Deletion analysis of the essentiality of penicillin-binding proteins 1A, 2B and 2X of Streptococcus pneumoniae

New Insights into Beta-Lactam Resistance of Streptococcus pneumoniae: Serine Protease HtrA Degrades Altered Penicillin-Binding Protein 2x

Reduced amounts of the essential penicillin-binding protein 2x (PBP2x) were detected in two cefotaxime-resistant Streptococcus pneumoniae laboratory mutants C405 and C606. These mutants contain two or four mutations in the penicillin-binding domain of PBP2x, respectively. The transcription of the pbp2x gene was not affected in both mutants; thus, the reduced PBP2x amounts were likely due to post-transcriptional regulation. The mutants carry a mutation in the histidine protein kinase gene ciaH, resulting in enhanced gene expression mediated by the cognate response regulator CiaR. Deletion of htrA, encoding a serine protease regulated by CiaR, or inactivation of HtrA proteolytic activity showed that HtrA is indeed responsible for PBP2x degradation in both mutants, and that this affects β-lactam resistance. Depletion of the PBP2x_C405 in different genetic backgrounds confirmed that HtrA degrades PBP2x_C405. A GFP-PBP2x_C405 fusion protein still localized at the septum in the absence of HtrA. The complementation studies in HtrA deletion strains showed that HtrA can be overexpressed in pneumococcal cells to specific levels, depending on the genetic background. Quantitative Western blotting revealed that the PBP2x amount in C405 strain was less than 20% compared to parental strain, suggesting that PBP2x is an abundant protein in S. pneumoniae R6 strain.

Class A PBPs: It is time to rethink traditional paradigms

Until recently, class A penicillin-binding proteins (aPBPs) were the only enzymes known to catalyze glycan chain polymerization from lipid II in bacteria. Hence, the discovery of two novel lipid II polymerases, FtsW and RodA, raises new questions and has consequently received a lot of attention from the research community. FtsW and RodA are essential and highly conserved members of the divisome and elongasome, respectively, and work in conjunction with their cognate class B PBPs (bPBPs) to synthesize the division septum and insert new peptidoglycan into the lateral cell wall. The identification of FtsW and RodA as peptidoglycan glycosyltransferases has raised questions regarding the role of aPBPs in peptidoglycan synthesis and fundamentally changed our understanding of the process. Despite their dethronement, aPBPs are essential in most bacteria. So, what is their function? In this review, we discuss recent progress in answering this question and present our own views on the topic.

Class A PBPs have a distinct and unique role in the construction of the pneumococcal cell wall

Peptidoglycan, the main structural component of the bacterial cell wall, is made of glycan strands cross-linked by short peptides. It has long been assumed that class A penicillin-binding proteins (PBPs) are the only enzymes capable of synthesizing glycan strands from lipid II. Recently, however, it was discovered that two non-PBP proteins, FtsW and RodA, constitute the core peptidoglycan polymerizing enzymes of the divisome and elongasome, respectively. What, then, is the role of class A PBPs in the construction of the bacterial cell wall? In contrast to previous assumptions, our results strongly suggest that class A PBPs are not an intrinsic part of the divisome and elongasome but have important autonomous roles in construction of the fully mature bacterial cell wall.

In oval-shaped Streptococcus pneumoniae, septal and longitudinal peptidoglycan syntheses are performed by independent functional complexes: the divisome and the elongasome. Penicillin-binding proteins (PBPs) were long considered the key peptidoglycan-synthesizing enzymes in these complexes. Among these were the bifunctional class A PBPs, which are both glycosyltransferases and transpeptidases, and monofunctional class B PBPs with only transpeptidase activity. Recently, however, it was established that the monofunctional class B PBPs work together with transmembrane glycosyltransferases (FtsW and RodA) from the shape, elongation, division, and sporulation (SEDS) family to make up the core peptidoglycan-synthesizing machineries within the pneumococcal divisome (FtsW/PBP2x) and elongasome (RodA/PBP2b). The function of class A PBPs is therefore now an open question. Here we utilize the peptidoglycan hydrolase CbpD that targets the septum of S. pneumoniae cells to show that class A PBPs have an autonomous role during pneumococcal cell wall synthesis. Using assays to specifically inhibit the function of PBP2x and FtsW, we demonstrate that CbpD attacks nascent peptidoglycan synthesized by the divisome. Notably, class A PBPs could process this nascent peptidoglycan from a CbpD-sensitive to a CbpD-resistant form. The class A PBP-mediated processing was independent of divisome and elongasome activities. Class A PBPs thus constitute an autonomous functional entity which processes recently formed peptidoglycan synthesized by FtsW/PBP2×. Our results support a model in which mature pneumococcal peptidoglycan is synthesized by three functional entities, the divisome, the elongasome, and bifunctional PBPs. The latter modify existing peptidoglycan but are probably not involved in primary peptidoglycan synthesis.

The cell cycle regulator GpsB functions as cytosolic adaptor for multiple cell wall enzymes

Bacterial growth and cell division requires precise spatiotemporal regulation of the synthesis and remodelling of the peptidoglycan layer that surrounds the cytoplasmic membrane. GpsB is a cytosolic protein that affects cell wall synthesis by binding cytoplasmic mini-domains of peptidoglycan synthases to ensure their correct subcellular localisation. Here, we describe critical structural features for the interaction of GpsB with peptidoglycan synthases from three bacterial species (Bacillus subtilis, Listeria monocytogenes and Streptococcus pneumoniae) and suggest their importance for cell wall growth and viability in L. monocytogenes and S. pneumoniae. We use these structural motifs to identify novel partners of GpsB in B. subtilis and extend the members of the GpsB interactome in all three bacterial species. Our results support that GpsB functions as an adaptor protein that mediates the interaction between membrane proteins, scaffolding proteins, signalling proteins and enzymes to generate larger protein complexes at specific sites in a bacterial cell cycle-dependent manner.

PBP2b plays a key role in both peripheral growth and septum positioning in Lactococcus lactis

Lactococcus lactis is an ovoid bacterium that forms filaments during planktonic and biofilm lifestyles by uncoupling cell division from cell elongation. In this work, we investigate the role of the leading peptidoglycan synthase PBP2b that is dedicated to cell elongation in ovococci. We show that the localization of a fluorescent derivative of PBP2b remains associated to the septal region and superimposed with structural changes of FtsZ during both vegetative growth and filamentation indicating that PBP2b remains intimately associated to the division machinery during the whole cell cycle. In addition, we show that PBP2b-negative cells of L. lactis are not only defective in peripheral growth; they are also affected in septum positioning. This septation defect does not simply result from the absence of the protein in the cell growth machinery since it is also observed when PBP2b-deficient cells are complemented by a catalytically inactive variant of PBP2b. Finally, we show that round cells resulting from β-lactam treatment are not altered in septation, suggesting that shape elongation as such is not a major determinant for selection of the division site. Altogether, we propose that the specific PBP2b transpeptidase activity at the septum plays an important role for tagging future division sites during L. lactis cell cycle.

