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.     

Inhibition of the binding of penicillin to the pneumococcal penicillin-binding proteins (PBPs) by exogenous cell wall peptides

Incubation of pneumococci with D-alanine-containing peptides naturally occurring in peptidoglycan protected cells against lysis and killing by beta-lactam antibiotics near MIC. Such peptides caused decreased binding of the antibiotic to penicillin-binding proteins (PBPs), primarily PBP 2B. This provides direct evidence in vivo for the hypothesis that beta-lactams act as substrate analogues and identifies PBP 2B as a killing target in pneumococci.     

Interspecies recombinational events during the evolution of altered PBP 2x genes in penicillin-resistant clinical isolates of Streptococcus pneumoniae

Penicillin resistance in pneumococci is due to the appearance of high molecular-weight penicillin-binding proteins (PBPs) that have reduced affinity for the antibiotic. We have compared the PBX 2x genes (pbpX) of one penicillin-susceptible and five penicillin-resistant clinical isolates of Streptococcus pneumoniae isolated from various parts of the world. All of the resistant isolates contained a low-affinity form of PBP 2x. The 2 kb region of the two penicillin-susceptible isolates differed at only eight nucleotide sites (0.4%) and resulted in one single amino acid difference in PBP 2x. In contrast, the sequences of the PBP 2x genes from the resistant isolates differed overall from those of the susceptible isolates at between 7 and 18% of nucleotide sites and resulted in between 27 and 86 amino acid substitutions in PBP 2x. The altered PBP 2x genes consisted of regions that were similar to those of susceptible strains (less than 3% diverged), alternating with regions that were very different (18-23% diverged). The presence of highly diverged regions within the PBP 2x genes of the resistant isolates contrasts with the uniformity of the sequences of the amylomaltase genes from the same isolates, and with the uniformity of the PBP 2x genes in the two susceptible isolates. It suggests that the altered PBP 2x genes have arisen by localized interspecies recombinational events involving the PBP 2x genes of closely related streptococci, as has been suggested to occur for altered PBP 2b genes (Dowson et al., 1989b). The PBP 2x genes from the resistant isolates could transform the susceptible strain R6 to increased levels of resistance to beta-lactam antibiotics, indicating that the altered forms of PBP 2x in the resistant isolates contribute to their resistance to penicillin.     

Methicillin-resistant strains of Staphylococcus aureus; presence of identical additional penicillin-binding protein in all strains examined

Abstract The methicillin-resistant strain of Staphylococcus aureus MR-1 previously reported to possess a penicillin-binding protein 3 (PBP 3) with a decreased affinity for β-lactam antibiotics was re-examined and, in common with other resistant strains, found to contain an additional PBP (PBP 2'). Expression of the additional protein, which has a very low affinity for β-lactams, was not influenced by temperature or osmolarity of the medium in contrast with strains examined previously. It was the only PBP still available to bind radioactive β-lactams and therefore still active enzymically when strain MR-1 was grown in the presence of concentrations of β-lactam antibiotics sufficient to kill sensitive strains of S. aureus . Penicillin-peptides derived by partial proteolysis of PBP 2'-penicillin complexes of MR-1 and 3 other methicillin-resistant strains appeared to be identical and different from the penicillin-peptides derived from PBP 1, PBP 2 and PBP 3, each of which gave rise to a unique series of peptides containing covalently-bound penicillin.     

Effects of Low PBP2b Levels on Cell Morphology and Peptidoglycan Composition in Streptococcus pneumoniae R6

Streptococcus pneumoniae produces two class B penicillin-binding proteins, PBP2x and PBP2b, both of which are essential. It is generally assumed that PBP2x is specifically involved in septum formation, while PBP2b is dedicated to peripheral cell wall synthesis. However, little experimental evidence exists to substantiate this belief. In the present study, we obtained evidence that strongly supports the view that PBP2b is essential for peripheral peptidoglycan synthesis. Depletion of PBP2b expression gave rise to long chains of cells in which individual cells were compressed in the direction of the long axis and looked lentil shaped. This morphological change is consistent with a role for pneumococcal PBP2b in the synthesis of the lateral cell wall. Depletion of PBP2x, on the other hand, resulted in lemon-shaped and some elongated cells with a thickened midcell region. Low PBP2b levels gave rise to changes in the peptidoglycan layer that made pneumococci sensitive to exogenously added LytA during logarithmic growth and refractory to chain dispersion upon addition of LytB. Interestingly, analysis of the cell wall composition of PBP2b-depleted pneumococci revealed that they had a larger proportion of branched stem peptides in their peptidoglycan than the corresponding undepleted cells. Furthermore, MurM-deficient mutants, i.e., mutants lacking the ability to synthesize branched muropeptides, were found to require much higher levels of PBP2b to sustain growth than those required by MurM-proficient strains. These findings might help to explain why increased incorporation of branched muropeptides is required for high-level beta-lactam resistance in S. pneumoniae.     

