The signal molecule for beta-lactamase induction in Enterobacter cloacae is the anhydromuramyl-pentapeptide

Beyond Susceptible and Resistant, Part I: Treatment of Infections Due to Gram-Negative Organisms With Inducible β-Lactamases

Inactivation of β-lactams by the action of β-lactamase enzymes is the most common mode of resistance to these drugs among Gram-negative organisms. The genomes of some key clinical pathogens such as Enterobacter and Pseudomonas encode AmpC, an inducible chromosomal β-lactamase. The potent activity of AmpC against broad-spectrum β-lactams complicates treatment of organisms with this gene. Antibiotic exposure can select for mutants expressing high levels of this enzyme, leading to the emergence of resistant isolates and failure of therapy, even when the initial isolate is fully susceptible. The risk of selecting for resistant organisms varies according to the particular β-lactam used for treatment. This article reviews the microbiology of these enzymes, summarizes clinical data on the frequency emergence of resistance, and discusses considerations for antimicrobial treatment of these organisms.

Pseudomonas aeruginosa β-lactamase induction requires two permeases, AmpG and AmpP

Background

In Enterobacteriaceae, β-lactam antibiotic resistance involves murein recycling intermediates. Murein recycling is a complex process with discrete steps taking place in the periplasm and the cytoplasm. The AmpG permease is critical to this process as it transports N-acetylglucosamine anhydrous N-acetylmuramyl peptides across the inner membrane. In Pseudomonadaceae, this intrinsic mechanism remains to be elucidated. Since the mechanism involves two cellular compartments, the characterization of transporters is crucial to establish the link.

Results

Pseudomonas aeruginosa PAO1 has two ampG paralogs, PA4218 (ampP) and PA4393 (ampG). Topology analysis using β-galactosidase and alkaline phosphatase fusions indicates ampP and ampG encode proteins which possess 10 and 14 transmembrane helices, respectively, that could potentially transport substrates. Both ampP and ampG are required for maximum expression of β-lactamase, but complementation and kinetic experiments suggest they act independently to play different roles. Mutation of ampG affects resistance to a subset of β-lactam antibiotics. Low-levels of β-lactamase induction occur independently of either ampP or ampG. Both ampG and ampP are the second members of two independent two-gene operons. Analysis of the ampG and ampP operon expression using β-galactosidase transcriptional fusions showed that in PAO1, ampG operon expression is β-lactam and ampR-independent, while ampP operon expression is β-lactam and ampR-dependent. β-lactam-dependent expression of the ampP operon and independent expression of the ampG operon is also dependent upon ampP.

Conclusions

In P. aeruginosa, β-lactamase induction occurs in at least three ways, induction at low β-lactam concentrations by an as yet uncharacterized pathway, at intermediate concentrations by an ampP and ampG dependent pathway, and at high concentrations where although both ampP and ampG play a role, ampG may be of greater importance. Both ampP and ampG are required for maximum induction. Similar to ampC, ampP expression is inducible in an ampR-dependent manner. Importantly, ampP expression is autoregulated and ampP also regulates expression of ampG. Both AmpG and AmpP have topologies consistent with functions in transport. Together, these data suggest that the mechanism of β-lactam resistance of P. aeruginosa is distinct from well characterized systems in Enterobacteriaceae and involves a highly complicated interaction between these putative permeases and known Amp proteins.

ampG gene of Pseudomonas aeruginosa and its role in β-lactamase expression

In enterobacteria, the ampG gene encodes a transmembrane protein (permease) that transports 1,6-GlcNAc-anhydro-MurNAc and the 1,6-GlcNAc-anhydro-MurNAc peptide from the periplasm to the cytoplasm, which serve as signal molecules for the induction of ampC β-lactamase. The role of AmpG as a transporter is also essential for cell wall recycling. Pseudomonas aeruginosa carries two AmpG homologues, AmpG (PA4393) and AmpGh1 (PA4218), with 45 and 41% amino acid sequence identity, respectively, to Escherichia coli AmpG, while the two homologues share only 19% amino acid identity. In P. aeruginosa strains PAO1 and PAK, inactivation of ampG drastically repressed the intrinsic β-lactam resistance while ampGh1 deletion had little effect on the resistance. Further, deletion of ampG in an ampD-null mutant abolished the high-level β-lactam resistance that is associated with the loss of AmpD activity. The cloned ampG gene is able to complement both the P. aeruginosa and the E. coli ampG mutants, while that of ampGh1 failed to do so, suggesting that PA4393 encodes the only functional AmpG protein in P. aeruginosa. We also demonstrate that the function of AmpG in laboratory strains of P. aeruginosa can effectively be inhibited by carbonyl cyanide m-chlorophenylhydrazone (CCCP), causing an increased sensitivity to β-lactams among laboratory as well as clinical isolates of P. aeruginosa. Our results suggest that inhibition of the AmpG activity is a potential strategy for enhancing the efficacy of β-lactams against P. aeruginosa, which carries inducible chromosomal ampC, especially in AmpC-hyperproducing clinical isolates.

