Inducible or constitutive expression of resistance in clinical isolates of streptococci and enterococci cross-resistant to erythromycin and lincomycin

Regulation of erm(T) MLSB phenotype expression in the emergent emm92 type group A Streptococcus

In the last decade, invasive group A Streptococcus (iGAS) infections have doubled in the US, with equivalent increases in MLSB (macrolide, lincosamide, and streptogramin B)-resistance. The emm92-type isolates carrying the erm(T) gene have been associated with an alarming emergence of iGAS infections in people who inject drugs or experience homelessness. Our goal was to elucidate the mechanisms behind inducible (iMLSB) and constitutive (cMLSB) resistance in emm92 isolates. Sequence analysis identified polymorphisms in the erm(T) regulatory region associated with cMLSB resistance. RT-qPCR and RNAseq revealed increased erm(T) mRNA levels in iMLSB isolates in response to erythromycin exposure, while cMLSB isolates exhibited high erm(T) expression independent from antibiotic exposure. Transcription results were coupled with shifting levels of ribosomal methylation. A homology model of the ErmT enzyme identified structural elements and residues conserved in methyltransferases. Delayed growth of iMLSB isolates cultured with erythromycin and increased clindamycin resistance in cMLSB isolates were observed.

Epitranscriptional m6A modification of rRNA negatively impacts translation and host colonization in Staphylococcus aureus

Macrolides, lincosamides, and streptogramin B (MLS) are structurally distinct molecules that are among the safest antibiotics for prophylactic use and for the treatment of bacterial infections. The family of erythromycin resistance methyltransferases (Erm) invariantly install either one or two methyl groups onto the N6,6-adenosine of 2058 nucleotide (m6A2058) of the bacterial 23S rRNA, leading to bacterial cross-resistance to all MLS antibiotics. Despite extensive structural studies on the mechanism of Erm-mediated MLS resistance, how the m6A epitranscriptomic mark affects ribosome function and bacterial physiology is not well understood. Here, we show that Staphylococcus aureus cells harboring m6A2058 ribosomes are outcompeted by cells carrying unmodified ribosomes during infections and are severely impaired in colonization in the absence of an unmodified counterpart. The competitive advantage of m6A2058 ribosomes is manifested only upon antibiotic challenge. Using ribosome profiling (Ribo-Seq) and a dual-fluorescence reporter to measure ribosome occupancy and translational fidelity, we found that specific genes involved in host interactions, metabolism, and information processing are disproportionally deregulated in mRNA translation. This dysregulation is linked to a substantial reduction in translational capacity and fidelity in m6A2058 ribosomes. These findings point to a general "inefficient translation" mechanism of trade-offs associated with multidrug-resistant ribosomes.

Molecular basis and evolutionary cost of a novel macrolides/lincosamides resistance phenotype in Staphylococcus haemolyticus

Staphylococcus haemolyticus (S. haemolyticus) is a coagulase-negative Staphylococcus that has become one of the primary causes of nosocomial infection. After a long period of antibiotic use, S. haemolyticus has developed multiple resistance phenotypes for macrolides and lincosamides. Herein, we evaluated four S. haemolyticus clinical isolates, of which three had antibiotic resistance patterns reported previously. The fourth isolate was resistant to both erythromycin and clindamycin in the absence of erythromycin induction. This novel phenotype, known as constitutive macrolides-lincosamides-streptogramins resistance, has been reported in other bacteria but has not been previously reported in S. haemolyticus. Investigation of the isolate demonstrated a deletion in the methyltransferase gene ermC, upstream leader peptide. This deletion resulted in constitutive MLS resistance based on whole-genome sequencing and experimental verification. Continuous expression of ermC was shown to inhibit the growth of S. haemolyticus, which turned out to be the fitness cost with no MLS pressure. In summary, this study is the first to report constitutive MLS resistance in S. haemolyticus, which provides a better understanding of MLS resistance in clinical medicine. IMPORTANCE This study identified a novel phenotype of macrolides/lincosamides resistance in Staphylococcus haemolyticus which improved a better guidance for clinical treatment. It also clarified the mechanistic basis for this form of antibiotic resistance that supplemented the drug resistance mechanism of Staphylococcus. In addition, this study elaborated on a possibility that continuous expression of some resistance genes was shown to inhibit the growth of bacteria themselves, which turned out to be the fitness cost in the absence of antibiotic pressure.

The Plasmidomic Landscape of Clinical Methicillin-Resistant Staphylococcus aureus Isolates from Malaysia

Methicillin-resistant Staphylococcus aureus (MRSA) is a priority nosocomial pathogen with plasmids playing a crucial role in its genetic adaptability, particularly in the acquisition and spread of antimicrobial resistance. In this study, the genome sequences of 79 MSRA clinical isolates from Terengganu, Malaysia, (obtained between 2016 and 2020) along with an additional 15 Malaysian MRSA genomes from GenBank were analyzed for their plasmid content. The majority (90%, 85/94) of the Malaysian MRSA isolates harbored 1-4 plasmids each. In total, 189 plasmid sequences were identified ranging in size from 2.3 kb to ca. 58 kb, spanning all seven distinctive plasmid replication initiator (replicase) types. Resistance genes (either to antimicrobials, heavy metals, and/or biocides) were found in 74% (140/189) of these plasmids. Small plasmids (<5 kb) were predominant (63.5%, 120/189) with a RepL replicase plasmid harboring the ermC gene that confers resistance to macrolides, lincosamides, and streptogramin B (MLS_B) identified in 63 MRSA isolates. A low carriage of conjugative plasmids was observed (n = 2), but the majority (64.5%, 122/189) of the non-conjugative plasmids have mobilizable potential. The results obtained enabled us to gain a rare view of the plasmidomic landscape of Malaysian MRSA isolates and reinforces their importance in the evolution of this pathogen.