PBP2a in β-Lactam-Resistant Laboratory Mutants and Clinical Isolates: Disruption Versus Reduced Penicillin Affinity

Alterations in PBP2a have been recognized in cefotaxime-resistant laboratory mutants and β-lactam-resistant clinical isolates of Streptococcus pneumoniae. DNA sequencing revealed fundamental differences between these two settings. Internal stop codons in pbp2a occurred in all three laboratory mutants analyzed, caused by a mutation in pbp2a of mutant C604, and tandem duplications within pbp2a resulting in premature stop codons in another two mutants C403 and C406. In contrast, mosaic PBP2a genes were observed in several penicillin-resistant clinical isolates from South Africa, the Czech Republic, Hungary, and in the clone Poland^23F-16, with sequence blocks diverging from sensitive strains by over 4%. Most of these pbp2a variants except pbp2a from the South African strain contained sequences related to pbp2a of Streptococcus mitis B6, confirming that this species serves as reservoir for penicillin-resistance determinants.

Identification of EloR (Spr1851) as a regulator of cell elongation in Streptococcus pneumoniae

In a screen for mutations suppressing the lethal loss of PBP2b in Streptococcus pneumoniae we identified Spr1851 (named EloR), a cytoplasmic protein of unknown function whose inactivation removed the requirement for PBP2b as well as RodA. It follows from this that EloR and the two elongasome proteins must be part of the same functional network. This network also includes StkP, as this serine/threonine kinase phosphorylates EloR on threonine 89 (T89). We found that ΔeloR cells, and cells expressing the phosphoablative form of EloR (EloR^T89A ), are significantly shorter than wild-type cells. Furthermore, the phosphomimetic form of EloR (EloR^T89E ) is not tolerated unless the cell in addition acquires a truncated MreC or non-functional RodZ protein. By itself, truncation of MreC as well as inactivation of RodZ gives rise to less elongated cells, demonstrating that the stress exerted by the phosphomimetic form of EloR is relieved by suppressor mutations that reduce or abolish the activity of the elongasome. Of note, it was also found that loss of elongasome activity caused by truncation of MreC elicits increased StkP-mediated phosphorylation of EloR. Together, the results support a model in which phosphorylation of EloR stimulates cell elongation, while dephosphorylation has an inhibitory effect.

Promoter Identification and Transcription Analysis of Penicillin-Binding Protein Genes in Streptococcus pneumoniae R6

Penicillin-binding proteins (PBPs) are membrane-associated enzymes, which are involved in the last two steps of peptidoglycan biosynthesis, and some of them are key players in cell division. Furthermore, they are targets of β-lactams, the most widely used antibiotics. Nevertheless, very little is known about the expression and regulation of PBP genes. Using transcriptional mapping, we now determined the promoter regions of PBP genes from the laboratory strain Streptococcus pneumoniae R6 and examined the expression profile of these six promoters. The extended −10 region is highly conserved and complies with a σ^A-type promoter consensus sequence. In contrast, the −35 region is poorly conserved, indicating the possibility for differential PBP regulation. All PBP promoters were constitutively expressed and highly active during the exponential and early stationary growth phase. However, the individual expression of PBP promoters varied approximately fourfold, with pbp1a being the highest and pbp3 the lowest. Furthermore, the deletion of one nucleotide in the spacer region of the PBP3 promoter reduced pbp3 expression ∼10-fold. The addition of cefotaxime above the minimal inhibitory concentration (MIC) did not affect PBP expression in the penicillin-sensitive R6 strain. No evidence for regulation of S. pneumoniae PBP genes was obtained.

Biological and Epidemiological Features of Antibiotic-Resistant Streptococcus pneumoniae in Pre- and Post-Conjugate Vaccine Eras: a United States Perspective

Streptococcus pneumoniae inflicts a huge disease burden as the leading cause of community-acquired pneumonia and meningitis. Soon after mainstream antibiotic usage, multiresistant pneumococcal clones emerged and disseminated worldwide. Resistant clones are generated through adaptation to antibiotic pressures imposed while naturally residing within the human upper respiratory tract. Here, a huge array of related commensal streptococcal strains transfers core genomic and accessory resistance determinants to the highly transformable pneumococcus. β-Lactam resistance is the hallmark of pneumococcal adaptability, requiring multiple independent recombination events that are traceable to nonpneumococcal origins and stably perpetuated in multiresistant clonal complexes. Pneumococcal strains with elevated MICs of β-lactams are most often resistant to additional antibiotics. Basic underlying mechanisms of most pneumococcal resistances have been identified, although new insights that increase our understanding are continually provided. Although all pneumococcal infections can be successfully treated with antibiotics, the available choices are limited for some strains. Invasive pneumococcal disease data compiled during 1998 to 2013 through the population-based Active Bacterial Core surveillance program (U.S. population base of 30,600,000) demonstrate that targeting prevalent capsular serotypes with conjugate vaccines (7-valent and 13-valent vaccines implemented in 2000 and 2010, respectively) is extremely effective in reducing resistant infections. Nonetheless, resistant non-vaccine-serotype clones continue to emerge and expand.

The function of the transmembrane and cytoplasmic domains of pneumococcal penicillin-binding proteins 2x and 2b extends beyond that of simple anchoring devices

The biosynthesis of cell-wall peptidoglycan is a complex process that involves six different penicillin-binding proteins (PBPs) in Streptococcus pneumoniae. Two of these, PBP2x and PBP2b, are monofunctional transpeptidases that catalyse the formation of peptide cross-links between adjacent glycan strands. Both of them are bitopic membrane proteins with a small cytoplasmic and a large extracellular domain. PBP2x and PBP2b are essential for septal and peripheral peptidoglycan synthesis, respectively. Although several studies have investigated the properties of their extracellular catalytic domains, it is not known whether the role of their N-terminal non-catalytic domains extends beyond that of being simple anchoring devices. We therefore decided to use reciprocal domain swapping and mutational analysis to gain more information about the biological function of the membrane anchors and cytoplasmic tails of PBP2x and PBP2b. In the case of PBP2x both domains are essential, but neither the membrane anchor nor the cytoplasmic domain of PBP2x appear to serve as major localization signals. Instead, our results suggest that they are involved in interactions with other components of the divisome. Mutations of conserved amino acids in the cytoplasmic domain of PBP2x resulted in loss of function, underlining the importance of this region. The cytoplasmic domain of PBP2b could be swapped with the corresponding domain from PBP2x, whereas replacement of the PBP2b transmembrane domain with the corresponding PBP2x domain gave rise to slow-growing cells with grossly abnormal morphology. When both domains were exchanged simultaneously the cells were no longer viable.

Penicillin-binding protein 2x of Streptococcus pneumoniae: the mutation Ala707Asp within the C-terminal PASTA2 domain leads to destabilization

Streptococcus pneumoniae penicillin-binding protein 2x (PBP2x) is an enzyme involved in the last stages of peptidoglycan assembly and essential for bacterial growth and survival. PBP2x localizes to the division site, a process that depends on its Penicillin-Binding Protein And Serine-Threonine-kinase Associated (PASTA) domains, which was previously demonstrated via GFP-PBP2x in living cells. During this study a mutant strain was isolated in which the GFP-PBP2x fusion protein did not localize at division sites and it contained reduced amounts of the full-length GFP-PBP2x. We now show that this defect is due to a point mutation within the C-terminal PASTA2 domain of PBP2x. The mutant protein was analyzed in detail in terms of beta-lactam binding, functionality, and localization in live cells. We demonstrate that the mutation affects the GFP-tagged PBP2x variant severely and renders it susceptible to the protease/chaperone HtrA.