NagZ inactivation prevents and reverts beta-lactam resistance, driven by AmpD and PBP 4 mutations, in Pseudomonas aeruginosa

AmpC hyperproduction is the most frequent mechanism of resistance to penicillins and cephalosporins in Pseudomonas aeruginosa and is driven by ampD mutations or the recently described inactivation of dacB, which encodes the nonessential penicillin-binding protein (PBP) PBP 4. Recent work showed that nagZ inactivation attenuates beta-lactam resistance in ampD mutants. Here we explored whether the same could be true for the dacB mutants with dacB mutations alone or in combination with ampD mutations. The inactivation of nagZ restored the wild-type beta-lactam MICs and ampC expression of PAO1 dacB and ampD mutants and dramatically reduced the MICs (for example, the MIC for ceftazidime dropped from 96 to 4 microg/ml) and the level of ampC expression (from ca. 1,000-fold to ca. 50-fold higher than that for PAO1) in the dacB-ampD double mutant. On the other hand, nagZ inactivation had little effect on the inducibility of AmpC. The NagZ inhibitor O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino-N-phenylcarbamate attenuated the beta-lactam resistance of the AmpC-hyperproducing strains, showing a greater effect on the dacB mutant (reducing the ceftazidime MICs from 24 to 6 microg/ml) than the ampD mutant (reducing the MICs from 8 to 4 microg/ml). Additionally, nagZ inactivation in the dacB mutant blocked the overexpression of creD (blrD), which is a marker of the activation of the CreBC (BlrAB) regulator involved in the resistance phenotype. Finally, through population analysis, we show that the inactivation of nagZ dramatically reduces the capacity of P. aeruginosa to develop ceftazidime resistance, since spontaneous mutants were not obtained at concentrations > or = 8 microg/ml (the susceptibility breakpoint) for the nagZ mutant but were obtained with wild-type PAO1. Therefore, NagZ is envisaged to be a candidate target for preventing and reverting beta-lactam resistance in P. aeruginosa.

Crystal structure of the AmpR effector binding domain provides insight into the molecular regulation of inducible ampc beta-lactamase

Hyperproduction of AmpC beta-lactamase (AmpC) is a formidable mechanism of resistance to penicillins and cephalosporins in Gram-negative bacteria such as Pseudomonas aeruginosa and Enterobacteriaceae. AmpC expression is regulated by the LysR-type transcriptional regulator AmpR. ampR and ampC genes form a divergent operon with overlapping promoters to which AmpR binds and regulates the transcription of both genes. AmpR induces ampC by binding to one member of the family of 1,6-anhydro-N-acetylmuramyl peptides, which are cytosolic catabolites of peptidoglycan that accumulate during beta-lactam challenge. To gain structural insights into AmpR regulation, we determined the crystal structure of the effector binding domain (EBD) of AmpR from Citrobacter freundii up to 1.83 A resolution. The AmpR EBD is dimeric and each monomer comprises two subdomains that adopt alpha/beta Rossmann-like folds. Located between the monomer subdomains is a pocket that was found to bind the crystallization buffer molecule 2-(N-morpholino)ethanesulfonic acid. The pocket, together with a groove along the surface of subdomain I, forms a putative effector binding site into which a molecule of 1,6-anhydro-N-acetylmuramyl pentapeptide could be modeled. Amino acid substitutions at the base of the interdomain pocket either were found to render AmpR incapable of inducing ampC (Thr103Val, Ser221Ala and Tyr264Phe) or resulted in constitutive ampC expression (Gly102Glu). While the substitutions that prevented ampC induction did not alter the overall AmpR EBD structure, circular dichroism spectroscopy revealed that the nonconservative Gly102Glu mutation affected EBD secondary structure, confirming previous work suggesting that Gly102Glu induces a conformational change to result in constitutive AmpC production.

Induction of beta-lactamase production in Aeromonas hydrophila is responsive to beta-lactam-mediated changes in peptidoglycan composition

We have studied the mechanism by which beta-lactam challenge leads to beta-lactamase induction in Aeromonas hydrophila through transposon-insertion mutagenesis. Disruption of the dd-carboxypeptidases/endopeptidases, penicillin-binding protein 4 or BlrY leads to elevated monomer-disaccharide-pentapeptide levels in A. hydrophila peptidoglycan and concomitant overproduction of beta-lactamase through activation of the BlrAB two-component regulatory system. During beta-lactam challenge, monomer-disaccharide-pentapeptide levels increase proportionately with beta-lactamase production and beta-lactamase induction is inhibited by vancomycin, which binds muro-pentapeptides. Taken together, these data strongly suggest that the Aeromonas spp. beta-lactamase regulatory sensor kinase, BlrB, responds to the concentration of monomer-disaccharide-pentapeptide in peptidoglycan.

AmpN-AmpG operon is essential for expression of L1 and L2 beta-lactamases in Stenotrophomonas maltophilia

AmpG is an inner membrane permease which transports products of murein sacculus degradation from the periplasm into the cytosol in Gram-negative bacteria. This process is linked to induction of the chromosomal ampC beta-lactamase gene in some members of the Enterobacteriaceae and in Pseudomonas aeruginosa. In this study, the ampG homologue of Stenotrophomonas maltophilia KJ was analyzed. The ampG homologue and its upstream ampN gene form an operon and are cotranscribed under the control of the promoter P(ampN). Expression from P(ampN) was found to be independent of beta-lactam exposure and ampN and ampG products. A DeltaampN allele exerted a polar effect on the expression of ampG and resulted in a phenotype of null beta-lactamase inducibility. Complementation assays elucidated that an intact ampN-ampG operon is essential for beta-lactamase induction. Consistent with ampG of Escherichia coli, the ampN-ampG operon of S. maltophilia did not exhibit a gene dosage effect on beta-lactamase expression. The AmpG permease of E. coli could complement the beta-lactamase inducibility of ampN or ampG mutants of S. maltophilia, indicating that both species have the same precursor of activator ligand(s) for beta-lactamase induction.

Antibacterial-resistant Pseudomonas aeruginosa: clinical impact and complex regulation of chromosomally encoded resistance mechanisms

Treatment of infectious diseases becomes more challenging with each passing year. This is especially true for infections caused by the opportunistic pathogen Pseudomonas aeruginosa, with its ability to rapidly develop resistance to multiple classes of antibiotics. Although the import of resistance mechanisms on mobile genetic elements is always a concern, the most difficult challenge we face with P. aeruginosa is its ability to rapidly develop resistance during the course of treating an infection. The chromosomally encoded AmpC cephalosporinase, the outer membrane porin OprD, and the multidrug efflux pumps are particularly relevant to this therapeutic challenge. The discussion presented in this review highlights the clinical significance of these chromosomally encoded resistance mechanisms, as well as the complex mechanisms/pathways by which P. aeruginosa regulates their expression. Although a great deal of knowledge has been gained toward understanding the regulation of AmpC, OprD, and efflux pumps in P. aeruginosa, it is clear that we have much to learn about how this resourceful pathogen coregulates different resistance mechanisms to overcome the antibacterial challenges it faces.

