Penicillin-binding proteins in beta-lactam-resistant streptococcus pneumoniae

Analysis of amino acid motif of penicillin-binding proteins 1a, 2b, and 2x in invasive Streptococcus pneumoniae nonsusceptible to penicillin isolated from pediatric patients in Casablanca, Morocco

OBJECTIVES

This study aimed to investigate the nature of the amino acid motifs found in PBPs of Streptococcus pneumoniae isolates in invasive diseases from pediatric patients at Casablanca, Morocco. Five penicillin-susceptible (PSSP), ten penicillin-intermediate (PISP), and fifteen penicillin-resistant S. pneumoniae (PRSP) were studied by PCR-RFLP and DNA sequencing of the pbp1a, - 2b, and - 2x genes.

RESULTS

There were no changes in the conserved motifs of PBP1a, PBP2b and PBP2x for PSSP strains. Substitution close to PBP1a conserved motifs were found in all PRSP isolates and six/five PISP. Analysis of PBP2b showed that all but one of the 10 PISP strains and all PRSP had substitutions. Substitution close to PBP2x motifs showed that all but three of the 10 PISP strains and all PRSP had substitutions in tow conserved motifs. A total of 6, 11 and 10 genotypes were found after analysis of pbp1a, pbp2b, and pbp2x, respectively. The penicillin-nonsusceptible S. pneumoniae isolated in Casablanca share most amino acid substitutions of those reported worldwide, but they occurred among pneumococci with low level resistance to b-lactams.

Molecular characterization of penicillin non-susceptible Streptococcus pneumoniae isolated before and after pneumococcal conjugate vaccine implementation in Casablanca, Morocco

Background

Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide, especially among children and the elderly. The ability to effectively treat pneumococcal infection has been compromised due to the acquisition of antibiotic resistance, particularly to β-lactam drugs. This study aimed to describe the prevalence and molecular evolution of penicillin non-susceptible S. pneumoniae (PNSP) isolated from invasive diseases before and after pneumococcal conjugate vaccine implementation in Casablanca, Morocco.

Methods

Isolates were obtained from the Microbiology Laboratory of Ibn Rochd University Hospital Centre of Casablanca. Serogrouping was done by Pneumotest Kit and serotyping by the Quellung capsular swelling. Antibiotic susceptibility pattern was determined by disk diffusion and E-test methods. The PNSP were analyzed by pulsed-field gel electrophoresis (PFGE) and by genotyping of pbp1a, pbp2b, and pbp2x genes.

Results

A total of 361 S. pneumoniae isolates were collected from 2007 to 2014. Of these isolates, 58.7% were obtained before vaccination (2007–2010) and 41.3% after vaccination (2011–2014). Of the 361 isolates, 80 were PNSP (22.2%). Generally, the proportion of PNSP between pre- and post-vaccination periods were 31 and 13% (p = 0.009), respectively. The proportion of PNSP isolated from pediatric and adult (age > 14 years) patients decreased from 34.5 to 22.9% (p = 0.1) and from 17.7 to 10.2% (p = 0.1) before and after vaccine implementation, respectively. The leading serotypes of PNSP were 14 (33 vs. 57%) and 19A (18 vs. 14%) before and after vaccination among children. For adults, serotypes 19A (53%) and 23F (24%) were the dominant serotypes in the pre-vaccination period, while serotype 14 (22%) was the most prevalent after vaccination. There were 21 pbp genotypes in the pre-vaccination period vs. 12 for post-vaccination period. PFGE clustering showed six clusters of PNSP grouped into three clusters specific to pre-vaccination period (clusters I, II and III), two clusters specific to post-period (clusters V and VI) and a cluster (IV) that contained clones belonging to the two periods of vaccination.

Conclusion

Our observations demonstrate a high degree of genetic diversity among PNSP. Genetic clustering among PNSP strains showed that they spread mainly by a restricted number of PNSP clones with vaccine serotypes. PFGE clustering combined with pbp genotyping revealed that vaccination can change the population structure of PNSP.

Deciphering the distance to antibiotic resistance for the pneumococcus using genome sequencing data

Advances in genome sequencing technologies and genome-wide association studies (GWAS) have provided unprecedented insights into the molecular basis of microbial phenotypes and enabled the identification of the underlying genetic variants in real populations. However, utilization of genome sequencing in clinical phenotyping of bacteria is challenging due to the lack of reliable and accurate approaches. Here, we report a method for predicting microbial resistance patterns using genome sequencing data. We analyzed whole genome sequences of 1,680 Streptococcus pneumoniae isolates from four independent populations using GWAS and identified probable hotspots of genetic variation which correlate with phenotypes of resistance to essential classes of antibiotics. With the premise that accumulation of putative resistance-conferring SNPs, potentially in combination with specific resistance genes, precedes full resistance, we retrogressively surveyed the hotspot loci and quantified the number of SNPs and/or genes, which if accumulated would confer full resistance to an otherwise susceptible strain. We name this approach the ‘distance to resistance’. It can be used to identify the creep towards complete antibiotics resistance in bacteria using genome sequencing. This approach serves as a basis for the development of future sequencing-based methods for predicting resistance profiles of bacterial strains in hospital microbiology and public health settings.

Penicillin-Binding Protein Transpeptidase Signatures for Tracking and Predicting β-Lactam Resistance Levels in Streptococcus pneumoniae

β-Lactam antibiotics are the drugs of choice to treat pneumococcal infections. The spread of β-lactam-resistant pneumococci is a major concern in choosing an effective therapy for patients. Systematically tracking β-lactam resistance could benefit disease surveillance. Here we developed a classification system in which a pneumococcal isolate is assigned to a "PBP type" based on sequence signatures in the transpeptidase domains (TPDs) of the three critical penicillin-binding proteins (PBPs), PBP1a, PBP2b, and PBP2x. We identified 307 unique PBP types from 2,528 invasive pneumococcal isolates, which had known MICs to six β-lactams based on broth microdilution. We found that increased β-lactam MICs strongly correlated with PBP types containing divergent TPD sequences. The PBP type explained 94 to 99% of variation in MICs both before and after accounting for genomic backgrounds defined by multilocus sequence typing, indicating that genomic backgrounds made little independent contribution to β-lactam MICs at the population level. We further developed and evaluated predictive models of MICs based on PBP type. Compared to microdilution MICs, MICs predicted by PBP type showed essential agreement (MICs agree within 1 dilution) of >98%, category agreement (interpretive results agree) of >94%, a major discrepancy (sensitive isolate predicted as resistant) rate of <3%, and a very major discrepancy (resistant isolate predicted as sensitive) rate of <2% for all six β-lactams. Thus, the PBP transpeptidase signatures are robust indicators of MICs to different β-lactam antibiotics in clinical pneumococcal isolates and serve as an accurate alternative to phenotypic susceptibility testing.

