returned for modification 1 September 2005/ Accepted 7 February 2006

Top Abstract Introduction Materials and Methods Results Discussion References

 
Pseudomonas aeruginosa, a gram-negative bacterium of human pathogens, is noted for its environmental versatility, enormous metabolic capacity, and resistance to antibiotics. Overexpression of the outer membrane protein OprH and increased resistance to polycationic peptide antibiotics (e.g., polymyxin B) mediated by the PhoPQ two-component system on induction of a putative lipopolysaccharide (LPS) modification operon (PA3552-PA3559) have been reported as part of the adaptive responses to magnesium limitation in P. aeruginosa. Induction of the oprH-phoPQ operon and the LPS modification operon by exogenous spermidine was revealed from GeneChip analysis during studies of polyamine metabolism and was confirmed by the lacZ fusions of affected promoters. From the results of MIC measurements, it was found that addition of spermidine or other polyamines to the growth medium increased the MIC values of multiple antibiotics, including polycationic antibiotics, aminoglycosides, quinolones, and fluorescent dyes. MIC values of these compounds in the transposon insertion mutants of oprH, phoP, phoQ, and pmrB were also determined in the presence and absence of spermidine. The results showed that the spermidine effect on cationic peptide antibiotic and quinolone resistance was diminished in the phoP mutant only. The spermidine effect on antibiotics was not influenced by magnesium concentrations, as demonstrated by MICs and oprH::lacZ fusion studies in the presence of 20 µM or 2 mM magnesium. Furthermore, in spermidine uptake mutants, MICs of cationic peptide antibiotics and fluorescent dyes, but not of aminoglycosides and quinolones, were increased by spermidine. These results suggested the presence of a complicated molecular mechanism for polyamine-mediated resistance to multiple antibiotics in P. aeruginosa.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Pseudomonas aeruginosa is an important gram-negative human pathogenic bacterium that causes nosocomial infections. In particular, the organism is a predominant respiratory pathogen among cystic fibrosis patients producing chronic pulmonary infection and progressive deterioration in lung infection (23, 24, 25).

Currently, antibiotic regimens to treat P. aeruginosa infections include a two-drug combination of ß-lactams and aminoglycosides or, alternatively, quinolones and cationic peptide antibiotics (7, 21). In the latest reports, the effectiveness of the ß-lactam and aminoglycoside combination is 70% to 98% in the United States and the United Kingdom (11). Failure of the treatment is mostly due to intrinsic and acquired resistance to a single and/or multiple antibiotics (11). Unfortunately, the resistance rate is increasing annually as reported by National Surveillance of Antimicrobial Resistance in clinical isolates of P. aeruginosa, in which nationwide (United States) multidrug resistance to ß-lactams, aminoglycosides, and quinolones has gradually increased, with a rate of 4% to 14% from 1993 to 2002 (22).

Antibiotic resistance of P. aeruginosa is caused by environmental factors such as exposure to subinhibitory concentrations of antibiotics or limiting concentrations of divalent cations (4, 11). It has long been reported that limiting concentrations of divalent Mg⁺⁺ or Ca⁺⁺ ions are involved in resistance to cationic peptide antibiotics (e.g., polymyxin B) in Salmonella enterica serovar Typhimurium and P. aeruginosa (4, 15). Molecular mechanisms of this resistance phenomenon in association with lipopolysaccharide (LPS) modification have been studied extensively in S. enterica serovar Typhimurium. The PhoPQ and PmrAB two-component regulatory systems have been reported to play roles in the induction of the LPS modification system pmrHFIJKLM operon, which is required for the biosynthesis of lipid A with 4-aminoarabinose to create a more positively charged LPS (4, 18). Homologues of serovar Typhimurium PhoPQ, PmrAB, and PmrHFIJKLM have also been reported in P. aeruginosa (15, 16).

