Acquisition of five high-Mr penicillin-binding protein variants during transfer of high-level beta-lactam resistance from Streptococcus mitis to Streptococcus pneumoniae

Developing multispecies quorum-sensing modulators based on the Streptococcus mitis competence-stimulating peptide

Bacteria utilize quorum sensing (QS) to coordinate many group behaviors. As such, QS has attracted significant attention as a potential mean to attenuate bacterial infectivity without introducing selective pressure for resistance development. Streptococcus mitis, a human commensal, acts as a genetic diversity reservoir for Streptococcus pneumoniae, a prevalent human pathogen. S. mitis possesses a typical comABCDE competence regulon QS circuitry; however, the competence-stimulating peptide (CSP) responsible for QS activation and the regulatory role of the competence regulon QS circuitry in S. mitis are yet to be explored. We set out to delineate the competence regulon QS circuitry in S. mitis, including confirming the identity of the native CSP signal, evaluating the molecular mechanism that governs CSP interactions with histidine kinase receptor ComD leading to ComD activation, and defining the regulatory roles of the competence regulon QS circuitry in initiating various S. mitis phenotypes. Our analysis revealed important structure-activity relationship insights of the CSP signal and facilitated the development of novel CSP-based QS modulators. Our analysis also revealed the involvement of the competence regulon in modulating competence development and biofilm formation. Furthermore, our analysis revealed that the native S. mitis CSP signal can modulate QS response in S. pneumoniae. Capitalizing on this crosstalk, we developed a multispecies QS modulator that activates both the pneumococcus ComD receptors and the S. mitis ComD-2 receptor with high potencies. The novel scaffolds identified herein can be utilized to evaluate the effects temporal QS modulation has on S. mitis as it inhabits its natural niche.

Loss of Pde1 function acts as an evolutionary gateway to penicillin resistance in Streptococcus pneumoniae

Streptococcus pneumoniae is a major human pathogen and rising resistance to β-lactam antibiotics, such as penicillin, is a significant threat to global public health. Mutations occurring in the penicillin-binding proteins (PBPs) can confer high-level penicillin resistance but other poorly understood genetic factors are also important. Here, we combined strictly controlled laboratory experiments and population analyses to identify a new penicillin resistance pathway that is independent of PBP modification. Initial laboratory selection experiments identified high-frequency pde1 mutations conferring S. pneumoniae penicillin resistance. The importance of variation at the pde1 locus was confirmed in natural and clinical populations in an analysis of >7,200 S. pneumoniae genomes. The pde1 mutations identified by these approaches reduce the hydrolytic activity of the Pde1 enzyme in bacterial cells and thereby elevate levels of cyclic-di-adenosine monophosphate and penicillin resistance. Our results reveal rapid de novo loss of function mutations in pde1 as an evolutionary gateway conferring low-level penicillin resistance. This relatively simple genomic change allows cells to persist in populations on an adaptive evolutionary pathway to acquire further genetic changes and high-level penicillin resistance.

Gaps in the wall: understanding cell wall biology to tackle amoxicillin resistance in Streptococcus pneumoniae

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, and one of the main pathogens responsible for otitis media infections in children. Amoxicillin (AMX) is a broad-spectrum β-lactam antibiotic, used frequently for the treatment of bacterial respiratory tract infections. Here, we discuss the pneumococcal response to AMX, including the mode of action of AMX, the effects on autolysin regulation, and the evolution of resistance through natural transformation. We discuss current knowledge gaps in the synthesis and translocation of peptidoglycan and teichoic acids, major constituents of the pneumococcal cell wall and critical to AMX activity. Furthermore, an outlook of AMX resistance research is presented, including the development of natural competence inhibitors to block evolution via horizontal gene transfer, and the use of high-throughput essentiality screens for the discovery of novel cotherapeutics.

Interspecies recombination, not de novo mutation, maintains virulence after β-lactam resistance acquisition in Streptococcus pneumoniae

As opposed to de novo mutation, β-lactam resistance in S. pneumoniae is often conferred via homologous recombination during horizontal gene transfer. We hypothesize that β-lactam resistance in pathogenic streptococci is restricted to naturally competent species via intra-/interspecies recombination due to in vivo fitness trade-offs of de novo penicillin-binding protein (PBP) mutations. We show that de novo mutant populations have abrogated invasive disease capacity and are difficult to evolve in vivo. Conversely, serially transformed recombinant strains efficiently integrate resistant oral streptococcal DNA, gain penicillin resistance and tolerance, and retain virulence in mice. Large-scale changes in pbp2X, pbp2B, and non-PBP-related genes occur in recombinant isolates. Our results indicate that horizontal transfer of β-lactam resistance engenders initially favorable or minimal cost changes in vivo compared with de novo mutation(s), underscoring the importance of recombination in the emergence of β-lactam resistance and suggesting why some pathogenic streptococci lacking innate competence remain universally susceptible.

The acquisition of clinically relevant amoxicillin resistance in Streptococcus pneumoniae requires ordered horizontal gene transfer of four loci

Understanding how antimicrobial resistance spreads is critical for optimal application of new treatments. In the naturally competent human pathogen Streptococcus pneumoniae, resistance to β-lactam antibiotics is mediated by recombination events in genes encoding the target proteins, resulting in reduced drug binding affinity. However, for the front-line antibiotic amoxicillin, the exact mechanism of resistance still needs to be elucidated. Through successive rounds of transformation with genomic DNA from a clinically resistant isolate, we followed amoxicillin resistance development. Using whole genome sequencing, we showed that multiple recombination events occurred at different loci during one round of transformation. We found examples of non-contiguous recombination, and demonstrated that this could occur either through multiple D-loop formation from one donor DNA molecule, or by the integration of multiple DNA fragments. We also show that the final minimum inhibitory concentration (MIC) differs depending on recipient genome, explained by differences in the extent of recombination at key loci. Finally, through back transformations of mutant alleles and fluorescently labelled penicillin (bocillin-FL) binding assays, we confirm that pbp1a, pbp2b, pbp2x, and murM are the main resistance determinants for amoxicillin resistance, and that the order of allele uptake is important for successful resistance evolution. We conclude that recombination events are complex, and that this complexity contributes to the highly diverse genotypes of amoxicillin-resistant pneumococcal isolates.