IMPORTANCE

The human pathogen Streptococcus pneumoniae is a leading cause of morbidity and mortality worldwide. β-Lactam antibiotics such as penicillin and ceftriaxone are the drugs of choice to treat pneumococcal infections. Some pneumococcal strains have developed β-lactam resistance through altering their penicillin-binding proteins (PBPs) and have become a major concern in choosing effective patient therapy. To systematically track and predict β-lactam resistance, we obtained the sequence signatures of PBPs from a large collection of clinical pneumococcal isolates using whole-genome sequencing data and found that these "PBP types" were predictive of resistance levels. Our findings can benefit the current era of strain surveillance when whole-genome sequencing data often lacks detailed resistance information. Using PBP positions that we found are always substituted within highly resistant strains may lead to further refinements. Sequence-based predictions are accurate and may lead to the ability to extract critical resistance information from nonculturable clinical specimens.

Computational studies on the resistance of penicillin-binding protein 2B (PBP2B) of wild-type and mutant strains of Streptococcus pneumoniae against β-lactam antibiotics

Mutations within transpeptidase domain of penicillin-binding protein 2B of the strains of Streptococcus pneumoniae leads to resistance against β-lactam antibiotics. To uncover the important residues responsible for sensitivity and resistance, the recently determined three dimensional structures of penicillin-binding protein 2B of both wild-type R6 (sensitive) and mutant 5204 (resistant) strains along with the predicted structures of other mutant strains G54, Hungary19A-6 and SP195 were considered for the interaction study with β-lactam antibiotics using induced-fit docking of Schrödinger. Associated binding energies of the complexes and their intermolecular interactions in the binding site clearly show that the wild-type R6 as sensitive, mutant strains 5204 and G54 as highly resistant, and the mutant strains Hungary19A-6 and SP195 as intermediate resistant. The study also reveals that the mutant strains Hungary19A-6 and SP195 exhibit intermediate resistant because of the existence of mutations till the intermediate 538th and 516th positions, respectively, and not till the end of the C-terminus. Furthermore, our investigations show that if the mutations are extended till the end of the C terminus, then the antibiotic resistance of induced-mutated strains increases from intermediate to high as in the strains 5204 and G54. The binding patterns obtained in the study are useful in designing potential inhibitors against multidrug resistant 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.

Pneumococci can persistently colonize adult patients with chronic respiratory disease

Streptococcus pneumoniae plays an important role in causing acute exacerbations in patients with chronic respiratory disease. However, few data are available regarding pneumococcal persistence in adult patients with chronic respiratory diseases. Fifty pneumococci recovered from sputum samples (1995 to 2010) from 13 adult patients with ≥ 3 episodes of acute exacerbation or pneumonia, with the same serotype and pulsed-field gel electrophoresis (PFGE) pattern, were studied. Multilocus sequence typing (MLST) loci, penicillin-binding protein (PBP) genes (pbp2x, pbp1a, pbp2b), and the quinolone-resistant determining regions (QRDRs) of parC, parE, and gyrA were PCR amplified and sequenced. The average time between the first and last episode was 582 days (standard deviation [SD], ± 362). All but two patients received multiple courses of β-lactam treatment, and all persistent strains were resistant to penicillin; however, the PBP sequences were stable over time apart from one variable nucleotide in pbp2x, observed among pneumococci isolated from three patients. In contrast, 7/11 patients treated with fluoroquinolones had fluoroquinolone-resistant pneumococci. In three patients, the initially fluoroquinolone-susceptible strain developed resistance after fluoroquinolone therapy, and in the remaining four patients, the persistent strain was fluoroquinolone resistant from the first episode. QRDR changes involved in fluoroquinolone resistance were frequently observed in persistent strains after fluoroquinolone treatment; however, the PBP sequences and MLST genotypes of these strains were stable over time.

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.

Pneumococcal interactions with epithelial cells are crucial for optimal biofilm formation and colonization in vitro and in vivo

The human nasopharynx is the main reservoir for Streptococcus pneumoniae (the pneumococcus) and the source for both horizontal spread and transition to infection. Some clinical evidence indicates that nasopharyngeal carriage is harder to eradicate with antibiotics than is pneumococcal invasive disease, which may suggest that colonizing pneumococci exist in biofilm communities that are more resistant to antibiotics. While pneumococcal biofilms have been observed during symptomatic infection, their role in colonization and the role of host factors in this process have been less studied. Here, we show for the first time that pneumococci form highly structured biofilm communities during colonization of the murine nasopharynx that display increased antibiotic resistance. Furthermore, pneumococcal biofilms grown on respiratory epithelial cells exhibited phenotypes similar to those observed during colonization in vivo, whereas abiotic surfaces produced less ordered and more antibiotic-sensitive biofilms. The importance of bacterial-epithelial cell interactions during biofilm formation was shown using both clinical strains with variable colonization efficacies and pneumococcal mutants with impaired colonization characteristics in vivo. In both cases, the ability of strains to form biofilms on epithelial cells directly correlated with their ability to colonize the nasopharynx in vivo, with colonization-deficient strains forming less structured and more antibiotic-sensitive biofilms on epithelial cells, an association that was lost when grown on abiotic surfaces. Thus, these studies emphasize the importance of host-bacterial interactions in pneumococcal biofilm formation and provide the first experimental data to explain the high resistance of pneumococcal colonization to eradication by antibiotics.

Ceftaroline fosamil: a new broad-spectrum cephalosporin

Ceftaroline fosamil, the prodrug of the active metabolite, ceftaroline, is a new, broad-spectrum cephalosporin recently approved in the USA for the treatment of acute bacterial skin and skin structure infections (ABSSSIs) and community-acquired bacterial pneumonia (CABP). Ceftaroline has potent in vitro activity against Gram-positive organisms, including methicillin-resistant Staphylococcus aureus and Streptococcus pneumoniae, as well as common Gram-negative organisms. The high affinity of ceftaroline for penicillin-binding proteins is responsible for the potent activity observed against clinically relevant pathogens. With respect to the treatment of CABP, the activity of ceftaroline against pathogens such as S. pneumoniae, S. aureus, Haemophilus influenzae and Moraxella catarrhalis demonstrates coverage across a broad range of pathogens typically encountered in clinical practice. Ceftaroline is also very active against common pathogens seen in ABSSSIs such as S. aureus (methicillin-susceptible S. aureus and methicillin-resistant S. aureus) and Streptococcus pyogenes. Ceftaroline exhibits a dose-proportional pharmacokinetic profile, similar to other renally excreted cephalosporins, and has a well-tolerated safety profile consistent with the cephalosporin class. Ceftaroline fosamil is compatible via Y-site administration with many other commonly administered parenteral drugs.