Targeting of PBP1 by β-lactams Determines recA/SOS Response Activation in Heterogeneous MRSA Clinical Strains

The SOS response, a conserved regulatory network in bacteria that is induced in response to DNA damage, has been shown to be associated with the emergence of resistance to antibiotics. Previously, we demonstrated that heterogeneous (HeR) MRSA strains, when exposed to sub-inhibitory concentrations of oxacillin, were able to express a homogeneous high level of resistance (HoR). Moreover, we showed that oxacillin appeared to be the triggering factor of a β-lactam-mediated SOS response through lexA/recA regulators, responsible for an increased mutation rate and selection of a HoR derivative. In this work, we demonstrated, by selectively exposing to β-lactam and non-β-lactam cell wall inhibitors, that PBP1 plays a critical role in SOS-mediated recA activation and HeR-HoR selection. Functional analysis of PBP1 using an inducible PBP1-specific antisense construct showed that PBP1 depletion abolished both β-lactam-induced recA expression/activation and increased mutation rates during HeR/HoR selection. Furthermore, based on the observation that HeR/HoR selection is accompanied by compensatory increases in the expression of PBP1,-2, -2a, and -4, our study provides evidence that a combination of agents simultaneously targeting PBP1 and either PBP2 or PBP2a showed both in-vitro and in-vivo efficacy, thereby representing a therapeutic option for the treatment of highly resistant HoR-MRSA strains. The information gathered from these studies contributes to our understanding of β-lactam-mediated HeR/HoR selection and provides new insights, based on β-lactam synergistic combinations, that mitigate drug resistance for the treatment of MRSA infections.     

Nucleotide sequences of the pbpX genes encoding the penicillin-binding proteins 2x from Streptococcus pneumoniae R6 and a cefotaxime-resistant mutant, C506

Development of penicillin resistance in Streptococcus pneumoniae is due to successive mutations in penicillin-binding proteins (PBPs) which reduce their affinity for beta-lactam antibiotics. PBP2x is one of the high-Mr PBPs which appears to be altered both in resistant clinical isolates, and in cefotaxime-resistant laboratory mutants. In this study, we have sequenced a 2564 base-pair chromosomal fragment from the penicillin-sensitive S. pneumoniae strain R6, which contains the PBP2x gene. Within this fragment, a 2250 base-pair open reading frame was found which coded for a protein having an Mr of 82.35kD, a value which is in good agreement with the Mr of 80-85 kD measured by SDS-gel electrophoresis of the PBP2x protein itself. The N-terminal region resembled an unprocessed signal peptide and was followed by a hydrophobic sequence that may be responsible for membrane attachment of PBP2x. The corresponding nucleotide sequence of the PBP2x gene from C504, a cefotaxime-resistant laboratory mutant obtained after five selection steps, contained three nucleotide substitutions, causing three amino acid alterations within the beta-lactam binding domain of the PBP2x protein. Alterations affecting similar regions of Escherichia coli PBP3 and Neisseria gonorrhoeae PBP2 from beta-lactam-resistant strains are known. The penicillin-binding domain of PBP2x shows highest homology with these two PBPs and S. pneumoniae PBP2b. In contrast, the N-terminal extension of PBP2x has the highest homology with E. coli PBP2 and methicillin-resistant Staphylococcus aureus PBP2'. No significant homology was detected with PBP1a or PBP1b of Escherichia coli, or with the low-Mr PBPs.     

A role for Ca2+ in the effect of very low frequency electromagnetic field on the blastogenesis of human lymphocytes

Methicillin-resistant clinical isolates of Staphylococcus aureus are intrinsically resistant to beta-lactam antibiotics in that the resistance mechanism is unrelated to the possession of beta-lactamases. We have demonstrated that a new, high-molecular-mass penicillin-binding protein (PBP) is present in these strains with a low affinity for beta-lactams and that its amount is regulated by the growth conditions. The new PBP from all strains that have been examined has an identical mobility on SDS gel electrophoresis and is the only PBP still present in an uncomplexed state with beta-lactams (and therefore the only functional PBP when these strains are grown in media containing concentrations of beta-lactam antibiotics sufficient to kill sensitive strains.     

