Macrolide resistance

Distribution and driving mechanisms of antibiotic resistance genes in urbanized watersheds

Antibiotic resistance genes (ARGs) have emerged as a global concern, posing significant threats to human health and safety. Understanding the contamination levels and driving mechanisms behind ARG proliferation is urgently needed. Urban watersheds, influenced by human activities, serve as critical reservoirs for ARGs; however, the impact of urbanization on ARG spread of and the underlying driving mechanisms remain unclear. This study evaluates the diversity and abundance of ARGs in water and sediment samples from the Jialing River watershed in Chongqing City, China. The obtained results indicate that aminoglycoside and multidrug ARGs are the primary contributors to ARG presence in both sediments and water. Additionally, the diversity and abundance of ARGs are higher in water than in sediments. ARGs in watershed show a significant positive correlation with mobile genetic elements (MGEs). While environmental factors in urbanized watersheds affect ARG abundance and distribution to some extent, they are not the primary drivers. Urbanization itself emerges as a prominent factor influencing ARG diversity and abundance in river basins. Specifically, livestock, healthcare, and agriculture are identified as the main social factors influencing ARG proliferation in the highly urbanized areas of the Jialing River watershed. Further investigation into other contributing social factors, such as industrial development, is warranted. This study reveals the factors driving ARG distribution in urbanized watersheds, providing a foundation for future efforts to maintain ecological health in these environments.

Impact of environment on transmission of antibiotic-resistant superbugs in humans and strategies to lower dissemination of antibiotic resistance

Antibiotics are the most efficient type of therapy developed in the twentieth century. From the early 1960s to the present, the rate of discovery of new and therapeutically useful classes of antibiotics has significantly decreased. As a result of antibiotic use, novel strains emerge that limit the efficiency of therapies in patients, resulting in serious consequences such as morbidity or mortality, as well as clinical difficulties. Antibiotic resistance has created major concern and has a greater impact on global health. Horizontal and vertical gene transfers are two mechanisms involved in the spread of antibiotic resistance genes (ARGs) through environmental sources such as wastewater treatment plants, agriculture, soil, manure, and hospital-associated area discharges. Mobile genetic elements have an important part in microbe selection pressure and in spreading their genes into new microbial communities; additionally, it establishes a loop between the environment, animals, and humans. This review contains antibiotics and their resistance mechanisms, diffusion of ARGs, prevention of ARG transmission, tactics involved in microbiome identification, and therapies that aid to minimize infection, which are explored further below. The emergence of ARGs and antibiotic-resistant bacteria (ARB) is an unavoidable threat to global health. The discovery of novel antimicrobial agents derived from natural products shifts the focus from chemical modification of existing antibiotic chemical composition. In the future, metagenomic research could aid in the identification of antimicrobial resistance genes in the environment. Novel therapeutics may reduce infection and the transmission of ARGs.

Epidemiology of Antimicrobial Resistance Genes in Staphyloccocus aureus Isolates from a Public Database in a One Health Perspective-Sample Characteristics and Isolates' Sources

Staphylococcus aureus is considered one of the most widespread bacterial pathogens for both animals and humans, being the causative agent of various diseases like food poisoning, respiratory tract infections, nosocomial bacteremia, and surgical site and cardiovascular infections in humans, as well as clinical and subclinical mastitis, dermatitis, and suppurative infections in animals. Thanks to their genetic flexibility, several virulent and drug-resistant strains have evolved mainly due to horizontal gene transfer and insurgence of point mutations. Infections caused by the colonization of such strains are particularly problematic due to frequently occurring antibiotic resistance, particulary methicillin-resistant S. aureus (MRSA), and are characterized by increased mortality, morbidity, and hospitalization rates compared to those caused by methicillin-sensitive S. aureus (MSSA). S. aureus infections in humans and animals are a prime example of a disease that may be managed by a One Health strategy. In fact, S. aureus is a significant target for control efforts due to its zoonotic potential, the frequency of its illnesses in both humans and animals, and the threat posed by S. aureus antibiotic resistance globally. The results of an epidemiological analysis on a worldwide public database (NCBI Pathogen Detection Isolate Browser; NPDIB) of 35,026 S. aureus isolates were described. We considered the diffusion of antibiotic resistance genes (ARGs), in both human and animal setting, and the results may be considered alarming. The result of this study allowed us to identify the presence of clusters with specific ARG patterns, and that these clusters are associated with different sources of isolation (e.g., human, non-human).

Phenotypic and Genotypic Characterization of Macrolide-Lincosamide-Streptogramin Resistance in Staphylococcus aureus Isolates from Bovine and Human

In this study, penicillin, oxacillin, and macrolide-lincosamide-streptogramin (MLS) resistance in S. aureus strains that were isolated from bovine mastitis cases, and human patients were investigated. Inducible clindamycin resistance (iML) was not found in 30 bovine isolates, while it was detected in 3 (10%) of 30 human isolates. MIC90 values of penicillin, oxacillin and macrolide-lincosamides (ML) were 2, 0.19, >256 µg/ml in bovine isolates and were 3, 3 and 0.19-1.5 µg/ml in human isolates, respectively. Streptogramin resistance was not found in both bovine and human isolates. Although the mecA gene was detected in all of the oxacillin resistant isolates, blaZ gene could not be detected in penicillin resistant isolates. The erm(B) gene was detected in 5 (38.6%) of 13 ML-resistant bovine isolates, and the mph(C) gene was detected in 2 (66.66%) of 3 human isolates. As a result, resistance to penicillin and oxacillin was found to be higher in human S. aureus isolates, while ML resistance was found to be higher in bovine isolates in this investigation. It was concluded that the presence of genes in extra-chromosomal elements associated to penicillin and macrolide resistance should be investigated. The data obtained from this study will contribute to the studies on antimicrobial susceptibility in the field of human and veterinary medicine.