In vitro reconstitution of peptidoglycan assembly from the Gram-positive pathogen Streptococcus pneumoniae

Understanding the molecular basis of bacterial cell wall assembly is of paramount importance in addressing the threat of increasing antibiotic resistance worldwide. Streptococcus pneumoniae presents a particularly acute problem in this respect, as it is capable of rapid evolution by homologous recombination with related species. Resistant strains selected by treatment with β-lactams express variants of the target enzymes that do not recognize the drugs but retain their activity in cell wall building, despite the antibiotics being mimics of the natural substrate. Until now, the crucial transpeptidase activity that is inhibited by β-lactams was not amenable to in vitro investigation with enzymes from Gram-positive organisms, including streptococci, staphylococci, or enterococci pathogens. We report here for the first time the in vitro assembly of peptidoglycan using recombinant penicillin-binding proteins from pneumococcus and the precursor lipid II. The two required enzymatic activities, glycosyl transferase for elongating glycan chains and transpeptidase for cross-linking stem-peptides, were observed. Most importantly, the transpeptidase activity was dependent on the chemical nature of the stem-peptide. Amidation of the second residue glutamate into iso-glutamine by the recently discovered amido-transferase MurT/GatD is required for efficient cross-linking of the peptidoglycan.

Interaction of Penicillin-Binding Protein 2x and Ser/Thr protein kinase StkP, two key players in Streptococcus pneumoniae R6 morphogenesis

Bacterial cell growth and division require the co-ordinated action of peptidoglycan biosynthetic enzymes and cell morphogenesis proteins. However, the regulatory mechanisms that allow generating proper bacterial shape and thus preserving cell integrity remain largely uncharacterized, especially in ovococci. Recently, the conserved eukaryotic-like Ser/Thr protein kinase of Streptococcus pneumoniae (StkP) was demonstrated to play a major role in cell shape and division. Here, we investigate the molecular mechanisms underlying the regulatory function(s) of StkP and show that it involves one of the essential actors of septal peptidoglycan synthesis, Penicillin-Binding Protein 2x (PBP2x). We demonstrate that StkP and PBP2x interact directly and are present in the same membrane-associated complex in S. pneumoniae. We further show that they both display a late-division localization pattern at the division site and that the positioning of PBP2x depends on the presence of the extracellular PASTA domains of StkP. We demonstrate that StkP and PBP2x interaction is mediated by their extracellular regions and that the complex formation is inhibited in vitro in the presence of cell wall fragments. These data suggest that the role of StkP in cell division is modulated by an interaction with PBP2x.

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.

From models to pathogens: how much have we learned about Streptococcus pneumoniae cell division?

Streptococcus pneumoniae is an oval-shaped Gram-positive coccus that lives in intimate association with its human host, both as a commensal and pathogen. The seriousness of pneumococcal infections and the spread of multi-drug resistant strains call for new lines of intervention. Bacterial cell division is an attractive target to develop antimicrobial drugs. This review discusses the recent advances in understanding S. pneumoniae growth and division, in comparison with the best studied rod-shaped models, Escherichia coli and Bacillus subtilis. To maintain their shape, these bacteria propagate by peripheral and septal peptidoglycan synthesis, involving proteins that assemble into distinct complexes called the elongasome and the divisome, respectively. Many of these proteins are conserved in S. pneumoniae, supporting the notion that the ovococcal shape is also achieved by rounds of elongation and division. Importantly, S. pneumoniae and close relatives with similar morphology differ in several aspects from the model rods. Overall, the data support a model in which a single large machinery, containing both the peripheral and septal peptidoglycan synthesis complexes, assembles at midcell and governs growth and division. The mechanisms generating the ovococcal or coccal shape in lactic-acid bacteria have likely evolved by gene reduction from a rod-shaped ancestor of the same group.

Molecular mechanisms of β-lactam resistance in Streptococcus pneumoniae

Alterations in the target enzymes for β-lactam antibiotics, the penicillin-binding proteins (PBPs), have been recognized as a major resistance mechanism in Streptococcus pneumoniae. Mutations in PBPs that confer a reduced affinity to β-lactams have been identified in laboratory mutants and clinical isolates, and document an astounding variability of sites involved in this phenotype. Whereas point mutations are selected in the laboratory, clinical isolates display a mosaic structure of the affected PBP genes, the result of interspecies gene transfer and recombination events. Depending on the selective β-lactam, different combinations of PBP genes and mutations within are involved in conferring resistance, and astoundingly in non-PBP genes as well.

Molecular dynamics simulation of the complex PBP-2x with drug cefuroxime to explore the drug resistance mechanism of Streptococcus suis R61

Drug resistance of Streptococcus suis strains is a worldwide problem for both humans and pigs. Previous studies have noted that penicillin-binding protein (PBPs) mutation is one important cause of β-lactam antibiotic resistance. In this study, we used the molecular dynamics (MD) method to study the interaction differences between cefuroxime (CES) and PBP2x within two newly sequenced Streptococcus suis: drug-sensitive strain A7, and drug-resistant strain R61. The MM-PBSA results proved that the drug bound much more tightly to PBP2x in A7 (PBP2x-A7) than to PBP2x in R61 (PBP2x-R61). This is consistent with the evidently different resistances of the two strains to cefuroxime. Hydrogen bond analysis indicated that PBP2x-A7 preferred to bind to cefuroxime rather than to PBP2x-R61. Three stable hydrogen bonds were formed by the drug and PBP2x-A7, while only one unstable bond existed between the drug and PBP2x-R61. Further, we found that the Gln569, Tyr594, and Gly596 residues were the key mutant residues contributing directly to the different binding by pair wise energy decomposition comparison. By investigating the binding mode of the drug, we found that mutant residues Ala320, Gln553, and Thr595 indirectly affected the final phenomenon by topological conformation alteration. Above all, our results revealed some details about the specific interaction between the two PBP2x proteins and the drug cefuroxime. To some degree, this explained the drug resistance mechanism of Streptococcus suis and as a result could be helpful for further drug design or improvement.

A new morphogenesis pathway in bacteria: unbalanced activity of cell wall synthesis machineries leads to coccus-to-rod transition and filamentation in ovococci

Bacteria display a variety of shapes, which have biological relevance. In most eubacteria, cell shape is maintained by the tough peptidoglycan (PG) layer of the cell wall, the sacculus. The organization of PG synthesis machineries, orchestrated by different cytoskeletal elements, determines the specific shapes of sacculi. In rod-shaped bacteria, the actin-like (MreB) and the tubuline-like (FtsZ) cytoskeletons control synthesis of the sidewall (elongation) and the crosswall (septation) respectively. Much less is known concerning cell morphogenesis in cocci, which lack MreB proteins. While spherical cocci exclusively display septal growth, ovococci additionally display peripheral growth, which is responsible of the slight longitudinal expansion that generates their ovoid shape. Here, we report that the ovococcus Lactococcus lactis has the ability to become rod-shaped. L. lactis IL1403 wild-type cells form long aseptate filaments during both biofilm and planktonic growth in a synthetic medium. Nascent PG insertion and the division protein FtsK localize in multiple peripheral rings regularly spaced along the filaments. We show that filamentation results from septation inhibition, and that penicillin-binding proteins PBP2x and PBP2b play a direct role in this process. We propose a model for filament formation in L. lactis, and discuss the possible biological role of such morphological differentiation.