Other possible environmental factors affecting antibiotic resistance in P. aeruginosa are polyamines. Polyamines (e.g., putrescine, spermidine, spermine, and cadaverine), derived from decarboxylation of arginine, ornithine, and lysine, are polycationic compounds found in all living organisms. Polyamines have also been known as a modulator in gene expression including DNA replication, transcription, translation, and protein activity in Escherichia coli, which directs optimal cell growth and viability as well as defense mechanisms against toxic environmental conditions (2, 3, 8, 26). P. aeruginosa can utilize many polyamines as sole carbon and/or nitrogen sources (13) and possibly as modulators to direct resistance to certain toxic environmental conditions. Although intensive research for polyamines has been done in recent decades, the actions of polyamines at the molecular levels in bacterial cells are largely unknown.

We have studied arginine metabolism and polyamine utilization in P. aeruginosa PAO1 (13) and have worked to understand the physiological roles of polyamines in this organism. In this report, we provide initial evidence from GeneChip experiments for the involvement of polyamines in antibiotic resistance. We substantiated this surprising finding and demonstrated that polyamines can trigger induction of resistance to cationic peptide antibiotics, aminoglycosides, quinolones, and dyes. Resistance to quinolones and cationic peptide antibiotics by exogenous spermidine appeared to be linked to the oprH-phoPQ operon.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Bacterial strains and culture conditions. Bacterial strains used in this study are listed in Table 1. Strains of P. aeruginosa and E. coli were routinely grown on Luria-Bertani (LB) agar plate or in LB broth at 37°C as rich medium. When indicated, P. aeruginosa strains were also grown on minimal medium P [MMP; 30 mM Na₂HPO₄, 14 mM KH₂PO₄, 4 µM FeSO₄, 1 mM MgSO₄, 20 mM (NH₄)₂SO₄] containing 20 mM glutamate or other supplements when indicated as the carbon source (13). For the antibiotic susceptibility testing of P. aeruginosa, MMP or cation-adjusted Mueller-Hinton (Oxoid, Ogdensburg, New York) broth was used. Appropriate concentrations of antibiotics or polyamines were supplemented when needed. All antibiotics and polyamines were purchased from Sigma (Sigma, St. Louis, MO). Solutions of these compounds were prepared by dissolving the compounds in sterile double-distilled water or solvent, as suggested by the manufacturer, and sterilized by filtering through 0.4-µm-pore-size disposable membranes (Millipore, Billerica, Mass.).

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TABLE 1. Bacterial strains and plasmids

RNA isolation, generation of cDNA, hybridization of GeneChip, and data analysis. P. aeruginosa PAO1 grown in MMP containing 20 mM glutamate as a carbon source and supplemented with spermidine or arginine (20 mM concentrations of each) was used to extract total RNA as described previously (12) and further purified by QIAGEN spin columns (Chatsworth, Calif.) after DNase I treatment. Preparation of cDNA, hybridization of GeneChip (Affymetrix, Santa Clara, CA 95051), and data analysis were performed as described previously (14).

Antibiotic susceptibility testing. Antibiotic susceptibility was tested by the standard broth dilution method according to the guidelines of the Clinical and Laboratory Standards Institute (formerly NCCLS) (19, 20). Briefly, each stock solution of antibiotics was added to MMP-glucose broth to achieve serial twofold dilutions between 0.015 and 1,024 µg/ml and dispensed into sterile 17- by 100-mm snap-cap Falcon culture tubes (1 ml/tube; Fisher Scientific). Fresh overnight cultures of P. aeruginosa were diluted in saline to an optical density at 600 nm of 0.09 to 0.1 (approximately 1 x 10⁸ viable cells per ml). A portion of the adjusted cell suspension (2 to 5 µl for 10⁵ cells) was inoculated to each MMP broth sample containing antibiotics as indicated. The cell cultures were then incubated overnight (12 to 14 h) at 37°C. The MIC was defined as the lowest concentration of each antibiotic that completely inhibited the growth of the inoculum. MICs were determined and confirmed by three independent experiments.

