Induction of a class I beta-lactamase from Citrobacter freundii in Escherichia coli requires active ftsZ but not ftsA or ftsQ products

Cell-Wall Recycling of the Gram-Negative Bacteria and the Nexus to Antibiotic Resistance

The importance of the cell wall to the viability of the bacterium is underscored by the breadth of antibiotic structures that act by blocking key enzymes that are tasked with cell-wall creation, preservation, and regulation. The interplay between cell-wall integrity, and the summoning forth of resistance mechanisms to deactivate cell-wall-targeting antibiotics, involves exquisite orchestration among cell-wall synthesis and remodeling and the detection of and response to the antibiotics through modulation of gene regulation by specific effectors. Given the profound importance of antibiotics to the practice of medicine, the assertion that understanding this interplay is among the most fundamentally important questions in bacterial physiology is credible. The enigmatic regulation of the expression of the AmpC β-lactamase, a clinically significant and highly regulated resistance response of certain Gram-negative bacteria to the β-lactam antibiotics, is the exemplar of this challenge. This review gives a current perspective to this compelling, and still not fully solved, 35-year enigma.

Providing β-lactams a helping hand: targeting the AmpC β-lactamase induction pathway

A major cause of the clinical failure of broad-spectrum β-lactam antibiotics against Pseudomonas aeruginosa and many Enterobacteriaceae species are chromosomal mutations that lead to the hyperproduction of AmpC β-lactamase. These mutations typically affect proteins within the peptidoglycan (PG) recycling pathway, as well as proteins that are modulated by metabolic intermediates of this pathway. Blocking PG recycling and associated sensing mechanisms with small-molecule inhibitors holds promise as a strategy for overcoming AmpC-mediated resistance that results from the selection of mutations during β-lactam therapy, or from the direct acquisition of infections by AmpC-producing mutants. Here we report on the structural and functional biology of potential drug targets within the Gram-negative PG recycling pathway and the utility of blocking PG recycling as a means of attenuating AmpC-mediated resistance in P. aeruginosa.

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.

The gene bolA regulates dacA (PBP5), dacC (PBP6) and ampC (AmpC), promoting normal morphology in Escherichia coli

The gene bolA has been shown to trigger the formation of osmotically stable round cells when overexpressed in stationary phase. We show that in poor growth conditions bolA is essential for normal cell morphology in stationary phase and under conditions of starvation. During exponential growth bolA promotes round morphology through a mechanism that is strictly dependent on the two main Escherichia colid,d-carboxypeptidases, PBP5 and PBP6. The results show that bolA controls the levels of transcription of dacA (PBP5), dacC (PBP6) and ampC (AmpC), a class C beta-lactamase, thus connecting for the first time penicillin binding proteins (PBPs) and beta-lactamases at the level of gene regulation. Furthermore, PBP5 and PBP6 are shown to be independently regulated and to have distinct effects on the peptidoglycan layer. The evidence presented demonstrates that bolA is a regulator of cell wall biosynthetic enzymes with different roles in cell morphology and cell division.

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.

Decreased production of AmpC-type beta-lactamases associated with the development of resistance to quinolones in Citrobacter freundii strains

The effect of fluoroquinolones in Citrobacter freundii strains that results in a decreased expression of cephalosporin-hydrolysing beta-lactamases was studied. Resistance to broad-spectrum cephalosporins and penicillins in two C. freundii clinical isolates was associated with moderate production of chromosomal AmpC-type-beta-lactamase in addition to changes in the outer membrane proteins profile with respect to wild-type C. freundii strains. Ten quinolone-resistant mutants were derived from the two clinical isolates using increasing fluoroquinolone concentrations. The level of susceptibility to cephalosporins and meropenem of these 10 mutants was increased and was associated with a 3.6-32% diminution in the hydrolyzing activity of their periplasmic extracts containing beta-lactamases on cephaloridine as compared with those from their parent strains. Susceptibility to cephalosporins and meropenem, as well as the expression of chromosomal AmpC-type-beta-lactamase in C. freundii strains, was influenced by the exposure to quinolones.

The convergence of murein recycling research with beta-lactamase research

The story of how investigation of Escherichia coli cell wall elongation evolved into a study of murein recycling and how this led to the discovery that ampG and ampD were required for both murein recycling and beta-lactamase regulation is chronicled. Preliminary information on two other genes believed to be involved in recycling, nagZ, the structural gene for beta-N-acetylglucosaminidase, and tpl, the presumed structural gene for the hypothetical tripeptide-adding enzyme, is presented. The possibility that recycling of murein fragments serves a signaling function is discussed.

