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J Med Microbiol 56 (2007), 809-814; DOI: 10.1099/jmm.0.47019-0
© 2007 Society for General Microbiology
ISSN 1473-5644

Role of AmpD, OprF and penicillin-binding proteins in ß-lactam resistance in clinical isolates of Pseudomonas aeruginosa

Simona Bratu, David Landman, Jyoti Gupta and John Quale

Division of Infectious Diseases, State University of New York Downstate Medical Center, 450 Clarkson Avenue, Brooklyn, NY 11203, USA

Correspondence
John Quale
jquale{at}downstate.edu

Received 21 October 2006
Accepted 22 February 2007

In this study, the mechanisms leading to increased chromosomal AmpC ß-lactamase expression and the contributory roles of the outer-membrane protein OprF and penicillin-binding proteins were analysed in 33 characterized clinical isolates of Pseudomonas aeruginosa. The genes ampD and ampE were analysed by PCR and DNA sequencing. Expression of the gene oprF was assessed using real-time RT-PCR, and penicillin-binding proteins were analysed using a chemiluminescence assay. Several of the isolates with increased ampC expression had major deletions affecting ampD, although in some isolates the mechanism of increased ampC expression could not be ascertained. Occasional isolates had increased expression of both ampC and oprF but remained susceptible to cephalosporins, suggesting that increased ß-lactamase activity could not offset increased outer-membrane permeability. There were no discernible changes in penicillin-binding proteins. Genomic deletions in ampD were observed in selected clinical isolates of P. aeruginosa with increased expression of the AmpC ß-lactamase. For some isolates, cephalosporin resistance was dependent upon the interplay of ampC and oprF expression.

Abbreviations: DFAM, 6-carboxyfluorescein; DTAM, 6-carboxytetramethylrhodamine.


   INTRODUCTION
TOP
 INTRODUCTION
METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Pseudomonas aeruginosa is an opportunistic pathogen, affecting patients in intensive care areas and those with cystic fibrosis. Both intrinsic and acquired resistance mechanisms contribute to antibiotic resistance, and multidrug resistance is increasingly common. Derepression of the chromosomal AmpC ß-lactamase or, less commonly, acquisition of a new ß-lactamase, are commonly observed in ß-lactam-resistant isolates. AmpR is a positive transcriptional regulator belonging to the LysR family of regulatory proteins (Bagge et al., 2002; Kong et al., 2005). Although laboratory-derived mutants of P. aeruginosa involving ampR have confirmed the regulatory role of this protein (Kong et al., 2005), mutations involving ampR are infrequently seen in clinical isolates overexpressing ampC (Campbell et al., 1997; Bagge et al., 2002; Juan et al., 2005; Quale et al., 2006). The gene ampD encodes a cytosolic N-acetyl-anhydromuramil-L-alanine amidase that represses ampC expression; altering mutations in this gene have been noted in only a few clinical isolates with derepressed ampC activity (Langaee et al., 2000; Juan et al., 2005). In studies involving the strain PAO1, inactivation of ampD resulted in partial derepression of ampC, and ampC remained inducible with cefoxitin (Juan et al., 2006). Two homologues of ampD have been described recently that are also involved in the regulation of ampC expression; loss of all three ampD-type genes led to very high-level stably derepressed expression of ampC (Juan et al., 2006).

It is also evident that other mechanisms may be involved in ß-lactam resistance in isolates of P. aerguinosa. The association between reduced production of the porin OprD and carbapenem resistance has been verified. However, the role of OprF in antibiotic resistance remains controversial. OprF is the most common outer-membrane protein in P. aeruginosa and is most closely related to OmpA of Escherichia coli (Woodruff & Hancock, 1989; Rawling et al., 1998; Brinkman et al., 2000). Although the pore-forming structure of OprF has been questioned (Gotoh et al., 1989; Yoshihara & Nakae, 1989), it appears that the N-terminal half of the molecule can form channels (Nikaido et al., 1991; Bellido et al., 1992; Rawling et al., 1998; Brinkman et al., 2000). OprF-deficient isolates, obtained by either chemical or insertional mutagenesis, demonstrate poor growth in low-osmolarity media and altered morphology (Gotoh et al., 1989; Woodruff & Hancock, 1989; Rawling et al., 1998). In one clinical isolate, loss of OprF following exposure to a fluoroquinolone was associated with a fourfold to tenfold increase in the MICs for ciprofloxacin, carbenicillin and ceftazidime but not imipenem (Piddock et al., 1992). However, diminished OprF had a minimal effect on antibiotic resistance in laboratory-derived strains and in another clinical isolate (Woodruff & Hancock, 1988; Gotoh et al., 1989; Chamberland et al., 1990). Increased permeability due to the disruption of membrane integrity by the loss of OprF may explain the minimal effect that OprF deficiency has on antibiotic susceptibility (Hancock & Woodruff, 1988; Woodruff & Hancock, 1988).