Molecular Mechanisms of Drug Resistance in Staphylococcus aureus

This paper discusses the mechanisms of S. aureus drug resistance including: (1) introduction. (2) resistance to beta-lactam antibiotics, with particular emphasis on the mec genes found in the Staphylococcaceae family, the structure and occurrence of SCCmec cassettes, as well as differences in the presence of some virulence genes and its expression in major epidemiological types and clones of HA-MRSA, CA-MRSA, and LA-MRSA strains. Other mechanisms of resistance to beta-lactam antibiotics will also be discussed, such as mutations in the gdpP gene, BORSA or MODSA phenotypes, as well as resistance to ceftobiprole and ceftaroline. (3) Resistance to glycopeptides (VRSA, VISA, hVISA strains, vancomycin tolerance). (4) Resistance to oxazolidinones (mutational and enzymatic resistance to linezolid). (5) Resistance to MLS-B (macrolides, lincosamides, ketolides, and streptogramin B). (6) Aminoglycosides and spectinomicin, including resistance genes, their regulation and localization (plasmids, transposons, class I integrons, SCCmec), and types and spectrum of enzymes that inactivate aminoglycosides. (7). Fluoroquinolones (8) Tetracyclines, including the mechanisms of active protection of the drug target site and active efflux of the drug from the bacterial cell. (9) Mupirocin. (10) Fusidic acid. (11) Daptomycin. (12) Resistance to other antibiotics and chemioterapeutics (e.g., streptogramins A, quinupristin/dalfopristin, chloramphenicol, rifampicin, fosfomycin, trimethoprim) (13) Molecular epidemiology of MRSA.

Synthesis and biological evaluation of antibacterial activity of novel clarithromycin derivatives incorporating 1,2,3-triazole moieties at the 4''- and 11-OH positions

Bacterial infection is still one of the diseases that threaten human health, and bacterial drug resistance is widespread worldwide. As a result, their eradication now largely relies on antibacterial drug discovery. Here, we reveal a novel approach to the development of 14-membered macrolide antibiotics by describing the design, synthesis, and evaluation of novel clarithromycin derivatives incorporating 1,2,3-triazole moieties at the 4''- and 11-OH positions. Using chemical synthesis, 35 clarithromycin derivatives were prepared, and their antibacterial properties were profiled. We found that compounds 8e-8h, 8l-8o, 8v, and 19d were as potent as azithromycin against Enterococcus faecalis ATCC29212. Furthermore, compounds 8c, 8d, 8n, and 8o showed slightly improved antibacterial activity (2-fold) against Acinetobacter baumannii ATCC19606 when compared with azithromycin and clarithromycin. In addition, compounds 8e, 8f, 8h, 8l, and 8v exhibited excellent antibacterial activity against Staphylococcus aureus ATCC43300, Staphylococcus aureus PR, and Streptococcus pneumoniae ER-2. These compounds were generally 64- to 128-fold more active than azithromycin, and 32- to 128-fold more active than clarithromycin. The results of molecular docking indicated that compound 8f may bind to the nucleotide residue A752 through hydrogen-bonding, hydrophobic, electrostatic, or π-π stacking interactions. The predicted ClogP data suggested that higher values of ClogP (>6.65) enhanced the antibacterial activity of compounds such as 8e, 8f, 8h, 8l, and 8v. The determination of the minimum bactericidal concentration showed that most of the tested compounds were bacteriostatic agents. From this study of bactericidal kinetics, we can conclude that compound 8f had a concentration- and time-dependent effect on the proliferation of Staphylococcus aureus ATCC43300. Finally, the results of the cytotoxicity assay showed that compound 8f exhibited no toxicity at the effective antibacterial concentration.

Increased risks of antibiotic resistant genes (ARGs) induced by chlorine disinfection in the reverse osmosis system for potable reuse of reclaimed water

The reverse osmosis (RO) process has been applied extensively in wastewater reclamation for industrial and potable reuse. To prevent biofouling, chlorine disinfection was usually used in pretreatment. However, this study found that chlorine disinfection could significantly increase risks of antibiotic resistant genes (ARGs) in the RO system. With the increase of chlorine concentration from 0 to 5 mg/L, the accumulative relative abundance of 14 common ARGs in the membrane foulants increased by 49.6%. Among these ARGs, tolC, acrA and acrB (resistance to multiple drugs) showed the highest increament after chlorine disinfection. Especially, the relative abundance of tolC in the group with 5 mg/L chlorine increased by 113.3% compared with the control group. These ARGs tended to be enriched in a few bacterial genus, including Candidatus, Thiomonas, Silanimonas, Xanthomonas and Pseudomonas. These results indicated that the foulants on RO membranes might become a potential sink of ARGs. Considering the possibility of membrane breach, the ARGs may contaminate the permeate and bring great biological risks.

Antimicrobial resistance, mechanisms and its clinical significance

Antimicrobial agents play a key role in controlling and curing infectious disease. Soon after the discovery of the first antibiotic, the challenge of antibiotic resistance commenced. Antimicrobial agents use different mechanisms against bacteria to prevent their pathogenesis and they can be classified as bactericidal or bacteriostatic. Antibiotics are one of the antimicrobial agents which has several classes, each with different targets. Consequently, bacteria are endlessly using methods to overcome the effectivity of the antibiotics by using distinct types of mechanisms. Comprehending the mechanisms of resistance is vital for better understanding and to continue use of current antibiotics. Which also helps to formulate synthetic antimicrobials to overcome the current mechanism of resistance. Also, encourage in prudent use and misuse of antimicrobial agents. Thus, decline in treatment costs and in the rate of morbidity and mortality. This review will be concentrating on the mechanism of actions of several antibiotics and how bacteria develop resistance to them, as well as the method of acquiring the resistance in several bacteria and how can a strain be resistant to several types of antibiotics. This review also analyzes the prevalence, major clinical implications, clinical causes of antibiotic resistance. Further, it evaluates the global burden of antimicrobial resistance, identifies various challenges and strategies in addressing the issue. Finally, put forward certain recommendations to prevent the spread and reduce the rate of resistance growth.