Molecular characterization of invasive penicillin non-susceptible Streptococcus pneumoniae from Denmark, 2001 to 2005

The aim of this study was to characterize invasive penicillin non-susceptible pneumococci (PNSP) isolated in Denmark from 2001 to 2005, describe the genetic changes in the penicillin-binding proteins (PBPs) and compare the isolates to the international recognized PNSP clones. All isolates were characterized using serotyping; 43 isolates were also characterized by multilocus sequence typing (MLST) and sequence determination of the PBP genes, pbp1a, pbp2b and pbp2x. In addition, isolates were tested for susceptibility to various antimicrobial agents. From 2001 to 2005, 178 invasive PNSP isolates were found among all invasive isolates. Of these, 13% were penicillin-resistant pneumococci (PRP). The PRP isolates were different to the penicillin intermediate-resistant pneumococci (PIRP) as they were co-resistant to a higher number of antimicrobial agents, and the serotype diversity was lower for the PRP isolates compared to the PIRP isolates. Changes in the PBPs were found to be the same as those observed in other studies. Most PRP isolates were found to belong to the clonal complex CC156 of which most were ST156 serotype 9V or 14. ST156 is a well known multiresistant widespread international PMEN (Pneumococcal Molecular Epidemiology Network) clone, Spain(9V)-3, which is also present as one of the dominant clones in our neighbour countries of Sweden, Norway and Germany.

Evolving trends in Streptococcus pneumoniae resistance: implications for therapy of community-acquired bacterial pneumonia

Pneumonia is a major infectious disease associated with significant morbidity, mortality and utilisation of healthcare resources. Streptococcus pneumoniae is the predominant pathogen in community-acquired pneumonia (CAP), accounting for 20-60% of bacterial cases. Emergence of multidrug-resistant S. pneumoniae has become a significant problem in the management of CAP. Although pneumococcal conjugate vaccine usage in children has led to significant decreases in morbidity and mortality due to S. pneumoniae in all age groups, disease management has been further complicated by the unexpected increase in resistant serotypes, such as 19A, in some regions. Until rapid and accurate diagnostic tests become available, initial treatment of CAP will remain empirical. Thus, selection of appropriate antimicrobial therapy for CAP must be based on prediction of the most likely pathogens and their local antimicrobial susceptibility patterns. This article reviews information on antimicrobial resistance patterns amongst S. pneumoniae and implications for managing CAP.

Cloning, expression, and characterization of a peculiar choline-binding beta-galactosidase from Streptococcus mitis

A Streptococcus mitis genomic DNA fragment carrying the SMT1224 gene encoding a putative beta-galactosidase was identified, cloned, and expressed in Escherichia coli. This gene encodes a protein 2,411 amino acids long with a predicted molecular mass of 268 kDa. The deduced protein contains an N-terminal signal peptide and a C-terminal choline-binding domain consisting of five consensus repeats, which facilitates the anchoring of the secreted enzyme to the cell wall. The choline-binding capacity of the protein facilitates its purification using DEAE-cellulose affinity chromatography, although its complete purification was achieved by constructing a His-tagged fusion protein. The recombinant protein was characterized as a monomeric beta-galactosidase showing a specific activity of around 2,500 U/mg of protein, with optimum temperature and pH ranges of 30 to 40 degrees C and 6.0 to 6.5, respectively. Enzyme activity is not inhibited by glucose, even at 200 mM, and remains highly stable in solution or immobilized at room temperature in the absence of protein stabilizers. In S. mitis, the enzyme was located attached to the cell surface, but a significant activity was also detected in the culture medium. This novel enzyme represents the first beta-galactosidase having a modular structure with a choline-binding domain, a peculiar property that can also be useful for some biotechnological applications.

Antimicrobial-resistant Streptococcus pneumoniae: trends and management

Management of pneumococcal infections has been challenged by the development of resistance and, more recently, the unexpected spread of resistant clones of serotypes, such as 19A, following the introduction of a conjugate pneumococcal vaccine for use in children in 2000. High-dose penicillin G and many other agents continue to be efficacious parenterally for pneumonia and bacteremia. However, treatment options for meningitis and for infections treated with oral agents, particularly in children, have been limited by resistance. Empiric treatment guidelines should reflect the emerging threats from increased drug resistance. Compliance with guidelines by physicians and patients is important to prevent further development of resistance as new classes of agents are unlikely to be available in the next decade.

Point mutation in the group B streptococcal pbp2x gene conferring decreased susceptibility to beta-lactam antibiotics

Beta-lactam antibiotics (BLAs) are the first-line agents used against group B streptococci (GBS) infection. A clonal set of four independent, invasive GBS isolates with elevated MICs to BLAs were identified that shared a pbp2x mutation (Q557E) corresponding to a resistance-conferring pneumococcal mutation. BLA sensitivity was restored through allelic replacement or complementation with the wild-type pbp2x.

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.

Structural and mechanistic basis of penicillin-binding protein inhibition by lactivicins

Beta-lactam antibiotics, including penicillins and cephalosporins, inhibit penicillin-binding proteins (PBPs), which are essential for bacterial cell wall biogenesis. Pathogenic bacteria have evolved efficient antibiotic resistance mechanisms that, in Gram-positive bacteria, include mutations to PBPs that enable them to avoid beta-lactam inhibition. Lactivicin (LTV; 1) contains separate cycloserine and gamma-lactone rings and is the only known natural PBP inhibitor that does not contain a beta-lactam. Here we show that LTV and a more potent analog, phenoxyacetyl-LTV (PLTV; 2), are active against clinically isolated, penicillin-resistant Streptococcus pneumoniae strains. Crystallographic analyses of S. pneumoniae PBP1b reveal that LTV and PLTV inhibition involves opening of both monocyclic cycloserine and gamma-lactone rings. In PBP1b complexes, the ring-derived atoms from LTV and PLTV show a notable structural convergence with those derived from a complexed cephalosporin (cefotaxime; 3). The structures imply that derivatives of LTV will be useful in the search for new antibiotics with activity against beta-lactam-resistant bacteria.

New Streptococcus pneumoniae clones in deceased wild chimpanzees

In wild chimpanzees in the Taï National Park, Côte d'Ivoire, sudden deaths which were preceded by respiratory problems had been observed since 1999. Two new clones of Streptococcus pneumoniae were identified in deceased apes on the basis of multilocus sequence typing analysis and ply, lytA, and pbp2x sequences. The findings suggest that virulent S. pneumoniae occurs in populations of wild chimpanzees with the potential to cause infections similar to those observed in humans.