Studies evaluating the role of penicillin-binding protein alterations on ß-lactam resistance in P. aeruginosa have also been conflicting. The high-molecular-mass penicillin-binding proteins (1a, 1b, 2 and 3) carry essential functions corresponding to those found in E. coli (Curtis et al., 1979; Noguchi et al., 1979). The low-molecular-mass penicillin-binding proteins function as DD-carboxypeptidases and are not essential for cell viability (Curtis et al., 1979; Noguchi et al., 1979, 1985). The anti-pseudomonal cephalosporins appear to bind preferentially to PBP3 (Pucci et al., 1991), whilst carbapenems bind to PBP2 (Quinn et al., 1986). Two different serotypes of P. aeruginosa isolated from a patient with cystic fibrosis that developed resistance to piperacillin following exposure to that antibiotic had reduced affinity for most penicillin-binding proteins, and one strain appeared to have an altered PBP3 (Godfrey et al., 1981). However, no changes in penicillin-binding proteins were observed in three clinical isolates that developed resistance to imipenem (Quinn et al., 1986).

In this report, we examined the mechanisms contributing to ß-lactam resistance in a large number of clinical isolates of P. aeruginosa. The roles of AmpD, OprF and penicillin-binding proteins were examined in a large number of previously characterized isolates (Quale et al., 2006).


   METHODS
TOP
 INTRODUCTION
 METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Thirty-three clinical isolates of P. aeruginosa with varying degrees of ß-lactam susceptibility were selected as described previously (Quale et al., 2006). Susceptibility testing and ribotyping were also performed as described previously (Quale et al., 2006). The isolates have been characterized for expression (measured by real-time RT-PCR) of ampC and oprD, and have had the regulatory gene ampR amplified and sequenced (Quale et al., 2006). For all experiments, P. aeruginosa ATCC 27853 was used as the control isolate.

ampD and ampE studies. To search for possible mutations affecting important genes involved in ampC expression, ampD and ampE were characterized. Template DNA was obtained using a DNeasy kit (Qiagen). The following primer pairs were used for amplification and sequencing: ampD forward, 5'-AACGACTGCAGCAATGTCAG-3', and ampD reverse, 5'-ATGCCGGGTCTTTTCTTTCT-3'; and ampE forward, 5'-GAAATCATGCGGACACGTT-3', and ampE reverse, 5'-AGGAAACATGACGTTCCTGG-3'. The PCR conditions were 94 °C for 15 min, followed by 32 cycles of 94 °C for 60 s, 37 °C for 60 s and 72 °C for 60 s, with a final extension at 72 °C for 10 min.

For isolates in which ampD could not be amplified, the following non-overlapping primers were employed: ampD2 forward, 5'-ACGTCATGTTTCCTCCTTGG-3', and ampD2 reverse, 5'-ATTAGAGTGGGGGCTCGTTC-3'. To evaluate further the ampD operon in isolates with non-amplifiable ampD, primers derived from the adjacent ampE gene (ampEfor 5'-GAGACTGTAGACCACCACCAGAAGGT-3') and PA4523 (PA4523rev 5'-ACAGCCTGTACCAGAAGATCGAACAG-3') were employed. Because of the possibility of a large insertion sequence (Bagge et al., 2002), PCR was performed using Accutaq DNA polymerase (Sigma).