Construction of lacZ promoter fusions and ß-galactosidase assays. Putative promoter regions for oprH-phoPQ, PA4773-pmrAB, and an operon for LPS modification (PA3552-PA3559) were amplified using the following pairs of PCR primers: 5'-CAGGCAGATCACGAGAAACAG-3' and 5'-GCCGACGAAGTTGTCGGCGGC-3' (380-bp fragment) for oprH; 5'-GATCGACTACCGCGGGGTGGT-3' and 5'-GCAGTCGAACAGCTCGATGAC-3' (469-bp fragment) for PA4773; and 5'-CGTCTACGGGCGCACCGCAGA-3' and 5'-GGCGGCGATTTCGTCCTCGCC-3' (498-bp fragment) for PA3552. Genomic DNA extracted from P. aeruginosa PAO1 (10) was used as the template for PCR amplification. Each PCR fragment amplified by Pfu DNA polymerase (Stratagene) was purified by QIAGEN spin columns (Chatsworth, Calif.), ligated into a broad-host-range transcriptional fusion vector (pQF50) on the SmaI site, and introduced into E. coli DH5 by transformation. The orientation of each insert in the clone was confirmed by nucleotide sequencing at the Biotechnology Core Facility of Georgia State University. The resulting plasmids were introduced into appropriate P. aeruginosa strains by transformation (10). For measurements of ß-galactosidase activities, cells were grown in MMP with supplements as indicated in the figures until the optical density at 600 nm reached 0.7 to 0.8. Cells were collected by centrifugation (5,000 x g), and cell pellets were suspended in 50 mM potassium phosphate buffer (pH 7.0). After passage through a French pressure cell at 8,000 lb/in², soluble cellular extracts were used for the measurement of ß-galactosidase activities with o-nitrophenyl-ß-D-galactopyranoside as the substrate (17). The protein concentration of each sample was determined by the method of Bradford (1) with bovine serum albumin as the standard.

Top Abstract Introduction Materials and Methods Results Discussion References

 
Induction of the oprH-phoPQ and PA3552-PA3559 operons by exogenous spermidine. During our search for genes involved in polyamine utilization of P. aeruginosa, induction by spermidine of the oprH-phoPQ operon encoding an outer membrane porin and a two-component regulatory system and the PA3552-PA3559 operon for LPS modification was found fortuitously from the GeneChip experiments. The expression profiles of these genes from cells grown in glutamate minimal medium were compared to those from cells grown in the presence of arginine or spermidine (Table 2). For the PA3552-PA3559 operon, exogenous spermidine induced significantly expression levels of all genes in this operon, with the relative increase in induction in the range of 6.2- to 10.3-fold. In comparison, exogenous arginine exhibited only 0.5- to 1.9-fold increase in induction. For the oprH-phoPQ operon, spermidine increased the induction levels of these genes by 16-, 4.5-, and 3.4-fold, and arginine only caused a marginal increase in induction (0.9- to 2.3-fold) of these genes. Since induction of these two operons has been reported as part of the resistance mechanism against cationic peptide antibiotics (e.g., polymyxin B) in P. aeruginosa (15, 16) and enteric bacteria (4, 18), data described above provided the first line of evidence to support the hypothesis that polyamines could be related to antibiotic susceptibility.

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TABLE 2. Gene expression profiles of oprH-phoPQ, PA3552-PA3559, and pmrAB in P. aeruginosa PAO1

The other known transcriptional regulator for resistance to cationic peptide antibiotics is the PmrAB two-component system (16). No significant relative increase in the expression of pmrA and pmrB was found in the presence of arginine or spermidine (Table 2) or for the upstream PA4773-PA4775 genes (data not shown).