Changing antimicrobial susceptibility patterns among Streptococcus pneumoniae and Haemophilus influenzae from Brazil: Report from the SENTRY Antimicrobial Surveillance Program (1998-2004)

Although antimicrobial resistance rates among Streptococcus pneumoniae and Haemophilus influenzae have increased significantly in most countries in the last years, most studies from Brazil report relatively low resistance rates among these pathogens. In this study, we analyzed the susceptibility patterns of S. pneumoniae and H. influenzae from Brazil during a 7-year period. A total of 829 S. pneumoniae and 718 H. influenzae consecutively collected from 1998 to 2004, mainly from respiratory tract and bloodstream infections, were susceptibility tested by broth microdilution methods against >30 drugs and the results were analyzed by year. Overall, 77.8% of S. pneumoniae strains were considered susceptible (MIC, < or =0.06 microg/ml) to penicillin. Resistance to penicillin (MIC, > or =2 microg/ml) and ceftriaxone (MIC, > or =4 microg/ml) were detected in 7.5 and 0.5% of strains, respectively. The fluoroquinolones, levofloxacin (MIC90) 1 microg/ml) and gatifloxacin (MIC90, 0.5 microg/ml), were active against 99.8% of the isolates tested. Among the other non-beta-lactam drugs tested, the rank order of susceptibility rates was chloramphenicol (98.9%) > clindamycin (96.4%) > erythromycin (90.6%) > tetracycline (69.8%) > trimethoprim/sulfamethoxazole (36.7%). Resistance to penicillin has increased markedly among S. pneumoniae isolates over 7 years (from 2.9 to 11.0%). Additionally, resistance rates against erythromycin, clindamycin, and tetracycline decreased among pneumococcal strains during the same period. S. pneumoniae recovered from pediatric patients (< or =5 years) showed increased penicillin and trimethoprim/sulfametroxazole resistance rates compared to older populations. The rate of ampicillin resistance among H. influenzae was 14.0%, which also corresponds with the beta -lactamase production rate. All H. influenzae isolates were susceptible to amoxicillin/clavulanate (MIC90, 1 microg/ml), ceftriaxone (MIC90, < or =0.008 microg/ml), cefepime (MIC90, 0.12 microg/ml), ciprofloxacin (MIC90, < or = 0.12microg/ml), levofloxacin (MIC90, < or =0.5 microg/ml), and gatifloxacin (MIC90, < or =0.03 microg/ml). Resistance to the antimicrobials tested remained very stable among H. influenzae isolates during the 7-year study period. The continued emerging antimicrobial resistances found in these pathogens (mainly S. pneumoniae) highlight the need for alternative agents for the treatment of infections caused by these species.

[Resistance of infectious agents involved in low respiratory tract infections in France]

This review concerning the major lower respiratory tract pathogens in France has for aim to describe the epidemiology of resistance to beta-lactams, macrolides, ketolides, and fluoroquinolones especially in Streptococcus pneumoniae and Haemophilus influenzae. It should also provide new insights on the mechanisms of acquired resistance and the level of resistance conferred, highlighting the related ecological impact. In the context of this XVth consensus conference, this review should contribute to the elaboration of guidelines for the treatment of lower respiratory tract infections in adults.

Molecular evolution perspectives on intraspecific lateral DNA transfer of topoisomerase and gyrase loci in Streptococcus pneumoniae, with implications for fluoroquinolone resistance development and spread

Fluoroquinolones are an important class of antibiotics for the treatment of infections arising from the gram-positive respiratory pathogen Streptococcus pneumoniae. Although there is evidence supporting interspecific lateral DNA transfer of fluoroquinolone target loci, no studies have specifically been designed to assess the role of intraspecific lateral transfer of these genes in the spread of fluoroquinolone resistance. This study involves a comparative evolutionary perspective, in which the evolutionary history of a diverse set of S. pneumoniae clinical isolates is reconstructed from an expanded multilocus sequence typing data set, with putative recombinants excluded. This control history is then assessed against networks of each of the four fluoroquinolone target loci from the same isolates. The results indicate that although the majority of fluoroquinolone target loci from this set of 60 isolates are consistent with a clonal dissemination hypothesis, 3 to 10% of the sequences are consistent with an intraspecific lateral transfer hypothesis. Also evident were examples of interspecific transfer, with two isolates possessing a parE-parC gene region arising from viridans group streptococci. The Spain 23F-1 clone is the most dominant fluoroquinolone-nonsusceptible clone in this set of isolates, and the analysis suggests that its members act as frequent donors of fluoroquinolone-nonsusceptible loci. Although the majority of fluoroquinolone target gene sequences in this set of isolates can be explained on the basis of clonal dissemination, a significant number are more parsimoniously explained by intraspecific lateral DNA transfer, and in situations of high S. pneumoniae population density, such events could be an important means of resistance spread.

Pneumococcal β-Lactam Resistance Due to a Conformational Change in Penicillin-binding Protein 2x

Streptococcus pneumoniae is a life-threatening human pathogen that is increasingly resistant to a wide array of drugs. Resistance to beta-lactams, the most widely used antibiotics, is correlated with tens of amino acid substitutions in their targets; that is, the penicillin-binding proteins (PBPs), resulting from multiple events of recombination. To discriminate relevant substitutions from those that are incidental to the recombination process, we report the exhaustive characterization of all the mutations in the transpeptidase domain of PBP2x from the highly resistant strain 5204. A semi-automated method combining biochemical and microbiological approaches singled out 6 mutations of 41 (15%) that are essential for high level resistance. The hitherto uncharacterized I371T, R384G, M400T, and N605T together with the previously studied T338M and M339F account for nearly all the loss of affinity of PBP2x for beta-lactams. Most interestingly, I371T and R384G cause the conformational change of a loop that borders the entrance of the active site cavity, hampering antibiotic binding. For the first time all the mutations of a PBP relevant to beta-lactam resistance have been identified, providing new mechanistic insights. Most notable is the relationship between the decreased susceptibility to beta-lactams and the dynamic behavior of a loop.

Crystal structure of penicillin-binding protein 1a (PBP1a) reveals a mutational hotspot implicated in beta-lactam resistance in Streptococcus pneumoniae

Streptococcus pneumoniae is a major human pathogen whose infections have been treated with beta-lactam antibiotics for over 60 years, but the proliferation of strains that are highly resistant to such drugs is a problem of worldwide concern. Beta-lactams target penicillin-binding proteins (PBPs), membrane-associated enzymes that play essential roles in the peptidoglycan biosynthetic process. Bifunctional PBPs catalyze both the polymerization of glycan chains (glycosyltransfer) and the cross-linking of adjacent pentapeptides (transpeptidation), while monofunctional enzymes catalyze only the latter reaction. Although S. pneumoniae has six PBPs, only three (PBP1a, PBP2x, PBP2b) are major resistance determinants, with PBP1a being the only bifunctional enzyme. PBP1a plays a key role in septum formation during the cell division cycle and its modification is essential for the development of high-level resistance to penicillins and cephalosporins. The crystal structure of a soluble form of pneumococcal PBP1a (PBP1a*) has been solved to 2.6A and reveals that it folds into three domains. The N terminus contains a peptide from the glycosyltransfer domain bound to an interdomain linker region, followed by a central, transpeptidase domain, and a small C-terminal unit. An analysis of PBP1a sequences from drug-resistant clinical strains in light of the structure reveals the existence of a mutational hotspot at the entrance of the catalytic cleft that leads to the modification of the polarity and accessibility of the mutated PBP1a active site. The presence of this hotspot in all variants sequenced to date is of key relevance for the development of novel antibiotherapies for the treatment of beta-lactam-resistant pneumococcal strains.

