Simultaneous deletion of D-alanine carboxypeptidase IB-C and penicillin-binding component IV in a mutant of Escherichia coli K12

Live-Cell Profiling of Penicillin-Binding Protein Inhibitors in Escherichia coli MG1655

Penicillin-binding proteins (PBPs) make up an essential class of bacterial enzymes that carry out the final steps of peptidoglycan synthesis and regulate the recycling of this polymeric structure. PBPs are an excellent drug target and have been the most clinically relevant antibacterial target since the 1940s with the introduction of β-lactams. Despite this, a large gap in knowledge remains regarding the individual function and regulation of each PBP homologue in most bacteria. This can be attributed to a lack of chemical tools and methods that enable the study of individual PBPs in an activity-dependent manner and in their native environment. The development of such methods in Gram-negative bacteria has been particularly challenging due to the presence of an outer membrane and numerous resistance mechanisms. To address this, we have developed an optimized live-cell assay for screening inhibitors of the PBPs in Escherichia coli MG1655. We utilized EDTA to permeabilize Gram-negative cells, enabling increased penetration of our readout probe, Bocillin-FL, and subsequent analysis of PBP-inhibition profiles. To identify scaffolds for future development of PBP-selective activity-based probes, we screened ten β-lactams, one diazabicyclooctane, and one monobactam for their PBP-selectivity profiles in E. coli MG1655. These results demonstrate the utility of our assay for the screening of inhibitors in live, non-hypersusceptible Gram-negative organisms.

Early midcell localization of Escherichia coli PBP4 supports the function of peptidoglycan amidases

Insertion of new material into the Escherichia coli peptidoglycan (PG) sacculus between the cytoplasmic membrane and the outer membrane requires a well-organized balance between synthetic and hydrolytic activities to maintain cell shape and avoid lysis. Since most bacteria carry multiple enzymes carrying the same type of PG hydrolytic activity, we know little about the specific function of given enzymes. Here we show that the DD-carboxy/endopeptidase PBP4 localizes in a PBP1A/LpoA and FtsEX dependent fashion at midcell during septal PG synthesis. Midcell localization of PBP4 requires its non-catalytic domain 3 of unknown function, but not the activity of PBP4 or FtsE. Microscale thermophoresis with isolated proteins shows that PBP4 interacts with NlpI and the FtsEX-interacting protein EnvC, an activator of amidases AmiA and AmiB, which are needed to generate denuded glycan strands to recruit the initiator of septal PG synthesis, FtsN. The domain 3 of PBP4 is needed for the interaction with NlpI and EnvC, but not PBP1A or LpoA. In vivo crosslinking experiments confirm the interaction of PBP4 with PBP1A and LpoA. We propose that the interaction of PBP4 with EnvC, whilst not absolutely necessary for mid-cell recruitment of either protein, coordinates the activities of PBP4 and the amidases, which affects the formation of denuded glycan strands that attract FtsN. Consistent with this model, we found that the divisome assembly at midcell was premature in cells lacking PBP4, illustrating how the complexity of interactions affect the timing of cell division initiation.

Peptidoglycan biosynthesis is a major target for antibacterials. The covalently closed peptidoglycan mesh, called sacculus, protects the bacterium from lysis due to its turgor. Sacculus growth is facilitated by the balanced activities of synthases and hydrolases, and disturbing this balance leads to cell lysis and bacterial death. Because of the large number and possible redundant functions of peptidoglycan hydrolases, it has been difficult to decipher their individual functions. In this paper we show that the DD-endopeptidase PBP4 localizes at midcell during septal peptidoglycan synthesis in Escherichia coli and is involved in the timing of the assembly and activation of the division machinery. This shows that inhibition of certain hydrolases could weaken the cells and might enhance antibiotic action.

Peptidoglycan hydrolases of Escherichia coli

The review summarizes the abundant information on the 35 identified peptidoglycan (PG) hydrolases of Escherichia coli classified into 12 distinct families, including mainly glycosidases, peptidases, and amidases. An attempt is also made to critically assess their functions in PG maturation, turnover, elongation, septation, and recycling as well as in cell autolysis. There is at least one hydrolytic activity for each bond linking PG components, and most hydrolase genes were identified. Few hydrolases appear to be individually essential. The crystal structures and reaction mechanisms of certain hydrolases having defined functions were investigated. However, our knowledge of the biochemical properties of most hydrolases still remains fragmentary, and that of their cellular functions remains elusive. Owing to redundancy, PG hydrolases far outnumber the enzymes of PG biosynthesis. The presence of the two sets of enzymes acting on the PG bonds raises the question of their functional correlations. It is difficult to understand why E. coli keeps such a large set of PG hydrolases. The subtle differences in substrate specificities between the isoenzymes of each family certainly reflect a variety of as-yet-unidentified physiological functions. Their study will be a far more difficult challenge than that of the steps of the PG biosynthesis pathway.