Polyamines induced antibiotic resistance. To understand whether polyamines were involved in antibiotic susceptibility in P. aeruginosa, MICs of various antibiotics and dyes were determined and compared in the cells grown with or without polyamines (spermine, spermidine, putrescine, and cadaverine) or polyamine precursors (agmatine, ornithine, arginine, and lysine). As shown in Table 3, MICs of cationic peptide antibiotics (polymyxin B and colistin) increased eightfold in the cells grown with spermidine but were virtually unchanged in the cells grown with other compounds. For aminoglycosides (kanamycin and gentamicin), quinolones (ciprofloxacin and norfloxacin), and dyes (ethidium bromide and acridine orange), MICs increased up to 16-fold in the cells grown with all polyamines and agmatine. In contrast, the MIC of tetracycline was not affected by the presence of polyamines or polyamine precursors.

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TABLE 3. Antibiotic susceptibility of P. aeruginosa PAO1 and mutant strains grown with polyamines or their precursors

In comparison to spermidine, exogenous spermine exhibited a similar effect on the MICs of these antibiotics, but the relative change in MICs was lower with spermine. This result could be due to the difference in polyamine concentrations applied in the study (1 mM spermine versus 20 mM spermidine). We have noticed that precipitates of unknown properties were formed when spermine and glutamate in higher concentrations were mixed in MMP.

To further substantiate the effects of polyamines on antibiotic resistance, the MIC of polymyxin B against PAO1 was determined in the presence of various concentrations of spermine and spermidine in Miller-Minton broth. As shown in Table 4, 0.6 mM spermidine can raise the MIC of polymyxin B by fourfold. Similarly, 0.3 mM spermine also caused a comparable increase in the MIC. A 16-fold increase in the MIC of polymyxin was observed with 20 mM spermidine or spermine.

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TABLE 4. Effect of polyamine concentrations on MIC of polymyxin B against P. aeruginosa PAO1

Analysis of the effects of polyamines on expression of the oprH-phoPQ, PA4773-pmrAB, and PA3552-PA3559 operons. The results of GeneChip experiments (Table 2) revealed that spermidine induced the oprH-phoPQ and PA3552-PA3559 operons but showed no effect on the PA4773-pmrAB operon. To substantiate this finding and to test the possible effects of other polyamines and their precursor compounds on the regulation of these operons, lacZ fusions of the corresponding promoters were constructed as described in Materials and Methods. These fusions were introduced into the wild-type P. aeruginosa PAO1 strain, and the expression profiles of these promoters in response to different compounds were determined by monitoring the activities of ß-galactosidase. T oprH promoter exhibited about 4.5-fold increase in induction in response to spermidine, and similar levels of induction were also observed in response to putrescine, cadaverine, and agmatine (Fig. 1). For The PA3552 promoter showed a fourfold increase in induction in the presence of spermidine, while no significant effect was detected in the presence of putrescine, cadaverine, and agmatine (Fig. 1). In contrast, PA4773 promoter activity was not affected by any polyamines or agmatine (data not shown).

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FIG. 1. Effects of polyamines on the induction of oprH::lacZ and PA3552::lacZ fusions. P. aeruginosa PAO1 containing each of the fusions was grown in either MMP or Miller-Hinton broth in the presence of exogenous supplements as indicated (final concentration, 20 mM), and ß-galactosidase activities were measured as described in Materials and Methods.

As described above, only 1 mM spermine can be added to the MMP medium without forming precipitates. Under this condition, the oprH promoter was not induced by spermine (data not shown) while the PA3552 promoter was induced by about fourfold. To analyze the possible effect of spermine on induction of the oprH promoter, experiments were carried out by growing cells in Miller-Hinton broth, to which 20 mM spermine and spermidine can be added without problem. As shown in Fig. 1, these two polyamines exhibited a comparable relative increase in induction of the oprH promoter.

Overall, these results were consistent with those of the GeneChip analysis (Table 2) regarding the induction effect of spermidine on the oprH-phoPQ and PA3552-PA3559 operons. These data also suggested a differential effect of polyamines on the induction of the oprH and PA3552 promoters.