Cell-division inhibitors: new insights for future antibiotics

The growing problem of antibiotic resistance has been exacerbated by the use of new drugs that are merely variants of older overused antibiotics. While it is naive to expect to restrain the spread of resistance without controlling antibacterial usage, the desperate need for drugs with novel targets has been recognized by health organizations, industry and academia alike. The wealth of knowledge available about the bacterial cell-division pathway has aided target-driven approaches to identify novel inhibitors. Here, we discuss the therapeutic potential of inhibiting bacterial cell division, and review the progress made in this exciting new area of antibacterial discovery.

Meta-analysis of antibiotic susceptibility and the genotype of penicillin-binding proteins in Streptococcus pneumoniae

To further understanding of the mechanisms of development of resistance to penicillin in Streptococcus pneumoniae, and the role of penicillin-binding proteins (PBPs) mutations to antibiotics resistance a meta-analysis was performed. Major databases, Pubmed, Current Contents, Biosis previews, Web of Science, were searched for studies that published within 1997 through to 2007, and reported the penicillin MIC and the alteration of PBP 1a, 2b and 2x (genes or proteins) of clinical S. pneumoniae isolates. Papers were reviewed by 2 persons and used standard criteria to enroll them. Meta-analysis was performed using a random-effects model. Overall, 20 studies were included in the meta-analysis. For the included 1771 clinical S. pneumoniae isolates, the susceptibility to penicillin decreased in inverse proportion to the presence of mutated pbp genes. The mutations of the conserved amino acid motifs STMK and SRNVP of PBP 1A, STMK and LKSG of PBP2X, and SSNT of PBP2B are critical for the penicillin resistance. Those motifs can be used as markers for the penicillin susceptibility of S. pneumoniae. These results are useful in helping define the mechanism of penicillin resistance in S. pneumoniae.

Promoter and transcription analysis of penicillin-binding protein genes in Streptococcus gordonii

An optimally cross-linked peptidoglycan requires both transglycosylation and transpeptidation, provided by class A and class B penicillin-binding proteins (PBPs). Streptococcus gordonii possesses three class A PBPs (PBPs 1A, 1B, and 2A) and two class B PBPs (PBPs 2B and 2X) that are important for penicillin resistance. High-level resistance (MIC, > or =2 microg/ml) requires mutations in class B PBPs. However, although unmutated, class A PBPs are critical to facilitate resistance development (M. Haenni and P. Moreillon, Antimicrob. Agents Chemother. 50:4053-4061, 2006). Thus, their overexpression might be important to sustain the drug. Here, we determined the promoter regions of the S. gordonii PBPs and compared them to those of other streptococci. The extended -10 box was highly conserved and complied with a sigma(A)-type promoter consensus sequence. In contrast, the -35 box was poorly conserved, leaving the possibility of differential PBP regulation. Gene expression in a penicillin-susceptible parent (MIC, 0.008 microg/ml) and a high-level-resistant mutant (MIC, 2 microg/ml) was monitored using luciferase fusions. In the absence of penicillin, all PBPs were constitutively expressed, but their expression was globally increased (1.5 to 2 times) in the resistant mutant. In the presence of penicillin, class A PBPs were specifically overexpressed both in the parent (PBP 2A) and in the resistant mutant (PBPs 1A and 2A). By increasing transglycosylation, class A PBPs could promote peptidoglycan stability when transpeptidase is inhibited by penicillin. Since penicillin-related induction of class A PBPs occurred in both susceptible and resistant cells, such a mutation-independent facilitating mechanism could be operative at each step of resistance development.

Mutations in penicillin-binding protein (PBP) genes and in non-PBP genes during selection of penicillin-resistant Streptococcus gordonii

Penicillin resistance in Streptococcus spp. involves multiple mutations in both penicillin-binding proteins (PBPs) and non-PBP genes. Here, we studied the development of penicillin resistance in the oral commensal Streptococcus gordonii. Cyclic exposure of bacteria to twofold-increasing penicillin concentrations selected for a progressive 250- to 500-fold MIC increase (from 0.008 to between 2 and 4 microg/ml). The major MIC increase (> or = 35-fold) was related to non-PBP mutations, whereas PBP mutations accounted only for a 4- to 8-fold additional increase. PBP mutations occurred in class B PBPs 2X and 2B, which carry a transpeptidase domain, but not in class A PBP 1A, 1B, or 2A, which carry an additional transglycosylase domain. Therefore, we tested whether inactivation of class A PBPs affected resistance development in spite of the absence of mutations. Deletion of PBP 1A or 2A profoundly slowed down resistance development but only moderately affected resistance in already highly resistant mutants (MIC = 2 to 4 microg/ml). Thus, class A PBPs might facilitate early development of resistance by stabilizing penicillin-altered peptidoglycan via transglycosylation, whereas they might be less indispensable in highly resistant mutants which have reestablished a penicillin-insensitive cell wall-building machinery. The contribution of PBP and non-PBP mutations alone could be individualized in DNA transformation. Both PBP and non-PBP mutations conferred some level of intrinsic resistance, but combining the mutations synergized them to ensure high-level resistance (> or = 2 microg/ml). The results underline the complexity of penicillin resistance development and suggest that inhibition of transglycosylase might be an as yet underestimated way to interfere with early resistance development.

Mutational analysis of class A and class B penicillin-binding proteins in Streptococcus gordonii

High-molecular-weight (HMW) penicillin-binding proteins (PBPs) are divided into class A and class B PBPs, which are bifunctional transpeptidases/transglycosylases and monofunctional transpeptidases, respectively. We determined the sequences for the HMW PBP genes of Streptococcus gordonii, a gingivo-dental commensal related to Streptococcus pneumoniae. Five HMW PBPs were identified, including three class A (PBPs 1A, 1B, and 2A) and two class B (PBPs 2B and 2X) PBPs, by homology with those of S. pneumoniae and by radiolabeling with [3H]penicillin. Single and double deletions of each of them were achieved by allelic replacement. All could be deleted, except for PBP 2X, which was essential. Morphological alterations occurred after deletion of PBP 1A (lozenge shape), PBP 2A (separation defect and chaining), and PBP 2B (aberrant septation and premature lysis) but not PBP 1B. The muropeptide cross-link patterns remained similar in all strains, indicating that cross-linkage for one missing PBP could be replaced by others. However, PBP 1A mutants presented shorter glycan chains (by 30%) and a relative decrease (25%) in one monomer stem peptide. Growth rate and viability under aeration, hyperosmolarity, and penicillin exposure were affected primarily in PBP 2B-deleted mutants. In contrast, chain-forming PBP 2A-deleted mutants withstood better aeration, probably because they formed clusters that impaired oxygen diffusion. Double deletion could be generated with any PBP combination and resulted in more-altered mutants. Thus, single deletion of four of the five HMW genes had a detectable effect on the bacterial morphology and/or physiology, and only PBP 1B seemed redundant a priori.