Slaughterhouse wastewater as a reservoir for extended-spectrum β-lactamase (ESBL)-producing, and colistin-resistant Klebsiella spp. and their impact in a "One Health" perspective

Klebsiella spp. are ubiquitous bacteria capable of colonizing humans and animals, and sometimes leading to severe infections in both. Due to their high adaptability against environmental/synthetic conditions as well as their potential in aquiring antimicrobial/metal/biocide resistance determinants, Klebsiella spp. are recognized as an emerging threat to public health, worldwide. Currently, scarce information on the impact of livestock for the spread of pathogenic Klebsiella spp. is available. Thus, the phenotypic and genotypic properties of extended-spectrum β-lactamase-producing, and colistin-resistant Klebsiella strains (n = 185) from process- and wastewater of two poultry and pig slaughterhouses as well as their receiving municipal wastewater treatment plants (mWWTPs) were studied to determine the diversity of isolates that might be introduced into the food-production chain or that are released into the environment by surviving the wastewater treatment. Selectively-isolated Klebsiella spp. from slaughterhouses including effluents and receiving waterbodies of mWWTPs were assigned to various lineages, including high-risk clones involved in human outbreaks, and exhibited highly heterogeneous antibiotic-resistance patterns. While isolates originating from poultry slaughterhouses showed the highest rate of colistin resistance (32.4%, 23/71), carbapenem-resistant Klebsiella spp. were only detected in mWWTP samples (n = 76). The highest diversity of resistance genes (n = 77) was detected in Klebsiella spp. from mWWTPs, followed by isolates from pig (n = 56) and poultry slaughterhouses (n = 52). Interestingly, no carbapenemase-encoding genes were detected and mobile colistin resistance genes were only obeserved in isolates from poultry and pig slaughterhouses. Our study provides in-depth information into the clonality of livestock-associated Klebsiella spp. and their determinants involved in antimicrobial resistance and virulence development. On the basis of their pathogenic potential and clinical importance there is a potential risk of colonization and/or infection of wildlife, livestock and humans exposed to contaminated food and/or surface waters.

Antibiotic-resistant bacteria, antibiotic resistance genes, and antibiotic residues in wastewater from a poultry slaughterhouse after conventional and advanced treatments

Slaughterhouse wastewater is considered a reservoir for antibiotic-resistant bacteria and antibiotic residues, which are not sufficiently removed by conventional treatment processes. This study focuses on the occurrence of ESKAPE bacteria (Enterococcus spp., S. aureus, K. pneumoniae, A. baumannii, P. aeruginosa, Enterobacter spp.), ESBL (extended-spectrum β-lactamase)-producing E. coli, antibiotic resistance genes (ARGs) and antibiotic residues in wastewater from a poultry slaughterhouse. The efficacy of conventional and advanced treatments (i.e., ozonation) of the in-house wastewater treatment plant regarding their removal was also evaluated. Target culturable bacteria were detected only in the influent and effluent after conventional treatment. High abundances of genes (e.g., bla_TEM, bla_CTX-M-15, bla_CTX-M-32, bla_OXA-48, bla_CMY and mcr-1) of up to 1.48 × 10⁶ copies/100 mL were detected in raw influent. All of them were already significantly reduced by 1–4.2 log units after conventional treatment. Following ozonation, mcr-1 and bla_CTX-M-32 were further reduced below the limit of detection. Antibiotic residues were detected in 55.6% (n = 10/18) of the wastewater samples. Despite the significant reduction through conventional and advanced treatments, effluents still exhibited high concentrations of some ARGs (e.g., sul1, ermB and bla_OXA-48), ranging from 1.75 × 10² to 3.44 × 10³ copies/100 mL. Thus, a combination of oxidative, adsorptive and membrane-based technologies should be considered.

Clinically Relevant Escherichiacoli Isolates from Process Waters and Wastewater of Poultry and Pig Slaughterhouses in Germany

Escherichia coli is frequently associated with multiple antimicrobial resistances and a major cause of bacterial extraintestinal infections in livestock and humans. However, data on the epidemiology of (i) multidrug-resistant (MDR) and (ii) extraintestinal pathogenic E. coli (ExPEC) in poultry and pig slaughterhouses in Germany is currently lacking. Selected E. coli isolates (n = 71) with phenotypic resistance to cephalosporins from two poultry and two pig slaughterhouses expressing high MDR rates (combined resistance to piperacillin, cefotaxime and/or ceftazidime, and ciprofloxacin) of 51.4% and 58.3%, respectively, were analyzed by whole-genome sequencing. They constituted a reservoir for 53 different antimicrobial resistance determinants and were assigned various sequence types, including high-risk clones involved in human infections worldwide. An ExPEC pathotype was detected in 17.1% and 5.6% of the isolates from poultry and pig slaughterhouses, respectively. Worryingly, they were recovered from scalding water and eviscerators, indicating an increased risk for cross-contaminations. Uropathogenic E. coli (UPEC) were detected in the effluent of an in-house wastewater treatment plant (WWTP) of a poultry slaughterhouse, facilitating their further dissemination into surface waters. Our study provides important information on the molecular characteristics of (i) MDR, as well as (ii) ExPEC and UPEC regarding their clonal structure, antimicrobial resistance and virulence factors. Based on their clinical importance and pathogenic potential, the risk of slaughterhouse employees’ exposure cannot be ruled out. Through cross-contamination, these MDR E. coli pathotypes may be introduced into the food chain. Moreover, inadequate wastewater treatment may contribute to the dissemination of UPEC into surface waters, as shown for other WWTPs.

Overcoming Acquired and Native Macrolide Resistance with Bicarbonate

The growing challenge of microbial resistance emphasizes the importance of new antibiotics or reviving strategies for the use of old ones. Macrolide antibiotics are potent bacterial protein synthesis inhibitors with a formidable capacity to treat life-threatening bacterial infections; however, acquired and intrinsic resistance limits their clinical application. In the work presented here, we reveal that bicarbonate is a potent enhancer of the activity of macrolide antibiotics that overcomes both acquired and intrinsic resistance mechanisms. With a focus on azithromycin, a highly prescribed macrolide antibiotic, and using clinically relevant pathogens, we show that physiological concentrations of bicarbonate overcome drug resistance by increasing the intracellular concentration of azithromycin. We demonstrate the potential of bicarbonate as a formulation additive for topical use of azithromycin in treating a murine wound infection caused by Pseudomonas aeruginosa. Further, using a systemic murine model of methicillin-resistant Staphylococcus aureus (MRSA) infection, we demonstrate the potential role of physiological bicarbonate, naturally abundant in the host, to enhance the activity of azithromycin against macrolide-resistant MRSA. In all, our findings suggest that macrolide resistance, observed in the clinical microbiology laboratory using standard culturing techniques, is a poor predictor of efficacy in the clinic and that observed resistance should not necessarily hamper the use of macrolides. Whether as a formulation additive for topical use or as a natural component of host tissues, bicarbonate is a powerful potentiator of macrolides with the capacity to overcome drug resistance in life-threatening bacterial infections.