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.

The signal transducer encoded by ampG is essential for induction of chromosomal AmpC beta-lactamase in Escherichia coli by beta-lactam antibiotics and 'unspecific' inducers

Chemical mutagenesis of the AmpC beta-lactamase-hyperinducible Escherichia coli strain SN0301/pNu305 carrying the cloned ampC and ampR genes from Citrobacter freundii OS60 gave four independent mutants in which beta-lactamase was no longer inducible, or was inducible only to a low level, by beta-lactam antibiotics. The genes ampC, ampR, ampD and ampE, which were essential for beta-lactamase induction, were functional in these mutants. In all four mutants, the sites of mutation were mapped to 9.9 min on the E. coli chromosome. Complementation with wild-type ampG restored inducibility of beta-lactamase to wild-type levels. The nucleotide sequence of all four mutant ampG alleles (ampG1, ampG3, ampG4 and ampG5) was determined. In three of the mutants, a single base exchange led to an amino acid change from glycine to aspartate at different sites in the deduced amino acid sequence. In the fourth mutant (ampG4), with low-level inducibility, the nucleotide sequence was identical to wild-type ampG. Spontaneous back-mutation of the chromosomal ampG1 mutant resulted in restoration of wild-type inducibility and a return to the wild-type ampG sequence. Unspecific induction by components of the growth medium was also dependent on intact ampG function.

Overproduction, solubilization, purification and DNA-binding properties of AmpR from Citrobacter freundii

AmpR belongs to the LysR family of prokaryotic DNA-binding transcriptional regulators and controls induction of the enterobacterial ampC beta-lactamase gene. The ampR gene of Citrobacter freundii was deregulated by employing the polymerase chain reaction to introduce an efficient ribosome-binding sequence and suitable restriction enzyme sites for cloning into a chemically inducible tac-promoter expression vector. When induced in Escherichia coli, the modified ampR gene rapidly overproduced the AmpR protein as an insoluble aggregate. The AmpR protein could be solubilized with 1.32 M guanidine/HCl and remained soluble when dialyzed against 0.5 M NaCl. The solubility properties of AmpR were exploited to selectively precipitate and resolubilize the protein in a nearly homogenous state. AmpR was then purified by a single gel-filtration chromatography step which demonstrated that AmpR exists in solution as a monodisperse homodimeric protein. Several milligrams of purified AmpR could be obtained routinely from a 1-1 culture of induced bacteria. A DNA-binding assay buffer containing 300 mM potassium glutamate and 30% glycerol was found to stabilize AmpR and used to demonstrate sequence-specific DNA-binding. Additionally, purified AmpR binds a half-operator DNA with an inverted-repeat sequence which competes with binding by the wild-type operator. These findings are discussed in terms of the helix-turn-helix DNA-binding motif, whereby AmpR is proposed to interact with its wild-type operator as a dimer of dimers.

Peptidoglycan tripeptide content and cross-linking are altered in Enterobacter cloacae induced to produce AmpC beta-lactamase by glycine and D-amino acids

Induction of AmpC beta-lactamase in Enterobacter cloacae ATCC 13047 by D-methionine, glycine, or D-tryptophan was accompanied by alterations in peptidoglycan composition and structure; in the case of D-methionine, it was also accompanied by morphologic changes. A decrease in peptidoglycan tripeptides was seen. With glycine, there was an increase in the proportion of diaminopimelic-diaminopimelic cross-links. The possible implications of these changes for beta-lactamase induction are discussed.

Coordinate regulation of murein peptidase activity and AmpC beta-lactamase synthesis in Escherichia coli

In the periplasmic space of Escherichia coli, the (L)-m-A2pm-(D)-m-A2pm peptide, the lipoprotein, and the AmpC beta-lactamase are controlled by growth rate. To explain this coordinate regulation, it is proposed that the AmpC protein functions as an LD-endopeptidase in addition to its known function as a beta-lactamase. As LD-peptides, DD-peptides and beta-lactams are structurally similar, LD-peptidases may belong to the larger family of DD-peptidases and serine beta-lactamases. In contrast to E. coli, many related bacteria possess an inducible AmpC protein. Several gene systems necessary for AmpC induction are known to affect various aspects of peptidoglycan metabolism. It is proposed that AmpC induction occurs indirectly via a recyclable cell wall peptide.