Contribution of penicillin-binding protein homologs to antibiotic resistance, cell morphology, and virulence of Listeria monocytogenes EGDe

Seven open reading frames, annotated as potential penicillin-binding-protein-encoding genes (lmo0441, lmo0540, lmo1438, lmo1892, lmo2039, lmo2229, and lmo2754), were targeted for insertional mutagenesis in Listeria monocytogenes EGDe. These genes were found to contribute in various degrees to beta-lactam resistance, cell morphology, or the virulence potential of this organism.

Analysis of mutations in the pbp genes of penicillin-non-susceptible pneumococci from Turkey

Sequence analysis of the pbp genes from 20 Streptococcus pneumoniae isolates from Turkey (eight with high-level penicillin-resistance, nine with low-level penicillin-resistance, and three that were penicillin-susceptible) was performed and phylogenetic trees were constructed. Most isolates clustered together within a single branch that was distinct from sequences deposited previously in GenBank, which suggests that these isolates have probably evolved following new recombination events. The most prominent active-site mutations, which have also been associated previously with resistance, were T371A in PBP1a, E481G followed by T451A in PBP2b, and T338A in PBP2x. All isolates also possessed a (570)SVES/TK(574) block in the PBP2b sequence, instead of the QLQPT sequence of R6, which is fairly uncommon in GenBank sequences. This is the first study to analyse alterations in the pbp sequences of pneumococci isolated in Turkey.

High Prevalence of Streptococcus pneumoniae with Mutations in pbp1a, pbp2x, and pbp2b Genes of Penicillin-Binding Proteins in the Nasopharynx in Children in Japan

OBJECTIVE

To evaluate the resistances of Streptococcus pneumoniae to beta-lactams developed by stepwise alterations in high-molecular-weight penicillin-binding proteins (PBPs) with a reduced binding affinity of beta-lactams. Among the numerous mutations in pbp genes that alter the affinity for beta-lactams, the decreased affinity of PBP1A, 2X and 2B is especially important in the development of resistances to beta-lactams.

STUDY DESIGN

Retrospective review.

METHODS

In this study, we investigated the mutations in pbp1a, pbp2x, and pbp2b genes evaluated by polymerase chain reaction (PCR) in 866 pneumococcal isolates collected from the nasopharynx of Japanese children with acute otitis media.

RESULTS

210 strains (24.3%) exhibited no mutations in the three pbp genes. 333 strains (38.5%) had mutations in the three pbp genes, 78 (9.0%) in two pbp genes, whereas 245 (28.3%) displayed mutations in only one pbp gene. Among the 656 strains with mutations in pbp genes, 620 (94.5%) strains had mutations in pbp2x. The annual prevalence of antimicrobial-resistant S. pneumoniae showed a gradual increase in strains with mutations in the three pbp genes and a parallel decrease in strains without mutations.

CONCLUSIONS

PCR-based genotyping can characterize the antimicrobial resistances in pneumococci along with minimal inhibitory concentrations (MICs). Physicians should pay attention to the recent increase in antimicrobial-resistant S. pneumoniae when treating pediatric acute otitis media.

Amino acid mutations essential to production of an altered PBP 2X conferring high-level beta-lactam resistance in a clinical isolate of Streptococcus pneumoniae

Altered penicillin-binding protein 2X (PBP 2X) is a primary beta-lactam antibiotic resistance determinant and is essential to the development of penicillin and cephalosporin resistance in the pneumococcus. We have studied the importance for resistance of 23 amino acid substitutions located in the transpeptidase domain (TD) of PBP 2X from an isolate with high-level resistance, isolate 3191 (penicillin MIC, 16 mug/ml; cefotaxime MIC, 4 microg/ml). Strain R6(2X/2B/1A/mur) (for which the MICs are as described for isolate 3191) was constructed by transforming laboratory strain R6 with all the necessary resistance determinants (altered PBPs 2X, 2B, and 1A and altered MurM) from isolate 3191. Site-directed mutagenesis was used to reverse amino acid substitutions in altered PBP 2X, followed by investigation of the impact of these reversions on resistance levels in R6(2X/2B/1A/mur). Of the 23 substitutions located in the TD of PBP 2X, reversals at six positions decreased the resistance levels in R6(2X/2B/1A/mur). Reversal of the Thr338Pro and Ile371Thr substitutions individually decreased the penicillin and cefotaxime MICs to 2 and 1 microg/ml, respectively, and individually displayed the greatest impact on resistance. To a lesser extent, reversal of the Leu364Phe, Ala369Val, Arg384Gly, and Tyr595Phe substitutions individually also decreased the penicillin and cefotaxime MICs. Reversal at all six positions collectively decreased both the penicillin and the cefotaxime MICs of R6(2X/2B/1A/mur) to 0.06 microg/ml. This study confirms the essential role of altered PBP 2X as a resistance determinant. Our data reveal that, for isolate 3191, the six amino acid substitutions described above are collectively essential to the production of an altered PBP 2X required for high-level resistance to penicillin and cefotaxime.

Single-step capsular transformation and acquisition of penicillin resistance in Streptococcus pneumoniae

The capsule (cps) locus of Streptococcus pneumoniae is flanked by the pbp2x and pbp1a genes, coding for penicillin-binding proteins, enzymes involved in cell wall synthesis that are targets for beta-lactams. This linkage suggested to us that selection for beta-lactam resistance might coselect for capsular transformants. The recombination event would then involve PBP genes, as well as the cps operon, and would change both the serotype and the resistance profile of the strain. We transformed beta-lactam-susceptible strain TIGR4 by using whole genomic DNA extracted from multidrug-resistant strain GA71, a serotype 19F variant of pneumococcal clone Spain(23F)-1, and selected beta-lactam-resistant transformants. Smooth colonies appearing on selective plates were subcultured, serotyped by the Quellung reaction, and genotyped to confirm the presence of the GA71 pbp2x-cps19-pbp1a locus in the TIGR4 genetic background by restriction fragment length polymorphism analysis of the whole locus and its flanking regions. The results showed that a new serotype, combined with resistance to beta-lactams, could emerge in a susceptible strain via a single transformation event. Quantitative analysis showed that transfer of the cps locus had occurred at an elevated rate in beta-lactam-selected transformants. This suggests that in natural settings selection by host immunity and selection by antibiotics may be interrelated because of "hitchhiking" effects due to linkage of resistance determinants and the capsule locus.

Growth and division of Streptococcus pneumoniae: localization of the high molecular weight penicillin-binding proteins during the cell cycle

The bacterial peptidoglycan, the main component of the cell wall, is synthesized by the penicillin-binding proteins (PBPs). We used immunofluorescence microscopy to determine the cellular localization of all the high molecular weight PBPs of the human pathogen Streptococcus pneumoniae, for a wild type and for several PBP-deficient strains. Progression through the cell cycle was investigated by the simultaneous labelling of DNA and the FtsZ protein. Our main findings are: (i) the temporal dissociation of cell wall synthesis, inferred by the localization of PBP2x and PBP1a, from the constriction of the FtsZ-ring; (ii) the localization of PBP2b and PBP2a at duplicated equatorial sites indicating the existence of peripheral peptidoglycan synthesis, which implies a similarity between the mechanism of cell division in bacilli and streptococci; (iii) the abnormal localization of some class A PBPs in PBP-defective mutants which may explain the apparent redundancy of these proteins in S. pneumoniae.