Differential responses of Escherichia coli cells expressing cytoplasmic domain mutants of penicillin-binding protein 1b after impairment of penicillin-binding proteins 1a and 3

Penicillin-binding protein 1b (PBP1b) is the major high-molecular-weight PBP in Escherichia coli. Although it is coded by a single gene, it is usually found as a mixture of three isoforms which vary with regard to the length of their N-terminal cytoplasmic tail. We show here that although the cytoplasmic tail seems to play no role in the dimerization of PBP1b, as was originally suspected, only the full-length protein is able to protect the cells against lysis when both PBP1a and PBP3 are inhibited by antibiotics. This suggests a specific role for the full-length PBP1b in the multienzyme peptidoglycan-synthesizing complex that cannot be fulfilled by either PBP1a or the shorter PBP1b proteins. Moreover, we have shown by alanine-stretch-scanning mutagenesis that (i) residues R(11) to G(13) are major determinants for correct translocation and folding of PBP1b and that (ii) the specific interactions involving the full-length PBP1b can be ascribed to the first six residues at the N-terminal end of the cytoplasmic domain. These results are discussed in terms of the interactions with other components of the murein-synthesizing complex.     

Penicillin-binding protein 2b of Streptococcus pneumoniae in piperacillin-resistant laboratory mutants.

In Streptococcus pneumoniae, alterations in penicillin-binding protein 2b (PBP 2b) that reduce the affinity for penicillin binding are observed during development of beta-lactam resistance. The development of resistance was now studied in three independently obtained piperacillin-resistant laboratory mutants isolated after several selection steps on increasing concentrations of the antibiotic. The mutants differed from the clinical isolates in major aspects: first-level resistance could not be correlated with alterations in the known PBP genes, and the first PBP altered was PBP 2b. The point mutations occurring in the PBP 2b genes were characterized. Each mutant contained one single point mutation in the PBP 2b gene. In one mutant, this resulted in a mutation of Gly-617 to Ala within one of the homology boxes common to all PBPs, and in the other two cases, the same Gly-to-Asp substitution at the end of the penicillin-binding domain had occurred. The sites affected were homologous to those determined previously in the S. pneumoniae PBP 2x of mutants resistant to cefotaxime, indicating that, in both PBPs, similar sites are important for interaction with the respective beta-lactams.     

Cell wall modifications during osmotic stress in Lactobacillus casei

AIMS: To study the modification of the cell wall of Lactobacillus casei ATCC 393 grown in high salt conditions. METHODS AND RESULTS: Differences in the overall structure of cell wall between growth in high salt (MRS + 1 mol l(-1) NaCl; N condition) and control (MRS; C condition) conditions were determined by transmission electronic microscopy and analytical procedures. Lactobacillus casei cells grown in N condition were significantly larger than cells grown under unstressed C condition. Increased sensitivity to mutanolysin and antibiotics with target in the cell wall was observed in N condition. Purified cell wall also showed the increased sensitivity to lysis by mutanolysin. Analysis of peptidoglycan (PG) from stressed cells showed that modification was at the structural level in accordance with a decreased PG cross-link involving penicillin-binding proteins (PBP). Nine PBP were first described in this species and these proteins were expressed in low percentages or presented a modified pattern of saturation with penicillin G (Pen G) during growth in high salt. Three of the essential PBP were fully saturated in N condition at lower Pen G concentrations than in C condition, suggesting differences in functionality in vivo. CONCLUSIONS: The results show that growth in high salt modified the structural properties of the cell wall. SIGNIFICANCE AND IMPACT OF STUDY: Advances in understanding the adaptation to high osmolarity, in particular those involving sensitivity to lysis of lactic acid bacteria.     