Practical implications of erythromycin resistance gene diversity on surveillance and monitoring of resistance

Use of antibiotics in human and animal medicine has applied selective pressure for the global dissemination of antibiotic-resistant bacteria. Therefore, it is of interest to develop strategies to mitigate the continued amplification and transmission of resistance genes in environmental reservoirs such as farms, hospitals and watersheds. However, the efficacy of mitigation strategies is difficult to evaluate because it is unclear which resistance genes are important to monitor, and which primers to use to detect those genes. Here, we evaluated the diversity of one type of macrolide antibiotic resistance gene (erm) in one type of environment (manure) to determine which primers would be most informative to use in a mitigation study of that environment. We analyzed all known erm genes and assessed the ability of previously published erm primers to detect the diversity. The results showed that all known erm resistance genes group into 66 clusters, and 25 of these clusters (40%) can be targeted with primers found in the literature. These primers can target 74%–85% of the erm gene diversity in the manures analyzed.

Seasonal variation of macrolide resistance gene abundances in the South Fork Iowa River Watershed

The Midwestern United States is dominated by agricultural production with high concentrations of swine, leading to application of swine manure onto lands with artificial subsurface drainage. Previous reports have indicated elevated levels of antibiotic resistance genes (ARGs) in surface water and groundwater around confined animal feeding operations which administer antimicrobials. While previous studies have examined the occurrence of ARGs around confined swine feeding operations, little information is known how their transport from tile-drained fields receiving swine manure application impacts downstream environments. To further our knowledge in this area, water samples were collected from five locations in the agriculturally dominated South Fork Iowa River Watershed with approximately 840,000 swine present in the 76,000ha basin. Samples were collected monthly from three stream sites and two main artificial subsurface drainage outlets. Samples were analyzed for macrolide resistance genes ermB, ermF and 16S rRNAgene abundance using qPCR. Abundance of erm genes ranged from below limits of quantification to >10⁷ copies 100mL^-1 water. Eighty-nine percent of stream water samples contained one of these two ARGs. Results indicate significantly more ermB and ermF in main drainage outlets than stream samples when normalized by 16S rRNA abundance (p<0.0001). Both artificial drainage locations revealed temporal trends for ermB and ermF abundance when normalized to 16S rRNA abundance. The higher resistance gene concentrations identified in artificial drainage samples occurring mid-Spring and late-Fall are likely due to manure application.

Temporal succession of soil antibiotic resistance genes following application of swine, cattle and poultry manures spiked with or without antibiotics

Land application of animal manure is a common agricultural practice potentially leading to dispersal and propagation of antibiotic resistance genes (ARGs) in environmental settings. However, the fate of resistome in agro-ecosystems over time following application of different manure sources has never been compared systematically. Here, soil microcosm incubation was conducted to compare effects of poultry, cattle and swine manures spiked with or without the antibiotic tylosin on the temporal changes of soil ARGs. The high-throughput quantitative PCR detected a total of 185 unique ARGs, with Macrolide-Lincosamide-Streptogramin B resistance as the most frequently encountered ARG type. The diversity and abundance of ARGs significantly increased following application of manure and manure spiked with tylosin, with more pronounced effects observed in the swine and poultry manure treatments than in the cattle manure treatment. The level of antibiotic resistance gradually decreased over time in all manured soils but was still significantly higher in the soils treated with swine and poultry manures than in the untreated soils after 130 days' incubation. Tylosin-amended soils consistently showed higher abundances of ARGs than soils treated with manure only, suggesting a strong selection pressure of antibiotic-spiked manure on soil ARGs. The relative abundance of ARGs had significantly positive correlations with integrase and transposase genes, indicative of horizontal transfer potential of ARGs in manure and tylosin treated soils. Our findings provide evidence that application of swine and poultry manures might enrich more soil ARGs than cattle manure, which necessitates the appropriate treatment of raw animal manures prior to land application to minimise the spread of environmental ARGs.

A Novel erm(44) Gene Variant from a Human Staphylococcus saprophyticus Isolate Confers Resistance to Macrolides and Lincosamides but Not Streptogramins

A novel erm(44) gene variant, erm(44)_v, has been identified by whole-genome sequencing in a Staphylococcus saprophyticus isolate from the skin of a healthy person. It has the particularity to confer resistance to macrolides and lincosamides but not to streptogramin B when expressed in S. aureus The erm(44)_v gene resides on a 19,400-bp genomic island which contains phage-associated proteins and is integrated into the chromosome of S. saprophyticus.

Fate and transport of tylosin-resistant bacteria and macrolide resistance genes in artificially drained agricultural fields receiving swine manure

Application of manure from swine treated with antibiotics introduces antibiotics and antibiotic resistance genes to soil with the potential for further movement in drainage water, which may contribute to the increase in antibiotic resistance in non-agricultural settings. We compared losses of antibiotic-resistant Enterococcus and macrolide-resistance (erm and msrA) genes in water draining from plots with or without swine manure application under chisel plow and no till conditions. Concentrations of ermB, ermC and ermF were all >10(9)copies g(-1) in manure from tylosin-treated swine, and application of this manure resulted in short-term increases in the abundance of these genes in soil. Abundances of ermB, ermC and ermF in manured soil returned to levels identified in non-manured control plots by the spring following manure application. Tillage practices yielded no significant differences (p>0.10) in enterococci or erm gene concentrations in drainage water and were therefore combined for further analysis. While enterococci and tylosin-resistant enterococci concentrations in drainage water showed no effects of manure application, ermB and ermF concentrations in drainage water from manured plots were significantly higher (p<0.01) than concentrations coming from non-manured plots. ErmB and ermF were detected in 78% and 44%, respectively, of water samples draining from plots receiving manure. Although ermC had the highest concentrations of the three genes in drainage water, there was no effect of manure application on ermC abundance. MsrA was not detected in manure, soil or water. This study is the first to report significant increases in abundance of resistance genes in waters draining from agricultural land due to manure application.