The oprH-phoPQ knockout mutants exhibited different antibiotic susceptibility profiles. P. aeruginosa MPAO1 and its isogenic transposon insertion mutants of oprH, phoP, phoQ, pmrB, and PA4773 were obtained from the University of Washington Genome Center (http://www.genome.washington.edu/UWGC/), and MICs of a variety of antibiotics against these strains were determined in cultures with or without spermidine (Table 5). MICs of tetracycline for transposon insertion mutants were much higher than for the parent strain MPAO1 due to the presence of a tetracycline resistance cassette carried by the transposon. As revealed from these experiments, the pmrB and PA4773 mutants had the same susceptibility profiles as strain MPAO1 (data not shown). Significant changes in antibiotic resistance profiles were observed in the oprH, phoP, and phoQ mutants as described below.

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TABLE 5. Antibiotic susceptibility of P. aeruginosa MPAO1 and its mutant strains grown with or without spermidine

Consistent with what was observed in strain PAO1 (Table 3), spermidine also caused similar relative increases in the MICs of cationic peptide antibiotics (polymyxin B and colistin), aminoglycosides (kanamycin and gentamicin), quinolones (ciprofloxacin and norfloxacin), and dyes (ethidium bromide and acridine orange) for strain MPAO1 (Table 5). However, it was noted that for strain MPAO1, the MICs of kanamycin and quinolones in the absence of spermidine were 4-fold lower and 16-fold higher, respectively, than the MICs for PAO1.

The MICs of cationic peptide antibiotics for the oprH and phoQ mutants were two- to eightfold higher than for the phoP mutant and the parent strain MPAO1 when tested in the absence of spermidine. As with results for strain MPAO1, the addition of spermidine also increased the MICs of cationic peptide antibiotics for the oprH and phoQ mutants; however, this effect of spermidine was completely abolished for the phoP mutant. These results indicated that PhoP, the response regulator of the PhoPQ two-component system, is essential for resistance induced by spermidine to cationic peptide antibiotics.

The MICs of quinolones for the oprH, phoP, and phoQ mutants were significantly lower than for the parent strain MPAO1 in the absence of spermidine. However, exogenous spermidine still exerted its effect on increasing the MICs of quinolones for the oprH and phoQ mutants but not for the phoP mutant. Similar to what was concluded for cationic peptide antibiotics, these results indicated that PhoP is important for the spermidine effect on induced resistance to quinolones.

An increase in the MIC value in the presence of spermidine was also found for the MICs of aminoglycosides and fluorescent dyes for the oprH, phoP, and phoQ mutants. It should be noted that P. aeruginosa possesses intrinsic resistance to kanamycin due to the presence of an aminoglycoside 3'-phosphotransferase (6), which might be the reason that the spermidine effect was more obvious with gentamicin (four- to eightfold fold) than with kanamycin (twofold). Consistent with what was reported previously (15), we also observed that the MIC of kanamycin increased by fourfold for the oprH, phoP, and phoQ mutants.

Magnesium has no effect on polyamine-mediated antibiotic resistance. It has been reported that P. aeruginosa grown in low magnesium (e.g., 20 µM) medium exhibited increased resistance to cationic antibiotics and aminoglycosides, and expression of oprH-phoPQ and the PA3552-PA3559 operon was also induced under this condition (22). To test whether magnesium limitation affects polyamine-mediated antibiotic resistance, the MICs of cationic peptide antibiotics, aminoglycosides, quinolones, tetracycline, and dyes were measured in spermidine-supplemented medium with either limiting (20 µM) or high (2 mM) concentrations of magnesium. Magnesium concentrations had no effect on spermidine-induced increases in MICs (data not shown). Magnesium limitation alone was able to increase the MICs of cationic peptides and aminoglycosides but showed no effect on other compounds tested (Table 3). These data suggested that the physiological responses triggered by magnesium limitation are different from those of polyamines.

The possible effect of magnesium on expression of the oprH promoter was analyzed by measurements of ß-galactosidase activities from the oprH::lacZ transcriptional fusion. As shown in Fig. 2, spermidine induction of the oprH promoter activity was not affected by magnesium concentrations. In the absence of spermidine, the oprH promoter was significantly induced by magnesium limitation, as reported previously (20).