Antibacterial-resistant Pseudomonas aeruginosa: clinical impact and complex regulation of chromosomally encoded resistance mechanisms

Treatment of infectious diseases becomes more challenging with each passing year. This is especially true for infections caused by the opportunistic pathogen Pseudomonas aeruginosa, with its ability to rapidly develop resistance to multiple classes of antibiotics. Although the import of resistance mechanisms on mobile genetic elements is always a concern, the most difficult challenge we face with P. aeruginosa is its ability to rapidly develop resistance during the course of treating an infection. The chromosomally encoded AmpC cephalosporinase, the outer membrane porin OprD, and the multidrug efflux pumps are particularly relevant to this therapeutic challenge. The discussion presented in this review highlights the clinical significance of these chromosomally encoded resistance mechanisms, as well as the complex mechanisms/pathways by which P. aeruginosa regulates their expression. Although a great deal of knowledge has been gained toward understanding the regulation of AmpC, OprD, and efflux pumps in P. aeruginosa, it is clear that we have much to learn about how this resourceful pathogen coregulates different resistance mechanisms to overcome the antibacterial challenges it faces.

Pseudomonas aeruginosa - a phenomenon of bacterial resistance

Pseudomonas aeruginosa is one of the leading nosocomial pathogens worldwide. Nosocomial infections caused by this organism are often hard to treat because of both the intrinsic resistance of the species (it has constitutive expression of AmpC beta-lactamase and efflux pumps, combined with a low permeability of the outer membrane), and its remarkable ability to acquire further resistance mechanisms to multiple groups of antimicrobial agents, including beta-lactams, aminoglycosides and fluoroquinolones. P. aeruginosa represents a phenomenon of bacterial resistance, since practically all known mechanisms of antimicrobial resistance can be seen in it: derepression of chromosomal AmpC cephalosporinase; production of plasmid or integron-mediated beta-lactamases from different molecular classes (carbenicillinases and extended-spectrum beta-lactamases belonging to class A, class D oxacillinases and class B carbapenem-hydrolysing enzymes); diminished outer membrane permeability (loss of OprD proteins); overexpression of active efflux systems with wide substrate profiles; synthesis of aminoglycoside-modifying enzymes (phosphoryltransferases, acetyltransferases and adenylyltransferases); and structural alterations of topoisomerases II and IV determining quinolone resistance. Worryingly, these mechanisms are often present simultaneously, thereby conferring multiresistant phenotypes. This review describes the known resistance mechanisms in P. aeruginosa to the most frequently administrated antipseudomonal antibiotics: beta-lactams, aminoglycosides and fluoroquinolones.

An externally tunable bacterial band-pass filter

The current paradigm for tuning synthetic biological systems is through re-engineering system components. Biological systems designed with the inherent ability to be tuned by external stimuli will be more versatile. We engineered Escherichia coli cells to behave as an externally tunable band-pass filter for enzyme activity and small molecules. The band's location can be positioned within a range of 4 orders of magnitude simply by the addition of compounds to the growth medium. Inclusion in the genetic network of an enzyme-substrate pair that functions as an attenuator is a generalizable strategy that enables this tunability. The genetic circuit enabled bacteria growth to be patterned in response to chemical gradients in nonintuitive ways and facilitated the isolation of engineered allosteric enzymes. The application of this strategy to other biological systems will increase their utility for biotechnological applications and their usefulness as a tool for gaining insight into nature's underlying design principles.

Inactivation of the glycoside hydrolase NagZ attenuates antipseudomonal beta-lactam resistance in Pseudomonas aeruginosa

The overproduction of chromosomal AmpC beta-lactamase poses a serious challenge to the successful treatment of Pseudomonas aeruginosa infections with beta-lactam antibiotics. The induction of ampC expression by beta-lactams is mediated by the disruption of peptidoglycan (PG) recycling and the accumulation of cytosolic 1,6-anhydro-N-acetylmuramyl peptides, catabolites of PG recycling that are generated by an N-acetyl-beta-D-glucosaminidase encoded by nagZ (PA3005). In the absence of beta-lactams, ampC expression is repressed by three AmpD amidases encoded by ampD, ampDh2, and ampDh3, which act to degrade these 1,6-anhydro-N-acetylmuramyl peptide inducer molecules. The inactivation of ampD genes results in the stepwise upregulation of ampC expression and clinical resistance to antipseudomonal beta-lactams due to the accumulation of the ampC inducer anhydromuropeptides. To examine the role of NagZ on AmpC-mediated beta-lactam resistance in P. aeruginosa, we inactivated nagZ in P. aeruginosa PAO1 and in an isogenic triple ampD null mutant. We show that the inactivation of nagZ represses both the intrinsic beta-lactam resistance (up to 4-fold) and the high antipseudomonal beta-lactam resistance (up to 16-fold) that is associated with the loss of AmpD activity. We also demonstrate that AmpC-mediated resistance to antipseudomonal beta-lactams can be attenuated in PAO1 and in a series of ampD null mutants using a selective small-molecule inhibitor of NagZ. Our results suggest that the blockage of NagZ activity could provide a strategy to enhance the efficacies of beta-lactams against P. aeruginosa and other gram-negative organisms that encode inducible chromosomal ampC and to counteract the hyperinduction of ampC that occurs from the selection of ampD null mutations during beta-lactam therapy.