Antimicrobial Susceptibility of Invasive Streptococcus pyogenes Isolates in Germany during 2003-2013

A nationwide laboratory-based surveillance study of invasive S. pyogenes infections was conducted in Germany. Invasive isolates (n = 1,281) were obtained between 2003 and 2013. All isolates were susceptible to penicillin, cefotaxime and vancomycin. Tetracycline showed the highest rate of resistant or intermediate resistant isolates with 9.8%, followed by macrolides (4.0%), trimethoprim/sulfamethoxazole (SXT) (1.9%), levofloxacin (1.3%), chloramphenicol (0.9%) and clindamycin (0.7%). The most prominent trends were the appearance of levofloxacin non-susceptible isolates since 2011, and an increase of SXT non-susceptibility since 2012.

Macrolide-resistant Streptococcus pyogenes: prevalence and treatment strategies

Although penicillin remains the first-choice treatment for Streptococcus pyogenes infection, macrolides are important alternatives for allergic patients and lincosamides are recommended together with β-lactams in invasive infections. S. pyogenes may exhibit macrolide resistance because of active efflux (mef genes) or target modification (erm genes), the latter conferring cross resistance to lincosamides and streptogramin B. Worldwide, resistance is restricted to a limited number of genetic lineages, despite resistance genes being encoded on mobile genetic elements. For reasons that are not completely clear, resistance and the associated phenotypes are highly variable across countries. Although resistance remains high in several countries, particularly in Asia, an overall decreasing trend of resistance has been noted in recent years, mostly in Europe. This decrease is not always accompanied by declines in macrolide consumption, suggesting significant roles of other factors in determining the dynamics of macrolide-resistant clones. Continued surveillance is needed to obtain further insights into the forces governing macrolide resistance in S. pyogenes.

High frequency of the 23S rRNA A2058G mutation of Treponema pallidum in Shanghai is associated with a current strategy for the treatment of syphilis

The preferred drugs for the treatment of syphilis, benzathine and procaine penicillin, have not been available in Shanghai for many years, and currently, the incidence of syphilis is increasing. Alternative antibiotics for patients with syphilis during the benzathine and procaine penicillin shortage include macrolides. The failure of macrolide treatment in syphilis patients has been reported in Shanghai, but the reason for this treatment failure remains unclear. We used polymerase chain reaction technology to detect a 23S rRNA A2058G mutation in Treponema pallidum in 109 specimens from syphilis patients. The use of azithromycin/erythromycin in the syphilis patients and the physicians' prescription habits were also assessed based on two questionnaires regarding the use of macrolides. A total of 104 specimens (95.4%) were positive for the A2058G mutation in both copies of the 23S rRNA gene, indicating macrolide resistance. A questionnaire provided to 122 dermatologists showed that during the penicillin shortage, they prescribed erythromycin and azithromycin for 8.24±13.95% and 3.21±6.37% of their patients, respectively, and in the case of penicillin allergy, erythromycin and azithromycin were prescribed 15.24±22.89% and 7.23±16.60% of the time, respectively. A second questionnaire provided to the syphilis patients showed that 150 (33.7%), 106 (23.8%) and 34 (7.6%) individuals had used azithromycin, erythromycin or both, respectively, although the majority did not use the drugs for syphilis treatment. Our findings suggest that macrolide resistance in Treponema pallidum is widespread in Shanghai. More than half of the syphilis patients had a history of macrolide use for other treatment purposes, which may have led to the high prevalence of macrolide resistance. Physicians in China are advised to not use azithromycin for early syphilis.

Macrolide resistance conferred by rRNA mutations in field isolates of Mannheimia haemolytica and Pasteurella multocida

OBJECTIVES

To determine how resistance to macrolides is conferred in field isolates of Pasteurella multocida and Mannheimia haemolytica that lack previously identified resistance determinants for rRNA methylation, efflux and macrolide-modifying enzymes.

METHODS

Isolates of P. multocida and M. haemolytica identified as being highly resistant (MICs >64 mg/L) to the macrolides erythromycin, gamithromycin, tilmicosin, tildipirosin and tulathromycin were screened by multiplex PCR for the previously identified resistance genes erm(42), msr(E) and mph(E). Strains lacking these determinants were analysed by genome sequencing and primer extension on the rRNAs.

RESULTS

Macrolide resistance in one M. haemolytica isolate was conferred by the 23S rRNA mutation A2058G; resistance in three P. multocida isolates were caused by mutations at the neighbouring nucleotide A2059G. In each strain, all six copies of the rrn operons encoded the respective mutations. There were no mutations in the ribosomal protein genes rplD or rplV, and no other macrolide resistance mechanism was evident.

CONCLUSIONS

High-level macrolide resistance can arise from 23S rRNA mutations in P. multocida and M. haemolytica despite their multiple copies of rrn. Selective pressures from exposure to different macrolide or lincosamide drugs presumably resulted in consolidation of either the A2058G or the A2059G mutation.

Evaluation of the automated Vitek 2 system for detection of various mechanisms of macrolide and lincosamide resistance in Staphylococcus aureus

We evaluated the performance of the automated Vitek 2 system against disk diffusion for susceptibility testing of Staphylococcus aureus strains showing various resistance mechanisms to macrolides and lincosamides (ML). The Vitek 2 system showed 100% concordance with the D-zone test in detection of the most common resistance mechanisms to ML, including methylase and efflux systems.