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FIG. 2. Effects of magnesium concentrations on spermidine-dependent induction of the oprH promoter. The oprH::lacZ fusion was introduced into P. aeruginosa PAO1, and ß-galactosidase activities from the cells grown under the indicated culture conditions were measured as described in Materials and Methods.

Roles of spermidine uptake in spermidine-mediated antibiotic resistance. Two mutant strains of P. aeruginosa PAO1 that were blocked in the polyamine transport system (spuF::Tc and spuH::Tc) were employed to determine whether spermidine uptake is required for its effects on antibiotic resistance and induction of the oprH promoter. Utilization of spermidine as the sole source of carbon was abolished in these mutants (13). As shown in Table 3, the MICs of cationic peptide antibiotics and dyes for both mutant strains were fully increased by spermidine to the same levels as for the parental strain. In contrast, spermidine exerted no effect on the MICs of aminoglycosides and quinolones for the mutant strains.

Expression of the oprH::lacZ transcriptional fusion was monitored in cells grown under different concentrations of spermidine. As shown in Fig. 3, induction by spermidine in the wild-type strain PAO1 was concentration dependent. In the spuF mutant, exogenous spermidine still exerted an induction effect on the oprH promoter, but the level of induction by 20 mM spermidine was approximately two-thirds of that in the parental strain.

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FIG. 3. Effects of spermidine uptake on induction of ß-galactosidase activities. The oprH::lacZ fusion was expressed in P. aeruginosa PAO1 (dark gray bars) and its mutant strain (spuF::Tc) (light gray bars) in the presence of different concentrations of spermidine.

Top Abstract Introduction Materials and Methods Results Discussion References

 
In this study, we presented solid evidence to demonstrate that polyamines (putrescine, spermidine, spermine, and cadaverine) and agmatine (the precursor compound of putrescine) represent a new group of environmental factors affecting antibiotic resistance in P. aeruginosa. In general, very little information is currently available regarding the effects of environmental factors on antibiotic resistance in bacteria, contrary to the enormous number of reports on mechanisms of drug inactivation, target modifications, uptake, or efflux. The magnesium limitation effect probably was the only case in this category that is being studied extensively at the molecular level (4, 15, 16, 18). Although polyamine supplementation and magnesium limitation exhibited similar phenotypes of antibiotic resistance, these two factors exerted their effects on cells through different routes, as indicated by the resistance spectrum, induction patterns, and genetic responses.

The resistance spectrum triggered by polyamine supplementation was significantly broader than that of magnesium limitation. Magnesium-mediated resistance included cationic peptide antibiotics and aminoglycosides (15). We demonstrated (Table 3) that the MICs of aminoglycosides, quinolones, and fluorescent dyes were increased to different levels by all polyamines and that the MICs of cationic peptide antibiotics were increased significantly by spermidine and spermine. These results also suggested that the spermidine- and/or spermine-responsive molecular mechanisms may be different from those of other polyamines.

Spermidine-mediated resistance was independent of magnesium concentration. It has been reported that the magnesium-mediated response was only induced by low concentrations of this divalent cation. In contrast, spermidine-mediated resistance required 5 mM to 20 mM concentrations this polyamine, and this response was not affected by the concentrations of magnesium (20 µM versus 2 mM). The concentration required for the induction of resistance may be clinically relevant to the efficiency of antibiotic treatment. The physiological concentrations of magnesium and polyamines are in the millimolar ranges in eukaryotic cells and tissues (3). These levels of polyamines, but not magnesium, would be enough to induce resistance in P. aeruginosa against a variety of antibiotics.