Beta-lactam antibiotics: from antibiosis to resistance and bacteriology

This review focuses on the era of antibiosis that led to a better understanding of bacterial morphology, in particular the cell wall component peptidoglycan. This is an effort to take readers on a tour de force from the concept of antibiosis, to the serendipity of antibiotics, evolution of beta-lactam development, and the molecular biology of antibiotic resistance. These areas of research have culminated in a deeper understanding of microbiology, particularly in the area of bacterial cell wall synthesis and recycling. In spite of this knowledge, which has enabled design of new even more effective therapeutics to combat bacterial infection and has provided new research tools, antibiotic resistance remains a worldwide health care problem.

Penicillin-binding protein 4 of Escherichia coli: molecular cloning of the dacB gene, controlled overexpression, and alterations in murein composition

The penicillin-binding protein 4 (PBP4), from Escherichia coli, a DD-carboxypeptidase/DD-endopeptidase, was purified in an enzymatically active form to homogeneity by affinity chromatography on 6-aminopenicillanic acid/Sepharose and heparin/Sepharose. Polyclonal antibodies raised against the pure protein were used to identify and isolate PBP4 overproducing clones from an E. coli expression library, which was established on the basis of a temperature-inducible runaway replication plasmid. Three positive clones were isolated, one of which carried the intact structural gene dacB that codes for PBP4, on a 1.9kb SmaI-EcoRI fragment, whereas the other two carried truncated forms of this gene. The direction of transcription was determined. The PBP4 overproducing strain, when grown in rich medium, tolerated 160-fold overexpression. After disrupting cells by sonication, the majority (80%) of the overproduced PBP4 was detected in the 100,000 X g supernatant. Southern blotting analysis using the cloned dacB gene as a probe revealed that, in contrast to that described by Takeda et al. (1981), the plasmid pLC18-38 of the Clarke-Carbon collection does not code for PBP4. The overall composition of murein, synthesized in vitro or in vivo by the PBP4 overproducing strain, as determined by high-performance liquid chromatography analysis, suggests that PBP4 is not involved in transpeptidation but exclusively catalyses a DD-carboxypeptidase and DD-endopeptidase reaction.

Properties of cell wall-associated DD-carboxypeptidase of Enterococcus hirae (Streptococcus faecium) ATCC 9790 extracted with alkali

DD-Carboxypeptidase (DD-CPase) activity of Enterococcus hirae (Streptococcus faecium) ATCC 9790 was extracted from intact bacteria and from the insoluble residue (crude cell wall fraction) of mechanically disrupted bacteria by a brief treatment at pH 10.0 (10 mM glycine-NaOH) at 0 degrees C or by extraction with any of several detergents. Extractions with high salt concentrations failed to remove DD-CPase activity from the crude wall fraction. In contrast to N-acetylmuramoylhydrolase (both muramidase 2 and muramidase 1) activities, DD-CPase activity failed to bind to insoluble cell walls or peptidoglycan matrices. Thus, whereas muramidase 1 and muramidase 2 activities can be considered to be cell wall proteins, the bulk of the data are consistent with the interpretation that the DD-CPase of this species is a membrane protein that is sometimes found in the cell wall fraction, presumably because of hydrophobic interactions with other proteins and cell wall polymers. The binding of [14C]penicillin to penicillin-binding protein 6 (43 kilodaltons) was proportional to DD-CPase activity. Kinetic parameters were also consistent with the presence of only one DD-CPase (penicillin-binding protein 6) in E. hirae.

Nucleotide sequence of the pbpA gene and characteristics of the deduced amino acid sequence of penicillin-binding protein 2 of Escherichia coli K12

We have determined the nucleotide sequence of the pbpA gene encoding penicillin-binding protein (PBP) 2 of Escherichia coli. The coding region for PBP 2 was 1899 base pairs in length and was preceded by a possible promoter sequence and two open reading frames. The primary structure of PBP 2, deduced from the nucleotide sequence, comprised 633 amino acid residues. The relative molecular mass was calculated to be 70867. The deduced sequence agreed with the NH2-terminal sequence of PBP 2 purified from membranes, suggesting that PBP 2 has no signal peptide. The hydropathy profile suggested that the NH2-terminal hydrophobic region (a stretch of 25 non-ionic amino acids) may anchor PBP 2 in the cytoplasmic membrane as an ectoprotein. There were nine homologous segments in the amino acid sequence of PBP 2 when compared with PBP 3 of E. coli. The active-site serine residue of PBP 2 was predicted to be Ser-330. Around this putative active-site serine residue was found the conserved sequence of Ser-Xaa-Xaa-Lys, which has been identified in all of the other E. coli PBPs so far studied (PBPs 1A, 1B, 3, 5 and 6) and class A and class C beta-lactamases. In the higher-molecular-mass PBPs 1A, 1B, 2 and 3, Ser-Xaa-Xaa-Lys-Pro was conserved. In the putative peptidoglycan transpeptidase domain there were six amino acid residues, which are common only in the PBPs of higher molecular mass.