Isolation of Group A Streptococci with Reduced In Vitro β-Lactam Susceptibility Harboring Amino Acid Substitutions in Penicillin-Binding Proteins in Japan

Streptococcus pyogenes (group A Streptococcus [GAS]) has long been regarded as being susceptible to β-lactams. However, amino acid substitutions in penicillin-binding protein 2X (PBP2X) conferring reduced in vitro β-lactam susceptibility have been indicated since 2019 in the United States and Iceland. Here, we report the first isolation of Streptococcus pyogenes possessing the PBP2X substitution conferring reduced in vitro β-lactam susceptibility in Asia; however, the MICs were below the susceptible breakpoint of the CLSI.

The role of interspecies recombination in the evolution of antibiotic-resistant pneumococci

Multidrug-resistant Streptococcus pneumoniae emerge through the modification of core genome loci by interspecies homologous recombinations, and acquisition of gene cassettes. Both occurred in the otherwise contrasting histories of the antibiotic-resistant S. pneumoniae lineages PMEN3 and PMEN9. A single PMEN3 clade spread globally, evading vaccine-induced immunity through frequent serotype switching, whereas locally circulating PMEN9 clades independently gained resistance. Both lineages repeatedly integrated Tn916-type and Tn1207.1-type elements, conferring tetracycline and macrolide resistance, respectively, through homologous recombination importing sequences originating in other species. A species-wide dataset found over 100 instances of such interspecific acquisitions of resistance cassettes and flanking homologous arms. Phylodynamic analysis of the most commonly sampled Tn1207.1-type insertion in PMEN9, originating from a commensal and disrupting a competence gene, suggested its expansion across Germany was driven by a high ratio of macrolide-to-β-lactam consumption. Hence, selection from antibiotic consumption was sufficient for these atypically large recombinations to overcome species boundaries across the pneumococcal chromosome.

The Cell Wall of Streptococcus pneumoniae

Streptococcus pneumoniae has a complex cell wall that plays key roles in cell shape maintenance, growth and cell division, and interactions with components of the human host. The peptidoglycan has a heterogeneous composition with more than 50 subunits (muropeptides)-products of several peptidoglycan-modifying enzymes. The amidation of glutamate residues in the stem peptide is needed for efficient peptide cross-linking, and peptides with a dipeptide branch prevail in some beta-lactam-resistant strains. The glycan strands are modified by deacetylation of N-acetylglucosamine residues and O-acetylation of N-acetylmuramic acid residues, and both modifications contribute to pneumococcal resistance to lysozyme. The glycan strands carry covalently attached wall teichoic acid and capsular polysaccharide. Pneumococci are unique in that the wall teichoic acid and lipoteichoic acid contain the same unusually complex repeating units decorated with phosphoryl choline residues, which anchor the choline-binding proteins. The structures of lipoteichoic acid and the attachment site of wall teichoic acid to peptidoglycan have recently been revised. During growth, pneumococci assemble their cell walls at midcell in coordinated rounds of cell elongation and division, leading to the typical ovococcal cell shape. Cell wall growth depends on the cytoskeletal FtsA and FtsZ proteins and is regulated by several morphogenesis proteins that also show patterns of dynamic localization at midcell. Some of the key regulators are phosphorylated by StkP and dephosphorylated by PhpP to facilitate robust selection of the division site and plane and to maintain cell shape.

Molecular characterization of serogroup 19 Streptococcus pneumoniae in the Czech Republic in the post-vaccine era

Purpose. The aim of this study was to characterize serogroup 19 isolates resistant to macrolides and/or penicillin found among pneumococci recovered from cases of invasive and respiratory tract disease in the Czech Republic in 2014.Methods. Pneumococcal isolates of serotypes 19A (n=26) and 19F (n=10) that were non-susceptible to penicillin and/or macrolides and had been collected in 2014 were analysed using multi-locus sequence typing (MLST). Four isolates representing the major clones were subjected to whole-genome sequencing (WGS).Results. The penicillin-susceptible macrolide-resistant isolates of serotype 19A were mainly associated with sequence type (ST) 416 belonging to clonal complex (CC) 199, and the penicillin-resistant isolates were of serotype 19F belonging to ST1464 (CC 320). WGS revealed the presence of pilus 1, in association with pilus 2, in serotype19F isolates belonging to CC 320. Another adhesin, pneumococcal serine-rich protein (PsrP), was only present in serotype 19A isolates of ST416. Analysis of the penicillin-binding proteins (PBPs) of serotype 19F penicillin-resistant isolates (ST1464 and ST271) performed on PBP1a, 2b and 2x identified a large number of mutations in comparison to the reference strain, R6. Both isolates contained a unique PBP profile; however, they were highly similar to PBP sequences of the Taiwan19F-14 reference strain. The Pbp2b sequences of both 19F isolates showed the lowest similarity to those of the Taiwan19F-14 strain (91 % similarity), while they were also found to be distantly related to each other (94 % similarity).Conclusions. WGS revealed specific virulence factors in antibiotic-resistant pneumococcal clones that spread rapidly in the post-vaccine era in the Czech Republic.

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.