Amplified products underwent bidirectional sequencing using an automated fluorescent dye-terminator sequencing system (Applied Biosystems). Sequences were compared with those of the PAO1 strain of P. aeruginosa using the BLAST program from the National Center for Biotechnology Information.

oprF expression studies. Expression of oprF was determined using real-time RT-PCR. An overnight culture was diluted 1 : 100 in Mueller–Hinton broth and grown to late exponential phase. RNA was isolated and treated with DNase using an RNeasy kit (Qiagen). A total of 2 µl RNA adjusted to a concentration of 25 µg ml–1 was used for all RT-PCR experiments; the housekeeping gene rpoD was used as the normalizing gene (Quale et al., 2006). Primers for oprF expression were oprF forward (5'-CTTCGACAAGTCCAAGGTCA-3') and oprF reverse (5'-AAGTGGACGGGTACTGCTTC-3') and the probe for detection was 5'-DFAM-CGCTGACATCAAGAACCTGGCTG-DTAM-3'. Primer and probe concentrations were adjusted to provide amplification efficiencies of ~90–110 % for all experiments. Real-time RT-PCR experiments were performed using the Brilliant QRT-PCR master mix (Stratagene). Samples were run in triplicate; virtually all individual results were within 0.5 Ct (cycle threshold) units of the averaged triplicate value. Controls run without reverse transcriptase confirmed the absence of contaminating DNA in any of the samples. Carboxy-X-rhodamine was included as a reference dye. Normalized expression of oprF was calibrated against corresponding mRNA expression by P. aeruginosa ATCC 27853; results are given as the relative expression of the mRNA compared with P. aeruginosa ATCC 27853.

Penicillin-binding protein analysis. Penicillin-binding proteins were assessed using a chemiluminescence assay, as described previously (Quale et al., 2003). Briefly, isolates were grown to late exponential phase in brain heart infusion broth. Cells were harvested by centrifugation, sonicated and cellular debris was removed by centrifugation. Membranes were collected by ultracentrifugation (100 000 g for 40 min at 4 °C) and resuspended in 10 mM phosphate buffer (pH 7.0). Penicillin-binding proteins were bound with biotinylated ampicillin (200 µg ml–1) for 10 min, and the reaction stopped by the addition of penicillin (120 mg ml–1) and 20 % aqueous Sarkosyl. Following SDS-PAGE on a 12 % resolving gel, the proteins were transferred to a nitrocellulose membrane using the Trans-Blot SD transfer cell (Bio-Rad). Following washes in PBS and skimmed milk, the membrane was incubated in a 1 : 5000 dilution of streptavidin–horseradish peroxidase solution. Penicillin-binding proteins were visualized using the Opti-4CN detection kit (Bio-Rad).


   RESULTS AND DISCUSSION
TOP
 INTRODUCTION
 METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
A total of 33 clinical isolates was examined; susceptibility and ribotyping data are given in Table 1Go. Ten isolates were non-susceptible to ceftazidime and had increased expression of ampC, defined as a tenfold or greater increase over the control. One other isolate (UL517) was intermediate in its susceptibility to ceftazidime and did not have increased expression of ampC. A total of 3 isolates were susceptible to ceftazidime with increased expression of ampC, and the remaining 19 isolates were susceptible to ceftazidime without increased ampC expression (Table 1Go).


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  		Table 1. Ribotyping, susceptibility data, AmpD, and AmpE identification, and relative expression of ampC, oprD and oprF in 33 clinical isolates of P. aeruginosa

 
ampD and ampE studies

Most of the 33 clinical isolates had one or two amino acid substitutions involving ampD and ampE; these did not seem to affect expression of ampC, as they were found in isolates with and without increased expression of ampC and were found in the control strain. Of the ten ceftazidime-resistant isolates with increased ampC expression, six (all from the same ribotype) had a deletion affecting the initial segment of ampD (Table 1Go). Attempts at amplification of the ampD operon using primers derived from the adjacent genes were also unsuccessful, suggesting genetic rearrangement involving this region. It is noteworthy that these six isolates did not have an inducible AmpC, consistent with constitutively derepressed mutants (Quale et al., 2006). A seventh isolate (BK553) in this group possessed a unique mutation causing an Asp173->His173 change in AmpD; the significance of this mutation is unknown. One unique isolate (DB513) that was susceptible to ceftazidime, despite markedly increased expression of ampC, had completely non-amplifiable ampD and ampE genes. Overall, of the 13 isolates with increased expression of ampC, nearly half did not have changes in AmpD to explain the increased ß-lactamase activity, results similar to those in another report involving clinical isolates (Juan et al., 2005).