Antigenic relationships among penicillin-binding proteins 1 from members of the families Pasteurellaceae and Enterobacteriaceae

Penicillin-binding proteins (PBPs) from Haemophilus influenzae RD purified by a combination of affinity chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and electroelution were used to immunize rabbits to obtain specific antisera. Antisera directed against PBP 1 (1b) of H. influenzae cross-reacted with representative organisms of the family Pasteurellaceae and with many members of the family Enterobacteriaceae but not with other gram-negative organisms. Immunization with purified PBP 3 of H. influenzae produced antisera that reacted with PBP 1 (1b) of H. influenzae and showed the same cross-reactive pattern with other species as the anti-PBP 1 antiserum. A 24,000-molecular-weight polypeptide of H. influenzae, not radiolabeled by [35S]penicillin, reacted with antisera against purified PBPs 1 (1a, 1b), 2, and 3. The results suggest that antigenic epitopes are shared among similar PBPs from related species and even among different PBPs within the same species.

Amino acid substitutions that reduce the affinity of penicillin-binding protein 3 of Escherichia coli for cephalexin

The location of amino acid substitutions that allow an enzyme to discriminate between the binding of its normal substrate and a substrate analogue may be used to identify regions of the polypeptide that fold to form the substrate binding site. We have isolated a large number of cephalexin-resistant mutants of Escherichia coli in which the resistance is due to the production of altered forms of penicillin-binding protein 3 that have reduced affinity for the antibiotic. Using three mutagens, and a variety of selection procedures, we obtained only five classes of mutants which could be distinguished by their patterns of cross-resistance to other beta-lactam antibiotics. The three classes of mutants that showed the highest levels of resistance to cephalexin were cross-resistant to several other cephalosporins but not to penicillins or to the monobactam, aztreonam. The penicillin-binding protein 3 gene from 46 independent mutants was cloned and sequenced. Each member of the five classes of cephalexin-resistant mutants had the same amino acid substitution in penicillin-binding protein 3. The mutants that showed the highest levels of resistance to cephalexin had alterations of either Thr-308 to Pro, Val-344 to Gly, or Asn-361 to Ser. The Thr-308 to Pro substitution had occurred within the beta-lactam-binding site since the adjacent residue (Ser-307) has been shown to be acylated by benzylpenicillin. The Asn-361 to Ser change occurred in a region that showed substantial similarity to regions in both penicillin-binding protein 1A and 1B and may also define a residue that is located within the beta-lactam-binding site in the three-dimensional structure of the enzyme.

Increased amounts of a novel penicillin-binding protein in a strain of methicillin-resistant Staphylococcus aureus exposed to nafcillin

In addition to the four typical penicillin-binding proteins (PBPs), a strain of heterogeneously methicillin-resistant Staphylococcus aureus produced an extra 78-kD PBP (PBP 2a) that had a low affinity for nafcillin and penicillin. Addition of nafcillin to cultures of this strain caused a rapid increase in the amount of this PBP in cell membranes. This increase occurred at subinhibitory concentrations of drug within minutes of exposure, and was blocked by inhibitors of protein and RNA synthesis. This suggests that the synthesis of PBP 2a can be stimulated by exposure to beta-lactam antibiotics. This process may, in part, explain the heterogeneity in methicillin-resistant S. aureus.

The nucleotide sequences of the ponA and ponB genes encoding penicillin-binding protein 1A and 1B of Escherichia coli K12

Penicillin-binding proteins 1A and 1B of Escherichia coli are the major peptidoglycan transglycosylase-transpeptidases that catalyse the polymerisation and insertion of peptidoglycan precursors into the bacterial cell wall during cell elongation. The nucleotide sequence of a 2764-base-pair fragment of DNA that contained the ponA gene, encoding penicillin-binding protein 1A, was determined. The sequence predicted that penicillin-binding protein 1A had a relative molecular mass of 93 500 (850 amino acids). The amino-terminus of the protein had the features of a signal peptide but it is not known if this peptide is removed during insertion of the protein into the cytoplasmic membrane. The nucleotide sequence of a 2758-base-pair fragment of DNA that contained the ponB gene, encoding penicillin-binding protein 1B, was also determined. Penicillin-binding protein 1B consists of two major components which were shown to result from the use of alternative sites for the initiation of translation. The large and small forms of penicillin-binding protein 1B were predicted to have relative molecular masses of 94 100 and 88 800 (844 and 799 amino acids). The amino acid sequences of penicillin-binding proteins 1A and 1B could be aligned if two large gaps were introduced into the latter sequence and the two proteins then showed about 30% identity. The amino acid sequences of the proteins showed no extensive similarity to the sequences of penicillin-binding proteins 3 or 5, or to the class A or class C beta-lactamases. Two short regions of amino acid similarity were, however, found between penicillin-binding proteins 1A and 1B and the other penicillin-binding proteins and beta-lactamases. One of these included the predicted active-site serine residue which was located towards the middle of the sequences of penicillin-binding proteins 1A, 1B and 3, within the conserved sequence Gly-Ser-Xaa-Xaa-Lys-Pro. The other region was 19-40 residues to the amino-terminal side of the active-site serine and may be part of a conserved penicillin-binding site in these proteins.