ICESag37, a Novel Integrative and Conjugative Element Carrying Antimicrobial Resistance Genes and Potential Virulence Factors in Streptococcus agalactiae

ICESag37, a novel integrative and conjugative element carrying multidrug resistance and potential virulence factors, was characterized in a clinical isolate of Streptococcus agalactiae. Two clinical strains of S. agalactiae, Sag37 and Sag158, were isolated from blood samples of new-borns with bacteremia. Sag37 was highly resistant to erythromycin and tetracycline, and susceptible to levofloxacin and penicillin, while Sag158 was resistant to tetracycline and levofloxacin, and susceptible to erythromycin. Transfer experiments were performed and selection was carried out with suitable antibiotic concentrations. Through mating experiments, the erythromycin resistance gene was found to be transferable from Sag37 to Sag158. SmaI-PFGE revealed a new SmaI fragment, confirming the transfer of the fragment containing the erythromycin resistance gene. Whole genome sequencing and sequence analysis revealed a mobile element, ICESag37, which was characterized using several molecular methods and in silico analyses. ICESag37 was excised to generate a covalent circular intermediate, which was transferable to S. agalactiae. Inverse PCR was performed to detect the circular form. A serine family integrase mediated its chromosomal integration into rumA, which is a known hotspot for the integration of streptococcal ICEs. The integration site was confirmed using PCR. ICESag37 carried genes for resistance to multiple antibiotics, including erythromycin [erm(B)], tetracycline [tet(O)], and aminoglycosides [aadE, aphA, and ant(6)]. Potential virulence factors, including a two-component signal transduction system (nisK/nisR), were also observed in ICESag37. S1-PFGE analysis ruled out the existence of plasmids. ICESag37 is the first ICESa2603 family-like element identified in S. agalactiae carrying both resistance and potential virulence determinants. It might act as a vehicle for the dissemination of multidrug resistance and pathogenicity among S. agalactiae.

The Expression of Antibiotic Resistance Methyltransferase Correlates with mRNA Stability Independently of Ribosome Stalling

Members of the Erm methyltransferase family modify 23S rRNA of the bacterial ribosome and render cross-resistance to macrolides and multiple distantly related antibiotics. Previous studies have shown that the expression of erm is activated when a macrolide-bound ribosome stalls the translation of the leader peptide preceding the cotranscribed erm. Ribosome stalling is thought to destabilize the inhibitory stem-loop mRNA structure and exposes the erm Shine-Dalgarno (SD) sequence for translational initiation. Paradoxically, mutations that abolish ribosome stalling are routinely found in hyper-resistant clinical isolates; however, the significance of the stalling-dead leader sequence is largely unknown. Here, we show that nonsense mutations in the Staphylococcus aureus ErmB leader peptide (ErmBL) lead to high basal and induced expression of downstream ErmB in the absence or presence of macrolide concomitantly with elevated ribosome methylation and resistance. The overexpression of ErmB is associated with the reduced turnover of the ermBL-ermB transcript, and the macrolide appears to mitigate mRNA cleavage at a site immediately downstream of the ermBL SD sequence. The stabilizing effect of antibiotics on mRNA is not limited to ermBL-ermB; cationic antibiotics representing a ribosome-stalling inducer and a noninducer increase the half-life of specific transcripts. These data unveil a new layer of ermB regulation and imply that ErmBL translation or ribosome stalling serves as a “tuner” to suppress aberrant production of ErmB because methylated ribosome may impose a fitness cost on the bacterium as a result of misregulated translation.

Resistance to Macrolide Antibiotics in Public Health Pathogens

Macrolide resistance mechanisms can be target-based with a change in a 23S ribosomal RNA (rRNA) residue or a mutation in ribosomal protein L4 or L22 affecting the ribosome's interaction with the antibiotic. Alternatively, mono- or dimethylation of A2058 in domain V of the 23S rRNA by an acquired rRNA methyltransferase, the product of an erm (erythromycin ribosome methylation) gene, can interfere with antibiotic binding. Acquired genes encoding efflux pumps, most predominantly mef(A) + msr(D) in pneumococci/streptococci and msr(A/B) in staphylococci, also mediate resistance. Drug-inactivating mechanisms include phosphorylation of the 2'-hydroxyl of the amino sugar found at position C5 by phosphotransferases and hydrolysis of the macrocyclic lactone by esterases. These acquired genes are regulated by either translation or transcription attenuation, largely because cells are less fit when these genes, especially the rRNA methyltransferases, are highly induced or constitutively expressed. The induction of gene expression is cleverly tied to the mechanism of action of macrolides, relying on antibiotic-bound ribosomes stalled at specific sequences of nascent polypeptides to promote transcription or translation of downstream sequences.

Inducible Expression of both ermB and ermT Conferred High Macrolide Resistance in Streptococcus gallolyticus subsp. pasteurianus Isolates in China

Streptococcus gallolyticus subsp. pasteurianus is an under-recognized pathogen and zoonotic agent causing opportunistic infections in humans. Despite increasing recognition of this subspecies as a cause for human infectious diseases, limited information is known about its antibiotic resistance mechanism. In this study, we aim to identify the molecular mechanism underlying the high macrolide resistance of six S. gallolyticus subsp. pasteurianus isolates from dead ducklings collected in several natural outbreaks in China during 2010–2013. All isolates exhibited multi-drug resistance including high macrolide resistance (MIC ≥ 1024 mg/L for erythromycin, and 512 mg/L for clarithromycin). Efflux-encoding mefA and mefE genes were not detectable in these isolates. The presence of 23S rRNA mutations in specific isolates did not significantly change macrolide MICs. No nucleotide substitutions were found in genes encoding ribosomal proteins L4 or L22. The ermB and ermT genes were found in the genomes of all isolates. These two genes were acquired independently in one highly virulent isolate AL101002, and clustered with Tn916 and IS1216, respectively. The expression of both ermB and ermT in all isolates was erythromycin inducible and yielded comparable macrolide MICs in all six isolates. Taken together, inducible expression of both ermB and ermT conferred high macrolide resistance in these S. gallolyticus subsp. pasterianus isolates. Our findings reveal new macrolide resistance features in S. gallolyticus subsp. pasteurianus by both ermB and ermT.

Constitutive and Inducible Expression of the rRNA Methylase Gene erm(B) in Campylobacter

Macrolides are the antimicrobials of choice for treating human campylobacteriosis. The recent emergence of erm(B) in Campylobacter bacteria threatens the utility of this class of antibiotics. Here we report the constitutive and inducible expression of erm(B) in Campylobacter isolates derived from diarrheal patients and food-producing animals. Constitutive expression of erm(B) was associated with insertion and deletion in the regulatory region of the gene, providing the first documentation of the differential expression of erm(B) in Campylobacter bacteria.

Enterococcal endocarditis revisited

The Enterococcus species is the third main cause of infective endocarditis (IE) worldwide, and it is gaining relevance, especially among healthcare-associated cases. Patients with enterococcal IE are older and have more comorbidities than other types of IE. Classical treatment options are limited due to the emergence of high-level aminoglycosides resistance (HLAR), vancomycin resistance and multidrug resistance in some cases. Besides, few new antimicrobial alternatives have shown real efficacy, despite some of them being recommended by major guidelines (including linezolid and daptomycin). Ampicillin plus ceftriaxone 2 g iv./12 h is a good option for Enterococcus faecalis IE caused by HLAR strains, but randomized clinical trials are essential to demonstrate its efficacy for non-HLAR EFIE and to compare it with ampicillin plus short-course gentamicin. The main mechanisms of resistance and treatment options are also reviewed for other enterococcal species.

Transcriptional attenuation controls macrolide inducible efflux and resistance in Streptococcus pneumoniae and in other Gram-positive bacteria containing mef/mel(msr(D)) elements

Macrolide resistance, emerging in Streptococcus pneumoniae and other Gram-positive bacteria, is increasingly due to efflux pumps encoded by mef/mel(msr) operons found on discrete mobile genetic elements. The regulation of mef/mel(msr) in these elements is not well understood. We identified the mef(E)/mel transcriptional start, localized the mef(E)/mel promoter, and demonstrated attenuation of transcription as a mechanism of regulation of macrolide-inducible mef-mediated macrolide resistance in S. pneumoniae. The mef(E)/mel transcriptional start site was a guanine 327 bp upstream of mef(E). Consensus pneumococcal promoter -10 (5′-TATACT-3′) and -35 (5′-TTGAAC-3′) boxes separated by 17 bp were identified 7 bp upstream of the start site. Analysis of the predicted secondary structure of the 327 5’ region identified four pairs of inverted repeats R1-R8 predicted to fold into stem-loops, a small leader peptide [MTASMRLR, (Mef(E)L)] required for macrolide induction and a Rho-independent transcription terminator. RNA-seq analyses provided confirmation of transcriptional attenuation. In addition, expression of mef(E)L was also influenced by mef(E)L-dependent mRNA stability. The regulatory region 5’ of mef(E) was highly conserved in other mef/mel(msr)-containing elements including Tn1207.1 and the 5612IQ complex in pneumococci and Tn1207.3 in Group A streptococci, indicating a regulatory mechanism common to a wide variety of Gram-positive bacteria containing mef/mel(msr) elements.

Deregulation of translation due to post-transcriptional modification of rRNA explains why erm genes are inducible

A key mechanism of bacterial resistance to macrolide antibiotics is the dimethylation of a nucleotide in the large ribosomal subunit by erythromycin resistance methyltransferases. The majority of erm genes are expressed only when the antibiotic is present and the erythromycin resistance methyltransferase activity is critical for the survival of bacteria. Although these genes were among the first discovered inducible resistance genes, the molecular basis for their inducibility has remained unknown. Here we show that erythromycin resistance methyltransferase expression reduces cell fitness. Modification of the nucleotide in the ribosomal tunnel skews the cellular proteome by deregulating the expression of a set of proteins. We further demonstrate that aberrant translation of specific proteins results from abnormal interactions of the nascent peptide with the erythromycin resistance methyltransferase-modified ribosomal tunnel. Our findings provide a plausible explanation why erm genes have evolved to be inducible and underscore the importance of nascent peptide recognition by the ribosome for generating a balanced cellular proteome.

Characterization of Enterococcus faecium with macrolide resistance and reduced susceptibility to quinupristin/dalfopristin in a Japanese hospital: detection of extensive diversity in erm(B)-regulator regions

Cross-resistance to macrolide, lincosamide, and streptogramin B (MLSB) antibiotics is mainly mediated by the erm (erythromycin ribosome methylation) genes that encode 23S rRNA methylases in enterococi, and various mechanisms are involved in the streptogramin B resistance. Prevalence of MLSB resistance and its genetic mechanisms were analyzed for a total of 159 strains of Enterococcus faecium isolated from clinical specimens in a university hospital in Japan from 1997 to 2006. Resistance to erythromycin (EM) and clindamycin was detected in 88.1% and 89.9% of all the strains examined, respectively, and expression of resistance was totally constitutive. Although none of the strain was resistant to quinupristin/dalfopristin (Q/D), 28 strains (17.6%) showed intermediate resistance to Q/D (MIC: 2 μg/ml). The erm(B) gene was detected in 139 strains (87.4%), and msrC was found in all the strains examined, whereas no other known MLSB resistance genes were identified. The erm(B) regulator region (RR) containing a coding region of the leader peptide was classified into 13 genetic variations (L1-L3, M, S1-S7, D, and R genotypes) in 56 strains. However, no relatedness was identified between the erm(B) RR genotype and EM resistance, or reduced susceptibility to Q/D, although most of Q/D-intermediate strains were assigned to the L1, L2, and S1 genotypes. Q/D-intermediate strains were classified into five multiple-locus variable-number tandem-repeat analysis (MLVA) types, including four types of clonal complex (CC)-C1, five sequence types (STs), including four STs of CC-17, and several resistance gene/virulence factor profiles. The present study revealed the occurrence of Q/D-intermediate E. faecium, which are composed of heterogeneous strains in Japan, and more genetic diversity in the erm(B) RRs than those reported previously.

Antimicrobial susceptibility, tetracycline and erythromycin resistance genes, and multilocus sequence typing of Streptococcus suis isolates from diseased pigs in China

Streptococcus suis (S. suis) is an emerging zoonotic pathogen causing significant economic losses in the swine industry. Here, we investigated the antimicrobial susceptibility, associated antibiotic-resistant determinants and sequence type (ST) of S. suis isolates from diseased pigs in China from 2008 to 2010. Serotype 2 was the most frequently observed strain (n=95) among the 106 S. suis strains collected, followed by serotypes 3 (n=3), 5 (n=3), 4 (n=2), 7 (n=1), 11 (n=1) and 28 (n=1). Multilocus sequence typing analysis revealed that ST1 (n=21) and ST7 (n=74) were the predominant STs, and serotype 2 was found to be significantly correlated with ST7 (P=0.017, Fisher's exact test) and CC1 (P=0.024, Fisher's exact test). The antimicrobial susceptibility results indicated that the antibiotic resistance rate was highest for tetracycline (99.1%), followed by azithromycin (68.9%), erythromycin (67.9%), clindamycin (67.9%), trimethoprim/sulfamethoxazole (16%), levofloxacin (2.8%), chloramphenicol (1.9%), cefaclor (0.9%) and ceftriaxone (0.9%). Antibiotic-resistant genes tet(M), tet(O), tet(O/W/32/O), tet(O/32/O), tet(S), tet(W), tet(L), tet(40), erm(B), mef(A/E) and msr(D) could be detected, and several tandem organizations of antibiotic resistance genes were also found in this study. In conclusion, S. suis strains isolated from diseased pigs in China were less diverse and multi-drug resistant.

Alterations in regulatory regions of erm(B) genes from clinical isolates of enterococci resistant to telithromycin

We determined rates of resistance to the ketolide telithromycin in 56 Enterococcus faecalis isolates and 44 Enterococcus faecium isolates collected from hospitals in Korea between 2005 and 2006. Twenty nine (51.8%) isolates of E. faecalis and 35 (79.5%) isolates of E. faecium were resistant to telithromycin (minimum inhibitory concentrations, ≥ 4 μg/mL). All of the telithromycin-resistant E. faecalis carried the erm(B) gene only. Of the telithromycin-resistant E. faecium, 29 resistant strains carried erm(B) only, the other six carried erm(A) and erm(B) together. The nucleotide sequence of the erm(B) regulatory regions from 29 E. faecalis and 29 E. faecium isolates with erm(B) only was analyzed. Five types of alterations were detected. The first and second types had point mutations that destabilize the secondary structure of erm(B) mRNA sequestering the translation initiation region of the structural gene. The third type was identical to erm(Bv1), a previously reported variant of erm(B) with different induction specificity. The fourth and fifth types had point mutations within the critical sequence for induction and a point mutation destabilizing the stem-loop of erm(B) mRNA sequestering the translation initiation region of the structural gene.

In vitro activity of ceftobiprole and seven other antimicrobial agents against invasive Streptococcus pneumoniae isolates in Spain

The in vitro activity of ceftobiprole was compared with that of seven antimicrobial agents against invasive Streptococcus pneumoniae isolated from adult patients (>15 years old). Characterization of erythromycin-resistant strains and serotype distribution of all pneumococci were also evaluated. Seventy invasive S. pneumoniae strains were isolated from December 2007 to January 2009. Serotyping was carried out by Quellung reaction. Antibiotic susceptibility was tested by broth microdilution (CLSI guidelines). The comparator agents were penicillin, cefotaxime, erythromycin, clindamycin, telithromycin, tetracycline and moxifloxacin. Phenotypic characterization of macrolide resistance was performed by the double disk method. Macrolide resistance genes [erm(B) and mef(A/E)] and the promoter of erm(B) were detected by PCR. Twenty-five different serotypes were detected of which 87% were non-PCV7 types. The percentages of resistance to erythromycin, clindamycin and tetracycline were 20%, 8.6% and 16%, respectively. A penicillin MIC ≥0.12 mg/L was observed in 14 of the 70 invasive pneumococci strains. The cefotaxime and ceftobiprole MIC(50)/MIC(90) of these 14 strains were 1/4 and 0.03/1 mg/L, respectively. Ceftobiprole showed higher in vitro activity than penicillin and cefotaxime with all isolates being inhibited by ≤1 mg/L. Its high in vitro activity should make ceftobiprole a very promising drug for the treatment of pneumococcal infections.

Transcriptional and translational control of the mlr operon, which confers resistance to seven classes of protein synthesis inhibitors

The methyltransferase genes erm(B) and cfr are adjacent to each other in the chromosome of methicillin-resistant Staphylococcus aureus strain CM05. Analyses of the transcriptional organization of the erm(B) and cfr genes in the chromosome of strain CM05 showed that the two genes are organized into an operon, designated mlr (for modification of the large ribosomal subunit), which is controlled by the erm(B) promoter. Analysis of the translation control and the inducibility of the erm(B) and cfr genes in the mlr operon showed that despite the presence of putative regulatory short open reading frames, both genes are expressed constitutively. The combined action of the two methyltransferases encoded in the mlr operon results in modification of two specific residues in 23S rRNA, A2058 and A2503, and renders cells resistant to all clinically useful antibiotics that target the large ribosomal subunit. Furthermore, simultaneous modification of both rRNA sites synergistically enhances resistance to 16-member-ring macrolides.

Telithromycin resistance in Streptococcus pneumoniae is conferred by a deletion in the leader sequence of erm(B) that increases rRNA methylation

A telithromycin-resistant clinical isolate of Streptococcus pneumoniae (strain P1501016) has been found to contain a version of erm(B) that is altered by a 136-bp deletion in the leader sequence. By allele replacement mutagenesis, a second strain of S. pneumoniae (PC13) with a wild-type erm(B) gene was transformed to the telithromycin-resistant phenotype by introduction of the mutant erm(B) gene. Whereas the wild-type PC13 strain showed slight telithromycin resistance only after induction by erythromycin (telithromycin MIC increased from 0.06 to 0.5 microg/ml), the transformed PC13 strain is constitutively resistant (MIC of 16 mug/ml). Expression of erm(B) was quantified by real-time reverse transcription-PCR in the presence of erythromycin or telithromycin; erm(B) expression was significantly higher in the transformed PC13 strain than the wild-type strain. Furthermore, the transformed strain had significantly higher levels of ribosomal methylation in the absence as well as in the presence of the antibiotics. Growth studies showed that the transformed PC13 strain had a shorter lag phase than the wild-type strain in the presence of erythromycin. Telithromycin resistance is conclusively shown to be conferred by the mutant erm(B) gene that is expressed at a constitutively higher level than the inducible wild-type gene. Elevated erm(B) expression results in a higher level of rRNA methylation that presumably hinders telithromycin binding to the ribosome.

Modes and modulations of antibiotic resistance gene expression

Since antibiotic resistance usually affords a gain of function, there is an associated biological cost resulting in a loss of fitness of the bacterial host. Considering that antibiotic resistance is most often only transiently advantageous to bacteria, an efficient and elegant way for them to escape the lethal action of drugs is the alteration of resistance gene expression. It appears that expression of bacterial resistance to antibiotics is frequently regulated, which indicates that modulation of gene expression probably reflects a good compromise between energy saving and adjustment to a rapidly evolving environment. Modulation of gene expression can occur at the transcriptional or translational level following mutations or the movement of mobile genetic elements and may involve induction by the antibiotic. In the latter case, the antibiotic can have a triple activity: as an antibacterial agent, as an inducer of resistance to itself, and as an inducer of the dissemination of resistance determinants. We will review certain mechanisms, all reversible, that bacteria have elaborated to achieve antibiotic resistance by the fine-tuning of the expression of genetic information.

Differences in the DNA sequence of the translational attenuator of several constitutively expressed erm(A) genes from clinical isolates of Streptococcus agalactiae

OBJECTIVES

To study the regulatory region of a constitutively expressed erm(A) gene in Streptococcus agalactiae clinical isolates.

METHODS

Thirty clinical isolates of S. agalactiae which were cross-resistant to erythromycin and clindamycin and with a clindamycin MIC higher than that of erythromycin were studied by PCR, sequencing and molecular typing.

RESULTS

PCR analysis revealed that all the strains harboured the erm(A) gene, either alone (26 isolates) or in combination with erm(B) (four isolates). Sequencing of the region upstream of erm(A) showed that all isolates possessed two types of genetic alteration. Most of the strains showed point mutations in the second leader peptide (mainly A137C) and, in four isolates (two clones), an insertion fragment with high homology to IS1381 and transposase genes was detected. Epidemiological analysis of strains indicated several clonal origins of isolates.

CONCLUSIONS

The mutations described here are thought to result in increased or constitutive expression of the erm(A) gene.

Macrolide-resistance mechanisms in Streptococcus pneumoniae isolates from Belgium

Of 233 erythromycin-resistant pneumococcal isolates collected in Belgium in 1999-2000, 89.7% carried the erm(B) gene, 6% the mef(A) gene, and 3.5%erm(B) plus mef(A). Two isolates contained neither erm(B) nor mef(A); one contained an erm(A) subclass erm(TR) gene, while the other contained an A2058G mutation in domain V of the 23S rRNA gene. Of 209 erm(B)-positive isolates, 191 had clindamycin MICs > 16 mg/L and 18 had MICs < or = 16 mg/L. Mef(A)-positive isolates all displayed the M resistance phenotype. Telithromycin remained active against erythromycin-resistant isolates, with the highest telithromycin MIC50 being found in mef(A)-positive isolates. No difference in the prevalence of different resistance mechanisms was observed compared to isolates collected in 1995-1997.

Telithromycin: The first ketolide antimicrobial

BACKGROUND

Telithromycin is the first of the ketolide antibacterials to receive US Food and Drug Administration (FDA) approval for clinical use. It is approved for the treatment of community-acquired pneumonia (CAP), acute exacerbations of chronic bronchitis (AECB), and acute maxillary sinusitis (AMS) in adults.

OBJECTIVE

This article reviews the mechanism of action, in vitro antimicrobial activity, pharmacokinetics and pharmacodynamics, clinical efficacy, safety, and drug-interaction profile of telithromycin.

METHODS

Relevant studies were identified through a search of the English-language literature indexed on MEDLINE (1990-March 2005) using the terms telithromycin and HMR 3647, a review of the reference lists of identified articles, and a review of the briefing document prepared by the manufacturer of telithromycin for presentation to the FDA Anti-infective Drugs Advisory Committee. A search of abstracts from the Interscience Conference on Antimicrobial Agents and Chemotherapy (2001-2004) also was performed.

RESULTS

The results of in vitro susceptibility studies suggest that telithromycin provides coverage against the key respiratory pathogens, both typical and atypical. In addition, telithromycin may be useful against multidrug-resistant strains of Streptococcus pneumoniae and against Haemophilus influenzae, irrespective of beta-lactamase production. In randomized, double-blind, comparative trials (against amoxicillin, amoxicillin/clavulanate, cefuroxime axetil, clarithromycin, moxifloxacin, or trovafloxacin), telithromycin had comparable efficacy to its comparators in the empiric treatment of CAP (4 studies), AECB (3 studies), and AMS (3 studies). Telithromycin is dosed at 800 mg (two 400-mg tablets) QD in community-acquired respiratory tract infections (RTIs). No dose adjustment is required in the elderly, patients with mild to moderate renal insufficiency, or patients with hepatic insufficiency. The majority of adverse events associated with telithromycin were mild to moderate, with gastrointestinal effects (diarrhea, nausea, vomiting) being the most commonly reported, followed by headache and dizziness. Telithromycin has been associated with elevations in hepatic transaminases and prolongation of the electrocardiographic QTc interval, although the significance of these findings is not known. Telithromycin is also a strong inhibitor of and substrate for the cytochrome P450 (CYP) 3A4 isozyme. Therefore, it is important to monitor for potential drug interactions with medications that prolong the QTc interval or are metabolized by the CYP system.

CONCLUSIONS

Telithromycin appears to be a useful option for the empiric treatment of community-acquired RTIs in adults. It may be particularly useful in the outpatient setting in areas with high rates of penicillin- and macrolide-resistant S pneumoniae; it may also be an alternative agent for patients who are allergic to beta-lactams and live in areas with a high prevalence of multidrug-resistant S pneumoniae or for those who have failed to respond to beta-lactam- or macrolide-based therapy.

Antipneumococcal activity of ceftobiprole, a novel broad-spectrum cephalosporin

Ceftobiprole (previously known as BAL9141), an anti-methicillin-resistant Staphylococcus aureus cephalosporin, was very highly active against a panel of 299 drug-susceptible and -resistant pneumococci, with MIC(50) and MIC(90) values (microg/ml) of 0.016 and 0.016 (penicillin susceptible), 0.06 and 0.5 (penicillin intermediate), and 0.5 and 1.0 (penicillin resistant). Ceftobiprole, imipenem, and ertapenem had lower MICs against all pneumococcal strains than amoxicillin, cefepime, ceftriaxone, cefotaxime, cefuroxime, or cefdinir. Macrolide and penicillin G MICs generally varied in parallel, whereas fluoroquinolone MICs did not correlate with penicillin or macrolide susceptibility or resistance. All strains were susceptible to linezolid, quinupristin-dalfopristin, daptomycin, vancomycin, and teicoplanin. Time-kill analyses showed that at 1x and 2x the MIC, ceftobiprole was bactericidal against 10/12 and 11/12 strains, respectively. Levofloxacin, moxifloxacin, vancomycin, and teicoplanin were each bactericidal against 10 to 12 strains at 2x the MIC. Azithromycin and clarithromycin were slowly bactericidal, and telithromycin was bactericidal against only 5/12 strains at 2x the MIC. Linezolid was mainly bacteriostatic, whereas quinupristin-dalfopristin and daptomycin showed marked killing at early time periods. Prolonged serial passage in the presence of subinhibitory concentrations of ceftobiprole failed to yield mutants with high MICs towards this cephalosporin, and single-passage selection showed very low frequencies of spontaneous mutants with breakthrough MICs towards ceftobiprole.

Mechanisms of resistance to telithromycin in Streptococcus pneumoniae

Reports of ketolide resistance remain scarce, however, a few laboratory-derived and clinical isolates of resistant Streptococcus pneumoniae have been documented. Mutations in key telithromycin-binding sites such as domains II and V of the 23S rRNA and ribosomal proteins L4 and L22, as well as mutations of the resistance determinant erm(B) are associated with elevated telithromycin MICs. Mutations in the secondary binding site of domain II coupled with ribosomal methylation may have serious resistance consequences should the domain II binding site be lost. Although ketolides are purported to maintain excellent activity against efflux-positive isolates, laboratory-derived telithromycin-resistant strains have been generated. As telithromycin usage increases, ketolide-resistant isolates of S. pneumoniae may well increase.