Understanding the acylation mechanisms of active-site serine penicillin-recognizing proteins: a molecular dynamics simulation study

Beta-lactam antibiotics inhibit enzymes involved in the last step of peptidoglycan synthesis. These enzymes, also identified as penicillin-binding proteins (PBPs), form a long-lived acyl-enzyme complex with beta-lactams. Antibiotic resistance is mainly due to the production of beta-lactamases, which are enzymes that hydrolyze the antibiotics and so prevent them reaching and inactivating their targets, and to mutations of the PBPs that decrease their affinity for the antibiotics. In this study, we present a theoretical study of several penicillin-recognizing proteins complexed with various beta-lactam antibiotics. Hybrid quantum mechanical/molecular mechanical potentials in conjunction with molecular dynamics simulations have been performed to understand the role of several residues, and pK(a) calculations have also been done to determine their protonation state. We analyze the differences between the beta-lactamase TEM-1, the membrane-bound PBP2x of Streptococcus pneumoniae, and the soluble DD-transpeptidase of Streptomyces K15.

Functional characterization of penicillin-binding protein 1b from Streptococcus pneumoniae

The widespread use of antibiotics has encouraged the development of drug resistance in pathogenic bacteria. In order to overcome this problem, the modification of existing antibiotics and/or the identification of targets for the design of new antibiotics is currently being undertaken. Bifunctional penicillin-binding proteins (PBPs) are membrane-associated molecules whose transpeptidase (TP) activity is irreversibly inhibited by beta-lactam antibiotics and whose glycosyltransferase (GT) activity represents a potential target in the antibacterial fight. In this work, we describe the expression and the biochemical characterization of the soluble extracellular region of Streptococcus pneumoniae PBP1b (PBP1b*). The acylation efficiency for benzylpenicillin and cefotaxime was characterized by stopped-flow fluorometry and a 40-kDa stable TP domain was generated after limited proteolysis. In order to analyze the GT activity of PBP1b*, we developed an electrophoretic assay which monitors the fluorescence signal from PBP1b*-bound dansylated lipid II. This binding was inhibited by the antibiotic moenomycin and was specific for the GT domain, since no signal was observed in the presence of the purified functional TP domain. Binding studies performed with truncated forms of PBP1b* demonstrated that the first conserved motif of the GT domain is not required for the recognition of lipid II, whereas the second motif is necessary for such interaction.

Penicillin-binding proteins in Streptococcus agalactiae: a novel mechanism for evasion of immune clearance

Group B streptococci (GBS) remain the most significant bacterial pathogen causing neonatal sepsis, pneumonia and meningitis in the USA despite CDC-recommended chemoprophylaxis strategies for preventing infection. To cause infection pathogens such as GBS must evade recognition and clearance by the host's immune system. Strategies for avoidance of opsonization and phagocytic killing include elaboration of antiopsonophagocytic capsules and surface proteins. During screening for mutants of GBS that were attenuated for virulence in a neonatal rat sepsis model, we identified a mutant with a transposon insertion in the ponA gene. ponA encodes an extra-cytoplasmic penicillin-binding protein PBP1a, a newly identified virulence trait for GBS that promotes resistance to phagocytic killing independent of capsular polysaccharide. Complementation analysis in vivo and in vitro confirmed that the altered phenotypes observed in the mutant were due to the transposon insertion in ponA. Deletion of PBP1a does not affect C3 deposition on GBS suggesting that mechanism by which PBP1a protects GBS from phagocytic killing is distinct from the antiopsonic activity of capsular polysaccharide. This is the first report describing expression of an antiphagocytic surface protein by GBS and represents a novel mechanism for evasion of immune recognition and clearance that may explain the decreased virulence observed in Gram-positive bacterial species for penicillin-binding protein mutants.

Penicillin-binding protein 1A, 2B, and 2X alterations in Canadian isolates of penicillin-resistant Streptococcus pneumoniae

Alterations within the penicillin-binding domain of penicillin-binding protein (PBP) genes pbp1a, pbp2b, and pbp2x were determined for 15 Canadian isolates of Streptococcus pneumoniae. All penicillin-nonsusceptible S. pneumoniae isolates showed a variety of PBP 2X substitutions and contained a Thr445-Ala change after the PBP 2B SSN motif. Only isolates for which penicillin MICs were > or =0.5 micro g/ml had PBP 1A alterations near the STMK and SRN motifs. Sequence analysis revealed identical PBP 1A, PBP 2B, and PBP 2X substitution patterns among all isolates for which penicillin MICs were > or =1 micro g/ml.

Kinetic and mechanistic studies of penicillin-binding protein 2x from Streptococcus pneumoniae

High molecular weight penicillin-binding proteins (PBPs) are bifunctional enzymes that build bacterial cell walls from the glycopeptide lipid II [GlcNAc-MurNAc(L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala)-pyrophosphate-undecaprenol] by a process involving disaccharide polymerization and peptide cross-linking. The latter reaction involves acyl-transfer chemistry in which the penultimate (D)Ala first acylates the active-site serine, with release of the terminal (D)Ala, and is then transferred to the epsilon-amine of a Lys on a neighboring pentapeptide chain. These enzymes also catalyze hydrolysis of specific thioester substrates and acylation by beta-lactam antibiotics. In this paper, we explore these latter two reactions and report mechanistic experiments on the reaction of Streptococcus pneumoniae PBP 2x with N-benzoyl-(D)Ala-thioacetic acid [Bz-(D)Ala-(S)Gly] and penicillin G. For these experiments, we used PBP 2x, a soluble form of PBP 2x in which the transmembrane domain was deleted. The following results are significant: (1) pH dependencies for acylation of PBP 2x by penicillin G and Bz-(D)Ala-(S)Gly are identical, suggesting that the same ionizable residues are involved in both reactions and that these residues play the same catalytic role in the two processes. On the basis of these results, we propose a mechanistic model that is also consistent with recently published structural data [Gordon, E., et al. (2000) J. Mol. Biol. 299, 477-485]. (2) Pre-steady-state experiments for the PBP 2x-catalyzed hydrolysis of Bz-(D)Ala-(S)Gly at pH 6.5 indicate that k(c) is principally rate-limited by acylation with some contribution from deacylation. The contribution of these steps to rate limitation is pH-dependent, with acylation entirely rate-limiting at pH values less than 5.5 and deacylation principally rate-limiting above pH 8.5. (3) Results of solvent isotope effect and proton inventory experiments for acylation suggest a complex process that is at least partially rate-limited by chemistry with some involvement of changes in solvation and/or enzyme conformation. (4) Analysis of activation parameters suggests that during the acylation of PBP 2x by penicillin G the inherent chemical stability of penicillin's amide bond, as manifested in the enthalpy of activation, is offset by a favorable entropy term that reflects penicillin's rotationally constrained bicyclic system, which presumably allows a less energetically demanding entry into the transition state for acylation.