On the process of cellular division in Escherichia coli: nucleotide sequence of the gene for penicillin-binding protein 3

We determined the nucleotide sequence of a DNA fragment containing the ftsI gene coding for the penicillin-binding protein 3 (PBP-3), an indispensable enzyme for cell division of Escherichia coli. The entire ftsI gene was within the 2.8 kilobase PvuII fragment derived from the chromosomal segment on pLC26-6 (Nishimura et al. 1977). The coding region for PBP-3 was identified by comparison with the N-terminal amino acid sequence of in vitro synthesized PBP-3. The structural gene for ftsI consisted of 1,764 base-pairs coding for a 588 amino acid residue-polypeptide with a molecular weight of 63,850. PBP-3 synthesized in vitro showed a lower mobility in SDS-gel electrophoresis than that of the authentic PBP-3, suggesting that the primary translation product of the ftsI gene may be processed to yield mature PBP-3.

Peptidoglycan biosynthesis in Neisseria gonorrhoeae strains sensitive and intrinsically resistant to beta-lactam antibiotics

Treatment of penicillin-sensitive and intrinsically resistant Neisseria gonorrhoeae strains with their respective inhibitory concentrations of penicillin caused rapid cell death. When the peptidoglycan syntheses of these two strains were examined in the presence of penicillin, the sensitive strain continued to make this cell wall polymer for an extended time, whereas the resistant strain underwent a rapid and marked depression in synthesis. Examination of the labeled sodium dodecyl sulfate-insoluble peptidoglycan made in the presence of inhibitory concentrations of penicillin revealed further differences. The primary effect on the penicillin-sensitive gonococcus was a slight change in peptide cross-linking and a sharp decline in the degree of O-acetylation. In contrast, the resistant strain exhibited a substantial decline in cross-linking, with a very moderate change in O-acetylation. The degree of saturation of the individual penicillin-binding proteins (PBPs) was assessed under these conditions. PBP 2, which exhibits a reduced affinity for penicillin in the resistant strain, appeared to be related to O-acetylation, whereas PBP 1 was implicated in the transpeptidation reaction.

Deletion of the penicillin-binding protein 6 gene of Escherichia coli

A strain of Escherichia coli with a deletion of the penicillin-binding protein 6 gene (dacC) has been constructed. The properties of this strain establish that the complete lack of penicillin-binding protein 6 has no marked effect on the growth of E. coli.

Interaction of beta-lactam antibiotics with penicillin-binding proteins from Bacillus megaterium

The binding properties of 25 beta-lactam antibiotics to Bacillus megaterium membranes have been studied. The affinities of the antibiotics for the penicillin-binding proteins (PBPs) are also reported. We found that PBP 4 has the highest affinity for nearly all the antibiotics studied whereas PBP 5 has the lowest affinity. Both PBP 4 and PBP 5 appear to be dispensable for the maintenance of bacterial growth and survival and appear to be DD-carboxypeptidases. Only the beta-lactam cefmetazol bound preferentially to PBP 5 and has been used to study the inhibition of DD-carboxypeptidase. Comparative studies with beta-lactam that simultaneously result in (a) binding to PBPs 1 and 3, (b) inhibition of cell growth and (c) lysis, stressed the importance of PBPs 1 and 3 for cell growth and survival.

Synthesis of penicillin-binding protein 6 by stationary-phase Escherichia coli

The level of penicillin-binding protein 6, a D-alanine carboxypeptidase I, was found to be 2- to 10-fold higher in stationary-phase cells than in exponentially growing cells of Escherichia coli. This increase appeared to be due to de novo synthesis rather than to an unmasking of preexisting material. There was no comparable change in the amount of any of the other six penicillin-binding proteins.

A role in vivo for penicillin-binding protein-4 of Staphylococcus aureus

The degree of cross-linking of the peptidoglycan of Staphylococcus aureus H and mutants lacking penicillin-binding proteins 1 and 4 was studied. No major changes were observed in organisms lacking protein 1 whereas loss of protein 4 was accompanied by a marked reduction in the degree of cross-linking and the absence of a membrane-bound 'model' transpeptidase activity. A similar effect was achieved when cultures of the staphylococci were treated with the beta-lactam antibiotic cefoxitin. At low concentrations (0.05 microgram ml-1) cefoxitin shows highest affinity for protein 4 to which it appears to bind irreversibly. Treatment of the mutant lacking protein 4 with this concentration of the antibiotic did not affect the degree of cross-linkage. The possibility that the decrease in cross-linkage was a consequence of DD-carboxypeptidase activity on peptidoglycan precursors was investigated. Although both S. aureus H and the mutants possessed such activity it was insensitive to benzylpenicillin and cefoxitin and the role of this enzyme(s) in peptidoglycan biosynthesis remains unknown. We conclude that in vivo protein 4 acts as a transpeptidase involved in the secondary cross-linking of peptidoglycan and this activity is necessary to achieve the high degree of cross-linkage observed in the peptidoglycan of staphylococci.