Novel resistance functions uncovered using functional metagenomic investigations of resistance reservoirs

Rates of infection with antibiotic-resistant bacteria have increased precipitously over the past several decades, with far-reaching healthcare and societal costs. Recent evidence has established a link between antibiotic resistance genes in human pathogens and those found in non-pathogenic, commensal, and environmental organisms, prompting deeper investigation of natural and human-associated reservoirs of antibiotic resistance. Functional metagenomic selections, in which shotgun-cloned DNA fragments are selected for their ability to confer survival to an indicator host, have been increasingly applied to the characterization of many antibiotic resistance reservoirs. These experiments have demonstrated that antibiotic resistance genes are highly diverse and widely distributed, many times bearing little to no similarity to known sequences. Through unbiased selections for survival to antibiotic exposure, functional metagenomics can improve annotations by reducing the discovery of false-positive resistance and by allowing for the identification of previously unrecognizable resistance genes. In this review, we summarize the novel resistance functions uncovered using functional metagenomic investigations of natural and human-impacted resistance reservoirs. Examples of novel antibiotic resistance genes include those highly divergent from known sequences, those for which sequence is entirely unable to predict resistance function, bifunctional resistance genes, and those with unconventional, atypical resistance mechanisms. Overcoming antibiotic resistance in the clinic will require a better understanding of existing resistance reservoirs and the dissemination networks that govern horizontal gene exchange, informing best practices to limit the spread of resistance-conferring genes to human pathogens.

Efficacy of in-feed medication with chlortetracycline in a farrow-to-finish herd against a clinical outbreak of respiratory disease in fattening pigs

The efficacy of chlortetracycline (CTC) in-feed medication to treat pigs with clinical respiratory disease was investigated in a farrow-to-finish pig herd infected with Mycoplasma hyopneumoniae, and with clinical respiratory disease in growing pigs. In total, 533 pigs were included. The animals were vaccinated against M hyopneumoniae and porcine circovirus type 2 at weaning. At onset of clinical respiratory disease, they were randomly allocated to one of the following treatment groups: chlortetracycline 1 (CTC1) (two consecutive weeks, 500 ppm), chlortetracycline 2 (CTC2) (two non-consecutive weeks, with a non-medicated week interval in between, 500 ppm) or tylosin (T) (three consecutive weeks, 100 ppm). Performance (daily weight gain, feed conversion ratio), pneumonia lesions at slaughter and clinical parameters (respiratory disease score) were assessed. Only numeric differences in favour of the CTC2 group were obtained for the performance and the clinical parameters. The prevalence of pneumonia lesions was 20.5, 13.1 and 23.0 per cent (P<0.05) for the CTC1, CTC2 and T groups, respectively. The study demonstrated that CTC, when administered at onset of clinical respiratory disease via the feed at a dose of 500 ppm during two alternative weeks, was able to decrease the prevalence of pneumonia lesions, and numerically reduce performance losses and clinical signs.

Multiplex PCR to identify macrolide resistance determinants in Mannheimia haemolytica and Pasteurella multocida

The bacterial pathogens Mannheimia haemolytica and Pasteurella multocida are major etiological agents in respiratory tract infections of cattle. Although these infections can generally be successfully treated with veterinary macrolide antibiotics, a few recent isolates have shown resistance to these drugs. Macrolide resistance in members of the family Pasteurellaceae is conferred by combinations of at least three genes: erm(42), which encodes a monomethyltransferase and confers a type I MLS(B) (macrolide, lincosamide, and streptogramin B) phenotype; msr(E), which encodes a macrolide efflux pump; and mph(E), which encodes a macrolide-inactivating phosphotransferase. Here, we describe a multiplex PCR assay that detects the presence of erm(42), msr(E), and mph(E) and differentiates between these genes. In addition, the assay distinguishes P. multocida from M. haemolytica by amplifying distinctive fragments of the 23S rRNA (rrl) genes. One rrl fragment acts as a general indicator of gammaproteobacterial species and confirms whether the PCR assay has functioned as intended on strains that are negative for erm(42), msr(E), and mph(E). The multiplex system has been tested on more than 40 selected isolates of P. multocida and M. haemolytica and correlated with MICs for the veterinary macrolides tulathromycin and tilmicosin, and the newer compounds gamithromycin and tildipirosin. The multiplex PCR system gives a rapid and robustly accurate determination of macrolide resistance genotypes and bacterial genus, matching results from microbiological methods and whole-genome sequencing.

Mechanism and diversity of the erythromycin esterase family of enzymes

Macrolide antibiotics such as azithromycin and erythromycin are mainstays of modern antibacterial chemotherapy, and like all antibiotics, they are vulnerable to resistance. One mechanism of macrolide resistance is via drug inactivation: enzymatic hydrolysis of the macrolactone ring catalyzed by erythromycin esterases, EreA and EreB. A genomic enzymology approach was taken to gain insight into the catalytic mechanisms and origins of Ere enzymes. Our analysis reveals that erythromycin esterases comprise a separate group in the hydrolase superfamily, which includes homologues of uncharacterized function found on the chromosome of Bacillus cereus, Bcr135 and Bcr136, whose three-dimensional structures have been determined. Biochemical characterization of Bcr136 confirms that it is an esterase that is, however, unable to inactivate macrolides. Using steady-state kinetics, homology-based structure modeling, site-directed mutagenesis, solvent isotope effect studies, pH, and inhibitor profiling performed in various combinations for EreA, EreB, and Bcr136 enzymes, we identified the active site and gained insight into some catalytic features of this novel enzyme superfamily. We rule out the possibility of a Ser/Thr nucleophile and show that one histidine, H46 (EreB numbering), is essential for catalytic function. This residue is proposed to serve as a general base in activation of a water molecule as the reaction nucleophile. Furthermore, we show that EreA, EreB, and Bcr136 are distinct, with only EreA inhibited by chelating agents and hypothesized to contain a noncatalytic metal. Detailed characterization of these esterases allows for a direct comparison of the resistance determinants, EreA and EreB, with their prototype, Bcr136, and for the discussion of their potential connections.