Expression and purification of a soluble form of penicillin-binding protein 2 from both penicillin-susceptible and penicillin-resistant Neisseria gonorrhoeae

Resistance to penicillin in non-β-lactamase-producing strains of Neisseria gonorrhoeae (CMRNG strains) is mediated in part by the production of altered forms of penicillin-binding protein 2 (PBP 2) that have a decreased affinity for penicillin. The reduction in the affinity of PBP 2 is largely due to the insertion of an aspartic acid residue (Asp-345a) into the amino acid sequence of PBP 2. Truncated forms of N. gonorrhoeae PBP 2, which differed only by the insertion of Asp-345a, were constructed by placing the region of the penA genes encoding the periplasmic domain of PBP 2 (amino acids 42–581) into an ATG expression vector. When the recombinant PBP 2 molecules were over-expressed in Escherichia coli, insoluble PBP 2 inclusion bodies, which could be isolated by low-speed centrifugation of cell lysates, were formed. These insoluble aggregates were solubilized and the truncated PBP 2 polypeptides were partially purified by cation-exchange chromatography and gel filtration in the presence of denaturant prior to the refolding of the enzyme in vitro. After renaturation, gel filtration was used to separate monomeric soluble PBP 2 from improperly folded protein aggregates and other protein contaminants. A 4-liter culture of induced E. coli cells yielded 1.4 mg of soluble PBP 2 or PBP 2′ (PBP 2 containing the Asp-345a insertion), both of which were estimated to be 99% pure. The affinity of soluble PBP 2′ for [³H]penicillin G was decreased fourfold relative to that of soluble PBP 2, and their affinities were found to be identical to the affinities of the full-length PBP 2 enzymes that were previously determined in N. gonorrhoeae membranes. Furthermore, soluble PBP 2 displayed a rank order of affinity for several other β-lactam antibiotics that was consistent with the rank order of affinities previously reported for the native molecules. On the basis of these results, both of these soluble PBPs should be suitable for crystallization and X-ray crystallographic analysis.     

Mutations affecting penicillin-binding proteins 2a, 2b and 3 in Bacillus subtilis alter cell shape and peptidoglycan metabolism

Bacillus subtilis mutants with altered penicillin-binding proteins (PBPs), or altered expression of PBPs, were isolated by screening for changes in susceptibility to beta-lactam antibiotics. Mutations affecting only PBPs 2a, 2b and 3 were isolated. Cell shape and peptidoglycan metabolism were examined in representative mutants. Cells of a PBP 2a mutant (UB8521) were usually twisted whereas PBP 2b (UB8524) and 3 (UB8525) mutants produced helices, particularly after growth at 41 degrees C. The PBP 2a mutant (UB8521) had a higher peptidoglycan synthetic activity than its parent strain whereas the opposite applied to the PBP 2b mutant UB8524. The PBP 3 mutant (UB8525) had a similar peptidoglycan synthetic activity to that of the parent strain when grown at 37 degrees C, but 40% higher activity after growth at 41 degrees C. The PBP 2a mutant (UB8521) exhibited the same wall thickening activity as the parent, but the PBP 2b and 3 mutants (UB8524 and UB8525) were partially defective in this respect. The changes in the susceptibility of PBP 2a, 2b and 3 mutants to beta-lactam antibiotics imply that these PBPs are killing targets, consistent with the fact that these PBPs are also important for shape determination and peptidoglycan synthesis.     

Separation of abnormal cell wall composition from penicillin resistance through genetic transformation of Streptococcus pneumoniae.

Compared with most penicillin-susceptible isolates of Streptococcus pneumoniae, penicillin-resistant clinical isolate Hun 663 contains mosaic penicillin-binding protein (PBP) genes encoding PBPs with reduced penicillin affinities, anomalous molecular sizes, and also cell walls of unusual chemical composition. Chromosomal DNA prepared from Hun 663 was used to transform susceptible recipient cells to donor level penicillin resistance, and a resistant transformant was used next as the source of DNA in the construction of a second round of penicillin-resistant transformants. The greatly reduced penicillin affinity of the high-molecular-weight PBPs was retained in all transformants through both genetic crosses. On the other hand, PBP pattern and abnormal cell wall composition, both of which are stable, clone-specific properties of strain Hun 663, were changed: individual transformants showed a variety of new, abnormal PBP patterns. Furthermore, while the composition of cell walls resembled that of the DNA donor in the first-round transformants, it became virtually identical to that of susceptible pneumococci in the second-round transformants. The findings indicate that genetic elements encoding the low affinity of PBPs and the penicillin resistance of the bacteria are separable from determinants that are responsible for the abnormal cell wall composition that often accompanies penicillin resistance in clinical strains of pneumococci.     