Competition of beta-lactam antibiotics for the penicillin-binding proteins of Neisseria gonorrhoeae

The affinities of nine structurally different beta-lactam antibiotics for the three major gonococcal penicillin-binding proteins (PBPs) were determined by using a competition assay with tritium-labeled penicillin and live, growing bacteria. Each determination was carried out in parallel in isogenic pairs of penicillin-susceptible (minimal inhibitory concentration of penicillin, 0.0075 microgram/ml) and intrinsically penicillin-resistant (minimal inhibitory concentration of penicillin, 0.5 microgram/ml) cells. Evidence is presented indicating that (i) PBP 3 may be a dispensable function; (ii) acquisition of resistance is accompanied by change in the beta-lactam antibiotic affinities of PBP 2 but not of PBP 1; (iii) PBP 2 appears to be the most important physiological target in the penicillin-susceptible strain; in the penicillin-resistant strain, PBP 1 seems to assume this role. The relative affinities of various beta-lactam antibiotics for the individual PBPs showed substantial variation with the antibiotic structure.

beta-Lactam-resistant Pseudomonas aeruginosa with modified penicillin-binding proteins emerging during cystic fibrosis treatment

The emergence of beta-lactam-resistant strains of Pseudomonas aeruginosa in a cystic fibrosis patient treated with high-dose tobramycin and piperacillin was studied. Two serotypes, M and K, were present before treatment and persisted, with changes in their beta-lactam resistance spectra, during treatment. The resistance was correlated with changes in the penicillin-binding proteins (PBPs) in both serotypes. In the low-level-resistant serotype K organism, PBP-3 either was absent or had lost the ability to bind [14C]penicillin G. Tow serotypes M strains, one with low- and one with high-level resistance to several antipseudomonal beta-lactam antibiotics, were isolated at progressively later stages of therapy. Several differences were noted between the PBP patterns of the resistant M and the susceptible M strains. The affinity for [14C]penicillin G was reduced in both resistant strains. PBP bands, with the exception of PBP-6 in the most resistant M type, were barely or not detectable at a [14C]penicillin G concentration of 39 microgram/ml. The graduated decrease in affinity for [14c]penicillin G was correlated with increasing beta-lactam resistance and with an increase in the quantity of the protein corresponding to PBP-6. The emergence of the low-level-resistant strains midway through, and of the highly resistant strain in the final stages of, the reported treatment strongly suggested that the resistance resulted from mutation in those strains present before treatment selected for by the high-dose piperacillin treatment.

Topographical distribution of penicillin-binding proteins in the Escherichia coli membrane

The penicillin-binding proteins (PBPs) found in the membranes of Escherichia coli X925 minicells (primarily cell ends or septa) were compared with those found in rod-shaped cells (primarily sidewalls) in an effort to determine whether certain PBPs are unevenly distributed over the bacterial cell membrane. The seven major PBPs of E. coli were all present in minicell membranes. PBP 1B was altered in minicells, however, appearing as two bands on sodium dodecyl sulfate-polyacrylamide gels rather than the usual three. PBP 2, which is needed for longitudinal growth of the cell but not for septum formation, was significantly reduced in minicell membranes. This observation is consistent with the fact that minicells contain very little sidewall material and raises the possibility that the specialized function of PBP 2 may be determined or regulated by its uneven topographical distribution in the membrane. None of the PBPs appeared to be selectively enriched in minicell membranes.

Sulfazecin and isosulfazecin, novel beta-lactam antibiotics of bacterial origin

In the long history of screening for antibiotics, fungi and actinomycetes have been the only producers of beta-lactam antibiotics although several phytopathogenic bacteria have been reported to produce toxins with a beta-lactam structure. We report here the first evidence that novel monocyclic beta-lactam antibiotics, sulfazecin and isosulfazecin, are produced by new species of Pseudomonas.

Properties of penicillin-binding proteins in Neisseria gonorrhoeae

The properties of penicillin-binding proteins (PBPs) of Neisseria gonorrhoeae were studied by comparing PBPs of clinical isolates of different penicillin susceptibility and by putting various beta-lactam antibiotics in competition with radioactive penicillin for PBP binding. Apparent molecular weights of the three major PBPs found were 87,000 (PBP 1), 59,000 (PBP 2), and 44,000 (PBP 3). Relative penicillin resistance was associated with decreased binding to PBP 2 and, to a lesser extent, to PBP 1. Cephaloridine and benzylpenicillin, which produced spheroplasts at minimal inhibitory concentrations, bound to all three PBPs. In contrast, antibiotics which produced a majority of enlarged but apparently intact cells bound only to PBP 2 (mecillinam) or to PBPs 2 and 3 (cephalexin) at their minimal inhibitory concentrations.

Purification of penicillin-binding protein 2 of Escherichia coli

Penicillin-binding protein 2 (PBP-2) of Escherichia coli K-12 was purified by covalent affinity chromatography using 6-aminopenicillanic acid covalently coupled to carboxymethyl-Sepharose (6-APA-CM-Sepharose). Purification of PBP-2 was accomplished by prebinding the methoxy cephalosporin, cefoxitin, to the Triton X-100-solubilized PBPs of E. coli and then incubating the PBPs with 6-APA-CM-Sepharose. Cefoxitin readily binds to all the E. coli PBPs except PBP-2 and, thus, in the presence of cefoxitin, only PBP-2 could bind to the 6-APA-CM-Sepharose. The purification of a mixture of all of the PBPs of E. coli by affinity chromatography is also described.