The genetic responses to spermidine and magnesium are different but overlapping. Spermidine supplementation or magnesium limitation (15, 16) caused cationic peptide resistance in P. aeruginosa. Cationic peptide antibiotics (e.g., polymyxin B) were reported to disrupt the structural organization of LPS, which increases permeability of the cationic peptide antibiotics and kills bacterial cells. For cell survival, a cascade of signal transduction was triggered to add 4-aminoarabinose residues to the lipid A portion of LPS. As a result, this modification makes LPS more positively charged, reducing the binding potential of cationic peptide antibiotics. In the wild-type strain of P. aeruginosa, this regulatory response worked only under very low magnesium conditions, mediated by the PhoPQ two-component system to transmit signals to the PA3552-PA3559 operon of LPS modification. It has been proposed that the PhoP response regulator by itself was active and phosphorylated by unknown mechanisms and that the membrane-anchored PhoQ sensor acts as a phosphatase to deactivate PhoP by the presence of magnesium (15, 16). In the case of spermidine, the results of GeneChip experiments and promoter fusion studies have indicated that this compound induced the expression of the oprH-phoPQ and PA3552-PA3559 operons regardless of the magnesium concentrations. In addition, the results of knockout mutant analyses showed that only PhoP but not PhoQ was essential for cationic resistance in the presence of spermidine. This observed delineation of PhoP and PhoQ was consistent with what has been reported by Hancock and coworkers (15, 16) and suggested interactions between the PhoPQ system and the spermidine-responsive regulatory elements yet to be identified. The PmrAB two-component system was excluded from this study as possible candidates for spermidine- or polyamine-responsive regulatory elements based on the results of GeneChip analysis, promoter fusions, and phenotypes of knockout mutants.

While the involvement of PhoPQ in causing resistance to cationic peptide antibiotics has been reported (15), we were surprised that PhoPQ also played a role in quinolone susceptibility (Table 4). In addition, we also observed that inactivation of PhoPQ can increase the MIC of kanamycin, which was consistent with what was reported previously (15), but produced no change in the MIC of another aminoglycoside, gentamicin. More work is required to elucidate the diverse molecular mechanisms of PhoPQ in the control of antibiotic susceptibility.

The complexity of polyamine-mediated antibiotic resistance was further reflected by the results of spermidine uptake mutants. Spermidine uptake was greatly diminished in these mutants as reported previously (13). Resistance in the presence of spermidine to aminoglycosides and quinolones was lost in these mutants (Table 3), suggesting that an increased intracellular concentration of spermidine, and perhaps polyamines in general, is required for the induction of resistance mechanisms for these antibiotics. Contrarily, resistance to cationic peptide antibiotics and dyes in the presence of spermidine was retained in the uptake-deficient mutants, and a much lower concentration of spermidine or spermine was sufficient to increase the MIC of polymyxin B significantly (Table 4). These results would suggest the presence of a spermidine-responsive sensor residing on the cytoplasmic membrane in control of the phosphorylation status of PhoP.

The presence of multiple efflux pumps is known to account for high levels of intrinsic antibiotic resistance in P. aeruginosa. It is reasonable to speculate that spermidine and other polyamines may be able to induce the expression of efflux pumps. We considered this hypothesis unlikely as no significant induction of any known efflux systems by spermidine can be identified from DNA microarray analysis (data not shown). However, polyamines might have a positive effect on the synthesis or the activity of efflux pumps by posttranscriptional regulation. In summary, the polyamine-mediated antibiotic resistance reported here warrants further studies to elucidate the molecular mechanisms of drug resistance of these potent compounds in P. aeruginosa.

 
This work was supported by National Science Foundation grants 0316005 and 0415608.

 
* Corresponding author. Mailing address: Department of Biology, Georgia State University, 24 Peachtree Center Avenue, Atlanta, GA 30303. Phone: (404) 651-2531. Fax: (404) 651-2509. E-mail: biocdl{at}langate.gsu.edu.

Top Abstract Introduction Materials and Methods Results Discussion References

 

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Antimicrobial Agents and Chemotherapy, May 2006, p. 1615-1622, Vol. 50, No. 5
0066-4804/06/$08.00+0     doi:10.1128/AAC.50.5.1615-1622.2006
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