Micromachined polymerase chain reaction system for multiple DNA amplification of upper respiratory tract infectious diseases

This paper presents a micro polymerase chain reaction (PCR) chip for the DNA-based diagnosis of microorganism genes and the detection of their corresponding antibiotic-resistant genes. The micro PCR chip comprises cheap biocompatible soda-lime glass substrates with integrated thin-film platinum resistors as heating/sensing elements, and is fabricated using micro-electro-mechanical-system (MEMS) techniques in a reliable batch-fabrication process. The heating and temperature sensing elements are made of the same material and are located inside the reaction chamber in order to ensure a uniform temperature distribution. This study performs the detection of several genes associated with upper respiratory tract infection microorganisms, i.e. Streptococcus pneumoniae, Haemopilus influenze, Staphylococcu aureus, Streptococcus pyogenes, and Neisseria meningitides, together with their corresponding antibiotic-resistant genes. The lower thermal inertia of the proposed micro PCR chip relative to conventional bench-top PCR systems enables a more rapid detection operation with reduced sample and reagent consumption. The experimental data reveal that the high heating and cooling rates of the system (20 and 10 degrees C/s, respectively) permit successful DNA amplification within 15 min. The micro PCR chip is also capable of performing multiple DNA amplification, i.e. the simultaneous duplication of multiple genes under different conditions in separate reaction wells. Compared with the large-scale PCR system, it is greatly advantageous for fast diagnosis of multiple infectious diseases. Multiplex PCR amplification of two DNA segments in the same well is also feasible using the proposed micro device. The developed micro PCR chip provides a crucial tool for genetic analysis, molecular biology, infectious disease detection, and many other biomedical applications.

Genetic analysis of pbp2x in clinical Streptococcus pneumoniae isolates in Quebec, Canada

OBJECTIVES

To investigate the nature of the amino acid motifs found in penicillin-binding protein (PBP) 2x of penicillin-resistant Streptococcus pneumoniae isolates across the province of Quebec (Canada), and to obtain preliminary information regarding the prevalence of these alterations.

METHODS

The pbp2x genomic region encompassing codons 178-703, which includes the entire region of the transpeptidase domain, was sequenced and compared for 52 clinical isolates comprising 20 penicillin-susceptible (PSSP), 20 penicillin-intermediate (PISP) and 12 penicillin-resistant (PRSP) pneumococci.

RESULTS

The degree of diversity within PBP2x correlated with increased resistance to beta-lactam antibiotics. There were an average of 5.0 +/- 1.8 mutations in PSSP, 37.9 +/- 4.4 in PISP, and 63.0 +/- 2.0 in PRSP isolates when compared with the control penicillin-susceptible strain R6. At least six distinct amino acid profiles were identified among PISP strains isolated in Quebec. In contrast, all PRSP isolates shared a similar pattern of altered amino acids compared with the sequence from susceptible strains.

CONCLUSIONS

These data will be useful in future studies to monitor the genetic changes associated with the emergence and spread of beta-lactam resistance in Quebec.

Mosaic genes and mosaic chromosomes-genomic variation in Streptococcus pneumoniae

The genome sequences of two strains of Streptococcus pneumoniae, one of the major human pathogens, are currently available: that of the nonencapsulated laboratory strain R6, the origin of which dates back to the early 20th century, and of the serotype 4 TIGR strain isolated recently. The two genomes are not only different in size (2 versus 2.16 Mb) but differ also by approximately 10% of their genes, many of which being organized in large clusters. Their strain-specific genes and gene clusters are described here. The R6 genome contains 69 kb organized in six large regions that are absent from the TIGR strain, which in turn contains an extra 157kb in twelve clusters compared to R6. In addition, the TIGR strain contains 13 clusters of 4 kb and larger that are not shared by a variety of genetically different S. pneumoniae strains. Many regions bear signs of gene transfer events such as the presence of insertion sequences, transposable elements, and putative site-specific integrases/recombinases. Three strain-specific regions are devoted to genes encoding proteins with the cell wall anchor motif LPXTG which are important for the interaction with host cells and appear to be highly variable, similar to cell wall-associated choline-binding proteins.

Antimicrobial susceptibility of Streptococcus pneumoniae in Latin America: results from five years of the SENTRY Antimicrobial Surveillance Program

A total of 1561 pneumococcal isolates were collected in 1997-2001, mainly from patients with community-acquired respiratory tract infections, and susceptibilities were tested by reference broth microdilution against 29 antimicrobial agents. In general, 69.3% of strains were considered susceptible (MIC < or = 0.06 mg/L) to penicillin. Resistance to penicillin (MIC > or = 2 mg/L) and cefotaxime (MIC > or = 4 mg/L) was found in 11.9% and 0.4% of isolates, respectively. The fluoroquinolones gatifloxacin (MIC90, 0.5 mg/L) and levofloxacin (MIC90, 1 mg/L) were active against > 99% of the isolates tested. Among the other non-beta-lactam drugs tested, the rank order of susceptibility was chloramphenicol (95.6%) > clindamycin (94.5%) > azithromycin (88.5%) > clarithromycin (87.5%) >tetracycline (79.5%) > trimethoprim + sulphamethoxazole (60.5%). The penicillin-non-susceptible isolates presented higher rates of resistance to other antimicrobial agents. The rank order of penicillin resistance rates among the seven participating countries was Mexico (25.0%) > Uruguay (19.2%) > Chile (18.3%) > Colombia = Argentina (9.9%) > Brazil (3.9%) > Venezuela (2.8%). The regional rate of penicillin resistance did not vary significantly over the years studied (p 0.339). Screening for the ermB and mefA genes by multiplex rapid cycle PCR on 23 erythromycin-resistant isolates collected during the year 2001 showed that 43.5% and 56.5%, respectively, were positive for ermB and mefA. Overall, the results indicated that antimicrobial susceptibilities of Streptococcus pneumoniae vary significantly among Latin American countries. Regional and local surveillance programmes are necessary to guide empirical therapy of pneumococcal infection in Latin American countries.

Streptococcus pneumoniae: epidemiology and patterns of resistance

Streptococcus pneumoniae is a leading cause of bacterial pneumonia, meningitis, otitis media, and sinusitis; it results in significant morbidity and mortality in patients with pneumonia and meningitis. The pneumococcus is a common colonizing bacterium in the respiratory tract; it is especially common in the respiratory tracts of children, where it is frequently exposed to antimicrobial agents. This exposure can lead to resistance. Penicillin nonsusceptibility is found in nearly 40% of strains causing disease in adults, although often these cases are treatable with appropriate dosing regimens of many oral and parenteral beta-lactam agents. In the United States resistance to macrolides is widespread--averaging approximately 28%--but geographically variable, ranging from 23% in the northwest to 30% in the northeast. Resistance to tetracyclines and trimethoprim-sulfamethoxazole are reported in approximately 20% and 35% of isolates, respectively, and resistance to multiple classes of agents is increasingly common. Amoxicillin, amoxicillin-clavulanate, respiratory fluoroquinolones, and clindamycin are currently the most effective agents for treatment of respiratory tract infections caused by S pneumoniae, with >90% of isolates in the United States being susceptible. Vancomycin is the only agent against which resistance has not emerged. Patient groups that are at increased risk for developing resistant pneumococcal infections have been identified and include patients with malignancies, human immunodeficiency virus infection, and sickle-cell disease. Judicious use of antimicrobials is the key to preventing the emergence of further resistance, particularly as few new classes of agents are likely to become available for clinical use in the short term.