New Aspects of the Interplay between Penicillin Binding Proteins, murM, and the Two-Component System CiaRH of Penicillin-Resistant Streptococcus pneumoniae Serotype 19A Isolates from Hungary

The Streptococcus pneumoniae clone Hungary^19A-6 expresses unusually high levels of β-lactam resistance, which is in part due to mutations in the MurM gene, encoding a transferase involved in the synthesis of branched peptidoglycan. Moreover, it contains the allele ciaH232, encoding the histidine kinase CiaH (M. Müller, P. Marx, R. Hakenbeck, and R. Brückner, Microbiology 157:3104–3112, 2011, https://doi.org/10.1099/mic.0.053157-0). High-level penicillin resistance primarily requires the presence of low-affinity (mosaic) penicillin binding protein (PBP) genes, as, for example, in strain Hu17, a closely related member of the Hungary^19A-6 lineage. Interestingly, strain Hu15 is β-lactam sensitive due to the absence of mosaic PBPs. This unique situation prompted us to investigate the development of cefotaxime resistance in transformation experiments with genes known to play a role in this phenotype, pbp2x, pbp1a, murM, and ciaH, and penicillin-sensitive recipient strains R6 and Hu15. Characterization of phenotypes, peptidoglycan composition, and CiaR-mediated gene expression revealed several novel aspects of penicillin resistance. The murM gene of strain Hu17 (murM_Hu17), which is highly similar to murM of Streptococcus mitis, induced morphological changes which were partly reversed by ciaH232. murM_Hu17 conferred cefotaxime resistance only in the presence of the pbp2x of strain Hu17 (pbp2x_Hu17). The ciaH232 allele contributed to a remarkable increase in cefotaxime resistance in combination with pbp2x_Hu17 and pbp1a of strain Hu17 (pbp1a_Hu17), accompanied by higher levels of expression of CiaR-regulated genes, documenting that ciaH232 responds to PBP1a_Hu17-mediated changes in cell wall synthesis. Most importantly, the proportion of branched peptides relative to the proportion of linear muropeptides increased in cells containing mosaic PBPs, suggesting an altered enzymatic activity of these proteins.

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.

Susceptibility to antimicrobials of mastitis-causing Staphylococcus aureus, Streptococcus uberis and Str. dysgalactiae from New Zealand and the USA as assessed by the disk diffusion test

OBJECTIVE

To compare the antimicrobial susceptibility patterns of three common mastitis pathogens (Staphylococcus aureus, Streptococcus uberis and Str. dysgalactiae) isolated from milk samples from New Zealand and the USA.

METHODS

A total of 182 S. aureus, 126 Str. uberis and 89 Str. dysgalactiae isolates from New Zealand (107, 106 and 41, respectively) and the USA (75, 20 and 48, respectively) were assessed using the disk diffusion test.

RESULTS

Susceptibility varied among the bacterial species. All isolates were susceptible to the amoxicillin-clavulanic acid combination. Resistance to lincomycin was most frequent (susceptibility of 8.6%) across all species. Non-susceptible (i.e. resistant or intermediate) isolates of S. aureus were identified for the three non-isoxazolyl penicillins (amoxicillin, ampicillin and penicillin: 20.6% and 36.0%) and lincomycin (99.9% and 94.6%) for NZ and the USA, respectively. Resistance to erythromycin (5.3%) and tetracyclines (6.7%) was detected only in isolates from the USA. There were differences in susceptibility between Str. uberis and Str. dysgalactiae; all streptococcal isolates demonstrated resistance to aminoglycosides (neomycin 52.4% and streptomycin 27.9%) and enrofloxacin (28%). Resistance of Str. dysgalactiae to tetracycline was almost 100.0% and to oxytetracycline 89.9%.

CONCLUSION AND CLINICAL RELEVANCE

Most of the isolates tested were susceptible to most of the antimicrobials commonly used for treatment of bovine mastitis, with the exception of the lincosamides. Susceptibility to a selected class-representative antimicrobial and at the genus level should be interpreted with caution. Differences between NZ and the USA confirm the value of national surveys to determine the susceptibility patterns of mastitis pathogens.

Transfer of penicillin resistance from Streptococcus oralis to Streptococcus pneumoniae identifies murE as resistance determinant

Beta-lactam resistant clinical isolates of Streptococcus pneumoniae contain altered penicillin-binding protein (PBP) genes and occasionally an altered murM, presumably products of interspecies gene transfer. MurM and MurN are responsible for the synthesis of branched lipid II, substrate for the PBP catalyzed transpeptidation reaction. Here we used the high-level beta-lactam resistant S. oralis Uo5 as donor in transformation experiments with the sensitive laboratory strain S. pneumoniae R6 as recipient. Surprisingly, piperacillin-resistant transformants contained no alterations in PBP genes but carried murEUo5 encoding the UDP-N-acetylmuramyl tripeptide synthetase. Codons 83-183 of murEUo5 were sufficient to confer the resistance phenotype. Moreover, the promoter of murEUo5 , which drives a twofold higher expression compared to that of S. pneumoniae R6, could also confer increased resistance. Multiple independent transformations produced S. pneumoniae R6 derivatives containing murEUo5 , pbp2xUo5 , pbp1aUo5 and pbp2bUo5 , but not murMUo5 sequences; however, the resistance level of the donor strain could not be reached. S. oralis Uo5 harbors an unusual murM, and murN is absent. Accordingly, the peptidoglycan of S. oralis Uo5 contained interpeptide bridges with one L-Ala residue only. The data suggest that resistance in S. oralis Uo5 is based on a complex interplay of distinct PBPs and other enzymes involved in peptidoglycan biosynthesis.

Molecular analysis of pbp2b in Streptococcus pneumonia isolated from clinical and normal flora samples

Streptococcus pneumoniae is an important bacterial pathogen responsible for respiratory infections, bacteraemia, and meningitis remains an important cause of disease and mortality in infants and younger children around the world, with penicillin being considered the drug of choice for the treatment of infections. However, penicillin-resistant S. pneumonia is now becoming endemic worldwide. In this study, a total of 80 pneumococcal isolates were collected from different clinical sources as well as normal flora. These isolates were subjected to antimicrobial susceptibility testing and MIC determination. The penicillin-binding proteins, pbp2b, were amplified by PCR, and they were sequenced. The genetic relationship of the penicillin-resistant isolates was performed by BOX PCR. Overall, 36 pneumococcal (45 %) isolates were found to be resistant to penicillin with different MICs. The majority of them (80 %) were intermediately resistant with MIC of 0.12-1 µg/ml, whereas 20 % of isolates were penicillin resistant with MICs of >2 µg/ml. The results identified seven groups which were based on the amino acid substitutions of pbp2b. Sequencing analysis revealed that the most prevalent mutation was the substitution of Adenine for Thymine at the position 445 which is next to the second PBP2b-conserved motif (SSN). This study indicates that resistance to penicillin appears to be dependent on specific mutations in pbp2b, and the substitution in S620 → T near to the third PBP2b-conserved motif appears to be important in developing highly antibiotic-resistant isolates. Moreover, there was a positive correlation between the mutations in pbp2b gene and MIC.