Kinetics of beta-lactam interactions with penicillin-susceptible and -resistant penicillin-binding protein 2x proteins from Streptococcus pneumoniae. Involvement of acylation and deacylation in beta-lactam resistance

Kinetic interactions of beta-lactam antibiotics such as penicillin-G and cefotaxime with normal, penicillin-susceptible PBP2x from Streptococcus pneumoniae and a penicillin-resistant PBP2x (PBP2x(R)) from a resistant clinical isolate (CS109) of the bacterium have been extensively characterized using electrospray mass spectrometry coupled with a fast reaction (quench flow) technique. Kinetic evidence for a two-step acylation of PBP2x by penicillin-G has been demonstrated, and the dissociation constant, K(d) of 0.9 mm, and the acylation rate constant, k(2) of 180 s(-1), have been determined for the first time. The millimolar range K(d) implies that the beta-lactam fits to the active site pocket of the penicillin-sensitive PBP rather poorly, whereas the extremely fast k(2) value indicates that this step contributes most of the binding affinity of the beta-lactam. The values of K(d) (4 mm) and k(2) (0.56 s(-1)) were also determined for PBP2x(R). The combined value of k(2)/K(d), known as overall binding efficiency, for PBP2x(R) (137 m(-1) s(-1)) was over 1000-fold slower than that for PBP2x (200,000 m(-1) s(-1)), indicating that a major part is played by the acylation steps in penicillin resistance. Most of the decreased binding efficiency of PBP2x(R) comes from the decreased ( approximately 300-fold) k(2). Kinetic studies of cefotaxime acylation of the two PBP2x proteins confirmed all of the above findings. Deacylation rate constants (k(3)) for the third step of the interactions were determined to be 8 x 10(-6) s(-1) for penicilloyl-PBP2x and 5.7 x 10(-4) s(-1) for penicilloyl-PBP2x(R), corresponding to over 70-fold increase of the deacylation rate for the resistant PBP2x(R). Similarly, over 80-fold enhancement of the deacylation rate was found for cefotaxime-PBP2x(R) complex (k(3) = 3 x 10(-4) s(-1)) as compared with that of cefotaxime-PBP2x complex (3.5 x 10(-6) s(-1)). This is the first time that such a significant increase of k(3) values was found for a beta-lactam-resistant penicillin-binding protein. These data indicate that the deacylation step also plays a role, which is much more important than previously thought, in PBP2x(R) resistance to beta-lactams.

The crystal structure of the penicillin-binding protein 2x from Streptococcus pneumoniae and its acyl-enzyme form: implication in drug resistance

Penicillin-binding proteins (PBPs), the primary targets for beta-lactam antibiotics, are periplasmic membrane-attached proteins responsible for the construction and maintenance of the bacterial cell wall. Bacteria have developed several mechanisms of resistance, one of which is the mutation of the target enzymes to reduce their affinity for beta-lactam antibiotics. Here, we describe the structure of PBP2x from Streptococcus pneumoniae determined to 2.4 A. In addition, we also describe the PBP2x structure in complex with cefuroxime, a therapeutically relevant antibiotic, at 2.8 A. Surprisingly, two antibiotic molecules are observed: one as a covalent complex with the active-site serine residue, and a second one between the C-terminal and the transpeptidase domains. The structure of PBP2x reveals an active site similar to those of the class A beta-lactamases, albeit with an absence of unambiguous deacylation machinery. The structure highlights a few amino acid residues, namely Thr338, Thr550 and Gln552, which are directly related to the resistance phenomenon.

Identification and characterization of the penicillin-binding protein 2a of Streptococcus pneumoniae and its possible role in resistance to beta-lactam antibiotics

To further understand the role of penicillin-binding protein 2a (PBP 2a) of Streptococcus pneumoniae in penicillin resistance, we confirmed the identity of the protein as PBP 2a. The PBP 2a protein migrated electrophoretically to a position corresponding to that of PBP 2x, PBP 2a, and PBP 2b of S. pneumoniae and was absent in a pbp2a insertional mutant of S. pneumoniae. We found that the affinities of PBP 2a for penicillins were lower than for cephalosporins and a carbapenem. When compared with other S. pneumoniae PBPs, PBP 2a exhibited lower affinities for beta-lactam antibiotics, especially penicillins. Therefore, PBP 2a is a low-affinity PBP for beta-lactam antibiotics in S. pneumoniae.

Gene disruption studies of penicillin-binding proteins 1a, 1b, and 2a in Streptococcus pneumoniae

The effects of inactivation of the genes encoding penicillin-binding protein 1a (PBP1a), PBP1b, and PBP2a in Streptococcus pneumoniae were examined. Insertional mutants did not exhibit detectable changes in growth rate or morphology, although a pbp1a pbp1b double-disruption mutant grew more slowly than its parent did. Attempts to generate a pbp1a pbp2a double-disruption mutant failed. The pbp2a mutants, but not the other mutants, were more sensitive to moenomycin, a transglycosylase inhibitor. These observations suggest that individually the pbp1a, pbp1b, and pbp2a genes are dispensable but that either pbp1a or pbp2a is required for growth in vitro. These results also suggest that PBP2a is a functional transglycosylase in S. pneumoniae.

Mutations in the active site of penicillin-binding protein PBP2x from Streptococcus pneumoniae. Role in the specificity for beta-lactam antibiotics

Penicillin-binding protein 2x (PBP2x) isolated from clinical beta-lactam-resistant strains of Streptococcus pneumoniae (R-PBP2x) have a reduced affinity for beta-lactam antibiotics. Their transpeptidase domain carries numerous substitutions compared with homologous sequences from beta-lactam-sensitive streptococci (S-PBP2x). Comparison of R-PBP2x sequences suggested that the mutation Gln552 --> Glu is important for resistance development. Mutants selected in the laboratory with cephalosporins frequently contain a mutation Thr550 --> Ala. The high resolution structure of a complex between S-PBP2x* and cefuroxime revealed that Gln552 and Thr550, which belong to strand beta3, are in direct contact with the cephalosporin. We have studied the effect of alterations at positions 552 and 550 in soluble S-PBP2x (S-PBP2x*) expressed in Escherichia coli. Mutation Q552E lowered the acylation efficiency for both penicillin G and cefotaxime when compared with S-PBP2x*. We propose that the introduction of a negative charge in strand beta3 conflicts with the negative charge of the beta-lactam. Mutation T550A lowered the acylation efficiency of the protein for cefotaxime but not for penicillin G. The in vitro data presented here are in agreement with the distinct resistance profiles mediated by these mutations in vivo and underline their role as powerful resistance determinants.

Mutational analysis of the Streptococcus pneumoniae bimodular class A penicillin-binding proteins

One group of penicillin target enzymes, the class A high-molecular-weight penicillin-binding proteins (PBPs), are bimodular enzymes. In addition to a central penicillin-binding-transpeptidase domain, they contain an N-terminal putative glycosyltransferase domain. Mutations in the genes for each of the three Streptococcus pneumoniae class A PBPs, PBP1a, PBP1b, and PBP2a, were isolated by insertion duplication mutagenesis within the glycosyltransferase domain, documenting that their function is not essential for cellular growth in the laboratory. PBP1b PBP2a and PBP1a PBP1b double mutants could also be isolated, and both showed defects in positioning of the septum. Attempts to obtain a PBP2a PBP1a double mutant failed. All mutants with a disrupted pbp2a gene showed higher sensitivity to moenomycin, an antibiotic known to inhibit PBP-associated glycosyltransferase activity, indicating that PBP2a is the primary target for glycosyltransferase inhibitors in S. pneumoniae.