Mechanisms of resistance among respiratory tract pathogens

Antimicrobial resistance among respiratory tract pathogens represents a significant health care threat. Identifying the antimicrobial agents that remain effective in the presence of resistance, and knowing why, requires a thorough understanding of the mechanisms of action of the various agents as well as the mechanisms of resistance demonstrated among respiratory tract pathogens. The primary goal of antimicrobial therapy is to eradicate the pathogen, via killing or inhibiting bacteria, from the site of infection; the defenses of the body are required for killing any remaining bacteria. Targeting a cellular process or function specific to bacteria and not to the host limits the toxicity to patients. Currently, there are four general cellular targets to which antimicrobials are targeted: cell wall formation and maintenance, protein synthesis, DNA replication, and folic acid metabolism. Resistance mechanisms among respiratory tract pathogens have been demonstrated for all four targets. In general, the mechanisms of resistance used by these pathogens fall into one of three categories: enzymatic inactivation of the antimicrobial, prevention of intracellular accumulation, and modification of the target site to which agents bind to exert an antimicrobial effect. Resistance to some agents can be overcome by modifying the dosage regimens (e.g., using high-dose therapy) or inhibiting the resistance mechanism (e.g., b-lactamase inhibitors), whereas other mechanisms of resistance can only be overcome by using an agent from a different class. Understanding the mechanisms of action of the various agents and the mechanisms of resistance used by respiratory tract pathogens can help clinicians identify the agents that will increase the likelihood of achieving optimal outcomes.

Unique variations of pbp2b sequences in penicillin-nonsusceptible Streptococcus pneumoniae isolates from Korea

pbp2b gene alterations were analyzed in 102 clinical isolates of Streptococcus pneumoniae (30 penicillin susceptible, 23 intermediate, and 49 resistant) from Korea. On the basis of PBP2B amino acid sequences, penicillin-nonsusceptible isolates of S. pneumoniae belonged to six groups, and 76% of the isolates in groups I to IV showed the same divergent block of amino acid alterations. Thirteen isolates (group II) also possessed a divergent block that was identical to that of Streptococcus oralis. The pbp2b genes of most Korean isolates showed novel mosaic mutations due to horizontal gene transfer. The Thr252 --> Ala substitution, previously thought to be associated only with penicillin-nonsusceptible strains, was also found in three penicillin-susceptible strains. On the basis of their pbp2b nucleotide sequences, all penicillin-nonsusceptible isolates can be detected by multiplex PCR, which can be used as a novel method for detection of antibiotic-resistant pneumococcal strains in clinical specimens.

Evolution of amoxicillin/clavulanate in the treatment of adults with acute bacterial rhinosinusitis and community-acquired pneumonia in response to antimicrobial-resistance patterns

Current treatment guidelines for community-acquired respiratory tract infections no longer depend solely on the characteristics of the patient and the clinical syndrome, but on those of the offending pathogen, including presence and level of antimicrobial resistance. The most common respiratory tract pathogens known to cause acute bacterial rhinosinusitis (ABRS) and community-acquired pneumonia (CAP) include Streptococcus pneumoniae and Haemophilus influenzae. The prevalence of antimicrobial resistance, especially b-lactum and macrolide resistance, among S pneumoniae and H influenzae has increased dramatically during the past 2 decades, diminishing the activity of many older antimicrobials against resistant organisms. A pharmacokinetically enhanced formulation of amoxicillin/clavulanate has been developed to fulfill the need for an oral b-lactam antimicrobial that achieves a greater time that the serum drug concentration exceeds the minimum inhibitory concentration (T > MIC) of antimicrobials against pathogens than conventional formulations to improve activity against S pneumoniae with reduced susceptibility to penicillin. The b-lactamase inhibitor clavulanate allows for coverage of b-lactamase-producing pathogens, such as H influenzae and M catarrhalis. This article reviews the rationale for, and evolution of, oral amoxicillin clavulanate for ABRS and CAP

Antimicrobial resistance among pediatric respiratory tract infections: clinical challenges

Considerable development of antimicrobial resistance has occurred in the major pediatric bacterial pathogens, Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. However, most of the respiratory infections that children suffer are viral and self-limiting, and only a small percentage of them will develop secondary bacterial infections with the pathogens listed. The challenge for rational antibiotic use is to determine which patients can be treated conservatively and which require antimicrobial intervention to avoid prolonged discomfort or development of permanent sequelae. The basis for rational use of antibiotic in the era of resistance in these major pathogens is to avoid overuse of antimicrobial agents, tailor treatment to identified pathogens as much as possible, and base empiric treatment on the disease being treated and the susceptibility of the probable pathogens at breakpoints based on pharmacokinetic and pharmacodynamic parameters. With appropriate dosing regimens based on these parameters and despite development of resistance, amoxicillin is still one of the most active oral agents against S. pneumoniae and non-beta-lactamase producing strains of H. influenzae, whereas amoxicillin-clavulanate is active against beta-lactamase-producing strains of H. influenzae and M. catarrhalis. Parenteral ceftriaxone and oral and parenteral fluoroquinolones are active against all 3 species, but fluoroquinolones should be used with utmost caution when all other options have been considered because of concerns about toxicity and development of resistance. Introduction of a 7-valent conjugate pneumococcal vaccine in the United States in 2000 reduced the prevalence of invasive pneumococcal disease in children younger than 2 years old, but, as of 2001, had not had a major impact on decreasing antimicrobial resistance.