Protective role of the capsule and impact of serotype 4 switching on Streptococcus mitis

The polysaccharide capsule surrounding Streptococcus pneumoniae is essential for virulence. Recently, Streptococcus mitis, a human commensal and a close relative of S. pneumoniae, was also shown to have a capsule. In this study, the S. mitis type strain switched capsule by acquisition of the serotype 4 capsule locus of S. pneumoniae TIGR4, following induction of competence for natural transformation. Comparison of the wild type with the capsule-switching mutant and with a capsule deletion mutant showed that the capsule protected S. mitis against phagocytosis by RAW 264.7 macrophages. This effect was enhanced in the S. mitis strain expressing the S. pneumoniae capsule, which showed, in addition, increased resistance against early clearance in a mouse model of lung infection. Expression of both capsules also favored survival in human blood, and the effect was again more pronounced for the capsule-switching mutant. S. mitis survival in horse blood or in a mouse model of bacteremia was not significantly different between the wild type and the mutant strains. In all models, S. pneumoniae TIGR4 showed higher rates of survival than the S. mitis type strain or the capsule-switching mutant, except in the lung model, in which significant differences between S. pneumoniae TIGR4 and the capsule-switching mutant were not observed. Thus, we identified conditions that showed a protective function for the capsule in S. mitis. Under such conditions, S. mitis resistance to clearance could be enhanced by capsule switching to serotype 4, but it was enhanced to levels lower than those for the virulent strain S. pneumoniae TIGR4.

Molecular docking and molecular dynamics studies on β-lactamases and penicillin binding proteins

Bacterial resistance to β-lactam antibiotics poses a serious threat to human health. Penicillin binding proteins (PBPs) and β-lactamases are involved in both antibacterial activity and mediation of β-lactam antibiotic resistance. The two major reasons for resistance to β-lactams include: (i) pathogenic bacteria expressing drug insensitive PBPs rendering β-lactam antibiotics ineffective and (ii) production of β-lactamases along with alteration of their specificities. Thus, there is an urgent need to develop newer β-lactams to overcome the challenge of bacterial resistance. Therefore the present study aims to identify the binding affinity of β-lactam antibiotics with different types of PBPs and β-lactamases. In this study, cephalosporins and carbapenems are docked into PBP2a of Staphylococcus aureus, PBP2b and PBP2x of Streptococcus pneumoniae and SHV-1 β-lactamase of Escherichia coli. The results reveal that Ceftobiprole can efficiently bind to PBP2a, PBP2b and PBP2x and not strongly to SHV-1 β-lactamase. Furthermore, molecular dynamics (MD) simulations are performed to refine the binding mode of the docked complex structure and to observe the differences in the stability of free PBP2x and Ceftobiprole bound PBP2x. MD simulation supports the greater stability of the Ceftobiprole-PBP2x complex compared to free PBP2x. This work demonstrates that potential β-lactam antibiotics can efficiently bind to different types of PBPs for circumventing β-lactam resistance and opens avenues for the development of newer antibiotics that can target bacterial pathogens.

Genomic analyses of DNA transformation and penicillin resistance in Streptococcus pneumoniae clinical isolates

Alterations in penicillin-binding proteins, the target enzymes for β-lactam antibiotics, are recognized as primary penicillin resistance mechanisms in Streptococcus pneumoniae. Few studies have analyzed penicillin resistance at the genome scale, however, and we report the sequencing of S. pneumoniae R6 transformants generated while reconstructing the penicillin resistance phenotypes from three penicillin-resistant clinical isolates by serial genome transformation. The genome sequences of the three last-level transformants T2-18209, T5-1983, and T3-55938 revealed that 16.2 kb, 82.7 kb, and 137.2 kb of their genomes had been replaced with 5, 20, and 37 recombinant sequence segments derived from their respective parental clinical isolates, documenting the extent of DNA transformation between strains. A role in penicillin resistance was confirmed for some of the mutations identified in the transformants. Several multiple recombination events were also found to have happened at single loci coding for penicillin-binding proteins (PBPs) that increase resistance. Sequencing of the transformants with MICs for penicillin similar to those of the parent clinical strains confirmed the importance of mosaic PBP2x, -2b, and -1a as a driving force in penicillin resistance. A role in resistance for mosaic PBP2a was also observed for two of the resistant clinical isolates.

In vitro characterization of the anti-bacterial activity of SQ109 against Helicobacter pylori

The most evident challenge to treatment of Helicobacter pylori, a bacterium responsible for gastritis, peptic ulcers and gastric cancer, is the increasing rate of resistance to all currently used therapeutic antibiotics. Thus, the development of novel therapies is urgently required. N-geranyl-N'-(2-adamantyl) ethane-1, 2-diamine (SQ109) is an ethylene diamine-based antitubercular drug that is currently in clinical trials for the treatment of tuberculosis (TB). Previous pharmacokinetic studies of SQ109 revealed that persistently high concentrations of SQ109 remain in the stomach 4 hours post oral administration in rats. This finding, combined with the need for new anti-Helicobacter therapies, prompted us to define the in vitro efficacy of SQ109 against H. pylori. Liquid broth micro-dilution was used for susceptibility studies to determine the antimicrobial activity of SQ109 against a total of 6 laboratory strains and 20 clinical isolates of H. pylori; the clinical isolates included a multi-drug resistant strain. All strains tested were susceptible to SQ109 with MIC and MBC ranges of 6-10 µM and 50-60 µM, respectively. SQ109 killing kinetics were concentration- and time-dependent. SQ109 killed H. pylori in 8-10 h at 140 µM (2MBCs) or 4-6 h at 200 µM (~3MBCs). Importantly, though the kinetics of killing were altered, SQ109 retained potent bactericidal activity against H. pylori at low pH. Additionally, SQ109 demonstrated robust thermal stability and was effective at killing slow growing or static bacteria. In fact, pretreatment of cultures with a bacteriostatic concentration of chloramphenicol (Cm) synergized the effects of typically bacteriostatic concentrations of SQ109 to the level of five-logs of bacterial killing. A molar-to-molar comparison of the efficacy of SQ109 as compared to metronidazole (MTZ), amoxicillin (AMX), rifampicin (RIF) and clarithromycin (CLR), revealed that SQ109 was superior to MTZ, AMX and RIF but not to CLR. Finally, the frequency of resistance to SQ109 was low and electron microscopy studies revealed that SQ109 interacted with bacterial inner membrane and cytoplasmic content(s). Collectively, our in vitro data demonstrate that SQ109 is an effective monotherapy against susceptible and multi-drug resistant strains of H. pylori and may be useful alone or in combination with other antibiotics for development as a new class of anti-Helicobacter drugs.