Structural and functional difference of pheromone binding proteins in discriminating chemicals in the gypsy moth, Lymantria dispar

Pheromone-binding proteins (PBPs) of the gypsy moth, Lymantria dispar L., play an important role in olfaction. Here structures of PBPs were first built by Homology Modeling, and each model of PBPs had seven α-helices and a large hydrophobic cavity including 25 residues for PBP1 and 30 residues for PBP2. Three potential semiochemicals were first screened by CDOCKER program based on the PBP models and chemical database. These chemicals were Palmitic acid n-butyl ester (Pal), Bis(3,4-epoxycyclohexylmethyl) adipate (Bis), L-trans-epoxysuccinyl-isoleucyl-proline methyl ester propylamide (CA-074). The analysis of chemicals docking the proteins showed one hydrogen bond was established between the residues Lys94 and (+)-Disparlure ((+)-D), and л-л interactions were present between Phe36 of PBP1 and (+)-D. The Lys94 of PBP1 formed two and three hydrogen bonds with Bis and CA-074, respectively. There was no residue of PBP2 interacting with these four chemicals except Bis forming one hydrogen bond with Lys121. After simulating the conformational changes of LdisPBPs at pH7.3 and 5.5 by constant pH molecular dynamics simulation in implicit solvent, the N-terminal sequences of PBPs was unfolded, only having five α-helices, and PBP2 had larger binding pocket at 7.3 than PBP1. To investigate the changes of α-helices at different pH, far-UV and near-UV circular dichroism showed PBPs consist of α-helices, and the tertiary structures of PBP1 and PBP2 were influenced at pH7.3 and 5.5. The fluorescence binding assay indicated that PBP1 and PBP2 have similarly binding affinity to (+)-D at pH 5.5 and 7.3, respectively. At pH 5.5, the dissociation constant of the complex between PBP1 and 2-decyl-1-oxaspiro [2.2] pentane (OXP1) was 0.68 ± 0.01 μM, for (+)-D was 5.32 ± 0.11 μM, while PBP2 with OXP1 and (+)-D were 1.88 ± 0.02 μM and 5.54 ± 0.04 μM, respectively. Three chemicals screened had higher affinity to PBP1 than (+)-D except Pal at pH5.5, and had lower affinity than (+)-D at pH7.3. To PBP2, these chemicals had lower affinity than the sex pheromone except Bis at pH 5.5 and pH 7.3. Only PBP1 had higher affinity with Sal than the sex pheromone at pH 5.5. Therefore, the structures of PBP1 and PBP2 had different changes at pH5.5 and 7.3, showing different affinity to chemicals. This study helps understanding the role of PBPs as well as in developing more efficient chemicals for pest control.     

Lysis of Escherichia coli by beta-lactams which bind penicillin-binding proteins 1a and 1b: inhibition by heat shock proteins.

The heat shock proteins (HSPs) of Escherichia coli were artificially induced in cells containing the wild-type rpoH+ gene under control of a tac promoter. At 30 degrees C, expression of HSPs produced cells that were resistant to lysis by cephaloridine and cefsulodin, antibiotics that bind penicillin-binding proteins (PBPs) 1a and 1b. This resistance could be reversed by the simultaneous addition of mecillinam, a beta-lactam that binds PBP 2. However, even in the presence of mecillinam, cells induced to produce HSPs were resistant to lysis by ampicillin, which binds all the major PBPs. Lysis of cells induced to produce HSPs could also be effected by imipenem, a beta-lactam known to lyse nongrowing cells. These effects suggest the existence of at least two pathways for beta-lactam-dependent lysis, one inhibited by HSPs and one not. HSP-mediated lysis resistance was abolished by a mutation in any one of five heat shock genes (dnaK, dnaJ, grpE, GroES, or groEL). Thus, resistance appeared to depend on the expression of the complete heat shock response rather than on any single HSP. Resistance to lysis was significant in the absence of the RelA protein, implying that resistance could not be explained by activation of the stringent response. Since many environmental stresses promote the expression of HSPs, it is possible that their presence contributes an additional mechanism toward development in bacteria of phenotypic tolerance to beta-lactam antibiotics.     