Amoxicillin/clavulanate potassium extended release tablets: a new antimicrobial for the treatment of acute bacterial sinusitis and community-acquired pneumonia

Community-acquired bacterial respiratory tract infections are among the most common health disorders requiring medical care and are associated with substantial morbidity, mortality, and direct and indirect costs. Recent increases in the prevalence of antimicrobial resistance have resulted in reduced susceptibility of the most common respiratory tract bacterial pathogens to a number of antimicrobials. Amoxicillin/clavulanate potassium extended release (ER) tablets (Augmentin XR, GlaxoSmithKline) is a new formulation of amoxicillin/clavulanate that retains activity against betalactamase-producing organisms whilst increasing the activity against Streptococcus pneumoniae through elevated and sustained plasma amoxicillin concentrations. The bilayer tablet provides immediate release of clavulanate and both immediate and sustained release of amoxicillin to maintain therapeutic concentrations of amoxicillin over longer periods of the dosing interval. In clinical trials of acute bacterial sinusitis (ABS) and community-acquired pneumonia (CAP), amoxicillin/clavulanate ER was shown to have excellent bacteriological and clinical success rates, even in patients infected with antimicrobial-resistant pathogens, and was found to be generally well tolerated. Amoxicillin/clavulanate ER is approved in the US for the treatment of patients with ABS or CAP caused by beta-lactamase-producing pathogens (ie, Haemophilus influenzae, Moraxella catarrhalis, Haemophilus parainfluenzae, Klebsiella pneumoniae, or methicillin-susceptible Staphylococcus aureus) and S. pneumoniae with reduced susceptibility to penicillin (penicillin minimum inhibitory concentration = 2.0 microg/ml).

Typing and Molecular Characterization ofStreptococcus pneumoniaewith Reduced Susceptibility to Cefotaxime Isolated in Latin America

The treatment of systemic infections, especially meningitis, caused by Streptococcus pneumoniae nonsusceptible to third-generation cephalosporins, is extremely difficult due to the paucity of therapeutic options. The main objective of this study was to characterize isolates of S. pneumoniae with reduced susceptibility to cefotaxime (MICs, > or = 1 microg/ml) by different typing methods and to evaluate whether clonal dissemination of this pathogen had occurred among Latin American medical centers. A total of 46 isolates collected from respiratory tract specimens, blood cultures, cerebrospinal fluid, eye, and other sources were analyzed. The isolates were collected from Latin American medical centers located in Argentina, Brazil, Chile, Colombia, Mexico, and Uruguay through two multicenter surveillance programs, in 1997 and 1998. Isolates were serotyped and molecular typed by pulsed-field gel electrophoresis (PFGE) and automated ribotyping. Antimicrobial susceptibilities were determined to 19 drugs by reference broth microdilution methods. Ten isolates (21.7%) had cefotaxime MICs > or = 2 microg/ml, whereas 36 (78.3%) had cefotaxime MIC results at 1 microg/ml. All isolates were susceptible to gatifloxacin, levofloxacin, and vancomycin. The isolates were distributed among five major serotypes (%): 23F (39.1%), 14 (32.6%), 19F (23.9%), 9V (2.2%), and 6B (2.2%). However, distinct molecular patterns were detected among isolates with a unique serotype. Six and four PFGE patterns were identified among isolates with serotype 23F and 19F, respectively. When PFGE and automated ribotyping analyses were combined, four clusters were identified. The largest cluster (10 isolates) was represented by isolates with ribotype 18-2, major PFGE pattern I, and serotype 14. ATCC 700671 (international clone Spain 9V-3) also showed ribotype 18-2. This clone was detected in four countries: Argentina, Brazil, Chile, and Uruguay. A second cluster (8 isolates) were characterized by isolates with ribotype 17-4, PFGE type D, and serotype 23F, similar to ATCC 700669 (international clone Spain23F-1). Isolates from this cluster were identified in three countries: Brazil, Chile, and Mexico. Our results indicated that clonal dissemination of S. pneumoniae with reduced susceptibility to cefotaxime has occurred in Latin America mainly among serogroups 14, 19F, and 23F.

The structural modifications induced by the M339F substitution in PBP2x from Streptococcus pneumoniae further decreases the susceptibility to beta-lactams of resistant strains

PBP2x is a primary determinant of beta-lactams resistance in Streptococcus pneumoniae. Altered PBP2x with multiple mutations have a reduced "affinity" for the antibiotics. An important polymorphism is found in PBP2x sequences from clinical resistant strains. To understand the mechanism of resistance, it is necessary to identify and characterize the relevant substitutions. Many similar PBP2x sequences from resistant isolates have the previously studied T338A mutation, adjacent to the active site Ser337. We report here the structural and functional analysis of the M339F substitution that is found in a subset of these sequences, originating from highly resistant strains. The M339F mutation causes a 4-10-fold reduction of the reaction rate with beta-lactams, depending on the molecular context. In addition, release of the inactivated antibiotic from the active site is up to 3-fold faster as a result from the M339F mutation. These effects measured in vitro are correlated with the level of beta-lactam resistance in vivo conferred by several PBP2x variants. Thus, a single amino acid difference between similar PBP2x from clinical isolates can strongly modulate the degree of beta-lactam resistance. The crystal structure of the double mutant T338A/M339F solved to a resolution of 2.4 A shows a distortion of the active site and a reorientation of the hydroxyl group of the active site Ser337, which can explain the kinetic effects of the mutations.

Antibiotic resistance

Through billions of years of evolution, microbes have developed myriad defense mechanisms designed to ensure their survival. This protection is readily transferred to their fellow life forms via transposable elements. Despite very early warnings, humans have chosen to abuse the gift of antibiotics and have created a situation where all microorganisms are resistant to some antibiotics and some microorganisms are resistant to all antibiotics. When antibiotics are used, six events may occur with only one being beneficial: when the antibiotic aids the host defenses to gain control and eliminate the infection. Alternatively, the antibiotic may cause toxicity or allergy, initiate a superinfection with resistant bacteria, promote microbial chromosomal mutations to resistance, encourage resistance gene transfer to susceptible species, or promote the expression of dormant resistance genes.

Bacterial resistance to penicillin G by decreased affinity of penicillin-binding proteins: a mathematical model

Streptococcus pneumoniae and Neisseria meningitidis have very similar mechanisms of resistance to penicillin G. Although penicillin resistance is now common in S. pneumoniae, it is still rare in N. meningitidis. Using a mathematical model, we studied determinants of this difference and attempted to anticipate trends in meningococcal resistance to penicillin G. The model predicted that pneumococcal resistance in a population similar to that of France might emerge after 20 years of widespread use of beta-lactam antibiotics; this period may vary from 10 to 30 years. The distribution of resistance levels became bimodal with time, a pattern that has been observed worldwide. The model suggests that simple differences in the natural history of colonization, interhuman contact, and exposure to beta-lactam antibiotics explain major differences in the epidemiology of resistance of S. pneumoniae and N. meningitidis.

Antibiotic tolerance in pneumococci

When bacteria such as Staphylococcus aureus and Streptococcus pneumoniae are exposed to lytic antibiotics such as penicillin and vancomycin, a self-induced killing process is initiated in the organism. This killing occurs via both non-lytic and lytic processes. Recent data suggest that the non-lytic killing system, which might affect the cytoplasmic membrane, secondarily activates murein hydrolases that eventually lyse the cell. Disturbances in this suicide pathway can lead to antibiotic tolerance, a process whereby the antibiotic still exerts its bacteriostatic effects but the self-induced killing system is impaired. In mutants obtained in vitro, signaling pathways have been affected that show either increased or decreased antibiotic-induced killing. Among clinical isolates of S. pneumoniae that are tolerant to penicillin and/or vancomycin, we do not yet know whether these signaling pathways are affected. We could, however, demonstrate that the activity of murein hydrolases is negatively controlled by the production of capsular polysaccharides in one vancomycin-tolerant isolate. Hence, type and level of capsular expression might constitute one factor that determines the degree of lysis, once the killing signal has been elicited by the antibiotic.