Crystal structures of bifunctional penicillin-binding protein 4 from Listeria monocytogenes

Penicillin-binding proteins (PBPs), which catalyze the biosynthesis of the peptidoglycan chain of the bacterial cell wall, are the major molecular target of bacterial antibiotics. Here, we present the crystal structures of the bifunctional peptidoglycan glycosyltransferase (GT)/transpeptidase (TP) PBP4 from Listeria monocytogenes in the apo-form and covalently linked to two β-lactam antibiotics, ampicillin and carbenicillin. The orientation of the TP domain with respect to the GT domain is distinct from that observed in the previously reported structures of bifunctional PBPs, suggesting interdomain flexibility. In this structure, the active site of the GT domain is occluded by the close apposition of the linker domain, which supports the hypothesis that interdomain flexibility is related to the regulation of GT activity. The acylated structures reveal the mode of action of β-lactam antibiotics toward the class A PBP4 from the human pathogen L. monocytogenes. Ampicillin and carbenicillin can access the active site and be acylated without requiring a structural rearrangement. In addition, the active site of the TP domain in the apo-form is occupied by the tartrate molecule via extensive hydrogen bond interactions with the catalytically important residues; thus, derivatives of the tartrate molecule may be useful in the search for new antibiotics to inhibit PBPs.

Genomic analysis and reconstruction of cefotaxime resistance in Streptococcus pneumoniae

OBJECTIVES

To identify non-penicillin-binding protein (PBP) mutations contributing to resistance to the third-generation cephalosporin cefotaxime in Streptococcus pneumoniae at the genome-wide scale.

METHODS

The genomes of two in vitro S. pneumoniae cefotaxime-resistant isolates and of two transformants serially transformed with the genomic DNA of cefotaxime-resistant mutants were determined by next-generation sequencing. A role in cefotaxime resistance for the mutations identified was confirmed by reconstructing resistance in a cefotaxime-susceptible background.

RESULTS

Analysis of the genome assemblies revealed mutations in genes coding for the PBPs 2x, 2a and 3, of which pbp2x was the only mutated gene common to all mutants. The transformation of altered PBP alleles into S. pneumoniae R6 confirmed the role of PBP mutations in cefotaxime resistance, but these were not sufficient to fully explain the levels of resistance. Thirty-one additional genes were found to be mutated in at least one of the four sequenced genomes. Non-PBP resistance determinants appeared to be mostly lineage specific. Mutations in spr1333, spr0981, spr1704 and spr1098, encoding a peptidoglycan N-acetylglucosamine deacetylase, a glycosyltransferase, an ABC transporter and a sortase, respectively, were implicated in resistance by transformation experiments and allowed the reconstruction of the full level of resistance observed in the parent resistant strains.

CONCLUSIONS

This whole-genome analysis coupled to functional studies has allowed the discovery of both known and novel cefotaxime resistance genes in S. pneumoniae.

The capsule of Streptococcus pneumoniae contributes to virulence in the insect model Manduca sexta

The polysaccharide capsule of Streptococcus pneumoniae is one of the most important virulence factors responsible for human infections and in mouse infection models as well. Larvae of Manduca sexta were used as an alternative animal model in order to test the impact of the pneumococcal capsule on virulence in the insect host. The unencapsulated S. pneumoniae strain R6 was able to cause disease and induce killing in the larvae, and similar results were obtained with related commensal species. However, using the same dose of S. pneumoniae, encapsulated strains including the type 2 D39 strain, the progenitor of R6, and genetically unrelated S. pneumoniae strains of serotype 2, 4, 6B, 23F and 19A, all had increased virulence potential compared to the R6 strain. Between 20 and 70% of the larvae were affected after 96 h compared to 12% observed with R6. Two type 6B S. pneumoniae strains were more virulent compared to the other strains. S. pneumoniae R6 transformants producing the type 6B capsule showed a similar elevated disease potential, confirming the contribution of the pneumococcal polysaccharide capsule to virulence in M. sexta.

Streptococcus pneumoniae R6 interspecies transformation: genetic analysis of penicillin resistance determinants and genome-wide recombination events

Interspecies gene transfer has been implicated as the major driving force for the evolution of penicillin resistance in Streptococcus pneumoniae. Genomic alterations of S. pneumoniae R6 introduced during four successive transformations with DNA of the high-level penicillin-resistant Streptococcus mitis B6 with beta-lactam selection have now been determined and the contribution of genes to high resistance levels was analysed genetically. Essential for high level resistance to penicillins of the transformant CCCB was the combination of murM(B) (6) and the 3' region of pbp2b(B) (6) . Sequences of both genes were detected in clinical isolates of S. pneumoniae, confirming the participation of S. mitis in the global gene pool of beta-lactam resistance determinants. The S. mitis PBP1b gene which contains an authentic stop codon within the transpeptidase domain is now shown to contribute only marginal to resistance, but it is possible that the presence of its transglycosylase domain is important in the context of cognate PBPs. The genome sequence of CCCB revealed 36 recombination events, including deletion and acquisition of genes and repeat elements. A total of 78 genes were affected representing 67 kb or 3.3% of the genome, documenting extensive alterations scattered throughout the genome.

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.