Phenotypic and genotypic characterization of macrolide resistant Streptococcus pneumoniae recovered from adult patients with community-acquired pneumonia in an Argentinian teaching hospital

Streptococcus pneumoniae isolates (n = 262) were recovered from adult patients with community-acquired pneumonia. Erythromycin-resistance levels increased from 9% (1997-1998) to 16% (2000-2002). Sampling for resistance mechanisms prevalent within 19 erythromycin-resistant S. pneumoniae showed mef(E) in 13/19 isolates while 4/19 carried the erm(B) gene (3/19 cMLS(B) and 1/19 iMLS(B) phenotype). MIC ranges for erythromycin and clindamycin were 0.5-16 mg/l and <0.008-0.063 mg/l for the M phenotype, 128-512 mg/l and 128-256 mg/l for the cMLS(B) phenotype, and 4 and <0.008 mg/l for the iMLS(B) phenotype. This is the first report studying the prevalence of macrolide resistance determinants in S. pneumoniae in our country.

Macrolide resistance in Streptococci and Haemophilus influenzae

Antimicrobial resistance is a growing problem among pathogens from respiratory tract infections. b-Lactam resistance rates are escalating among Streptococcus pneumoniae and Haemophilus influenzae. Macrolides are increasingly used for the treatment of respiratory tract infections, but their utility is compromised by intrinsic and acquired resistance. This article analyses macrolide-resistance mechanisms and their worldwide distributions in S pneumoniae, S pyogenes, and H influenzae.

Heterogeneity of macrolide-lincosamide-streptogramin B resistance phenotypes in enterococci

We determined the macrolide resistance phenotypes of 241 clinical isolates of erythromycin-resistant enterococci (MICs, > or = 1 microg/ml), including 147 Enterococcus faecalis strains and 94 Enterococcus faecium strains, collected from a hospital in Seoul, Korea, between 1999 and 2000. By the erythromycin (40 micro g)-josamycin (100 microg) double-disk test, 93 strains were assigned to the constitutive macrolide, lincosamide, and streptogramin B (MLS(B)) resistance (cMLS(B)) phenotype, and the remaining 148 strains were assigned to the inducible MLS(B) resistance (iMLS(B)) phenotype. Of the strains with the iMLS(B) phenotype, 36 exhibited a reversibly inducible MLS(B) (riMLS(B)) phenotype, i.e., blunting of the erythromycin zone of inhibition, which indicates that the 16-membered-ring macrolide josamycin is a more effective inducer than the 14-membered-ring macrolide erythromycin. Sequence analysis of the regulatory regions of the erm(B) genes from all of the strains exhibiting the riMLS(B) phenotype revealed not only erm(Bv) [where v represents variant; previously erm(AMR)] (n = 13), as reported previously, but also three kinds of erm(B) variants, which were designated erm(Bv1) (n = 17), erm(Bv2) (n = 3), and erm(Bv3) (n = 3), respectively. In lacZ reporter gene assays of these variants, the 16-membered-ring macrolide tylosin had stronger inducibility than erythromycin at > or = 0.1 microg/ml. These findings highlight the versatility of erm(B) in induction specificity.

Macrolide and lincosamide resistance in the gram-positive nasal and tonsillar flora of pigs

Macrolide and lincosamide resistance phenotypes and the presence of the erm(A), erm(B), erm(C), and mef(A) genes were determined in 344 bacterial strains belonging to 34 species and nine genera, isolated from the tonsils and nasal cavities of 2-week- and 6-week-old piglets, derived from four different farms. These piglets had never before been treated with macrolides or lincosamides. Macrolide and lincosamide resistance was most frequently present in Streptococcus and Enterococcus strains, of which over two-thirds were resistant. These genera were followed in decreasing order of resistance frequency by Lactobacillus, Rothia, Staphylococcus, Arcanobacterium, Actinomyces, Pediococcus strains. Only five infrequently occurring species did not show resistance. This high frequency of resistance in nontreated piglets indicates that resistant strains circulate in the herds. In streptococci, enterococci, and Lactobacillus strains, resistance was most often encoded by the erm(B) gene and in staphylococci by erm(A) or erm(C). The erm(B) gene was sporadically detected in other bacterial genera (Actinomyces, Rothia, Aerococcus, Pediococcus). The sequence of the erm(B) gene of 29 strains of 11 pigs originating from the four different farms was determined. This sequence was identical in 12 strains and only differed by 1-6 nucleotides in the other strains, indicating that exchanges of resistance genes might occur between bacterial species and genera belonging to the nasal or tonsillar flora of piglets.

Antimicrobial susceptibility and macrolide resistance inducibility of Streptococcus pneumoniae carrying erm(A), erm(B), or mef(A)

Erythromycin-resistant Streptococcus pneumoniae isolates from young carriers were tested for their antimicrobial susceptibility; additionally, inducibility of macrolide and clindamycin resistance was investigated in pneumococci carrying erm(A), erm(B), or mef(A). Of 125 strains tested, 101 (81%) were multidrug resistant. Different levels of induction were observed with erythromycin, miocamycin, and clindamycin in erm(B) strains; however, in erm(A) strains only erythromycin was an inducer. Induction did not affect macrolide MICs in mef(A) strains.

Unusual inducible cross resistance to macrolides, lincosamides, and streptogramins B by methylase production in clinical isolates of Staphylococcus aureus

Clinical strains of Staphylococcus aureus UCN7 and UCN8 were inducibly resistant to erythromycin, clindamycin, lincomycin, and quinupristin. This unusual inducible MLS(B) resistance was due to the presence of an erm(A) or an erm(B) gene, which both encode a ribosomal methylase, in S. aureus UCN8 and UCN7, respectively. The inducible cross resistance expressed by S. aureus UCN8 was associated with an 83-bp deletion in the attenuator of the erm(A) gene that removed the second of the two leader peptides and several inverted repeats. The presence of an inducible erm(B) gene in S. aureus UCN7 conferred a cross-resistance MLS(B) phenotype, similar to that usually observed in streptococci. Therefore, in S. aureus, besides the classical inducible MLS(B) phenotype characterized by inducible resistance to 14- to 15-membered ring macrolides, an additional type of inducible cross resistance to macrolides, lincosamides, and streptogramins B due to variants of erm(A) or erm(B) genes exist.

Fluorescence assay for studying the ability of macrolides to induce production of ribosomal methylase

A screening assay to test the inducing capacity of macrolides by fusing the attenuator of the inducible erm(B) gene from Streptococcus pneumoniae HM28 with the gfpmut1 gene has been designed. Fluorescence was detected under UV light around disks impregnated with inducer macrolides (erythromycin or azithromycin) but not with noninducer ketolides. Induction could be quantified by fluorometry.

Molecular analysis of streptogramin resistance in enterococci

The new semi-synthetic streptogramin antibiotic combination quinupristin/dalfopristin (Synercid) is a promising alternative for a treatment of infections with multiple resistant gram-positive pathogens, e.g. glycopeptide- and multi-resistant Enterococcus faecium. Streptogramins consist of two unrelated compounds, a streptogramin A and B, which act synergistically when given in combination. Mechanisms conferring resistance against both components are essential for resistance against the combination in E. faecium. In this species resistance to streptogramin A compounds is mediated via related acetyltransferases VatD and VatE. Resistance against streptogramins B is either encoded by the widespread ermB gene cluster conferring resistance to macrolide-lincosamide-streptogramin B antibiotics or via expression of the vgbA gene, which encodes a staphylococcal-type lactonase. E. faecalis is intrinsically resistant to streptogramins. Due to a wide use of streptogramins (virginiamycins S/M) in commercial animal farming a reservoir of streptogramin-resistant E. faecium isolates had already been selected. Determinants for streptogramin resistance are localized on plasmids that can be transferred into an E. faecium recipient both in vitro in filter-matings and in vivo in the digestive tracts of rats. Hybridization and sequencing experiments revealed a linkage of resistance determinants for streptogramins A and B on definite plasmid fragments.

Antimicrobial susceptibility of Streptococcus species isolated from clinical mastitis in dairy cows

The antimicrobial susceptibility was determined for 50 Streptococcus uberis, 42 S. dysgalactiae subsp. dysgalactiae and eight S. agalactiae strains isolated from cow mastitis. Only 27% of the strains were susceptible to all antimicrobial compounds tested. Resistance to tetracycline was most frequent (particularly for S. dysgalactiae strains), then macrolide and/or lincomycin resistance. High level resistance to streptomycin and kanamycin was detected. All S. dysgalactiae and S. agalactiae strains were susceptible to beta-lactams but 44% of the S. uberis strains showed an elevated penicillin G MIC. All strains were susceptible to chloramphenicol and rifampicin.

Mechanisms of resistance to macrolides and lincosamides: nature of the resistance elements and their clinical implications

Resistance to macrolides and lincosamides is increasingly reported in clinical isolates of gram-positive bacteria. The multiplicity of mechanisms of resistance, which include ribosomal modification, efflux of the antibiotic, and drug inactivation, results in a variety of phenotypes of resistance. There is controversy concerning the clinical relevance of in vitro macrolide resistance. Recent data, however, have shown that eradication of bacteria correlates with clinical outcome of acute otitis media in children and that macrolide therapy results in delayed eradication of macrolide-resistant pneumococci. These results support the need for in vitro detection of macrolide resistance and correct interpretation of susceptibility tests to guide therapy.

High prevalence of inducible erythromycin resistance among Streptococcus bovis isolates in Taiwan

Susceptibilities to 13 antimicrobial agents were determined by measurement of MICs for 60 isolates of Streptococcus bovis from blood cultures. Thirty-eight isolates (63.3%) had high-level resistance to erythromycin (MICs, >or=128 microg/ml). Among the 38 erythromycin-resistant strains, 21 isolates (55%) had inducible resistance to macrolides-lincosamides-streptogramin B (iMLS isolates) and 17 (45%) had constitutive resistance to macrolides-lincosamides-streptogramin B (cMLS isolates). Tetracycline resistance was also found among all of the erythromycin-resistant strains. None of the strains displayed resistance to penicillin, chloramphenicol, or vancomycin. Detection of erythromycin resistance genes by PCR and sequencing indicated that all 17 cMLS isolates were positive for the ermB gene and that 7 of 21 iMLS isolates carried the ermB gene and the remaining 14 iMLS isolates carried the ermT gene. Sequence analysis of amplified partial ermB fragments (594 bp) from S. bovis isolates revealed a 99.8% nucleotide identity and a 100% amino acid homology compared with the sequences from gene banks. The sequences of amplified fragments with primers targeted for ermC were shown to be very similar to that of ermGT (ermT) from Lactobacillus reuteri (98.5% nucleotide identity). This is the first report to describe the detection of the ermT class of erythromycin resistance determinants in S. bovis. The high rate of inducible erythromycin resistance among S. bovis isolates in Taiwan was not reported before. The iMLS S. bovis isolates were shown to be heterogeneous by randomly amplified polymorphic DNA analysis. These results indicate that the prevalence of inducible erythromycin resistance in S. bovis in Taiwan is very high and that most of the resistant strains carry the ermT or the ermB gene.

In vitro activity of telithromycin against Spanish Streptococcus pneumoniae isolates with characterized macrolide resistance mechanisms

The susceptibilities to telithromycin of 203 Streptococcus pneumoniae isolates prospectively collected during 1999 and 2000 from 14 different geographical areas in Spain were tested and compared with those to erythromycin A, clindamycin, quinupristin-dalfopristin, penicillin G, cefotaxime, and levofloxacin. Telithromycin was active against 98.9% of isolates (MICs, < or =0.5 microg/ml), with MICs at which 90% of isolates are inhibited being 0.06 microg/ml, irrespective of the resistance genotype. The corresponding values for erythromycin were 61.0% (MICs, < or =0.25 microg/ml) and >64 microg/ml. The erm(B) gene (macrolide-lincosamide-streptogramin B resistance phenotype) was detected in 36.4% (n = 74) of the isolates, which corresponded to 93.6% of erythromycin-intermediate and -resistant isolates, whereas the mef(A) gene (M phenotype [resistance to erythromycin and susceptibility to clindamycin and spiramycin without blunting]) was present in only 2.4% (n = 5) of the isolates. One of the latter isolates also carried erm(B). Interestingly, in one isolate for which the erythromycin MIC was 2 microg/ml, none of these resistance genes could be detected. Erythromycin MICs for S. pneumoniae erm(B)-positive isolates were higher (range, 0.5 to >64 microg/ml) than those for erm(B)- and mef(A)-negative isolates (range, 0.008 to 2 microg/ml). The corresponding values for telithromycin were lower for both groups, with ranges of 0.004 to 1 and 0.002 to 0.06 microg/ml, respectively. The erythromycin MIC was high for a large number of erm(B)-positive isolates, but the telithromycin MIC was low for these isolates. These results indicate the potential usefulness of telithromycin for the treatment of infections caused by erythromycin-susceptible and -resistant S. pneumoniae isolates when macrolides are indicated.

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.