Combinations of macrolide resistance determinants in field isolates of Mannheimia haemolytica and Pasteurella multocida

Respiratory tract infections in cattle are commonly associated with the bacterial pathogens Mannheimia haemolytica and Pasteurella multocida. These infections can generally be successfully treated in the field with one of several groups of antibiotics, including macrolides. A few recent isolates of these species exhibit resistance to veterinary macrolides with phenotypes that fall into three distinct classes. The first class has type I macrolide, lincosamide, and streptogramin B antibiotic resistance and, consistent with this, the 23S rRNA nucleotide A2058 is monomethylated by the enzyme product of the erm(42) gene. The second class shows no lincosamide resistance and lacks erm(42) and concomitant 23S rRNA methylation. Sequencing of the genome of a representative strain from this class, P. multocida 3361, revealed macrolide efflux and phosphotransferase genes [respectively termed msr(E) and mph(E)] that are arranged in tandem and presumably expressed from the same promoter. The third class exhibits the most marked drug phenotype, with high resistance to all of the macrolides tested, and possesses all three resistance determinants. The combinations of erm(42), msr(E), and mph(E) are chromosomally encoded and intermingled with other exogenous genes, many of which appear to have been transferred from other members of the Pasteurellaceae. The presence of some of the exogenous genes explains recent reports of resistance to additional drug classes. We have expressed recombinant versions of the erm(42), msr(E), and mph(E) genes within an isogenic Escherichia coli background to assess their individually contributions to resistance. Our findings indicate what types of compounds might have driven the selection for these resistance determinants.

Antimicrobial characterisation of solithromycin (CEM-101), a novel fluoroketolide: activity against staphylococci and enterococci

Solithromycin (CEM-101) is a novel fluoroketolide with high potency against Gram-positive and Gram-negative bacteria commonly associated with community-acquired respiratory tract infections and skin and skin-structure infections. In this study, solithromycin and comparator antimicrobials were tested against a contemporary collection of Staphylococcus aureus, coagulase-negative staphylococci, Enterococcus faecalis, Enterococcus faecium and other Enterococcus spp. collected in the SENTRY Antimicrobial Surveillance Program. Solithromycin was active against S. aureus [minimum inhibitory concentration for 50% of the organisms (MIC(50))=0.12 μg/mL] and was two-fold more active than telithromycin (MIC(50)=0.25 μg/mL). Solithromycin was more potent against methicillin (oxacillin)-susceptible S. aureus [MIC(50)=0.06 μg/mL and MIC for 90% of the organisms (MIC(90))=0.12 μg/mL) compared with methicillin (oxacillin)-resistant S. aureus (MIC(50)=0.12 μg/mL and MIC(90)>16 μg/mL). Solithromycin activity was reduced amongst heterogeneous vancomycin-intermediate S. aureus and vancomycin-resistant S. aureus (MIC(50)>16 μg/mL). Against strains with defined susceptibilities to erythromycin, clindamycin and telithromycin, solithromycin showed potent inhibition against all combinations (MIC(50)=0.06 μg/mL) except those with non-susceptibility to telithromycin (>2 μg/mL) (MIC(50)>16 μg/mL). The solithromycin MIC(50) for E. faecium (1 μg/mL) was four-fold higher than the MIC(50) for E. faecalis (0.25 μg/mL). In summary, solithromycin demonstrated high potency against many Staphylococcus and Enterococcus spp. isolated from contemporary infections worldwide.

A novel Erm monomethyltransferase in antibiotic-resistant isolates of Mannheimia haemolytica and Pasteurella multocida

Mannheimia haemolytica and Pasteurella multocida are aetiological agents commonly associated with respiratory tract infections in cattle. Recent isolates of these pathogens have been shown to be resistant to macrolides and other ribosome-targeting antibiotics. Direct analysis of the 23S rRNAs by mass spectrometry revealed that nucleotide A2058 is monomethylated, consistent with a Type I erm phenotype conferring macrolide-lincosamide resistance. The erm resistance determinant was identified by full genome sequencing of isolates. The sequence of this resistance determinant, now termed erm(42), has diverged greatly from all previously characterized erm genes, explaining why it has remained undetected in PCR screening surveys. The sequence of erm(42) is, however, completely conserved in six independent M. haemolytica and P. multocida isolates, suggesting relatively recent gene transfer between these species. Furthermore, the composition of neighbouring chromosomal sequences indicates that erm(42) was acquired from other members of the Pasteurellaceae. Expression of recombinant erm(42) in Escherichia coli demonstrated that the enzyme retains its properties as a monomethyltransferase without any dimethyltransferase activity. Erm(42) is a novel addition to the Erm family: it is phylogenetically distant from the other Erm family members and it is unique in being a bona fide monomethyltransferase that is disseminated between bacterial pathogens.

Prevalence of resistance phenotypes and genotypes to macrolide, lincosamide and streptogramin antibiotics in Gram-positive cocci isolated in Tunisian Bone Marrow Transplant Center

To investigate the prevalence of resistance to macrolide, lincosamide and streptogramin (MLS) antibiotics in Gram-positive cocci isolated in a Bone Marrow Transplant Center of Tunisia, we tested the antibiotic susceptibility of 172 clinical isolates of Staphylococcus epidermidis, Streptococcus mitis and Enterococcus faecium to macrolide erythromycin and spiramycin, the lincosamide clindamycin and the streptogramin pristinamycin. These three groups of organisms were mostly resistant to macrolides and lincosamide, but were commonly susceptible to pristinamycin. The resistance phenotypes of erythromycin-resistant isolates were determined by the five-disc test with erythromycin, spiramycin, lincomycin, clindamycin and pristinamycin, which showed that most exhibited constitutive MLS resistance. In order to determine the prevalence of the resistance genotypes and the resistance mechanisms, the prevalence of the erythromycin resistance methylase (erm) (A), erm(B), erm(C), msr(A) and macrolide efflux (mef) (A) genes in the erythromycin-resistant isolates was identified by polymerase chain reaction (PCR) analysis. The resistance was due mainly to the presence of ermB in E. faecium (80%), ermC in S. epidermidis (53%) and mefA in S. mitis (65%).