The adaptive potential during nasopharyngeal colonisation of Streptococcus pneumoniae

Adaptation to host defences and antimicrobials is critical for Streptococcus pneumoniae (the pneumococcus) during colonisation of the nasopharynx--its only ecological habitat. The pneumococcus is highly transformable with the genome between different strains varying widely in both gene content and gene sequence. Thus, mixed strains colonising together will expand the genetic reservoir--"supragenome" for this highly transformable microorganism, increasing its adaptive potential. The extent of the phenotypic and genotypic diversity of strains co-colonising in the nasopharynx was determined. In contrast to most carriage studies, which characterise single colonies, a systematic analysis of up to 20 colonies per colonisation was undertaken in Tanzanian children for 12 months. The serotype was determined by conventional serology and confirmed by DNA-based methods. The antibiotype for penicillin and co-trimoxazole was determined from the minimum inhibitory concentration determined by E-test. As representative of the genotype of strains the sequence types (STs) was determined by multilocus sequence typing (MLST). Of 61 colonisation events studied, seven (11.5%) had strains expressing multiple serotypes, with a maximum of five serotypes detected. Four colonisation events also had co-colonisation of penicillin and/or co-trimoxazole susceptible and non-susceptible pneumococci. Sequence typing revealed that 58% were unique to our cohort. Simultaneous colonisation of up to six STs with two expressing serotype 6B was seen. Re-isolation of either the same or different strains of serotype 6B was seen. Genetically related single-locus and double-locus variants were identified in our cohort that differed by multiple nucleotides. Multiple colony characterisation revealed phenotypic and genetic evidence of microevolution and a greater diversity of pneumococcal strains colonising together than previously observed, thus increasing the potential to adapt in response to the host environment during colonisation.

Cell surface proteins in S. pneumoniae, S. mitis and S. oralis

BACKGROUND AND OBJECTIVES

Streptococcus pneumoniae, a major human pathogen, is closely related to the commensal species S. mitis and S. oralis. S. pneumoniae surface proteins are implicated in virulence and host interaction of this species, but many of them have recently been detected in S. mitis B6 in silico. We tested for the presence of such genes usinga set of eight S. mitis and eleven S. oralis strains from different geographic locations.

MATERIALS AND METHODS

An oligonucleotide microarray was designed based on the genomes of S. pneumoniae R6 and TIGR4 as well as S. mitis B6 to include 63 cell surface proteins. The S. pneumoniae genes encoding neuraminidases, hyaluronidase and pneumolysin were also included. In addition to comparative genomic hybridization experiments, homologues were identified in silico in the genome of S. oralis Uo5.

RESULTS AND CONCLUSIONS

The results document that many S. pneumoniae related surface proteins are ubiquitously present among the Mitis group of streptococci. All 19 samples hybridized with the pavA probe representing a gene important for adherence and invasion of S. pneumoniae. Only eight genes were not recognized in any strain, including the S. pneumoniae PcpC gene as the only virulence gene of the S. pneumoniae core genome.The fact that only 12 out of 26 genes present in the S. oralis Uo5 genome could be detected by microarray analysis confirms the sequence variation of surface components.

What are the properties that make an antibiotic acceptable for therapy of community-acquired pneumonia?

The factors associated with identifying an appropriate dose and schedule of an antimicrobial agent for treatment of hospitalized, seriously ill patients with pneumonia are straightforward. Information is required about the potency of the agent for typical pathogens likely to be encountered with pneumonia. It is helpful to understand the utility of the agent against pathogens that express resistance to older antimicrobial agents. The agent must be able to gain access to the site of infection at the dose and schedule chosen and at concentrations high enough to attain microbiologically effective targets [e.g. 1 or 2 log(10) (cfu/g) bacterial cell kill, accounting for between-patient variability]. Finally, therapeutic concentrations should be attained quickly at the primary site of infection to optimize clinical outcomes. When considering all of these factors, it is expected that ceftaroline fosamil will be a valuable addition to the therapeutic armamentarium for management of community-acquired pneumonia.

Emergence of a unique penicillin-resistant Streptococcus pneumoniae serogroup 35 strain

We analyzed seven Streptococcus pneumoniae serogroup 35 isolates by pulsed-field gel electrophoresis of the genome and pbp2b gene nucleotide sequences. Three penicillin-susceptible strains and one penicillin-intermediate-resistant strain exhibited 100% identity to prototype R6. Two resistant strains and one other intermediate strain differed from them and contained a unique sequence.

Evolution of the capsular gene locus of Streptococcus pneumoniae serogroup 6

Streptococcus pneumoniae expressing serogroup 6 capsules frequently causes pneumococcal infections and the evolutionary origins of the serogroup 6 strains have been extensively studied. However, these studies were performed when serogroup 6 had only two known members (serotypes 6A and 6B) and before the two new members (serotypes 6C and 6D) expressing wciN_β were found. We have therefore reinvestigated the evolutionary origins of serogroup 6 by examining the profiles of the capsule gene loci and the multilocus sequence types (MLSTs) of many serogroup 6 isolates from several continents. We confirmed that there are two classes of cps locus sequences for serogroup 6 isolates. In our study, class 2 cps sequences were limited to a few serotype 6B isolates. Neighbour-joining analysis of cps sequence profiles showed a distinct clade for 6C and moderately distinct clades for class 1 6A and 6B sequences. The serotype 6D cps profile was found within the class 1 6B clade, suggesting that it was created by recombination between 6C and 6B cps loci. Interestingly, all 6C isolates also had a unique wzy allele with a 6 bp deletion. This suggests that serotype switching to 6C involves the transfer of a large (>4 kb) gene segment that includes both the wciN_β allele and the ‘short’ wzy allele. The MLST studies of serotype 6C isolates suggest that the 6C cps locus is incorporated into many different pneumococcal genomic backgrounds but that, interestingly, 6C cps may have preferentially entered strains of the same genomic backgrounds as those of serotype 6A.

