Emergence of a pneumococcal clone with cephalosporin resistance and penicillin susceptibility

Allogenous Selection of Mutational Collateral Resistance: Old Drugs Select for New Resistance Within Antibiotic Families

Allogeneous selection occurs when an antibiotic selects for resistance to more advanced members of the same family. The mechanisms of allogenous selection are (a) collateral expansion, when the antibiotic expands the gene and gene-containing bacterial populations favoring the emergence of other mutations, inactivating the more advanced antibiotics; (b) collateral selection, when the old antibiotic selects its own resistance but also resistance to more modern drugs; (c) collateral hyper-resistance, when resistance to the old antibiotic selects in higher degree for populations resistant to other antibiotics of the family than to itself; and (d) collateral evolution, when the simultaneous or sequential use of antibiotics of the same family selects for new mutational combinations with novel phenotypes in this family, generally with higher activity (higher inactivation of the antibiotic substrates) or broader spectrum (more antibiotics of the family are inactivated). Note that in some cases, collateral selection derives from collateral evolution. In this article, examples of allogenous selection are provided for the major families of antibiotics. Improvements in minimal inhibitory concentrations with the newest drugs do not necessarily exclude “old” antibiotics of the same family of retaining some selective power for resistance to the newest agents. If this were true, the use of older members of the same drug family would facilitate the emergence of mutational resistance to the younger drugs of the family, which is frequently based on previously established resistance traits. The extensive use of old drugs (particularly in low-income countries and in farming) might be significant for the emergence and selection of resistance to the novel members of the family, becoming a growing source of variation and selection of resistance to the whole family. In terms of future research, it could be advisable to focus antimicrobial drug discovery more on the identification of new targets and new (unique) classes of antimicrobial agents, than on the perpetual chemical exploitation of classic existing ones.

New Mutations of Penicillin-Binding Proteins in Streptococcus agalactiae Isolates from Cattle with Decreased Susceptibility to Penicillin

Streptococcus agalactiae is a causal agent of bovine mastitis and is treated by β-lactam antibiotics (BLAs). Compared to penicillin-resistant S. agalactiae from humans, resistant strains in bovine are rarely reported. In this study, we aimed to investigate BLA resistance and mutations in penicillin-binding proteins (PBPs) of S. agalactiae in central and northeast China. The minimum inhibitory concentrations (MICs) of 129 penicillin-resistant S. agalactiae isolates from cows with mastitis were determined, and the related PBP genes were detected and sequenced. All strains were unsusceptible to penicillin G and mostly resistant to ampicillin, cefalexin, and ceftiofur sodium. One hundred twenty-nine strains were divided into 4 clonal groups and 8 sequence types by multilocus sequence typing analysis. We found a set of new substitutions in PBP1B, PBP2B, and PBP2X from most strains isolated from three provinces. The strains with high PBP mutations showed a broader unsusceptible spectrum and higher MICs than those with few or single mutation. Our research indicates unpredicted mutations in the PBP genes of S. agalactiae isolated from cows with mastitis treated by BLAs. This screening is the first of S. agalactiae from cattle.

An evolutionary model to predict the frequency of antibiotic resistance under seasonal antibiotic use, and an application to Streptococcus pneumoniae

The frequency of resistance to antibiotics in Streptococcus pneumoniae has been stable over recent decades. For example, penicillin non-susceptibility in Europe has fluctuated between 12% and 16% without any major time trend. In spite of long-term stability, resistance fluctuates over short time scales, presumably in part due to seasonal fluctuations in antibiotic prescriptions. Here, we develop a model that describes the evolution of antibiotic resistance under selection by multiple antibiotics prescribed at seasonally changing rates. This model was inspired by, and fitted to, published data on monthly antibiotics prescriptions and frequency of resistance in two communities in Israel over 5 years. Seasonal fluctuations in antibiotic usage translate into small fluctuations of the frequency of resistance around the average value. We describe these dynamics using a perturbation approach that encapsulates all ecological and evolutionary forces into a generic model, whose parameters quantify a force stabilizing the frequency of resistance around the equilibrium and the sensitivity of the population to antibiotic selection. Fitting the model to the data revealed a strong stabilizing force, typically two to five times stronger than direct selection due to antibiotics. The strong stabilizing force explains that resistance fluctuates in phase with usage, as antibiotic selection alone would result in resistance fluctuating behind usage with a lag of three months when antibiotic use is seasonal. While most antibiotics selected for increased resistance, intriguingly, cephalosporins selected for decreased resistance to penicillins and macrolides, an effect consistent in the two communities. One extra monthly prescription of cephalosporins per 1000 children decreased the frequency of penicillin-resistant strains by 1.7%. This model emerges under minimal assumptions, quantifies the forces acting on resistance and explains up to 43% of the temporal variation in resistance.

Subacute bacterial endocarditis caused by Cardiobacterium hominis: A case report

Cardiobacterium hominis is a fastidious organism that can rarely cause subacute bacterial endocarditis. This report describes a case involving a 47-year-old man with subacute endocarditis due to C hominis; the isolate was initally found to be resistant to third-generation cephalosporins, but was later found to be susceptible using an alternative method. The authors discuss the likely cause of this error and emphasize the importance of adherence to fully validated methods for the determination of antibiotic susceptibilities.

Cardiobacterium hominis, a member of the HACEK group of organisms, is an uncommon but important cause of subacute bacterial endocarditis. First-line therapy is a third-generation cephalosporin due to rare beta-lactamase production. The authors report a case involving endovascular infection due to C hominis that initially tested resistant to third-generation cephalosporins using an antibiotic gradient strip susceptibility method (nitrocephin negative), but later proved to be susceptible using broth microdilution reference methods (a ‘major’ error). There are limited studies to guide susceptibility testing and interpretive breakpoints for C hominis in the medical literature, and the present case illustrates some of the issues that may arise when performing susceptibility testing for fastidious organisms in the clinical microbiology laboratory.

Phenotypic and genotypic characterization of invasive Streptococcus pneumoniae clinical isolates

BACKGROUND

The emergence of infection caused by invasive penicillinnonsusceptible (PNS) and multidrug-resistant strains of Streptococcus pneumoniae has become a worldwide concern, necessitating the epidemiologic surveillance of such strains.

OBJECTIVES

One aim of this study was to identify clones of invasive PNS S pneumoniae among isolates in Riyadh, Saudi Arabia. The second aim was to compare these clones with international clones to track their spread in Saudi Arabia.

METHODS

The phenotypes of invasive isolates characterized as S pneumoniae were determined using susceptibility testing and serotyping (capsular test and E-test). The genotypes of PNS isolates were determined using random amplified polymorphic DNA analysis. The genetic relatedness of these local strains to the international widespread clones was investigated.

RESULTS

Of 296 S pneumoniae isolates identified using biochemical and culture characteristics, 89 (30.1%) were invasive. Susceptibility testing using the E-test revealed that 17 of the 89 invasive isolates (19.1%) were PNS. Most of the 89 isolates (89.9%) were resistant to sulfamethoxazole-trimethoprim; 32.6% and 23.6% of isolates were resistant to chloramphenicol and tetracycline, respectively. All of the isolates (100.0%) were fully susceptible to ceftriaxone and vancomycin. Capsular serotyping of the 89 isolates showed that 19A (18.0%), 613 (14.6%), 23F (13.5%), 9V (11.2%), 14 (6.7%), 19F (5.6%), and 18C (4.5%) were the most predominant serogroups/serotypes. The 17 PNS strains were confirmed on polymerase chain reaction to have penicillin resistance genes. Of these 17 strains, international clone 19A-a was the most predominant (41.2%), followed by 6B-a (17.6%), and 23F-a and 9V-a (each, 11.8%).

CONCLUSIONS

The present study identified the spread of the 4 most commonPNS S pneumoniae isolates (clones)-19A, 613, 23F, and 9V-to Riyadh, but identified no new clones among patients having invasive infection with S pneumoniae in Riyadh. This study emphasizes that international PNS clones have contributed to the prevalence and spread of PNS pneumococci among the clinical isolates in Saudi Arabia.

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.

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.

Combinations of PBPs and MurM protein variants in early and contemporary high-level penicillin-resistant Streptococcus pneumoniae isolates in Spain

OBJECTIVES

High-level penicillin resistance in Streptococcus pneumoniae requires extensive re-modulation of the penicillin-binding proteins (PBPs), and murM gene function is also required for the expression of resistance. In this work, we determined whether specific changes in PBPs were associated with specific MurM variants.

METHODS

Two collections of highly penicillin-resistant (MIC 2-8 mg/L) isolates, including 10 early (1997-1998) and 23 contemporary (2002-2004) isolates, were studied.

RESULTS

Most of the isolates belonged to clones Spain(6B)-2 (13 strains), Spain(23F)-1 (10 isolates) and Spain(14)-5 (20 isolates). Different protein variants of MurM (MA, MB5, MB6, MB9 and MB10), PBP1A (A-C), PBP2B (A-D) and PBP2X (A-C) were recognized, including two murM alleles not previously described. Particular [MurM-PBP1A-2B-2X] allelic combinations were predominant among the different clones, including [MA-B-B-B] for old (MIC 2 mg/L) and [MB10-C-A-B] for recent (MIC 4-8 mg/L) Spain(6B)-2 isolates, [MA-A-C-A] for Spain(23F)-1 and [MB5-A-A-A] in Spain(14)-5 isolates.

CONCLUSIONS

Although S. pneumoniae has a basic recombinational population structure, our results indicate remarkable conservation of PBPs and MurM protein types within each clone. This suggests that particular PBPs-MurM combinations tend to be preserved and may have an independent evolutionary history in particular clones.

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.

Clinical isolates of Streptococcus pneumoniae with different susceptibilities to ceftriaxone and cefotaxime

Ceftriaxone and cefotaxime are extended-spectrum cephalosporins previously demonstrated to possess very similar in vitro activities against Streptococcus pneumoniae. Anecdotal reports of isolates with divergent in vitro susceptibilities to ceftriaxone and cefotaxime have been published. To determine the prevalence of pneumococcal isolates with divergent ceftriaxone and cefotaxime susceptibilities, we tested 1,000 clinical isolates collected by U.S. laboratories in 2001-2002 by broth microdilution and E-test. The percentages of isolates susceptible to ceftriaxone and cefotaxime were significantly different by both broth microdilution (98.6 and 96.6%, respectively; P < 0.05) and E-test (98.3 and 95.8%; P < 0.001). The differences observed were due solely to the activities of the two agents against penicillin-resistant isolates. Twenty-six of 188 penicillin-resistant isolates (13.8%) demonstrated different ceftriaxone and cefotaxime MIC interpretative phenotypes when tested by broth microdilution; 18 isolates were concurrently ceftriaxone susceptible and cefotaxime intermediate, 6 were ceftriaxone intermediate and cefotaxime resistant, and 2 were ceftriaxone susceptible and cefotaxime resistant (1.1% of penicillin-resistant isolates; 0.2% of all isolates tested). Sixteen of the 26 isolates (65%) were from southern U.S. states. The 26 isolates had serogroups and serotypes (6, 9, 14, 19, and 23) commonly associated with penicillin-resistant isolates; SmaI pulsed-field gel electrophoresis identified 18 isolates (69%) dispersed among five subtype groups and 8 isolates that were unrelated to any of the other isolates. We conclude that certain isolates of penicillin-resistant pneumococci are less susceptible to cefotaxime than to ceftriaxone and that these isolates are not the result of the spread of a single clone. Whether such isolates have increased in prevalence over time remains unknown.

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.

Genotypes of invasive pneumococcal isolates recently recovered from Italian patients

We examined 73 recent invasive pneumococcal isolates within selected areas of Italy for genotypic variability. Thirty-three genomic macrorestriction types were found, three of which represented multiple serotypes. Restriction fragment patterns of pbp2b, pbp2x, and pspA were conserved within the majority of isolates that shared macrorestriction types. Of the nine macrorestriction types found among the 22 penicillin-nonsusceptible Streptococcus pneumoniae (PNSP) isolates, seven comprised isolates with allelic profiles showing five to seven allelic matches to profiles in the multilocus sequence typing database (www.mlst.net); however, three of the seven profiles represented serotypes not previously associated with these clonal clusters. Two PNSP macrorestriction types represented new clones with unique allelic profiles. Allelic profiles obtained from isolates of 3 of the 25 macrorestriction types found among the 51 penicillin-susceptible S. pneumoniae (PSSP) isolates were closely related to previously described profiles. One PSSP isolate was a novel type 24F isolate related to the multiresistant clone France(9V)-3. This work reports new PNSP strains and new serotype-clone associations.

Analysis of penicillin-binding protein genes of clinical isolates of Streptococcus pneumoniae with reduced susceptibility to amoxicillin

The recent emergence of pneumococcal isolates exhibiting an unusual resistance phenotype of higher amoxicillin MICs in relation to the penicillin MICs prompted an analysis of the pbp genes from three such strains isolated in France. For comparison, three amoxicillin-susceptible strains were included in the study. DNA sequence analysis of the pbp2x, pbp2b, and pbp1a genes revealed extensive sequence divergence in all six isolates compared to the sequences of the genes of penicillin-susceptible strain R6. With the exception of pbp2b, no amino acid mutations were unique to the resistant isolates. Transformation experiments with cloned pbp genes isolated from one of the resistant isolates demonstrated a stepwise development of amoxicillin resistance involving penicillin-binding proteins (PBPs) 2X, 2B, and 1A. Full resistance, equivalent to that of the donor strain, was achieved only when genomic DNA was transformed into R6(2x/2b/1a) mutants, suggesting that full resistance development in this isolate is mediated by a non-PBP determinant. Moreover, the recently identified murMN resistance determinant does not appear to have any impact on resistance in this isolate. This determinant (from the French isolate) was, however, able to transform an R6 mutant harboring pbp2x, pbp2b, and pbp1a genes from a Hungarian clone with an extremely high level of penicillin resistance so that it had increased levels of penicillin resistance. These results indicate that the development of high-level beta-lactam resistance is a complex process and that the involvement of MurMN in penicillin resistance appears to be dependent on specific mutations in PBPs 2X, 2B, and/or 1A. Furthermore, an additional (as yet unidentified) non-PBP-mediated resistance determinant is required for full resistance development in some pneumococci.