PCR-based ordered genomic libraries: a new approach to drug target identification for Streptococcus pneumoniae

Described here are the development and validation of a novel approach to identify genes encoding drug targets in Streptococcus pneumoniae. The method relies on the use of an ordered genomic library composed of PCR amplicons that were generated under error-prone conditions so as to introduce random mutations into the DNA. Since some of the mutations occur in drug target-encoding genes and subsequently affect the binding of the drug to its respective cellular target, amplicons containing drug targets can be identified as those producing drug-resistant colonies when transformed into S. pneumoniae. Examination of the genetic content of the amplicon giving resistance coupled with bioinformatics and additional genetic approaches could be used to rapidly identify candidate drug target genes. The utility of this approach was verified by using a number of known antibiotics. For drugs with single protein targets, amplicons were identified that rendered S. pneumoniae drug resistant. Assessment of amplicon composition revealed that each of the relevant amplicons contained the gene encoding the known target for the particular drug tested. Fusidic acid-resistant mutants that resulted from the transformation of S. pneumoniae with amplicons containing fusA were further characterized by sequence analysis. A single mutation was found to occur in a region of the S. pneumoniae elongation factor G protein that is analogous to that already implicated in other bacteria as being associated with fusidic acid resistance. Thus, in addition to facilitating the identification of genes encoding drug targets, this method could provide strains that aid future mechanistic studies.

Effects of amino acid alterations in penicillin-binding proteins (PBPs) 1a, 2b, and 2x on PBP affinities of penicillin, ampicillin, amoxicillin, cefditoren, cefuroxime, cefprozil, and cefaclor in 18 clinical isolates of penicillin-susceptible, -intermediate, and -resistant pneumococci

Amino acid alterations in or flanking conserved motifs making up the active binding sites of penicillin-binding proteins (PBPs) 1a, 2b, and 2x of pneumococci were correlated with changes in affinities of penicillin, ampicillin, amoxicillin, cefditoren, cefuroxime, cefprozil, and cefaclor for these PBPs. Four penicillin-susceptible (PSSP), eight penicillin-intermediate (PISP), and six penicillin-resistant (PRSP) pneumococci were studied by DNA sequencing of the penicillin-binding sites of the pbp1a, -2x, and -2b genes of strains and by determining 50% inhibitory concentrations of the seven agents for PBP1a, -2x, and -2b. Two PSSP strains had alterations in PBP2x (L(546)-->V) (one strain) or PBP2b (T(445)-->A) (one strain). All eight PISP strains had at least two alterations--T(338)-->P or A or H(394)-->Y in PBP2X and T(445)-->A in BPB2b. All PRSP strains had the same changes seen in PISP strains, as well as T(371)-->A or S substitutions in PBP1a. The two most resistant PRSP strains had a second change in PBP2x (M(339)-->F) in a conserved motif. The affinities of penicillin and ampicillin for all three PBPs were decreased for PRSP and most PISP strains. The affinity of amoxicillin for PBP1a and -2x was decreased only for PRSP. Cefaclor and cefprozil showed decreased affinity of PRSP but not PISP for all three PBPs. Cefuroxime showed decreased affinity of PISP and PRSP for PBP1a and -2x but no change for PBP2b. Cefditoren showed no difference in PBP affinity based on penicillin or cefditoren MICs, indicating a different PBP target for this agent. Overall, the MICs for and PBP affinities of the strains correlated with the changes found in the PBP active binding sites.

Antimicrobial resistance among respiratory pathogens collected in Thailand during 1999-2000

A multi-center surveillance study was conducted in Thailand during 1999-2000 to determine antimicrobial susceptibilities among the respiratory pathogens Streptococcus pneumoniae (n = 206), Haemophilus influenzae (n = 305), and Moraxella catarrhalis (n = 39). Of the S. pneumoniae isolates collected, 33.5% were penicillin-susceptible, 27.2% intermediate and 39.3% resistant. Expectedly, resistance rates to beta-lactams were higher among penicillin-resistant (ceftriaxone, 14.8%; amoxicillin-clavulanate, 42.0%; cefuroxime, 100%) than penicillin-susceptible (ceftriaxone, 0%; amoxicillin-clavulanate, 0%; cefuroxime, 0%) isolates. Likewise, azithromycin and clarithromycin resistances were 4.3% and 5.8% among penicillin-susceptible isolates, and 77.8% and 95.1% among penicillin-resistant isolates. All S. pneumoniae remained susceptible to vancomycin and 99.5% were susceptible to levofloxacin. Multidrug resistance (resistance to >3 antimicrobial classes) was present in 25.2% of pneumococcal isolates (n = 52), with resistance to azithromycin, penicillin and trimethoprim-sulfamethoxazole the most common phenotype (40/52 isolates; 77.0%). Among the isolates of H. influenzae, the prevalence of beta-lactamase production was 45.2%. All isolates of H. influenzae were susceptible to amoxicillin-clavulanate, azithromycin, ceftriaxone, cefuroxime and levofloxacin while 49.5% were resistant to trimethoprim-sulfamethoxazole. All 39 isolates of M. catarrhalis produced beta-lactamase. Azithromycin (MIC90, < or = 0.03 microg/ml) and levofloxacin (MIC90, 0.03 microg/ml) were the most active agents tested against M. catarrhalis. The results of this study may serve as a baseline for future studies to monitor antimicrobial susceptibilities among respiratory pathogens in Thailand.

Increase of the deacylation rate of PBP2x from Streptococcus pneumoniae by single point mutations mimicking the class A beta-lactamases

The class A beta-lactamases and the transpeptidase domain of the penicillin-binding proteins (PBPs) share the same topology and conserved active-site residues. They both react with beta-lactams to form acylenzymes. The stability of the PBP acylenzymes results in the inhibition of the transpeptidase function and the antibiotic activity of the beta-lactams. In contrast, the deacylation of the beta-lactamases is extremely fast, resulting in a high turnover of beta-lactam hydrolysis, which confers resistance to these antibiotics. In TEM-1 beta-lactamase from Escherichia coli, Glu166 is required for the fast deacylation and occupies the same spatial location as Phe450 in PBP2x from Streptococcus pneumoniae. To gain insight into the deacylation mechanism of both enzymes, Phe450 of PBP2x was replaced by various residues. The introduction of ionizable side chains increased the deacylation rate, in a pH-dependent manner, for the acidic residues. The aspartic acid-containing variant had a 110-fold faster deacylation at pH 8. The magnitude of this effect is similar to that observed in a naturally occurring variant of PBP2x, which confers increased resistance to cephalosporins.