Affinity of ceftaroline and other beta-lactams for penicillin-binding proteins from Staphylococcus aureus and Streptococcus pneumoniae

We compared the affinities of ceftaroline for all penicillin-binding proteins (PBPs) with those of ceftriaxone and cefotaxime in 6 Staphylococcus aureus and 7 Streptococcus pneumoniae isolates with various resistance phenotypes. Ceftaroline MICs were PBP1A, -1B, and -2A > PBP2B, and ceftaroline had >or=4-fold higher 50% inhibitory concentrations (IC(50)s) (0.1 to 4 microg/ml) for PBP2X, -2A, -2B, and -3 than those for the other cephalosporins tested. Among 3 penicillin-resistant S. pneumoniae strains, ceftaroline had a high affinity for PBP2X (IC(50), 0.1 to 1 microg/ml), a primary target for cephalosporin PBP binding activity, and high affinities for PBP2B (IC(50), 0.5 to 4 microg/ml) and PBP1A (IC(50), 0.125 to 0.25 microg/ml) as well, both of which are also known as major targets for PBP binding activity of cephalosporins. Ceftaroline PBP affinities in methicillin-susceptible S. aureus strains were greater than or equal to those of the 3 other beta-lactams tested. Ceftaroline bound to PBP2a in methicillin-resistant S. aureus (IC(50), 0.01 to 1 microg/ml) with up to 256-fold-higher affinity than those of other agents. Ceftaroline demonstrated very good PBP affinity against all S. aureus and S. pneumoniae strains tested, including resistant isolates.

The genome of Streptococcus mitis B6--what is a commensal?

Streptococcus mitis is the closest relative of the major human pathogen S. pneumoniae. The 2,15 Mb sequence of the Streptococcus mitis B6 chromosome, an unusually high-level beta-lactam resistant and multiple antibiotic resistant strain, has now been determined to encode 2100 genes. The accessory genome is estimated to represent over 40%, including 75 mostly novel transposases and IS, the prophage phiB6 and another seven phage related regions. Tetracycline resistance mediated by Tn5801, and an unusual and large gene cluster containing three aminoglycoside resistance determinants have not been described in other Streptococcus spp. Comparative genomic analyses including hybridization experiments on a S. mitis B6 specific microarray reveal that individual S. mitis strains are almost as distantly related to the B6 strain as S. pneumoniae. Both species share a core of over 900 genes. Most proteins described as pneumococcal virulence factors are present in S. mitis B6, but the three choline binding proteins PcpA, PspA and PspC, and three gene clusters containing the hyaluronidase gene, ply and lytA, and the capsular genes are absent in S. mitis B6 and other S. mitis as well and confirm their importance for the pathogenetic potential of S. pneumoniae. Despite the close relatedness between the two species, the S. mitis B6 genome reveals a striking X-alignment when compared with S. pneumoniae.

Emergence of a cell wall protease in the Streptococcus thermophilus population

Streptococcus thermophilus is perceived as a recently emerged food bacterium that evolved from a commensal ancestor by loss and gain of functions. Here, we provide data allowing a better understanding of this evolutionary scheme. A multilocus sequence typing approach that we developed showed that S. thermophilus diverges significantly from its potential ancestors of the salivarius group and displays a low level of allelic variability, confirming its likely recent emergence. An analysis of the origin and dissemination of the prtS gene was carried out within this evolutionary scheme. This gene encodes a protease that allows better growth in milk by facilitating casein breakdown to supply amino acids. The S. thermophilus protease exhibits 95% identity to the animal Streptococcus suis protein PrtS. Genomic analysis showed that prtS is part of an island flanked by two tandem insertion sequence elements and containing three other genes which present the best identities and synteny with the S. suis genome. These data indicate a potential origin for this "ecological" island in a species closely related to S. suis. The analysis of the distribution of the prtS gene in S. thermophilus showed that the gene is infrequent in historical collections but frequent in recent industrial ones. Moreover, this "ecological" island conferring an important metabolic trait for milk adaptation appears to have disseminated by lateral transfer in the S. thermophilus population. Taken together, these data support an evolutionary scheme of S. thermophilus where gene acquisition and selection by food producers are determining factors. The source and impact of genes acquired by horizontal gene transfer on the physiology and safety of strains should be addressed.

Versatility of choline metabolism and choline-binding proteins in Streptococcus pneumoniae and commensal streptococci

The pneumococcal choline-containing teichoic acids are targeted by cholinebinding proteins (CBPs), major surface components implicated in the interaction with host cells and bacterial cell physiology. CBPs also occur in closely related commensal species, Streptococcus oralis and Streptococcus mitis, and many strains of these species contain choline in their cell wall. Physiologically relevant CBPs including cell wall lytic enzymes are highly conserved between Streptococcus pneumoniae and S. mitis. In contrast, the virulence-associated CBPs, CbpA, PspA and PcpA, are S. pneumoniae specific and are thus relevant for the characteristic properties of this species.

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Human saliva can be separated by centrifugation into cell pellet and cell-free supernatant, which are called cellular phase and liquid phase in this study. While it is well documented that the cellular phase of saliva contains hundreds of oral bacteria species, little is known whether the liquid phase of saliva contains any information related to oral microbiota. In this study, we analyzed the bacterial nucleic acid contents of the liquid phase of saliva. Using primers universal to most eubacterial 16S rDNA, we detected large amounts of bacterial 16S rRNA and rDNA in the cell-free phase of saliva. Random sequencing analysis of forty PCR amplicons from the cell-free phase of saliva led to 15 operational taxonomic unit (OTU) groups. Furthermore, using denaturing gradient gel electrophoresis (DGGE), we compared 16S rRNA/rDNA profiles derived from liquid phases and cellular phases of saliva samples, and found positive correlations (Pearson Correlation=0.822, P<0.001) between these sample groups. These findings indicate that the liquid phase of saliva contains numerous bacterial 16S rRNA/rDNA molecules that have correlations with bacteria existing in the cellular phase.