Deletion of the penicillin-binding protein 5 gene of Escherichia coli

A strain of Escherichia coli that has a deletion of the entire dacA gene has been constructed. The complete lack of penicillin-binding protein 5 in this strain establishes that the activity of this protein is not essential for the growth of E. coli.

In vivo interaction of beta-lactam antibiotics with the penicillin-binding proteins of Streptococcus pneumoniae

The interactions of several beta-lactam antibiotics with the penicillin-binding proteins (PBPs) of Streptococcus pneumoniae have been studied using whole organisms treated with such antibiotics and subsequently with [3H]benzylpenicillin. Differences in chemical structure were shown to cause major and selective changes in the affinities of the beta-lactams for the PBPs Only 4 of the 28 compounds tested induced a specific morphological effect (enlargement of the equatorial region) under the particular conditions tested. In 12 of the 18 beta-lactams studied, a close correlation was found between the minimal inhibitory concentrations and the concentrations required to half-saturate PBP2b. However, such a correlation was no longer apparent when the bacteria were treated with the antibiotics at their minimal inhibitory concentrations. These findings are discussed in the context of various approaches that have been used to identify the growth-inhibitory targets of beta-lactam antibiotics in bacteria.

Affinity of cefoperazone for penicillin-binding proteins

Cefoperazone (T-1551, CFP) a new semisynthetic cephalosporin, has a broad spectrum of antibacterial activity. We investigated the affinity of CFP to penicillin-binding proteins (PBPs) and the inhibition of peptidoglycan synthesis by CFP. CFP had high affinities for Escherichia coli PBP-3, -1Bs, -2, and -1A, in descending order, and low affinities for PBP-4, -5, and -6. Similarly, CFP showed high affinity for Pseudomonas aeruginosa PBP-3, -1A, -1B, -2, and -4, in descending order. It is known that E. coli PBP-3 and P. aeruginosa PBP-3 participate in cell division. These results are in good agreement with the formation of filamentous cells of E. coli and P. aeruginosa treated with CFP. CFP had lower inhibitory activities on D-alanine carboxypeptidase IA and IB of E. coli than that of penicillin G, but its inhibitory activities on the cross-link formation in peptidoglycan synthesis were the same as those of penicillin G and higher than those of ampicillin.

Cefroxadine (CGP-9000), an orally active cephalosporin

Cefroxadine (GCP-9000; CXD), 7 beta[D-2-amino-2-(1,4-cyclohexadienyl)-acetamido]-3-methoxy-ceph-3-em-carboxylic acid, is a new orally active cephalosporin derivative. The spectrum of antibacterial activity of CXD is identical with that of cephalexin (CEX), but CXD was twofold more effective than CEX against Escherichia coli and Klebsiella pneumoniae, CXD was as stable to penicillinase as CEX, but it was hydrolyzed by cephalosporinase, with a relative rate of hydrolysis similar to that of CEX. The affinities of CXD and CEX to penicillin-binding proteins of E. coli were estimated; the affinity of CXD to penicillin-binding protein 1Bs was higher than that of CEX. Consistent with this, CXD had more intensive lytic activity than CEX. In vivo antibacterial activities of CXD and CEX were compared using systemic infections of mice with E. coli and K. pneumoniae; CXD was consistently more active than CEX.

Penicillin-binding proteins in bacteria

The penicilllin-binding proteins (PBPs) of several gram-positive and gram-negative bacteria have been examined. The results indicate that: (i) PBPs are membrane proteins with molecular weights ranging from 40,000 to 120,000. When extracted with Triton X-100 from sonicated cells, they appear to fall into two patterns: one found in rods and the other in spheres. A major difference is in the low-molecular-weight component, which is usually the major PBP in bacilli but a minor one in cocci. (ii) There is a wide variation in both the number and the amount of PBPs in different bacteria, and taxonomically related bacteria tend to have similar PBP patterns. These patterns often correlate with the affinity of PBPs for penicillin and other beta-lactam antibiotics. (iii) The low-molecular-weight component usually releases penicillin spontaneously with a half-life of 10 min or less. Most, but not all, PBPs release bound penicillin in the presence of neutral hydroxylamine (0.2 to 0.8 M).

Purification of two DD-carboxypeptidases/transpeptidases with different penicillin sensitivities from Proteus mirabilis

Two membrane-bound enzymes of Proteus mirabilis with the dual functions of peptidoglycan DD-carboxypeptidase and transpeptidase (named DD-carboxypeptidase/transpeptidase H and L) were isolated and purified by selective solubilization with the nonionic detergent Genapol X-100, affinity chromatography on matrix-bound ampicillin, and preparative isoelectric focusing in the presence of detergent. Purified enzymes H and L were, respectively, penicillin-binding proteins 4 and 5 among seven major penicillin-binding proteins present in P. mirabilis. The enzymes differed in the following properties. Enzyme H had an Mr of 49,000; isoelectric point at pH 8.2; high sensitivity to benzylpenicillin and permanent inactivation because of high stability of the enzyme-antibiotic complex EI* (half-life 300 min); fragmentation of benzylpenicillin with formation of phenylacetylglycine during the slow decay of EI*; it functioned as an endopeptidase on peptide-crosslinked side chains of peptidoglycan. Enzyme L had an Mr of 43 000; isoelectric point at pH 5.9; low sensitivity to benzylpenicillin and low stability of EI* (half-life 7.2 min) with rapid recovery of enzyme activity; no function as an endopeptidase. The properties of enzyme L were identical with those of the single active DD-carboxypeptidase found previously in the spheroplast L-form of P. mirabilis grown in the presence of benzylpenicillin. We conclude that the partial penicillin resistance of P. mirabilis, with growth as L-form and synthesis of peptide-crosslinked peptidoglycan, depends on the continuing fuction of enzyme L as a DD-carboxypeptidase and transpeptidase in the presence of the antibiotic.

Autolysis of Escherichia coli

Autolysis of unwashed exponential-phase Escherichia coli cells was efficiently promoted by first submitting them to a quick downshock with distilled water before an upshock with 0.5 M sodium acetate, pH 6.5. The association of these two osmotic shocks had a remarkable synergistic effect and led to significant decreases in turbidity and viability. Different factors influencing the rate of cell lysis were examined. A close correlation was established between autolysis and the degradation of peptidoglycan. Both phenomena were induced by the same shock treatment, followed similar kinetics, and were efficiently blocked by addition of divalent cations. Cell lysis was also inducible by a shock treatment with 10(-3) M ethylenediaminetetraacetic acid or ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid and blocked by the addition of divalent cations.

Affinities of penicillins and cephalosporins for the penicillin-binding proteins of Escherichia coli K-12 and their antibacterial activity

The affinities of a range of penicillins and cephalosporins for ther penicillin-binding proteins of Escherichia coli K-12 have been studied, and the results were compared with the antibacterial activity of the compounds against E. coli K-12 and an isogenic permeability mutant. Different penicillins and cephalosporins exhibited different affinities for the "essential" penicillin-binding proteins of E. coli K-12, in a manner which directly correlated with their observed effects upon bacterial morphology. Furthermore, the affinities of the compounds for their "primary" lethal penicillin-binding protein targets showed close agreement with their antibacterial activities against the permeability mutant.

Comparative studies of penicillin-binding proteins in Pseudomonas aeruginosa and Escherichia coli

Penicillin-binding proteins in Pseudomonas aeruginosa were compared with those of Escherichia coli. These in P. aeruginosa were found exclusively in the cytoplasmic membrane fraction (fraction soluble in sodium N-lauroyl sarcosinate). Sodium dodecyl sulfate/acrylamide gel electrophoresis of the proteins bound to [14C]penicillin G resulted in the separation of six major bands and several minor bands. The proteins in these bands are referred to as proteins 1A, 1B, 2, 3, 4 and 5 in order of increasing electrophoretical mobility. The electrophoretic mobilities and other properties of penicillin-binding proteins in P. aeruginosa and E. coli were compared and correlated. Fundamentally they seem to be very similar in the two bacteria, but proteins 1A and 1B in P. aeruginosa seem to correspond respectively to proteins 1B and 1A in E. coli, and protein 6 seems to be missing or present in only small amount in P. aeruginosa. In addition, the affinities of currently developed beta-lactam antibiotics to each protein of P. aeruginosa and E. coli were examined in relation to the morphological changes of the cells induced by these antibiotics and their antibacterial potencies. Mecillinam showed high affinity to only protein 2 in both P. aeruginosa and E. coli. At a minimal inhibitory concentration, it converted cells of both P. aeruginosa and E. coli from rods to spherical cells, although its minimal inhibitory concentration was much higher for P. aeruginosa than for E. coli.

Competition of beta-lactam antibiotics for the penicillin-binding proteins of Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella aerogenes, Proteus rettgeri, and Escherichia coli: comparison with antibacterial activity and effects upon bacterial morphology

The competition of a number of beta-lactam morphogenic probes for the penicillin-binding proteins (PBPs) of Pseudomonas aeruginosa, Enterobacter cloacae, Klebsiella aerogenes, Proteus rettgeri, and Escherichia coli has been studied. The results indicate that the various gram-negative bacteria have similar, but not identical, PBP patterns and that the individual proteins probably perform similar morphogenic functions as in E. coli K-12. Comparison of the 50% binding concentrations of the compounds for the various PBPs of the five strains with their antibacterial activity indicates that the different antibiotics are excluded to a greater or lesser degree by the outer membrane permeability barrier and that the exclusion is most pronounced in P. aeruginosa.

Behavior of penicillin-binding proteins in Escherichia coli upon heat and detergent treatments and partial purification of penicillin-binding proteins 1A and 1B

Penicillin-binding proteins differ greatly in heat sensitivity and sensitivity to detergents. The partial purification of penicillin-binding 1A and 1B proteins from Escherichia coli is described.

Inhibition of Escherichia coli K-12 by beta-lactam antibiotics with poor antibacterial activity: interaction of permeability and intrinsic activity against penicillin-binding proteins

The effect of methicillin, cloxacillin, 1078/1/1, penicillin G, and cephaloridine upon the penicillin-binding proteins of a permeability mutant of Escherichia coli K-12 and its isogenic wild type have been investigated. Comparison of the 50% inhibition values for the antibiotics against the penicillin-binding proteins of the two strains with the minimal inhibitory concentrations for the same compounds indicates that methicillin, cloxacillin, 1078/1/1, and to a lesser extent penicillin G, owe their poor antibacterial activity to exclusion from the bacterial cell, whereas cephaloridine is not excluded and is equally active against both the mutant and its wild type. The results further suggest that the lesion in the permeability mutant E. coli DC2 allows free access of all the compounds tested to the inner membrane target proteins.

Penicillin-binding proteins in Proteus species

Penicillin-binding proteins in three species of Proteus, Proteus mirabilis, P. morganii, and P. rettgeri, were investigated by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Penicillin-binding proteins in these Proteus species were compared with those in Escherichia coli K-12. An approximate correlation between penicillin-binding proteins in E. coli and those in Proteus species was shown by several criteria: electrophoretic mobilities; affinities of several beta-lactam antibiotics which show characteristic patterns of binding to penicillin-binding proteins in E. coli; relation between affinities of antibiotics to the proteins and effects on morphological changes in Proteus species; location of beta-lactamase activity among penicillin-binding proteins; and thermostability. The electrophoretic mobilities and several other characteristics of penicillin-binding proteins among the Proteus species examined were found to be similar from species to species and differed only slightly from those of E. coli.

Morphology of an Escherichia coli mutant with a temperature-dependent round cell shape

Mutants of Escherichia coli capable of growing in the presence of 10 microgram of mecillinam per ml were selected after intensive mutagenesis. Of these mutants, 1.4% formed normal, rod-shaped cells at 30 degrees C but grew as spherical cells at 42 degrees C. The phenotype of one of these rod(Ts) mutants was 88% cotransducible with lip (14.3 min), and all lip+ rod(Ts) transductants of a lip recipient had the following characteristics: (i) growth was relatively sensitive to mecillinam at 30 degrees C but relatively resistant to mecillinam at 42 degrees C; (ii) penicillin-binding protein 2 was present in membranes of cells grown at 30 degrees C in reduced amounts and was undetectable in the membranes of cells grown at 42 degrees C. The mecillinam resistance, penicillin-binding protein 2 defect, and rod phenotypes all cotransduced with lip with high frequency. Thus the mutation [rodA(Ts)] is most likely in the gene for penicillin-binding protein 2 and causes the organism to grow as a sphere at 42 degrees C, although it grows with normal rodlike morphology at 30 degrees C. At 42 degrees C, cells of this strain were round with many wrinkles on their surfaces, as revealed by scanning electron microscopy. In these round cells, chromosomes were dispersed or distributed peripherally, in contrast to normal rod-shaped cells which had centrally located, more condensed chromosomes. The round cells divided asymmetrically on solid agar, and it seemed that the plane of each successive division was perpendicular to the preceding one. On temperature shift-down in liquid medium many cells with abnormal morphology appeared before normal rod-shaped cells developed. Few abnormal cells were seen when cells were placed on solid medium during temperature shift-down. These pleiotropic effects are presumably caused by one or more mutations in the rodA gene.

New cephamycin antibiotic, CS-1170: binding affinity to penicillin-binding proteins and inhibition of peptidoglycan cross-linking reactions in Escherichia coli

The binding activity of CS-1170, a new cephamycin antibiotic, to penicillin-binding proteins (PBPs) in Escherichia coli and Proteus species and the potency of this antibiotic in vitro to inhibit enzymes involved in peptidoglycan cross-linking in E. coli were tested. Similar experiments were carried out with the 7alpha-H analog of CS-1170, R-45656, and the results were compared with those obtained with CS-1170. CS-1170 showed high affinities (compared with that of penicillin G) for E. coli PBP-1A, -1Bs, and -3, the PBPs of higher molecular weight, but not PBP-2. It also inhibited the in vitro peptidoglycan cross linking reaction and concomitant release of d-alanine at very low concentrations (approximately its minimal inhibitory concentration). This antibiotic also showed very high affinity for PBP-4, -5, and -6, the PBPs of lower molecular weight, and at extremely low concentrations it inhibited d-alanine carboxypeptidases IA and IB, corresponding to PBP-5/6 and PBP-4, respectively. CS-1170 seemed to be resistant to the beta-lactamase activity of PBP-5 and -6 in E. coli and Proteus species. R-45656 showed as high an affinity for PBP-1A, -1Bs, and -3 as CS-1170, but unlike CS-1170, it had low affinities for PBP-4, -5, and -6. The concentrations of R-45656 required for inhibition of d-alanine carboxypeptidases IA and IB were also much higher than those of CS-1170. R-45656 showed rather low activities in inhibiting the in vitro cross-linking reaction of peptidoglycan and concomitant release of d-alanine. Synergism was observed in 9 of 22 strains examined between CS-1170 and mecillinam, which bound specifically to PBP-2.