Penicillin-binding proteins of Thiobacillus versutus

The cytoplasmic membrane of Thiobacillus versutus was found to contain at least nine penicillin-binding proteins (PBPs) with apparent molecular weights as judged by sodium dodecyl sulphate polyacrylamide slab gel electrophoresis of 87000 (PBP1), 81000 (PBP2), 68000 (PBP3), 63000 (PBP4), 57000 (PBP5), 40000 (PBP6), 37000 (PBP70, 33000 (PBP8) and 31000 (PBP9). The PBP pattern of T. versutus was thus quite different from that of the Enterobacteria and the Pseudomonads. Also the properties of the PBPs of T. versutus such as affinity for various beta-lactam antibiotics, heat stability and release of bound penicillin were different from similar properties of Escherichia coli, Pseudomonas aeruginosa and other gram-negative bacteria.     

Interaction between two murein (peptidoglycan) synthases, PBP3 and PBP1B, in Escherichia coli

The murein (peptidoglycan) sacculus is an essential polymer embedded in the bacterial envelope. The Escherichia coli class B penicillin-binding protein (PBP) 3 is a murein transpeptidase and essential for cell division. In an affinity chromatography experiment, the bifunctional transglycosylase-transpeptidase murein synthase PBP1B was retained by PBP3-sepharose when a membrane fraction of E. coli was applied. The direct protein-protein interaction between purified PBP3 and PBP1B was characterized in vitro by surface plasmon resonance. The interaction was confirmed in vivo employing two different methods: by a bacterial two-hybrid system, and by cross-linking/co-immunoprecipitation. In the bacterial two-hybrid system, a truncated PBP3 comprising the N-terminal 56 amino acids interacted with PBP1B. Both synthases could be cross-linked in vivo in wild-type cells and in cells lacking FtsW or FtsN. PBP1B localized diffusely and in foci at the septation site and also at the side wall. Statistical analysis of the immunofluorescence signals revealed that the localization of PBP1B at the septation site depended on the physical presence of PBP3, but not on the activity of PBP3. These studies have demonstrated, for the first time, a direct interaction between a class B PBP (PBP3) and a class A PBP (PBP1B) in vitro and in vivo, indicating that different murein synthases might act in concert to enlarge the murein sacculus during cell division.     

Mosaic pbpX genes of major clones of penicillin-resistant Streptococcus pneumoniae have evolved from pbpX genes of a penicillin-sensitive Streptococcus oralis

Penicillin-resistant clinical isolates of Streptococcus pneumoniae contain mosaic penicillin-binding protein (PBP) genes that encode PBPs with decreased affinity for β-lactam antibiotics. The mosaic blocks are believed to be the result of gene transfer of homologous PBP genes from related penicillin-resistant species. We have now identified a gene homologous to the pneumococcal PBP2x gene (pbpX) in a penicillin-sensitive Streptococcus oralis isolate M3 from South Africa that diverged by almost 20% from pbpX of penicillin-sensitive pneumococci, and a central sequence block of a mosaic pbpX gene of Streptococcus mitis strain NCTC 10712. In contrast, it differed by only 2-4% of the 1 to 1.5 kb mosaic block in pbpX genes of three genetically unrelated penicillin-resistant S. pneumoniae isolates, two of them representing clones of serotype 6B and 23F, which are prevalent in Spain and are also already found in other countries. With low concentrations of cefotaxime, transformants of the sensitive S. pneumoniae R6 strain could be selected containing pbpX genes from either S. mitis NCTC 10712 or S. oralis M3, demonstrating that genetic exchange can already occur between β-lactam-sensitive species. These data are in agreement with the assumption that PBPs as penicillin-resistance determinants have evolved by the accumulation of point mutations in genes of sensitive commensal species.     

Two distinct transpeptidation reactions during murein synthesis in Escherichia coli.

Murein synthesized in ether-permeabilized cells of Escherichia coli deficient in individual penicillin-binding proteins (PBPs) and in the presence of certain beta-lactam antibiotics was analyzed by high-pressure liquid chromatography separation of the muramidase split products. PBP 1b was found to to be the major murein synthesizing activity that was poorly compensated for by PBP 1a. A PBP 2 mutant as well as mecillinam-inhibited cells showed increased activity in the formation of oligomeric muropeptides as well as UDP-muramylpeptidyl-linked muropeptides, the reaction products of transpeptidation, bypassing the lipid intermediate. In contrast, penicillin G and furazlocillin severely inhibited these reactions but stimulated normal dimer production. It is concluded that two distinct transpeptidases exist in E. coli: one, highly sensitive to penicillin G and furazlocillin, catalyzes the formation of hyper-cross-linked muropeptides, and a second one, quite resistant to these antibiotics, synthesizes muropeptide dimers.