Comparative antipneumococcal activities of sulopenem and other drugs

For 297 penicillin-susceptible, -intermediate, and -resistant pneumococcal strains, the sulopenem MIC(50)s were 0.008, 0.06, and 0.25, respectively, and the sulopenem MIC(90)s were 0.016, 0.25, and 0.5 microg/ml, respectively. The MIC(50)s of amoxicillin for the corresponding strains were 0.03, 0.25, and 2.0 microg/ml, respectively, and the MIC(90)s were 0.03, 1.0, and 8.0 microg/ml, respectively. The combination of amoxicillin and clavulanate gave MICs similar to those obtained with amoxicillin alone. The sulopenem MICs were similar to those of imipenem and meropenem. The MICs of ss-lactams increased with those of penicillin G, and among the quinolones tested, moxifloxacin had the lowest MICs. Additionally, 45 strains of drug-resistant type 19A pneumococci were tested by agar dilution and gave sulopenem MIC(50)s and MIC(90)s of 1.0 and 2.0 microg/ml, respectively. Both sulopenem and amoxicillin (with and without clavulanate) were bactericidal against all 12 strains tested at 2x MIC after 24 h. Thirty-one strains from 10 countries with various penicillin, amoxicillin, and carbapenems MICs, including those with the highest sulopenem MICs, were selected for sequencing analysis of the pbp1a, pbp2x, and pbp2b regions encoding the transpeptidase active site and MurM. We did not find any correlations between specific penicillin-binding protein-MurM patterns and changes in the MICs.

Binding of faropenem and other beta-lactam agents to penicillin-binding proteins of pneumococci with various beta-lactam susceptibilities

Faropenem demonstrated low MICs (< or = 1 microg/ml) for all penicillin-susceptible and nonsusceptible pneumococci and exhibited very strong abilities to bind to Streptococcus pneumoniae penicillin-binding proteins (PBPs), except for PBP2X. The lower faropenem affinity for PBP2X did not affect MICs for any strains tested, and only imipenem had lower MICs, with much lower binding affinities for all PBPs tested, than faropenem.

PBP active site flexibility as the key mechanism for beta-lactam resistance in pneumococci

Penicillin-binding proteins (PBPs), the main targets of beta-lactam antibiotics, are membrane-associated enzymes that catalyze the two last steps in the biosynthesis of peptidoglycan. In Streptococcus pneumoniae, a major human pathogen, the surge in resistance to such antibiotics is a direct consequence of the proliferation of mosaic PBP-encoding genes, which give rise to proteins containing tens of mutations. PBP2b is a major drug resistance target, and its modification is essential for the development of high levels of resistance to piperacillin. In this work, we have solved the crystal structures of PBP2b from a wild-type pneumococcal strain, as well as from a highly drug-resistant clinical isolate displaying 58 mutations. Although mutations are present throughout the entire PBP structure, those surrounding the active site influence the total charge and the polar character of the region, while those in close proximity to the catalytic nucleophile impart flexibility onto the beta3/beta4 loop area, which encapsulates the cleft. The wealth of structural data on pneumococcal PBPs now underlines the importance of high malleability in active site regions of drug-resistant strains, suggesting that active site "breathing" could be a common mechanism employed by this pathogen to prevent targeting by beta-lactams.

Molecular characteristics of penicillin-binding protein 2b, 2x, and 1a sequences in penicillin-nonsusceptible Streptococcus pneumoniae isolates in Shenyang, China

The aim of this sudy was to investigate the nature of the amino acid motifs found in penicillin-binding proteins (PBP) 2b, 2x, and 1a of penicillin-nonsusceptible Streptococcus pneumoniae isolates from Shenyang, China, and to obtain information regarding the prevalence of alterations within the motifs or in positions flanking the motifs. For 18 clinical isolates comprising 4 penicillin-susceptible S. pneumoniae, 5 penicillin-intermediate S. pneumoniae, and 9 penicillin-resistant S. pneumoniae. the DNA sequences of PBP2b, PBP2x, and PBP1a transpeptidase domains were determined and then genotyped by multilocus sequence typing. Sequence analysis revealed that most penicillin-nonsusceptible S. pneumoniae isolates (penicillin MIC ≥ 1.5 μg/mL and cefotaxime MIC ≥ 2 μg/mL) shared identical PBP2b, PBP2x, and PBP1a amino acid profiles. Most penicillin-resistant S. pneumoniae isolates were ST320 (4-16-19-15-6-20-1), the double-locus variant of the Taiwan19F-14 clone. This study will serve as a basis for future monitoring of genetic changes associated with the emergence and spread of β-lactam resistance in Shenyang, China.

Penicillin-binding proteins and beta-lactam resistance

A number of ways and means have evolved to provide resistance to eubacteria challenged by beta-lactams. This review is focused on pathogens that resist by expressing low-affinity targets for these antibiotics, the penicillin-binding proteins (PBPs). Even within this narrow focus, a great variety of strategies have been uncovered such as the acquisition of an additional low-affinity PBP, the overexpression of an endogenous low-affinity PBP, the alteration of endogenous PBPs by point mutations or homologous recombination or a combination of the above.

Penicillin-binding protein 2x of Streptococcus pneumoniae: three new mutational pathways for remodelling an essential enzyme into a resistance determinant

Mutations in the transpeptidase domain of penicillin-binding protein 2x (PBP2x) of Streptococcus pneumoniae that reduce the affinity to beta-lactams are important determinants of resistance to these antibiotics. We have now analyzed in vitro and in vivo properties of PBP2x variants from cefotaxime-resistant laboratory mutants and a clinical isolate. The patterns of two to four resistance-specific mutations present in each of the proteins, all of which are placed between 6.6 and 24 A around the active site, fall into three categories according to their positions in the three-dimensional structure. The first PBP2x group is characterized by mutations at the end of helix alpha 11 and carries the well-known T550A change and/or one mutation on the surface of the penicillin-binding domain in close contact with the C-terminal domain. All group I proteins display very low acylation efficiencies, <or=1700 M(-1) s(-1), for cefotaxime. The second class represented by PBP2x of the mutant C505 shows acylation efficiencies below 100 M(-1) s(-1) for both cefotaxime and benzylpenicillin and contains the mutation L403F at a critical site close to the active serine. PBP2x of the clinical isolate 669 reveals a third mutational pathway where at least the two mutations Q552E and S389L are important for resistance, and acylation efficiency is reduced for both beta-lactams to around 10,000 M(-1) s(-1). In each group, at least one mutation is located in close vicinity to the active site and mediates a resistance phenotype in vivo alone, whereas other mutations might exhibit secondary effects only in context with other alterations.