Diversity of substitutions within or adjacent to conserved amino acid motifs of penicillin-binding protein 2X in cephalosporin-resistant Streptococcus pneumoniae isolates

The sequence of an approximately 1.1-kb DNA fragment of the pbp2x gene, which encodes the transpeptidase domain, was determined for 35 clinical isolates of Streptococcus pneumoniae for which the cefotaxime (CTX) MICs varied. Strains with substitutions within a conserved amino acid motif changing STMK to SAFK and a Leu-to-Val change just before the KSG motif were highly resistant to CTX (MIC, >==2 microgram/ml). Strains with substitutions adjacent to SSN or KSG motifs had low-level resistance. The amino acid substitutions were plotted on the three-dimensional crystallographic structure of the transpeptidase domain of PBP2X. Transformants containing pbp2x from strains with high-level CTX resistance increased the CTX MIC from 0. 016 microgram/ml to 0.5 to 1.0 microgram/ml.

Genetic diversity of the streptococcal competence (com) gene locus

The com operon of naturally transformable streptococcal species contains three genes, comC, comD, and comE, involved in the regulation of competence. The comC gene encodes a competence-stimulating peptide (CSP) thought to induce competence in the bacterial population at a critical extracellular concentration. The comD and comE genes are believed to encode the transmembrane histidine kinase and response regulator proteins, respectively, of a two-component regulator, with the comD-encoded protein being a receptor for CSP. Here we report on the genetic variability of comC and comD within Streptococcus pneumoniae isolates. Comparative analysis of sequence variations of comC and comD shows that, despite evidence for horizontal gene transfer at this locus and the lack of transformability of many S. pneumoniae strains in the laboratory, there is a clear correlation between the presence of a particular comC allele and the cognate comD allele. These findings effectively rule out the possibility that the presence of noncognate comC and comD alleles may be responsible for the inability to induce competence in many isolates and indicate the importance of a functional com pathway in these isolates. In addition, we describe a number of novel CSPs from disease-associated strains of S. mitis and S. oralis. The CSPs from these isolates are much more closely related to those from S. pneumoniae than to most CSPs previously reported from S. mitis and S. oralis, suggesting that these particular organisms may be a potential source of DNA in recombination events generating the mosaic structures commonly reported in genes of S. pneumoniae that are under strong selective pressure.

Glycosyltransferase domain of penicillin-binding protein 2a from Streptococcus pneumoniae is membrane associated

Penicillin-binding proteins (PBPs) are bacterial cytoplasmic membrane proteins that catalyze the final steps of the peptidoglycan synthesis. Resistance to beta-lactams in Streptococcus pneumoniae is caused by low-affinity PBPs. S. pneumoniae PBP 2a belongs to the class A high-molecular-mass PBPs having both glycosyltransferase (GT) and transpeptide (TP) activities. Structural and functional studies of both domains are required to unravel the mechanisms of resistance, a prerequisite for the development of novel antibiotics. The extracellular region of S. pneumoniae PBP 2a has been expressed (PBP 2a*) in Escherichia coli as a glutathione S-transferase fusion protein. The acylation kinetic parameters of PBP 2a* for beta-lactams were determined by stopped-flow fluorometry. The acylation efficiency toward benzylpenicillin was much lower than that toward cefotaxime, a result suggesting that PBP 2a participates in resistance to cefotaxime and other beta-lactams, but not in resistance to benzylpenicillin. The TP domain was purified following limited proteolysis. PBP 2a* required detergents for solubility and interacted with lipid vesicles, while the TP domain was water soluble. We propose that PBP 2a* interacts with the cytoplasmic membrane in a region distinct from its transmembrane anchor region, which is located between Lys 78 and Ser 156 of the GT domain.

Identification of a structural determinant for resistance to beta-lactam antibiotics in Gram-positive bacteria

Streptococcus pneumoniae is the main causal agent of pathologies that are increasingly resistant to antibiotic treatment. Clinical resistance of S. pneumoniae to beta-lactam antibiotics is linked to multiple mutations of high molecular mass penicillin-binding proteins (H-PBPs), essential enzymes involved in the final steps of bacterial cell wall synthesis. H-PBPs from resistant bacteria have a reduced affinity for beta-lactam and a decreased hydrolytic activity on substrate analogues. In S. pneumoniae, the gene coding for one of these H-PBPs, PBP2x, is located in the cell division cluster (DCW). We present here structural evidence linking multiple beta-lactam resistance to amino acid substitutions in PBP2x within a buried cavity near the catalytic site that contains a structural water molecule. Site-directed mutation of amino acids in contact with this water molecule in the "sensitive" form of PBP2x produces mutants similar, in terms of beta-lactam affinity and substrate hydrolysis, to altered PBP2x produced in resistant clinical isolates. A reverse mutation in a PBP2x variant from a clinically important resistant clone increases the acylation efficiency for beta-lactams and substrate analogues. Furthermore, amino acid residues in contact with the structural water molecule are conserved in the equivalent H-PBPs of pathogenic Gram-positive cocci. We suggest that, probably via a local structural modification, the partial or complete loss of this water molecule reduces the acylation efficiency of PBP2x substrates to a point at which cell wall synthesis still occurs, but the sensitivity to therapeutic concentrations of beta-lactam antibiotics is lost.

Unconventional organization of the division and cell wall gene cluster of Streptococcus pneumoniae

The genes responsible for cell wall biosynthesis and cell division (dcw genes) were identified and sequenced in Streptococcus pneumoniae. The genetic organization of the dcw cluster in Streptococcus pneumoniae differed significantly from the clusters of other bacteria reported to date. In particular, the genes corresponding to the 2 min region of the Escherichia coli chromosome were found distributed in three genetically separate regions of the Streptococcus pneumoniae chromosome. The first region contained the expected ftsA and ftsZ cell division genes at one end and pbp2b, ddl and murF at the other end. The murD, murG and divIB genes, always found located upstream of ftsA, were found in a second region separated from the first. A third region contained the yllC, yllD, pbp2x and mraY genes. The chromosomal region downstream of ftsZ was also sequenced and characterized. In Streptococcus pneumoniae this region contains four ORFs, all of unknown function, and an ORF encoding the Bacillus subtilis DivIVA homologue. The gene order and the organization of this region was found to be conserved in Staphylococcus aureus, Streptococcus pyogenes and Bacillus subtilis, raising the possibility that previously unidentified loci may also be involved in division.

Identification, purification, and characterization of transpeptidase and glycosyltransferase domains of Streptococcus pneumoniae penicillin-binding protein 1a

Resistance to beta-lactam antibiotics in Streptococcus pneumoniae is due to alteration of penicillin-binding proteins (PBPs). S. pneumoniae PBP 1a belongs to the class A high-molecular-mass PBPs, which harbor transpeptidase (TP) and glycosyltransferase (GT) activities. The GT active site represents a new potential target for the generation of novel nonpenicillin antibiotics. The 683-amino-acid extracellular region of PBP 1a (PBP 1a*) was expressed in Escherichia coli as a GST fusion protein. The GST-PBP 1a* soluble protein was purified, and its domain organization was revealed by limited proteolysis. A protease-resistant fragment spanning Ser 264 to Arg 653 exhibited a reactivity profile against both beta-lactams and substrate analogues similar to that of the parent protein. This protein fragment represents the TP domain. The GT domain (Ser 37 to Lys 263) was expressed as a recombinant GST fusion protein. Protection by moenomycin of the GT domain against trypsin degradation was interpreted as an interaction between the GT domain and the moenomycin.