oprF expression studies

Most isolates had expression of oprF that approximated that of the control strain, P. aeruginosa ATCC 27853. The single isolate (UL517) that was non-susceptible to ceftazidime but without increased ampC expression had the lowest expression of oprF among the ceftazidime-resistant isolates (although it was comparable to many of the isolates in the ceftazidime-susceptible group). This isolate was intermediately susceptible to ceftazidime and aztreonam and resistant to cefepime, and did not overexpress efflux systems (Quale et al., 2006). Of the three isolates susceptible to ceftazidime with overexpression of ampC, two (AM609 and FI506) had markedly increased expression of oprF (6.4 and 8.1 times the control, respectively), suggesting that even increased ß-lactamase activity could not overcome the increased permeability. The third isolate (DB513) had a markedly increased ampC expression but only marginally increased oprF expression; this isolate was resistant to cefepime and aztreonam, and the MIC for ceftazidime was at the breakpoint for susceptibility. It appears that, for selected isolates, the balance between ampC and oprF expression can affect the susceptibility to cephalosporins, although this interplay does not fully explain all of the susceptibility results we obtained.

For the 26 isolates in which there was both oprD and oprF expression data, there was a marked decrease in oprD in the isolates non-susceptible to imipenem, as expected (0.21±0.15 versus 2.7±3.6, P=0.03). These isolates also tended to have greater expression of oprF, although this did not reach statistical significance (2.6±2.1 versus 1.5±1.5, P=0.15). There was no relationship with oprF expression and meropenem or ertapenem susceptibility.

Penicillin-binding protein analysis

The penicillin-binding proteins of 17 clinical isolates and P. aeruginosa 27853 were characterized (Fig. 1Go). All of the isolates appeared to have similar patterns regarding the essential high-molecular-mass proteins. Due to the close molecular masses of PBP4 and PBP5, there was poor resolution of these two non-essential proteins. Three isolates (including the control strain) had evidence of PBP6; the presence or absence of this protein did not appear to correlate with ß-lactam susceptibility.


Figure 1
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  		Fig. 1. Penicillin-binding protein patterns in 17 clinical isolates and in P. aeruginosa ATCC 27853 (lane 10).

 
Resistance to cephalosporins in isolates of P. aeruginosa revolves around ß-lactamase expression. Our findings suggest that expression of ampC generally correlates with cephalosporin resistance, although exceptions did occur. In some isolates, generally belonging to the same ribotype, major changes in ampD were found that could explain the increased expression of ampC, although none had the previously reported insertion sequence or point mutation (Bagge et al., 2002). Many of our isolates did not have a readily identifiable reason for increased ampC expression, which has been noted in other clinical isolates (Juan et al., 2005).

Our findings also suggested that, for some isolates, there is a relationship between ampC and oprF expression and cephalosporin susceptibility. Whilst most of our isolates had oprF expression comparable to the control strain, occasional isolates had markedly increased expression of oprF. For these isolates, increased expression of ampC did not result in cephalosporin resistance, suggesting that increased membrane permeability might offset the increase in ß-lactamase activity. A modest inverse relationship between oprF and oprD was also noted in our clinical isolates. In one report, disruption of the regulatory gene sigX resulted in diminished oprF expression and increased production of a 47 kDa outer-membrane protein (Brinkman et al., 1999); this may explain the slight decrease in the MIC of imipenem seen in one laboratory-derived OprF-deficient isolate (Woodruff & Hancock, 1988).

In conclusion, our results suggest the following: (i) rearrangements in ampD are observed in some isolates overexpressing ampC, although these isolates were generally clonally related, and for several unrelated isolates there was no apparent explanation for increased ampC expression; (ii) overexpression of oprF may offset increased ampC expression and isolates may retain susceptibility to cephalosporins; and (iii) penicillin-binding protein alterations did not appear to contribute to cephalosporin or carbapenem resistance.


   ACKNOWLEDGEMENTS
 
This work was supported by a grant from Merck & Co., Inc.


   REFERENCES
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Campbell, J. I. A., Ciofu, O. & Hoiby, N. (1997). Pseudomonas aeruginosa isolates from patients with cystic fibrosis have different ß-lactamase expression phenotypes but are homogeneous in the ampC–ampR genetic region. Antimicrob Agents Chemother 41, 1380–1384.[Abstract]

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