How bacteria consume their own exoskeletons (turnover and recycling of cell wall peptidoglycan)

SUMMARY

The phenomenon of peptidoglycan recycling is reviewed. Gram-negative bacteria such as Escherichia coli break down and reuse over 60% of the peptidoglycan of their side wall each generation. Recycling of newly made peptidoglycan during septum synthesis occurs at an even faster rate. Nine enzymes, one permease, and one periplasmic binding protein in E. coli that appear to have as their sole function the recovery of degradation products from peptidoglycan, thereby making them available for the cell to resynthesize more peptidoglycan or to use as an energy source, have been identified. It is shown that all of the amino acids and amino sugars of peptidoglycan are recycled. The discovery and properties of the individual proteins and the pathways involved are presented. In addition, the possible role of various peptidoglycan degradation products in the induction of beta-lactamase is discussed.

Benefit of having multiple ampD genes for acquiring beta-lactam resistance without losing fitness and virulence in Pseudomonas aeruginosa

The inactivation of ampD in Pseudomonas aeruginosa leads to a partially derepressed phenotype, characterized by a moderately high level basal ampC expression that is still further inducible, due to the presence of two additional ampD genes in this species (ampDh2 and ampDh3). The sequential inactivation of the three ampD genes was shown to lead to a stepwise upregulation of ampC expression, reaching full derepression in the triple mutant. To gain insight into the biological role of P. aeruginosa AmpD multiplicity, we determined the effects of the inactivation of the ampD genes on fitness and virulence. We show that, in contrast to what was previously documented for Salmonella spp., the inactivation of ampD in P. aeruginosa does not affect fitness or virulence in a mouse model of systemic infection. This lack of effect was demonstrated to be dependent on the presence of the additional ampD genes (ampDh2 and ampDh3), since the double and the triple ampD mutants completely lost their biological competitiveness and virulence; full ampC derepression and disruption of the AmpD peptidoglycan recycling system itself are both found to cause a major biological cost. Furthermore, among the ampD genes, ampDh3 is found to be the most relevant for virulence in P. aeruginosa. Therefore, as a consequence of the presence of additional ampD genes, partial ampC derepression mediated by ampD inactivation confers a biologically efficient resistance mechanism on P. aeruginosa.

Prolonged use of carbapenems and colistin predisposes to ventilator-associated pneumonia by pandrug-resistant Pseudomonas aeruginosa

OBJECTIVE

We present our experience with five cases of pandrug-resistant Pseudomonas aeruginosa ventilator-associated pneumonia (VAP) and analysis of risk factors.

DESIGN AND SETTING

Case-control study in a 15-bed intensive care unit (ICU).

PATIENTS AND PARTICIPANTS

The study included 5 cases and 20 controls. Each case patient was matched to four contemporary controls according to gender, prior hospital admissions, hospitalization duration, ICU admission cause, Acute Physiology and Chronic Health Evaluation (APACHE) II and Sequential Organ Function Assessment (SOFA) scores on ICU admission, and length of ICU stay, and mechanical ventilation duration until first VAP episode by a multidrug-resistant bacterium.

MEASUREMENTS AND RESULTS

Recorded variables included age, gender, daily APACHE II and SOFA scores, patient medication, treatment interventions, positive cultures and corresponding antibiograms, occurrence of infection, sepsis, and septic shock, other ICU-associated morbidity, length of ICU stay and mechanical ventilation, and patient outcome. Healthcare worker and environmental cultures, and a hand-disinfection survey were performed. Pandrug-resistant P. aeruginosa isolates belonged to the same genotype and were bla (VIM-1)-like gene positive. The outbreak resolved following reinforcement of infection-control measures (September 27). The sole independent predictor for pandrug-resistant P. aeruginosa VAP was combined use of carbapenem for more than 20 days and colistin use for and more than 13 days (odds ratio 76.0; 95% confidence interval 3.7-1487.6). An additional risk factor was more than 78 open suctioning procedures during 6-26 September (odds ratio 16.0; 95% confidence interval 1.4-185.4).

CONCLUSIONS

Prolonged carbapenem-colistin use predisposes to VAP by pandrug-resistant P. aeruginosa. Cross-transmission may be facilitated by open suctioning.

beta-Lactamase induction and cell wall recycling in gram-negative bacteria

beta-Lactams with the ability to induce beta-lactamase in gram-negative bacteria bind to essential penicillin-binding proteins (PBPs) after entering the periplasmic space. This leads to inactivation of transpeptidase activities and thereby a decrease in the number of peptide cross-links, allowing further degradation of murein by soluble lytic transglycosylases. If all DD-carboxypeptidases (PBP 4, 5, 6a and 6b) are inhibited as well, the degradation product aD-pentapeptide (N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic-acid-D-alanyl-D- alanine) accumulates, which is the case with inducing beta-lactams such as imipenem. These molecules in addition to tri- and tetrapeptides (N-acetylglucosaminyl-1,6-anhydro-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic-acid-[D-alanine]) which are the usual degradation products of peptidoglycan, are released into the cytoplasm and displace the UDP-pentapeptide (UDP-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelic-acid-D-alanyl-D-alanine) from the DNA-binding protein AmpR, converting it into an activator of AmpC beta-lactamase expression.

Stepwise upregulation of the Pseudomonas aeruginosa chromosomal cephalosporinase conferring high-level beta-lactam resistance involves three AmpD homologues

Development of resistance to the antipseudomonal penicillins and cephalosporins mediated by hyperproduction of the chromosomal cephalosporinase AmpC is a major threat to the successful treatment of Pseudomonas aeruginosa infections. Although ampD inactivation has been previously found to lead to a partially derepressed phenotype characterized by increased AmpC production but retaining further inducibility, the regulation of ampC in P. aeruginosa is far from well understood. We demonstrate that ampC expression is coordinately repressed by three AmpD homologues, including the previously described protein AmpD plus two additional proteins, designated AmpDh2 and AmpDh3. The three AmpD homologues are responsible for a stepwise ampC upregulation mechanism ultimately leading to constitutive hyperexpression of the chromosomal cephalosporinase and high-level antipseudomonal beta-lactam resistance, as shown by analysis of the three single ampD mutants, the three double ampD mutants, and the triple ampD mutant. This is achieved by a three-step escalating mechanism rendering four relevant expression states: basal-level inducible expression (wild type), moderate-level hyperinducible expression with increased antipseudomonal beta-lactam resistance (ampD mutant), high-level hyperinducible expression with high-level beta-lactam resistance (ampD ampDh3 double mutant), and very high-level (more than 1,000-fold compared to the wild type) derepressed expression (triple mutant). Although one-step inducible-derepressed expression models are frequent in natural resistance mechanisms, this is the first characterized example in which expression of a resistance gene can be sequentially amplified through multiple steps of derepression.

Molecular mechanisms of beta-lactam resistance mediated by AmpC hyperproduction in Pseudomonas aeruginosa clinical strains

The molecular mechanisms of beta-lactam resistance mediated by AmpC hyperproduction in natural strains of Pseudomonas aeruginosa were investigated in a collection of 10 isogenic, ceftazidime-susceptible and -resistant pairs of isolates, each sequentially recovered from a different intensive care unit patient treated with beta-lactams. All 10 ceftazidime-resistant mutants hyper-produced AmpC (beta-lactamase activities were 12- to 657-fold higher than those of the parent strains), but none of them harbored mutations in ampR or the ampC-ampR intergenic region. On the other hand, six of them harbored inactivating mutations in ampD: four contained frameshift mutations, one had a C-->T mutation, creating a premature stop codon, and finally, one had a large deletion, including the complete ampDE region. Complementation studies revealed that only three of the six ampD mutants could be fully trans-complemented with either ampD- or ampDE-harboring plasmids, whereas one of them could be trans-complemented only with ampDE and two of them (including the mutant with the deletion of the ampDE region and one with an ampD frameshift mutation leading to an ampDE-fused open reading frame) could not be fully trans-complemented with any of the plasmids. Finally, one of the four mutants with no mutations in ampD could be trans-complemented, but only with ampDE. Although the inactivation of AmpD is found to be the most frequent mechanism of AmpC hyperproduction in clinical strains, our findings suggest that for certain types of mutations, AmpE plays an indirect role in resistance and that there are other unknown genes involved in AmpC hyperproduction, with at least one of them apparently located close to the ampDE operon.

Membrane topology of the Escherichia coli AmpG permease required for recycling of cell wall anhydromuropeptides and AmpC beta-lactamase induction

Escherichia coli, and presumably most other gram-negative bacteria, possesses an efficient protein machinery for recycling its peptidoglycan during cell growth. The major recycled peptidoglycan product is N-acetylglucosamine-1,6-anhydro-N-acetylmuramic acid-tetrapeptide. Its uptake from the periplasm into the cytoplasm is carried out via the AmpG protein, an intrinsic membrane protein. In gram-negative bacteria carrying an ampC beta-lactamase-inducible gene on their chromosomes, the induction mechanism is directly linked to peptidoglycan recycling. After identification of the different putative hydrophobic segments by computing, the AmpG topology was experimentally determined by using beta-lactamase fusion. In the proposed model, AmpG contains 10 transmembrane segments and two large cytoplasmic loops.

Identification of rhtX and fptX, novel genes encoding proteins that show homology and function in the utilization of the siderophores rhizobactin 1021 by Sinorhizobium meliloti and pyochelin by Pseudomonas aeruginosa, respectively

Rhizobactin 1021 is a hydroxymate siderophore produced by the soil bacterium Sinorhizobium meliloti 2011. A regulon comprising rhtA, encoding the outer membrane receptor protein for the ferrisiderophore; the biosynthesis operon rhbABCDEF; and rhrA, the Ara-C-like regulator of the receptor and biosynthesis genes has been previously described. We report the discovery of a gene, located upstream of rhbA and named rhtX (for "rhizobactin transport"), which is required, in addition to rhtA, to confer the ability to utilize rhizobactin 1021 on a strain of S. meliloti that does not naturally utilize the siderophore. Rhizobactin 1021 is structurally similar to aerobactin, which is transported in Escherichia coli via the IutA outer membrane receptor and the FhuCDB inner membrane transport system. E. coli expressing iutA and fhuCDB was found to also transport rhizobactin 1021. We demonstrated that RhtX alone could substitute for FhuCDB to transport rhizobactin 1021 in E. coli. RhtX shows similarity to a number of uncharacterized proteins which are encoded proximal to genes that are either known to be or predicted to be involved in iron acquisition. Among these is PA4218 of Pseudomonas aeruginosa, which is located close to the gene cluster that functions in pyochelin biosynthesis and outer membrane transport. PA4218 was mutated by allelic replacement, and the mutant was found to have a pyochelin utilization-defective phenotype. It is proposed that PA4218 be named fptX (for "ferripyochelin transport"). RhtX and FptX appear to be members of a novel family of permeases that function as single-subunit transporters of siderophores.

Dynamics and spatial distribution of beta-lactamase expression in Pseudomonas aeruginosa biofilms

The development of resistance to beta-lactam antibiotics is a problem in the treatment of chronic Pseudomonas aeruginosa infection in the lungs of patients with cystic fibrosis. The main resistance mechanism is high-level expression of the chromosomally encoded AmpC beta-lactamase of P. aeruginosa cells growing in biofilms. Several genes have been shown to influence the level of ampC expression, but little is known about the regulation of ampC expression in P. aeruginosa biofilms. To study the expression of ampC in P. aeruginosa biofilms, we constructed a reporter that consisted of the fusion of the ampC promoter to gfp(ASV) encoding an unstable version of the green fluorescent protein. In vitro biofilms of P. aeruginosa were exposed to the beta-lactam antibiotics imipenem and ceftazidime. Sub-MICs of imipenem significantly induced the monitor system of the biofilm bacteria in the peripheries of the microcolonies, but the centers of the microcolonies remained uninduced. However, the centers of the microcolonies were physiologically active, as shown by experiments with another monitor construction consisting of an arabinose-inducible promoter fused to gfp(ASV). The whole biofilm was induced in the presence of increased imipenem concentrations. Ceftazidime induced the monitor system of the biofilm bacteria as well, but only bacteria in the peripheries of the microcolonies were induced in the presence of even very high concentrations. The experiments illustrate for the first time the dynamic and spatial distributions of beta-lactamase induction in P. aeruginosa cells growing in biofilms. Thus, our experiments show that P. aeruginosa cells growing in biofilms constitute a heterogeneous population unit which may create different antibiotic-selective environments for the bacteria in the biofilm.

Chromosomally-encoded resistance mechanisms of Pseudomonas aeruginosa: therapeutic implications

Pseudomonas aeruginosa is an important nosocomial pathogen that presents a difficult therapeutic challenge. Although P. aeruginosa has been shown to acquire resistance mechanisms encoded on plasmids, this pathogen comes armed with multiple chromosomally-encoded mechanisms of resistance that can provide impressive intrinsic resistance, as well as the potential to mutate to high-level multi-drug resistance. Recent analysis of the sequenced genome of P. aeruginosa PAO1 suggested that we have just started to unlock the resistance potential of this pathogen. One of the most serious threats to the usefulness of beta-lactams against P. aeruginosa is the chromosomal AmpC cephalosporinase. When AmpC production increases through mutational events, overproduction of this cephalosporinase provides high-level resistance to all beta-lactams except the carbapenems. Carbapenem resistance typically requires down-regulation of the outer membrane protein (OprD), which serves as the primary route of entry for carbapenems. Perhaps the most threatening of the resistance mechanisms encoded on the P. aeruginosa chromosome are the multi-drug efflux pumps. These pumps have the ability to extrude multiple classes of antibiotics from the periplasmic space, as well as the cytoplasm. Natural expression of efflux pumps in 'wild-type' cells plays an important role in the relatively decreased susceptibility of P. aeruginosa to antibiotics. However, the greatest therapeutic problems occur when these pumps are overproduced in mutants and high-level, multi-drug resistance develops. Although the development of infections with highly resistant strains of P. aeruginosa can present serious therapeutic challenges, the most troublesome threat associated with the chromosomally-encoded resistance mechanisms is the potential for high-level resistance to emerge during the course of therapy. When resistance emerges during therapy, clinical failure can occur and the therapeutic options for second-line therapy can become severely limited. Unfortunately, the emergence of resistance during therapy is not a rare event with P. aeruginosa and these three resistance mechanisms. Therefore, clinicians must be mindful of this threat when choosing an appropriate therapy, and usually appropriate therapy includes a combination of drugs. Since the standard combination of an aminoglycoside and a beta-lactam has been shown to be ineffective in preventing the emergence of some resistance problems, the search for more effective combinations must be a priority.

Substrate specificity of the AmpG permease required for recycling of cell wall anhydro-muropeptides

AmpG was originally identified as a gene required for induction of beta-lactamase. Subsequently, we found AmpG to be a permease required for recycling of murein tripeptide and uptake of anhydro-muropeptides. We have now studied the specificity of the AmpG permease. The principal requirement is for the presence of the disaccharide, N-acetylglucosaminyl-beta-1,4-anhydro-N-acetylmuramic acid (GlcNAc-anhMurNAc). These unique substrates for AmpG, which contain murein peptides linked to GlcNAc-anhMurNAc, are produced by turnover of the cell wall during logarithmic growth. AmpG permease is sensitive to carbonylcyanide m-chlorophenylhydrazone, demonstrating that AmpG permease is a single-component permease and that transport is dependent on the proton motive force.

Constitutive high expression of chromosomal beta-lactamase in Pseudomonas aeruginosa caused by a new insertion sequence (IS1669) located in ampD

The expression of chromosomal AmpC beta-lactamase in Pseudomonas aeruginosa is negatively regulated by the activity of an amidase, AmpD. In the present study we examined resistant clinical P. aeruginosa strains and several resistant variants isolated from in vivo and in vitro biofilms for mutations in ampD to find evidence for the genetic changes leading to high-level expression of chromosomal beta-lactamase. A new insertion sequence, IS1669, was found located in the ampD genes of two clinical P. aeruginosa isolates and several biofilm-isolated variants. The presence of IS1669 in ampD resulted in the expression of high levels of AmpC beta-lactamase. Complementation of these isolates with ampD from the reference P. aeruginosa strain PAO1 caused a dramatic decrease in the expression of AmpC beta-lactamase and a parallel decrease of the MIC of ceftazidime to a level comparable to that of PAO1. One highly resistant, constitutive beta-lactamase-producing variant contained no mutations in ampD, but a point mutation was observed in ampR, resulting in an Asp-135-->Asn change. An identical mutation of AmpR in Enterobacter cloacae has been reported to cause a 450-fold higher AmpC expression. However, in many of the isolates expressing high levels of chromosomal beta-lactamase, no changes were found in either ampD, ampR, or in the promoter region of ampD, ampR, or ampC. Our results suggest that multiple pathways may exist leading to increased antimicrobial resistance due to chromosomal beta-lactamase.

Role of the murein precursor UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala in repression of beta-lactamase induction in cell division mutants

Certain beta-lactam antibiotics induce the chromosomal ampC beta-lactamase of many gram-negative bacteria. The natural inducer, though not yet unequivocally identified, is a cell wall breakdown product which enters the cell via the AmpG permease component of the murein recycling pathway. Surprisingly, it has been reported that beta-lactamase is not induced by cefoxitin in the absence of FtsZ, which is required for cell division, or in the absence of penicillin-binding protein 2 (PBP2), which is required for cell elongation. Since these results remain unexplained, we examined an ftsZ mutant and other cell division mutants (ftsA, ftsQ, and ftsI) and a PBP2 mutant for induction of beta-lactamase. In all mutants, beta-lactamase was not induced by cefoxitin, which confirms the initial reports. The murein precursor, UDP-N-acetylmuramyl-L-Ala-gamma-D-Glu-meso-diaminopimelic acid-D-Ala-D-Ala (UDP-MurNAc-pentapeptide), has been shown to serve as a corepressor with AmpR to repress beta-lactamase expression in vitro. Our results suggest that beta-lactamase is not induced because the fts mutants contain a greatly increased amount of corepressor which the inducer cannot displace. In the PBP2(Ts) mutant, in addition to accumulation of corepressor, cell wall turnover and recycling were greatly reduced so that little or no inducer was available. Hence, in both cases, a high ratio of repressor to inducer presumably prevents induction.

BUT-1: a new member in the chromosomal inducible class C beta-lactamases family from a clinical isolate of Buttiauxella sp

An atypical Enterobacteriaceae strain with a beta-lactam susceptibility pattern of inducible cephalosporinase was isolated in Tenon Hospital (Paris, France) from a patient's skull wound infection. Identifications by the API-50CHE biochemical system and 16S rRNA gene sequencing concluded that it was a member of the Buttiauxella genus. The bla gene was cloned and sequenced. The deduced translated product was a 383-amino acid protein (BUT-1) with 75-78% identity with the chromosomal AmpC beta-lactamases of Citrobacter freundii, Enterobacter aerogenes, Enterobacter cloacae and Escherichia coli. The isoelectric point of 9.0 and the kinetic constants of BUT-1 were comparable with results described for other Ambler class C enzymes. bla(BUT-1) and the associated ampR transcriptional regulator gene were divergently transcribed from a common intercistronic region, a genetic organization already described for other inducible class C beta-lactamases. The deduced amino acid sequence of AmpR shared 85% and 81% identity with AmpR from E. cloacae and C. freundii respectively.

Chromosomal system for studying AmpC-mediated beta-lactam resistance mutation in Escherichia coli

In some enterobacterial pathogens, but not in Escherichia coli, loss-of-function mutations in the ampD gene are a common route to beta-lactam antibiotic resistance. We constructed an assay system for studying mechanism(s) of enterobacterial ampD mutation using the well-developed genetics of E. coli. We integrated the Enterobacter ampRC genes into the E. coli chromosome. These cells acquire spontaneous recombination- and SOS response-independent beta-lactam resistance mutations in ampD. This chromosomal system is useful for studying mutation mechanisms that promote antibiotic resistance.

AmpD is required for regulation of expression of NmcA, a carbapenem-hydrolyzing beta-lactamase of Enterobacter cloacae

To further elucidate the induction process of the carbapenem-hydrolyzing beta-lactamase of Ambler class A, NmcA, ampD genes of the wild-type (WT) strain and of ceftazidime-resistant mutants of Enterobacter cloacae NOR-1 were cloned and tested in transcomplementation experiments. Ceftazidime-resistant E. cloacae NOR-1 mutants exhibited derepressed expression of the AmpC-type cephalosporinase and of the carbapenem-hydrolyzing beta-lactamase NmcA. The ampD genes of Escherichia coli and E. cloacae WT NOR-1 transcomplemented the ceftazidime-resistant E. cloacae NOR-1 mutants to the WT level of beta-lactamase expression, while the mutated ampD alleles of E. cloacae NOR-1 failed to do so. The deduced E. cloacae NOR-1 WT AmpD protein exhibited 95 and 91% amino acid identity with the E. cloacae O29 and E. cloacae 14 WT AmpD proteins, respectively. Of the 12 ceftazidime-resistant E. cloacae NOR-1 strains, 3 had AmpD proteins with amino acid changes, while the others had truncated AmpD proteins. Most of these mutations were located outside the conserved regions that link the AmpD proteins to the cell wall hydrolases. AmpD from E. cloacae NOR-1 is involved in the regulation of expression of both beta-lactamases (NmcA and AmpC), suggesting that structurally unrelated genes may be under the control of an identical genetic system.

Hypersusceptibility of the Pseudomonas aeruginosa nfxB mutant to beta-lactams due to reduced expression of the ampC beta-lactamase

The Pseudomonas aeruginosa nfxB mutant lacking mexAB-oprM showed hypersusceptibility to 9 out of 24 beta-lactams tested. This hypersusceptibility was found for the nfxB mutant lacking mexAB-oprM-mexXY (N108) but not for the nfxB mutant lacking both mexAB-oprM-mexXY and ampC. The level of the AmpC beta-lactamase induction was reduced in N108. Thus, the reduced AmpC induction must be the cause of the hypersusceptibility.

beta -Lactamases: which ones are clinically important?

The introduction of a large array of beta-lactam antibiotics has spawned the emergence of an even larger variety of beta-lactamases designed to confer resistance to these agents. beta-lactamases are produced by both gram-positive and gram-negative bacteria, but their clinical importance is far greater among the gram-negatives. The virtual explosion in our knowledge about the variety of these enzymes can often create confusion and frustration among those not well versed in the field. In this paper, we attempt to focus the discussion of beta-lactamases on those enzymes that are of the greatest clinical importance, the Ambler Class A and C enzymes. We also discuss the growing importance of the Ambler Class B metallo beta-lactamases, which hydrolyze carbapenems and are increasing in prevalence in areas of significant carbapenem usage. Copyright 2000 Harcourt Publishers Ltd.

Inactivation of the ampD gene in Pseudomonas aeruginosa leads to moderate-basal-level and hyperinducible AmpC beta-lactamase expression

It has been shown in enterobacteria that mutations in ampD provoke hyperproduction of chromosomal beta-lactamase, which confers to these organisms high levels of resistance to beta-lactam antibiotics. In this study, we investigated whether this genetic locus was implicated in the altered AmpC beta-lactamase expression of selected clinical isolates and laboratory mutants of Pseudomonas aeruginosa. The sequences of the ampD genes and promoter regions from these strains were determined and compared to that of wild-type ampD from P. aeruginosa PAO1. Although we identified numerous nucleotide substitutions, they resulted in few amino acid changes. The phenotypes produced by these mutations were ascertained by complementation analysis. The data revealed that the ampD genes of the P. aeruginosa mutants transcomplemented Escherichia coli ampD mutants to the same levels of beta-lactam resistance and beta-lactamase expression as wild-type ampD. Furthermore, complementation of the P. aeruginosa mutants with wild-type ampD did not restore the inducibility of beta-lactamase to wild-type levels. This shows that the amino acid substitutions identified in AmpD do not cause the altered phenotype of AmpC beta-lactamase expression in the P. aeruginosa mutants. The effects of AmpD inactivation in P. aeruginosa PAO1 were further investigated by gene replacement. This resulted in moderate-basal-level and hyperinducible expression of beta-lactamase accompanied by high levels of beta-lactam resistance. This differs from the stably derepressed phenotype reported in AmpD-defective enterobacteria and suggests that further change at another unknown genetic locus may be causing total derepressed AmpC production. This genetic locus could also be altered in the P. aeruginosa mutants studied in this work.

ampR gene mutations that greatly increase class C beta-lactamase activity in Enterobacter cloacae

The ampC and ampR genes of Enterobacter cloacae GN7471 were cloned into pMW218 to yield pKU403. Four mutant plasmids derived from pKU403 (pKU404, pKU405, pKU406, and pKU407) were isolated in an AmpD mutant of Escherichia coli ML4953 by selection with ceftazidime or aztreonam. The beta-lactamase activities expressed by pKU404, pKU405, pKU406, and pKU407 were about 450, 75, 160, and 160 times higher, respectively, than that expressed by the original plasmid, pKU403. These mutant plasmids all carried point mutations in the ampR gene. In pKU404 and pKU405, Asp-135 was changed to Asn and Val, respectively. In both pKU406 and pKU407, Arg-86 was changed to Cys. The ease of selection of AmpR mutations at a frequency of about 10(-6) in this study strongly suggests that derepressed strains, such as AmpD or AmpR mutants, could frequently emerge in the clinical setting.

Problems related to determination of MICs of oximino-type expanded-spectrum cephems for Proteus vulgaris

During in vitro susceptibility testing of clinical isolates of Proteus vulgaris, we noted that the MICs of several expanded-spectrum cephems were much higher in the broth microdilution method than in the agar dilution method (termed the MIC gap phenomenon). Here we investigated the mechanism of the MIC gap phenomenon. Cephems with the MIC gap phenomenon were of the oximino type, such as cefotaxime, cefteram, and cefpodoxime, which serve as good substrates for inducible class A beta-lactamase (CumA) enzymes produced by P. vulgaris; this finding suggests a relationship between the MIC gap phenomenon and CumA. Since peptidoglycan recycling shares a system common to that inducing CumA, we analyzed the mechanism of the MIC gap phenomenon using P. vulgaris B317 and isogenic mutants with mutations in the peptidoglycan recycling and beta-lactamase induction systems. The MIC gap phenomenon was observed in the parent strain B317 but not in B317G (cumG-defective mutant; defective peptidoglycan recycling) and B317R (cumR-defective mutant; defective CumA transcriptional regulator). No beta-lactamase activity was detected in B317G and B317R. beta-Lactamase activity and the MIC gap phenomenon were restored in B317G/pMD301 (strain transcomplemented by a cloned cumG gene) and B317R/pMD501 (strain transcomplemented by a cloned cumR gene). MICs determined by the agar dilution method increased when lower agar concentrations were used. Our results indicated that the mechanism of the MIC gap phenomenon is related to peptidoglycan recycling and CumA induction systems. However, it remains unclear how beta-lactamase induction of P. vulgaris is suppressed on agar plates.

Role of penicillin-binding proteins in the initiation of the AmpC beta-lactamase expression in Enterobacter cloacae

Penicillin-binding proteins (PBPs) are involved in the regulation of beta-lactamase expression by determining the level of anhydromuramylpeptides in the periplasmatic space. It was hypothesized that one or more PBPs act as a sensor in the beta-lactamase induction pathway. We have performed induction studies with Escherichia coli mutants lacking one to four PBPs with DD-carboxypeptidase activity. Therefore, we conclude that a strong beta-lactamase inducer must inhibit all DD-carboxypeptidases as well as the essential PBPs 1a, 1b, and/or 2.

An ampD gene in Pseudomonas aeruginosa encodes a negative regulator of AmpC beta-lactamase expression

The ampD and ampE genes of Pseudomonas aeruginosa PAO1 were cloned and characterized. These genes are transcribed in the same orientation and form an operon. The deduced polypeptide of P. aeruginosa ampD exhibited more than 60% similarity to the AmpD proteins of enterobacteria and Haemophilus influenzae. The ampD product transcomplemented Escherichia coli ampD mutants to wild-type beta-lactamase expression.

Bacterial enzymatic resistance: beta-lactamases and aminoglycoside-modifying enzymes

Numerous novel beta-lactamases and aminoglycoside-modifying enzymes with altered substrate profiles continue to be identified. Plasmid-mediated transmission of many of these enzymes readily occurs due to inclusion of the encoding genes in mobile gene cassettes. Recent crystallographic determinations of the structures of metallo-beta-lactamases and aminoglycoside-modifying enzymes provide the opportunity for the rational design of inhibitors.