A bioinformatic approach to understanding antibiotic resistance in intracellular bacteria through whole genome analysis

Intracellular bacteria survive within eukaryotic host cells and are difficult to kill with certain antibiotics. As a result, antibiotic resistance in intracellular bacteria is becoming commonplace in healthcare institutions. Owing to the lack of methods available for transforming these bacteria, we evaluated the mechanisms of resistance using molecular methods and in silico genome analysis. The objective of this review was to understand the molecular mechanisms of antibiotic resistance through in silico comparisons of the genomes of obligate and facultative intracellular bacteria. The available data on in vitro mutants reported for intracellular bacteria were also reviewed. These genomic data were analysed to find natural mutations in known target genes involved in antibiotic resistance and to look for the presence or absence of different resistance determinants. Our analysis revealed the presence of tetracycline resistance protein (Tet) in Bartonella quintana, Francisella tularensis and Brucella ovis; moreover, most of the Francisella strains possessed the blaA gene, AmpG protein and metallo-beta-lactamase family protein. The presence or absence of folP (dihydropteroate synthase) and folA (dihydrofolate reductase) genes in the genome could explain natural resistance to co-trimoxazole. Finally, multiple genes encoding different efflux pumps were studied. This in silico approach was an effective method for understanding the mechanisms of antibiotic resistance in intracellular bacteria. The whole genome sequence analysis will help to predict several important phenotypic characteristics, in particular resistance to different antibiotics. In the future, stable mutants should be obtained through transformation methods in order to demonstrate experimentally the determinants of resistance in intracellular bacteria.

Molecular Mechanisms of Antibacterial Multidrug Resistance

Treatment of infections is compromised worldwide by the emergence of bacteria that are resistant to multiple antibiotics. Although classically attributed to chromosomal mutations, resistance is most commonly associated with extrachromosomal elements acquired from other bacteria in the environment. These include different types of mobile DNA segments, such as plasmids, transposons, and integrons. However, intrinsic mechanisms not commonly specified by mobile elements-such as efflux pumps that expel multiple kinds of antibiotics-are now recognized as major contributors to multidrug resistance in bacteria. Once established, multidrug-resistant organisms persist and spread worldwide, causing clinical failures in the treatment of infections and public health crises.

Synthesis and antibacterial activity of 6-O-heteroarylcarbamoyl-11,12-lactoketolides

A new series of erythromycin A derivatives, the 6-O-heteroarylcarbamoyl-11,12-lactoketolides, with activity against macrolide-resistant streptococci, are described. Structurally, these macrolide antibiotics are characterized by a heteroaryl side chain attached to the macrolactone core through a carbamate linkage at the C6 position, as well as 11,12-gamma-lactone and 3-keto functionalities. The synthesis and antibacterial activity of this new series of ketolides are discussed.

Multicenter study of the mechanisms of resistance and clonal relationships of Streptococcus agalactiae isolates resistant to macrolides, lincosamides, and ketolides in Spain

Macrolide, lincosamide, and ketolide mechanisms of resistance and clonal relationships were characterized in a collection of 79 resistant group B streptococcus isolates obtained from neonates or pregnant women. The erm(B), erm(TR), and mef(A) genes were present in 62%, 30.4%, and 3.8% of the isolates, respectively. There was considerable clonal diversity among them.

Synthesis, Characterization and In Vitro Antimicrobial Activity of Novel Sulfonylureas of 15-Membered Azalides

Three series of the novel sulfonylurea derivatives of 15-membered azalides, i.e. 9a-N-[N'-(aryl)sulfonylcarbamoyl] (4a-4f, 5a-5f), 9a-N-{N'-[(aryl)sulfonylcarbamoyl-gamma-aminopropyl]} (10a-10f, 11a, 11c) and 9a-N-{N'-(beta-cyanoethyl)-N'-[(aryl)sulfonylcarabamoyl-gamma-aminopropyl]} (14a-14f, 15a, 15b, 15f) derivatives of 9-deoxo-9-dihydro-9a-aza-9a-homoerythromycin A (2) and 5-O-desosaminyl-9-deoxo-9-dihydro-9a-aza-9a-homoerythronolide A (3) were prepared and their structures elucidated by NMR and IR spectroscopic methods and mass spectrometry. Minimal inhibitory concentration (MIC) of these compounds was determined on a panel of sensitive and resistant Gram-positive and Gram-negative bacterial strains. Several compounds of the series of 9a-N-[N'-(aryl)sulfonylcarbamoyl] derivatives that showed significant improvements in activity against inducible resistant Streptococcus pyogenes strain were suggested for further optimization.

Characterization and molecular analysis of macrolide-resistant Mycoplasma pneumoniae clinical isolates obtained in Japan

In recent years, Mycoplasma pneumoniae strains that are clinically resistant to macrolide antibiotics have occasionally been encountered in Japan. Of 76 strains of M. pneumoniae isolated in three different areas in Japan during 2000 to 2003, 13 strains were erythromycin (ERY) resistant. Of these 13 strains, 12 were highly ERY resistant (MIC, > or =256 microg/ml) and 1 was weakly resistant (MIC, 8 microg/ml). Nucleotide sequencing of domains II and V of 23S rRNA and ribosomal proteins L4 and L22, which are associated with ERY resistance, showed that 10 strains had an A-to-G transition at position 2063 (corresponding to 2058 in Escherichia coli numbering), 1 strain showed A-to-C transversion at position 2063, 1 strain showed an A-to-G transition at position 2064, and the weakly ERY-resistant strain showed C-to-G transversion at position 2617 (corresponding to 2611 in E. coli numbering) of domain V. Domain II and ribosomal proteins L4 and L22 were not involved in the ERY resistance of these clinical M. pneumoniae strains. In addition, by using our established restriction fragment length polymorphism technique to detect point mutations of PCR products for domain V of the 23S rRNA gene of M. pneumoniae, we found that 23 (24%) of 94 PCR-positive oral samples taken from children with respiratory infections showed A2063G mutation. These results suggest that ERY-resistant M. pneumoniae infection is not unusual in Japan.

The Tylosin-resistance Methyltransferase RlmAII (TlrB) Modifies the N-1 Position of 23S rRNA Nucleotide G748

The methyltransferase RlmA(II) (TlrB) confers resistance to the macrolide antibiotic tylosin in the drug-producing strain Streptomyces fradiae. The resistance conferred by RlmA(II) is highly specific for tylosin, and no resistance is conferred to other macrolide drugs, or to lincosamide and streptogramin B (MLS(B)) drugs that bind to the same region on the bacterial ribosome. In this study, the methylation site of RlmA(II) is identified unambiguously by liquid chromatography/electrospray ionization mass spectrometry as the N-1 position of 23S rRNA nucleotide G748. This position is contacted by the mycinose sugar moiety of tylosin, which is absent from the other drugs. The selective resistance to tylosin conferred by m(1)G748 illustrates how differences in drug structure facilitate the drug fit at the MLS(B)-binding site. This observation is of relevance for the rational design of novel antimicrobials targeting the MLS(B) site, especially if the antimicrobials are to be used against pathogens possessing m(1)G748.

Molecular basis of intrinsic macrolide resistance in the Mycobacterium tuberculosis complex

The intrinsic resistance of the Mycobacterium tuberculosis complex (MTC) to most antibiotics, including macrolides, is generally attributed to the low permeability of the mycobacterial cell wall. However, nontuberculous mycobacteria (NTM) are much more sensitive to macrolides than members of the MTC. A search for macrolide resistance determinants within the genome of M. tuberculosis revealed the presence of a sequence encoding a putative rRNA methyltransferase. The deduced protein is similar to Erm methyltransferases, which confer macrolide-lincosamide-streptogramin (MLS) resistance by methylation of 23S rRNA, and was named ErmMT. The corresponding gene, ermMT (erm37), is present in all members of the MTC but is absent in NTM species. Part of ermMT is deleted in some vaccine strains of Mycobacterium bovis BCG, such as the Pasteur strain, which lack the RD2 region. The Pasteur strain was susceptible to MLS antibiotics, whereas MTC species harboring the RD2 region were resistant to them. The expression of ermMT in the macrolide-sensitive Mycobacterium smegmatis and BCG Pasteur conferred MLS resistance. The resistance patterns and ribosomal affinity for erythromycin of Mycobacterium host strains expressing ermMT, srmA (monomethyltransferase from Streptomyces ambofaciens), and ermE (dimethyltransferase from Saccharopolyspora erythraea) were compared, and the ones conferred by ErmMT were similar to those conferred by SrmA, corresponding to the MLS type I phenotype. These results suggest that ermMT plays a major role in the intrinsic macrolide resistance of members of the MTC and could be the first example of a gene conferring resistance by target modification in mycobacteria.

Characteristic expression of three genes, msr(A), mph(C) and erm(Y), that confer resistance to macrolide antibiotics on Staphylococcus aureus

We have reported that the gene mph(C) (formally referred to as 'mphBM') is located on plasmid pMS97 342 bp downstream of the msr(A) gene. msr(A) specifies resistance to macrolides by ABC-transporter-mediated efflux, and mph(C) has 49% identity to the amino acid sequence of MPH(2')II, which encodes a phosphotransferase that inactivates some macrolide antibiotics. A strain of Staphylococcus aureus NCTC8325 containing plasmid pMS97 inactivated unlabeled and (14)C-labeled erythromycin when tested by bioautographic and radioautographic techniques. In addition to erythromycin, other 14-membered ring macrolides (except for ketolides), 15-membered ring macrolides and 16-membered ring macrolides, mycinamicin, rosamicin and YM133, were inactivated by the strain. Erythromycin inactivation products produced by the strain carrying pMS97 were completely different from those produced by Escherichia coli BM694 bearing plasmid pAT63, which contains the ereA gene encoding an esterase that hydrolyzes macrolide lactones. Constructs formed with the msr(A) and mph(C) genes, and with the msr(A), mph(C) and erm(Y) genes, showed erythromycin-inactivating activity, but another construct built with the mph(C) gene alone failed to show such activity. This result suggests that any region of the msr(A) gene is needed for the expression of mph(C).

Structural basis for the antibiotic activity of ketolides and azalides

The azalide azithromycin and the ketolide ABT-773, which were derived by chemical modifications of erythromycin, exhibit elevated activity against a number of penicillin- and macrolide-resistant pathogenic bacteria. Analysis of the crystal structures of the large ribosomal subunit from Deinococcus radiodurans complexed with azithromycin or ABT-773 indicates that, despite differences in the number and nature of their contacts with the ribosome, both compounds exert their antimicrobial activity by blocking the protein exit tunnel. In contrast to all macrolides studied so far, two molecules of azithromycin bind simultaneously to the tunnel. The additional molecule also interacts with two proteins, L4 and L22, implicated in macrolide resistance. These studies illuminated and rationalized the enhanced activity of the drugs against specific macrolide-resistant bacteria.

The ketolides: a critical review

Ketolides are a new class of macrolides designed particularly to combat respiratory tract pathogens that have acquired resistance to macrolides. The ketolides are semi-synthetic derivatives of the 14-membered macrolide erythromycin A, and retain the erythromycin macrolactone ring structure as well as the D-desosamine sugar attached at position 5. The defining characteristic of the ketolides is the removal of the neutral sugar, L-cladinose from the 3 position of the ring and the subsequent oxidation of the 3-hydroxyl to a 3-keto functional group. The ketolides presently under development additionally contain an 11, 12 cyclic carbamate linkage in place of the two hydroxyl groups of erythromycin A and an arylalkyl or an arylallyl chain, imparting in vitro activity equal to or better than the newer macrolides. Telithromycin is the first member of this new class to be approved for clinical use, while ABT-773 is presently in phase III of development. Ketolides have a mechanism of action very similar to erythromycin A from which they have been derived. They potently inhibit protein synthesis by interacting close to the peptidyl transferase site of the bacterial 50S ribosomal subunit. Ketolides bind to ribosomes with higher affinity than macrolides. The ketolides exhibit good activity against Gram-positive aerobes and some Gram-negative aerobes, and have excellent activity against drug-resistant Streptococcus pneumoniae, including macrolide-resistant (mefA and ermB strains of S. pneumoniae). Ketolides such as telithromycin display excellent pharmacokinetics allowing once daily dose administration and extensive tissue distribution relative to serum. