Inducible and constitutive resistance to macrolide antibiotics and lincomycin in clinically isolated strains of Streptococcus pyogenes

Mechanism of Macrolide-Induced Inhibition of Pneumolysin Release Involves Impairment of Autolysin Release in Macrolide-Resistant Streptococcus pneumoniae

Streptococcus pneumoniae is a leading cause of community-acquired pneumonia. Over the past 2 decades, macrolide resistance among S. pneumoniae organisms has been increasing steadily and has escalated at an alarming rate worldwide. However, the use of macrolides in the treatment of community-acquired pneumonia has been reported to be effective regardless of the antibiotic susceptibility of the causative pneumococci. Although previous studies suggested that sub-MICs of macrolides inhibit the production of the pneumococcal pore-forming toxin pneumolysin by macrolide-resistant S. pneumoniae (MRSP), the underlying mechanisms of the inhibitory effect have not been fully elucidated. Here, we show that the release of pneumococcal autolysin, which promotes cell lysis and the release of pneumolysin, was inhibited by treatment with azithromycin and erythromycin, whereas replenishing with recombinant autolysin restored the release of pneumolysin from MRSP. Additionally, macrolides significantly downregulated ply transcription followed by a slight decrease of the intracellular pneumolysin level. These findings suggest the mechanisms involved in the inhibition of pneumolysin in MRSP, which may provide an additional explanation for the benefits of macrolides on the outcome of treatment for pneumococcal diseases.

The complex resistomes of Paenibacillaceae reflect diverse antibiotic chemical ecologies

The ecology of antibiotic resistance involves the interplay of a long natural history of antibiotic production in the environment, and the modern selection of resistance in pathogens through human use of these drugs. Important components of the resistome are intrinsic resistance genes of environmental bacteria, evolved and acquired over millennia, and their mobilization, which drives dissemination in pathogens. Understanding the dynamics and evolution of resistance across bacterial taxa is essential to address the current crisis in drug-resistant infections. Here we report the exploration of antibiotic resistance in the Paenibacillaceae prompted by our discovery of an ancient intrinsic resistome in Paenibacillus sp. LC231, recovered from the isolated Lechuguilla cave environment. Using biochemical and gene expression analysis, we have mined the resistome of the second member of the Paenibacillaceae family, Brevibacillus brevis VM4, which produces several antimicrobial secondary metabolites. Using phylogenomics, we show that Paenibacillaceae resistomes are in flux, evolve mostly independent of secondary metabolite biosynthetic diversity, and are characterized by cryptic, redundant, pseudoparalogous, and orthologous genes. We find that in contrast to pathogens, mobile genetic elements are not significantly responsible for resistome remodeling. This offers divergent modes of resistome development in pathogens and environmental bacteria.

Antibiotic Resistance of Non-pneumococcal Streptococci and Its Clinical Impact

The taxonomy of streptococci has undergone major changes during the last two decades. The present classification is based on both phenotypic and genotypic data. Phylogenetic classification of streptococci is based on 16S rRNA sequences [1], and it forms the backbone of the overall classification system of streptococci. Phenotypic properties are also important, especially for clinical microbiologists. The type of hemolysis on blood agar, reaction with Lancefield grouping antisera, resistance to optochin, and bile solubility remain important for grouping of clinical Streptococcus isolates and therefore treatment options [2]. In the following chapter, two phenotypic classification groups, viridans group streptococci (VGS) and beta-hemolytic streptococci, will be discussed.

Inactivation of the lipopeptide antibiotic daptomycin by hydrolytic mechanisms

The lipopeptide daptomycin is a member of the newest FDA-approved antimicrobial class, exhibiting potency against a broad range of Gram-positive pathogens with only rare incidences of clinical resistance. Environmental bacteria harbor an abundance of resistance determinants orthologous to those in pathogens and thus may serve as an early-warning system for future clinical emergence. A collection of morphologically diverse environmental actinomycetes demonstrating unprecedented frequencies of daptomycin resistance and high levels of resistance by antibiotic inactivation was characterized to elucidate modes of drug inactivation. In vivo studies revealed that hydrolysis plays a key role, resulting in one or both of the following structural modifications: ring hydrolysis resulting in linearization (in 44% of inactivating isolates) or deacylation of the lipid tail (29%). Characterization of the mechanism in actinomycete WAC4713 (a Streptomyces sp. with an MIC of 512 μg/ml) demonstrated a constitutive resistance phenotype and established daptomycin's circularizing ester linkage to be the site of hydrolysis. Characterization of the hydrolase responsible revealed it to be likely a serine protease. These studies suggested that daptomycin is susceptible to general proteolytic hydrolysis, which was further supported by studies using proteases of diverse origin. These findings represent the first comprehensive characterization of daptomycin inactivation in any bacterial class and may not only presage a future mechanism of clinical resistance but also suggest strategies for the development of new lipopeptides.

Synergistic effect of erythromycin on polymorphonuclear cell antibacterial activity against erythromycin-resistant phenotypes of Streptococcus pyogenes

To evaluate the synergistic activity of erythromycin and human polymorphonuclear cells (PMNs) on the binomial erythromycin-resistant (ERY(R)) Streptococcus pyogenes/host, the phagocytic and bactericidal activities of PMNs against ERY(R) streptococcal strains (cMLS(B), M, and iMLS(B) A, B and C phenotypes) were assessed in the presence of the macrolide. The results showed that when erythromycin, PMNs and streptococci [both erythromycin-sensitive (ERY(S)) and ERY(R)] were simultaneously present in the culture medium, PMN phagocytic activity was similar to that of drug-free controls. In contrast, the results emphasised a significant high increase in intracellular killing by PMNs in the presence of erythromycin not only for ERY(S) streptococci but also for ERY(R)S. pyogenes with high (cMLS(B), iMLS(B) A and iMLS(B) B phenotypes) and moderate (M and iMLS(B) C phenotypes) erythromycin resistance compared with controls without drug. From literature data it emerged that, even if intracellularly concentrated, erythromycin is relatively inactive because of its instability. The results indicate that the enhanced intra-PMN streptococcal killing detected is mainly attributable to PMN bactericidal systems that synergise with intracellular erythromycin in eradicating ERY(R)S. pyogenes strains (both with high and moderate resistance). These data confirm that the antibiotic resistance detected in vitro does not always imply a failure of antimicrobial treatment.

Antibiotic Resistance of Non-Pneumococcal Streptococci and Its Clinical Impact

Viridans streptococci (VGS) form a phylogenetically heterogeneous group of species belonging to the genus Streptococcus (1). However, they have some common phenotypic properties. They are alfa- or non-haemolytic. They can be differentiated from S. pneumoniae by resistance to optochin and the lack of bile solubility (2). They can be differentiated from the Enterococcus species by their inability to grow in a medium containing 6.5% sodium chloride (2). Earlier, so-called nutritionally variant streptococci were included in the VGS but based on the molecular data they have now been removed to a new genus Abiotrophia (3) and are not included in the discussion below. VGS belong to the normal microbiota of the oral cavities and upper respiratory tracts of humans and animals. They can also be isolated from the female genital tract and all regions of the gastrointestinal tract (2, 3). Several species are included in VGS and are listed elsewhere (2, 3). Clinically the most important species belonging to the VGS are S. mitis, S. sanguis and S. oralis.

Utilisation of macrolides and the development of Streptococcus pyogenes resistance to erythromycin

OBJECTIVES

The aim of the study was to evaluate the relationship between the development of the resistance of Streptococcus pyogenes strains to erythromycin and the utilisation of macrolides in the Olomouc region, Czech Republic.

METHOD

During the period 1997-2001, data for utilisation of macrolides was obtained from the database of the regional General Health Insurance Company and expressed indefined daily doses per 1,000 patients per day. S. pyogenes strains were isolated from community patients suffering from acute bacterial tonsillitis. Their susceptibility to antibiotics was assessed by the disk diffusion method.

RESULTS

Utilisation of macrolides increased by 13% in the period 1997-2001; their utilisation represented 11.40% of total antibiotic prescription in 1997 and 15.48% in 2001. Occurrence of erythromycin-resistant S. pyogenes strains increased significantly from 14% in 1997 to 32% in the year 2001 (P < 0.01). In 2000, macrolides consumption decreased non-significantly, but with no concomitant decrease in erythromycin-resistant S. pyogenes strains occurrence. Absolute susceptibility of S. pyogenes, the most important bacterial pathogen in community-acquired bacterial tonsillitis, to penicillin contrasts with increasing macrolide resistance.

CONCLUSIONS

The study documents the influence of increased utilisation of macrolides on bacterial resistance. Penicillin should be a first-choice antibiotic in acute bacterial tornsillitis; macrolides should only be reserved for patients allergic to penicillins.

A novel efflux system in inducibly erythromycin-resistant strains of Streptococcus pyogenes

Streptococcus pyogenes strains inducibly resistant (iMLS phenotype) to macrolide, lincosamide, and streptogramin B (MLS) antibiotics can be subdivided into three phenotypes: iMLS-A, iMLS-B, and iMLS-C. This study focused on inducibly erythromycin-resistant S. pyogenes strains of the iMLS-B and iMLS-C types, which are very similar and virtually indistinguishable in a number of phenotypic and genotypic features but differ clearly in their degree of resistance to MLS antibiotics (high in the iMLS-B type and low in the iMLS-C type). As expected, the iMLS-B and iMLS-C test strains had the erm(A) methylase gene; the iMLS-A and the constitutively resistant (cMLS) isolates had the erm(B) methylase gene; and a control M isolate had the mef(A) efflux gene. mre(A) and msr(A), i.e., other macrolide efflux genes described in gram-positive cocci, were not detected in any test strain. With a radiolabeled erythromycin method for determination of the intracellular accumulation of the drug in the absence or presence of an efflux pump inhibitor, active efflux of erythromycin was observed in the iMLS-B isolates as well as in the M isolate, whereas no efflux was demonstrated in the iMLS-C isolates. By the triple-disk (erythromycin plus clindamycin and josamycin) test, performed both in normal test medium and in the same medium supplemented with the efflux pump inhibitor, under the latter conditions iMLS-B and iMLS-C strains were no longer distinguishable, all exhibiting an iMLS-C phenotype. In conjugation experiments with an iMLS-B isolate as the donor and a Rif(r) Fus(r) derivative of an iMLS-C isolate as the recipient, transconjugants which shared the iMLS-B type of the donor under all respects, including the presence of an efflux pump, were obtained. These results indicate the existence of a novel, transferable efflux system, not associated with mef(A) or with other known macrolide efflux genes, that is peculiar to iMLS-B strains. Whereas the low-level resistance of iMLS-C strains to MLS antibiotics is apparently due to erm(A)-encoded methylase activity, the high-level resistance of iMLS-B strains appears to depend on the same methylase activity plus the new efflux system.

Differentiation of resistance phenotypes among erythromycin-resistant Pneumococci

Laboratory differentiation of erythromycin resistance phenotypes is poorly standardized for pneumococci. In this study, 85 clinical isolates of erythromycin-resistant (MIC > or = 1 microg/ml) Streptococcus pneumoniae were tested for the resistance phenotype by the erythromycin-clindamycin double-disk test (previously used to determine the macrolide resistance phenotype in Streptococcus pyogenes strains) and by MIC induction tests, i.e., by determining the MICs of macrolide antibiotics without and with pre-exposure to 0.05 microg of erythromycin per ml. By the double-disk test, 65 strains, all carrying the erm(AM) determinant, were assigned to the constitutive macrolide, lincosamide, and streptogramin B resistance (cMLS) phenotype, and the remaining 20, all carrying the mef(E) gene, were assigned to the recently described M phenotype; an inducible MLS resistance (iMLS) phenotype was not found. The lack of inducible resistance to clindamycin was confirmed by determining clindamycin MICs without and with pre-exposure to subinhibitory concentrations of erythromycin. In macrolide MIC and MIC-induction tests, whereas homogeneous susceptibility patterns were observed among the 20 strains assigned to the M phenotype by the double-disk test, two distinct patterns were recognized among the 65 strains assigned to the cMLS phenotype by the same test; one pattern (n = 10; probably that of the true cMLS isolates) was characterized by resistance to rokitamycin also without induction, and the other pattern (n = 55; designated the iMcLS phenotype) was characterized by full or intermediate susceptibility to rokitamycin without induction turning to resistance after induction, with an MIC increase by more than three dilutions. A triple-disk test, set up by adding a rokitamycin disk to the erythromycin and clindamycin disks of the double-disk test, allowed the easy differentiation not only of pneumococci with the M phenotype from those with MLS resistance but also, among the latter, of those of the true cMLS phenotype from those of the iMcLS phenotype. While distinguishing MLS from M resistance in pneumococci is easily and reliably achieved, the differentiation of constitutive from inducible MLS resistance is far more uncertain and is strongly affected by the antibiotic used to test inducibility.

In vitro activity of the new ketolide telithromycin compared with those of macrolides against Streptococcus pyogenes: influences of resistance mechanisms and methodological factors

One hundred and seven clinical isolates of Streptococcus pyogenes, 80 susceptible to macrolides and 27 resistant to erythromycin A (MIC >0.5 microgram/ml), were examined. The erythromycin A-lincomycin double-disk test assigned 7 resistant strains to the M-phenotype, 8 to the inducible macrolide, lincosamide, and streptogramin B resistance (iMLS(B)) phenotype, and 12 to the constitutive MLS(B) resistance (cMLS(B)) phenotype. MICs of erythromycin A, clarithromycin, azithromycin, roxithromycin, and clindamycin were determined by a broth microdilution method. MICs of telithromycin were determined by three different methods (broth microdilution, agar dilution, and E-test methods) in an ambient air atmosphere and in a 5 to 6% CO(2) atmosphere. Erythromycin A resistance genes were investigated by PCR in the 27 erythromycin A-resistant isolates. MICs of erythromycin A and clindamycin showed six groups of resistant strains, groups A to F. iMLS(B) strains (A, B, and D groups) are characterized by two distinct patterns of resistance correlated with genotypic results. A- and B-group strains were moderately resistant to 14- and 15-membered ring macrolides and highly susceptible to telithromycin. All A- and B-group isolates harbored erm TR gene, D-group strains, highly resistant to macrolides and intermediately resistant to telithromycin (MICs, 1 to 16 microgram/ml), were all characterized by having the ermB gene. All M-phenotype isolates (C group), resistant to 14- and 15-membered ring macrolides and susceptible to clindamycin and telithromycin, harbored the mefA gene. All cMLS(B) strains (E and F groups) with high level of resistance to macrolides, lincosamide, and telithromycin had the ermB gene. The effect of 5 to 6% CO(2) was remarkable on resistant strains, by increasing MICs of telithromycin from 1 to 6 twofold dilutions against D-E- and F-group isolates.

Phenotypes and genotypes of erythromycin-resistant Streptococcus pyogenes strains in Italy and heterogeneity of inducibly resistant strains

A total of 387 clinical strains of erythromycin-resistant (MIC, >/=1 microg/ml) Streptococcus pyogenes, all isolated in Italian laboratories from 1995 to 1998, were examined. By the erythromycin-clindamycin double-disk test, 203 (52.5%) strains were assigned to the recently described M phenotype, 120 (31.0%) were assigned to the inducible macrolide, lincosamide, and streptogramin B resistance (iMLS) phenotype, and 64 (16.5%) were assigned to the constitutive MLS resistance (cMLS) phenotype. The inducible character of the resistance of the iMLS strains was confirmed by comparing the clindamycin MICs determined under normal testing conditions and those determined after induction by pregrowth in 0.05 microg of erythromycin per ml. The MICs of erythromycin, clarithromycin, azithromycin, josamycin, spiramycin, and the ketolide HMR3004 were then determined and compared. Homogeneous susceptibility patterns were observed for the isolates of the cMLS phenotype (for all but one of the strains, HMR3004 MICs were 0.5 to 8 microg/ml and the MICs of the other drugs were >128 microg/ml) and those of the M phenotype (resistance only to the 14- and 15-membered macrolides was recorded, with MICs of 2 to 32 microg/ml). Conversely, heterogeneous susceptibility patterns were observed in the isolates of the iMLS phenotype, which were subdivided into three distinct subtypes designated iMLS-A, iMLS-B, and iMLS-C. The iMLS-A strains (n = 84) were highly resistant to the 14-, 15-, and 16-membered macrolides and demonstrated reduced susceptibility to low-level resistance to HMR3004. The iMLS-B strains (n = 12) were highly resistant to the 14- and 15-membered macrolides, susceptible to the 16-membered macrolides (but highly resistant to josamycin after induction), and susceptible to HMR3004 (but intermediate or resistant after induction). The iMLS-C strains (n = 24) had lower levels of resistance to the 14- and 15-membered macrolides (with erythromycin MICs increasing two to four times after induction), were susceptible to the 16-membered macrolides (but resistant to josamycin after induction), and remained susceptible to HMR3004, also after induction. The erythromycin resistance genes in 100 isolates of the different groups were investigated by PCR. All cMLS and iMLS-A isolates tested had the ermAM (ermB) gene, whereas all iMLS-B and iMLS-C isolates had the ermTR gene (neither methylase gene was found in isolates of other groups). The M isolates had only the macrolide efflux (mefA) gene, which was also found in variable proportions of cMLS, iMLS-A, iMLS-B, and iMLS-C isolates. The three iMLS subtypes were easily differentiated by a triple-disk test set up by adding a josamycin disk to the erythromycin and clindamycin disks of the conventional double-disk test. Tetracycline resistance was not detected in any isolate of the iMLS-A subtype, whereas it was observed in over 90% of both iMLS-B and iMLS-C isolates.

Erythromycin resistance genes in group A streptococci in Finland. The Finnish Study Group for Antimicrobial Resistance

Streptococcus pyogenes isolates (group A streptococcus) of different erythromycin resistance phenotypes were collected from all over Finland in 1994 and 1995 and studied; they were evaluated for their susceptibilities to 14 antimicrobial agents (396 isolates) and the presence of different erythromycin resistance genes (45 isolates). The erythromycin-resistant isolates with the macrolide-resistant but lincosamide- and streptogramin B-susceptible phenotype (M phenotype) were further studied for their plasmid contents and the transferability of resistance genes. Resistance to antimicrobial agents other than macrolides, clindamycin, tetracycline, and chloramphenicol was not found. When compared to our previous study performed in 1990, the rate of resistance to tetracycline increased from 10 to 93% among isolates with the inducible resistance (IR) phenotype of macrolide, lincosamide, and streptogramin B (MLSB) resistance. Tetracycline resistance was also found among 75% of the MLSB-resistant isolates with the constitutive resistance (CR) phenotype. Resistance to chloramphenicol was found for the first time in S. pyogenes in Finland; 3% of the isolates with the IR phenotype were resistant. All the chloramphenicol-resistant isolates were also resistant to tetracycline. Detection of erythromycin resistance genes by PCR indicated that, with the exception of one isolate with the CR phenotype, all M-phenotype isolates had the macrolide efflux (mefA) gene and all the MLSB-resistant isolates had the erythromycin resistance methylase (ermTR) gene; the isolate with the CR phenotype contained the ermB gene. No plasmid DNA could be isolated from the M-phenotype isolates, but the mefA gene was transferred by conjugation.

A novel erythromycin resistance methylase gene (ermTR) in Streptococcus pyogenes

Erythromycin resistance among streptococci is commonly due to target site modification by an rRNA-methylating enzyme, which results in coresistance to macrolide, lincosamide, and streptogramin B antibiotics (MLSB resistance). Genes belonging to the ermAM (ermB) gene class are the only erythromycin resistance methylase (erm) genes in Streptococcus pyogenes with MLSB resistance that have been sequenced so far. We identified a novel erm gene, designated ermTR, from an erythromycin-resistant clinical strain of S. pyogenes (strain A200) with an inducible type of MLSB resistance. The nucleotide sequence of ermTR is 82.5% identical to ermA, previously found, for example, in Staphylococcus aureus and coagulase-negative staphylococci. Our finding provides the first sequence of an erm gene other than ermAM that mediates MLSB resistance in S. pyogenes.

Antibacterial activity of RU 64004 (HMR 3004), a novel ketolide derivative active against respiratory pathogens

The antibacterial activity of RU 64004, a new ketolide, was evaluated against more than 600 bacterial strains and was compared with those of various macrolides and pristinamycin. RU 64004 had good activity against multiresistant pneumococci, whether they were erythromycin A resistant or not, including penicillin-resistant strains. RU 64004 inhibited 90% of pneumococci resistant to erythromycin A and penicillin G at 0.6 and 0.15 microg/ml, respectively. Unlike macrolides, RU 64004 did not induce the phenotype of resistance to macrolides-lincosamides-streptogramin B. Its good antibacterial activity against multiresistant pneumococci ran in parallel with its well-balanced activity against all bacteria involved in respiratory infections (e.g., Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pyogenes). In contrast to all comparators (14- and 16-membered-ring macrolides and pristinamycin), RU 64004 displayed high therapeutic activity in animals infected with all major strains, irrespective of the phenotypes of the strains. The results suggest that RU 64004 has potential for use in the treatment of infections caused by respiratory pathogens including multiresistant pneumococci.

New macrolides active against Streptococcus pyogenes with inducible or constitutive type of macrolide-lincosamide-streptogramin B resistance

Macrolide-resistant bacteria can be classified as inducibly resistant or constitutively resistant. Inducibly resistant bacteria are resistant to 14-membered macrolides, such as erythromycin and clarithromycin (A-56268), but are susceptible to the 16-membered macrolides, such as tylosin and spiramycin, as well as to clindamycin. Constitutively resistant bacteria are resistant to macrolide-lincosamide-streptogramin B antibiotics. In this study, the MICs of several erythromycin and clarithromycin analogs against macrolide-susceptible and macrolide-resistant Streptococcus pyogenes strains were determined. Four 11,12-carbamate analogs of clarithromycin had lower MICs than erythromycin did against S. pyogenes with the inducible or constitutive type of macrolide-lincosamide-streptogramin B resistance. Five 11,12-carbonate analogs of erythromycin with modifications at the 4" position of cladinose had lower MICs than did erythromycin against S. pyogenes with the constitutive type of resistance, and one of these compounds, which had a naphthyl-glycyl substitution at the 4" position, had a lower MIC than erythromycin against both the inducibly resistant and constitutively resistant strains. Two analogs of erythromycin with a modification on the 4" position of cladinose had lower MICs than erythromycin did against the constitutively resistant organisms but not against the inducibly resistant organisms. Thus, 14-membered macrolides can be modified so as to confer a low MIC when tested in vitro.

Novel mechanism for plasmid-mediated erythromycin resistance by pNE24 from Staphylococcus epidermidis

We describe an unusual type of erythromycin resistance (Emr) mediated by a plasmid designated pNE24 from Staphylococcus epidermidis. This 26.5-kilobase plasmid encodes resistance strictly to 14-membered macrolide antibiotics, erythromycin, and oleandomycin. Resistance to other macrolide-lincosamide-streptogramin B (MLS) antibiotics was not observed even after a prior induction stimulus with various MLS antibiotics. Plasmid pNE24 was found to express resistance constitutively and manifested a low to intermediate MIC (62.5 micrograms/ml) for erythromycin. The resistance gene, designated erpA, appears to mediate resistance by altering the permeability of the host cell for erythromycin, because the measured uptake of 14C-labeled erythromycin by strain 958-2 (containing pNE24) was lower than for the erythromycin-susceptible, isogenic strain 958-1. No inactivation of erythromycin in overnight broth culture supernatants could be detected. In addition, no significant loss in binding affinity between [14C]erythromycin and ribosome could be detected for ribosomes isolated from strain 958-2 relative to 958-1, indicating that pNE24 probably does not produce a modification of the bacterial ribosome. No other selectable marker was found associated with pNE24; however, a 60,000-dalton protein was present only in the membrane fractions of cells (958-2) containing pNE24 and may play a role in mediating resistance to erythromycin.

Development of high-level streptomycin resistance affected by a plasmid in lactic streptococci

Some lactose-negative (Lac-) mutants of Streptococcus lactis C2 and ML3 exhibited development of very high level streptomycin resistance after incubation with subinhibitory concentrations of the drug for 18 to 22 h. These drug-resistant mutants showed no loss of resistance even after 6 months of subculturing in broth without any drug. The parental Lac+ strains did not show mutation to high-level streptomycin resistance. The Lac+ characteristic of the parental strain was conjugally transferred to Lac- derivatives of C2 and ML3, showing the ability to mutate to high-level resistance. When transconjugants were analyzed for this characteristic, they showed both mutable and nonmutable Lac+ types. The results suggested that genetic information for mutation to high-level streptomycin resistance in lactic streptococci resides on the chromosome, and its expression is affected by a plasmid. The plasmid profiles of strains C2, ML3, C2 Lac-, ML3 Lac-, and two kinds of transconjugants confirmed the presence of a plasmid of approximately 5.5 megadaltons in strains showing no mutation to high-level streptomycin resistance, while strains missing such a plasmid exhibited high-level streptomycin resistance after incubation with subinhibitory concentrations of the drug.

A complex attenuator regulates inducible resistance to macrolides, lincosamides, and streptogramin type B antibiotics in Streptococcus sanguis

Macrolide-lincosamide-streptogramin B resistance specified by Streptococcus sanguis plasmid pAM77 involves an adenine methylase, whose synthesis, demonstrable both phenotypically and by analysis of methionine-labeled proteins made in Bacillus subtilis minicells, is inducible by erythromycin, lincomycin, and streptogramin type B antibiotics. Localization of the methylase structural gene, including its control region in DNA fragments obtained with restriction endonucleases, has been deduced from DNA blot experiments with characterized target and probe DNAs from other streptococci, combined with DNA sequence analysis and comparison of the putative streptococcal methylase sequence with that of a cognate methylase in staphylococcal plasmid pE194. The streptococcal methylase migrates electrophoretically in polyacrylamide gels with the mobility of a 29,000-dalton protein. The sequence organization of the putative streptococcal methylase mRNA leader sequence partially resembles its staphylococcal counterpart and can support a similar mechanism of secondary structure rearrangement leading to methylase synthesis. The deduced 5' leader sequence preceding the pAM77 methylase structural gene sequence comprises approximately 155 nucleotides within which one can identify a putative control peptide 36 amino acid residues in length (in contrast to 19 in the pE194 peptide) and at least 14 possible classes of overlapping inverted complementary repeat sequences (in contrast to 3 in the pE194 control region), one of which can sequester the sequence AGGAG 7 nucleotides upstream from the putative (methionine) start codon of the streptococcal methylase. Comparison of the pAM77 and pE194 methylase amino acid sequences and their respective nucleotide sequences shows 51% conservation of amino acid residues (124 of 244) and 59% conservation of nucleotide residues (433 of 738), which suggests a common origin for the two methylase structural gene sequences. Differences in mRNA base composition associated with conserved amino acid residues occur mostly in the third nucleotide ("wobble") position of codons and may reflect adaptation of methylase genes to optimal expression in host cells with differing codon use patterns.

Plasmids, drug resistance, and gene transfer in the genus Streptococcus

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A family of r-determinants in Streptomyces spp. that specifies inducible resistance to macrolide, lincosamide, and streptogramin type B antibiotics

Inducible resistance to macrolide, lincosamide, and streptogramin type B antibiotics in Streptomyces spp. comprises a family of diverse phenotypes in which characteristic subsets of the macrolide-lincosamide-streptogramin antibiotics induce resistance mediated by mono- or dimethylation of adenine, or both, in 23S ribosomal ribonucleic acid. In these studies, diverse patterns of induction specificity in Streptomyces and associated ribosomal ribonucleic acid changes are described. In Streptomyces fradiae NRRL 2702 erythromycin induced resistance to vernamycin B, whereas in Streptomyces hygroscopicus IFO 12995, the reverse was found: vernamycin B induced resistance to erythromycin. In a Streptomyces viridochromogenes (NRRL 2860) model system studied in detail, tylosin induced resistance to erythromycin associated with N6-monomethylation of 23S ribosomal ribonucleic acid, whereas in Staphylococcus aureus, erythromycin induced resistance to tylosin mediated by N6-dimethylation of adenine. Inducible macrolide-lincosamide-streptogramin resistance was found in S. fradiae NRRL 2702 and S. hygroscopicus IFO 12995, which synthesize the macrolides tylosin and maridomycin, respectively, as well as in the lincosamide producer Streptomyces lincolnensis NRRL 2936 and the streptogramin type B producer Streptomyces diastaticus NRRL 2560. A wide range of different macrolides including chalcomycin, tylosin, and cirramycin induced resistance when tested in an appropriate system. Lincomycin was active as inducer in S. lincolnensis, the organism by which it is produced, and streptogramin type B antibiotics induced resistance in S. fradiae, S. hygroscopicus, and the streptogramin type B producer S. diastaticus. Patterns of adenine methylation found included (i) lincomycin-induced monomethylation in S. lincolnensis (and constitutive monomethylation in a mutant selected with maridomycin), (ii) concurrent equimolar levels of adenine mono- plus dimethylation in S. hygroscopicus, (iii) monomethylation in S. fradiae (and dimethylation in a mutant selected with erythromycin), and (iv) adenine dimethylation in S. diastaticus induced by ostreogrycin B.

Macrolide-lincosamide-streptogramin resistance patterns in Clostridium perfringens from animals

Different patterns of resistance against commonly used macrolide, lincosamide, and streptogramin antibiotics were found in Clostridium perfringens of animal origin. The patterns were designated as (i) macrolide-lincosamide-streptogramin group B generalized resistance, (ii) macrolide-lincosamide generalized resistance, (iii) macrolide-lincosamide inducible resistance, and (iv) macrolide-lincosamide-streptogramin low-level generalized resistance. The strains of the fourth pattern were able to inactivate pristinamycin and virginiamycin. The macrolide-susceptible strains showed a bimodal distribution of lincomycin and clindamycin susceptibility levels. The susceptible strains were inhibited by 0.25 micrograms of lincomycin per ml and 0.03 micrograms of clindamycin per ml. The low-level resistant strains were inhibited at concentrations of 2 to 4 micrograms of lincomycin per ml and 0.5 to 2 micrograms of clindamycin per ml.

Genetic study of plasmid-associated zonal resistance to lincomycin in Streptococcus pyogenes

The phenomenon of zonal resistance to lincomycin, which is characteristic of most clinical isolates with lincomycin resistance in Streptococcus pyogenes, has been studied. These strains grow within a defined concentration range of lincomycin (approximately 60 to 200 microgram/ml), or at lincomycin concentrations below the minimal inhibitory concentration for susceptible strains. It is shown that the zonal growth phenomenon is a stable phenotype and results from induction of resistance only within the zonal concentration range of lincomycin. These strains also possess inducible resistance to erythromycin which is nonzonal in character. One-step mutations to constitutive resistance have been isolated which are of two types: constitutive for lincomycin or for erythromycin, but not for both. Those strains with constitutive erythromycin resistance retain their zonal resistance for lincomycin. Mutants doubly constitutive for both lincomycin and erythromycin can be obtained by a second mutational step from either of the singly constitutive mutants. Satellite deoxyribonucleic acid has been shown to be present in the zonal resistant strains. A plasmid, pSM10419, of 14.9 megadaltons, has been isolated from one of the doubly constitutive mutants and used to jointly transform Streptococcus sanguis strain Challis to constitutive resistance to both lincomycin and erythromycin. From this, a multicopy plasmid of reduced size, pSM10 (5.4 megadaltons), which retains its resistance phenotype, has been isolated and mapped with restriction endonucleases HindIII (three sites), EcoRI (one site), KpnI (one site), and HpaI (one site). The staphylococcal plasmid pC221 (2.9 megadaltons; chloramphenicol resistant) has been fused to pSM10 at the EcoRI site resulting in a chimeric plasmid, pSM10221 (8.3 megadaltons), which retains resistance to chloramphenicol, erythromycin, and lincomycin. pSM10 is therefore suggestive as an effective cloning vehicle for the genus Streptococcus.

Identification of tetracycline-resistant R-plasmids in Streptococcus agalactiae (group B)

In this report, 30 tetracycline-resistant clinical isolates of group B Streptococcus were examined to assess the extent to which tetracycline resistance is plasmid mediated. Of these, 27 showed no physical or genetic evidence of plasmid-mediated resistance; however, one conjugative and two small (3.5 X 10(6)-dalton) multicopy non-self-transmissible tetracycline resistance plasmids were identified. The conjugative plasmid was transmissible to Streptococcus faecalis as well as to Streptococcus agalactiae (group B). The two nonconjugative plasmids were readily mobilized by a number of sex factors into these same two backgrounds and, in addition, readily transformed Streptococcus sanguis Challis to tetracycline resistance. Due to readily available sites for several site-specific endonuycleases, these small, multicopy plasmids should prove useful as cloning vehicles in this host system.

Erythromycin and clindamycin resistance in Corynebacterium diphtheriae from skin lesions

Erythromycin- and clindamycin-resistant Corynebacterium diphtheriae isolates were recovered from skin lesions. Resistance to erythromycin and clindamycin was induced by a subinhibitory concentration (0.03 microgram/ml) of erythromycin. Clindamycin (0.07 microgram/ml) was a more effective inducer of its own resistance than of erythromycin resistance. Erythromycin-inducible cross-resistance to vernamycin B alpha was demonstrated in disk diffusion tests.

Macrolide resistance in Staphylococcus aureus: induction of macrolide-resistant protein synthesis

Induction of resistance to macrolide-, lincosamide-, and streptogramin B-type antibiotics in Staphylococcus aureus was studied by monitoring the appearance of erythromycin A (EM)-resistant [(14)C]leucine incorporation. Examination of the induction process revealed saturation kinetics and a time course much like that reported for penicillinase in gram-positive bacteria. Induction kinetics in exponentially growing cells were sigmoidal and appeared to reach a maximum and constant rate when growth reached stationary phase. Since the induction of EM-resistant colony-forming ability was complete within 60 min, ribosome modification cannot be limited to a fraction of the population and must occur in essentially every cell. However, EM-resistant growth was expressed in cells where less than half the [(14)C]leucine-incorporating activity was resistant to EM. This suggests that resistance requires that only a threshold level of ribosome modification be exceeded and that, once exceeded, resistance is dominant to sensitivity.

Macrolide resistance in Staphylococcus aureus: inducers of macrolide resistance

Several macrolide-, lincosamide-, and streptogramin B-type (MLS) antibiotics were tested as inducers of erythromycin A (EM)-resistant [(14)C]leucine incorporation. Only 14-membered-ring macrolides having a glycosidically linked 6-deoxy sugar at the C-3 position of the lactone ring and the structurally dissimilar lincosamide, celesticetin, showed inducer activity. Modifications of EM at the C-4'' position of cladinose can apparently destroy the inducer property but do not affect the inhibitory properties of the antibiotic. The findings clearly show that inducer and inhibitor activities can be dissociated and are consistent with the concept that distinct binding/receptor sites are utilized for inhibition of ribosome function and induction of resistance.

Systematic difference in the methylation of ribosomal ribonucleic acid from gram-positive and gram-negative bacteria

A survey of gram-positive and gram-negative organisms was performed to compare the distributionof N6-methylated adenine. It was found that (i) all the gram-positive strains tested, Staphylococcus aureus, Sarcina lutea, Bacillus stearothermophilus, Bacillus subtilis, and Bacillus megaterium, contain neither N6-monomethyl adenine (m6A) nor N6-dimethyladenine (m26A) in 23S ribosomal ribonucleic acid (rRNA). In the case of S. aureus and Streptococcus pyogenes, strains which are clinically resistant to erythromycin contain m26A. (ii) The gram-negative strains Salmonella typhimurium, Escherichia coli, Proteus mirabilis, Pseudomonas aeruginosa, and Klebsiella pneumoniae all contain m6A but not m23A in 23S rRNA. These observations suggest the existence of at least one systematic structural difference between the ribosomes of the two classes of bacteria. Because of the demonstrated relationship between N6-dimethylation of adenine in 23S rRNA and clinical resistance to macrolide, lincosamide, and streptogramin B-type antibiotics in staphylococci and streptococci, the observed systematic differences found in rRNA methylation combined with greater cellular permeability may be related to the relatively greater efficacy of macrolide, lincosamide, and streptogramin B-type antibiotics in treating infections caused by gram-positive organisms.

Plasmid-determined resistance to erythromycin: comparison of strains of streptococcus faecalis and streptococcus pyogenes with regard to plasmid hmology and resistance inducibility

Streptococcus faecalis strains DS-5 and Streptococcus pyogenes strain AC-1 both have a 17 million dalton plasmid that determines resistance to erythromycin, lincomycin, and vernamycin B(alpha). The results of deoxyribonucleic acid-deoxyribonucleic acid hybridization experiments indicate that the two plasmids are about 95% homologous. It was also shown that erythromycin resistance is inducible in AC-1 and constitutive in DS-5.

Genetics of resistance to macrolide antibiotics and lincomycin in natural isolates of Streptococcus pyogenes

Of 5 clinically isolated strains of Streptococcus pyogenes, 3 showed high-level resistance to erythromycin and lincomycin that was inducible by subinhibitory concentrations of these drugs (IR strains) while 2 strains exhibited constitutive erythromycin and lincomycin resistance (CR strains) which was expressed without prior exposure to low drug concentrations. The CR strain 15346 showed spontaneous loss of resistance whereas resistance in the other strains was quite stable even under curing conditions. The IR strain 13234 was found to be polylysogenic for at least 4 different phages designated P13234ma, mi, mu, and mo. Phage mo, antigenically distinct from the other three, was shown to mediate the transfer of the resistance determinant ERL1 of strain 13234. ERL1 if borne by appropriate strains was also transducible by the virulent phage A25. ERL1 behaved as a discrete genetic unit in transduction experiments, was not linked to either of two chromosomal regions governing resistance to antibiotics that affect the ribosome, could be transferred to recombination deficient hosts, represented a relatively large UV inactivation target, and showed no stimulation of transduction by low UV doses. These findings suggest that resistance to erythromycin and lincomycin in certain natural isolates of S. pyogenes is specified by, or under the control of, a plasmid.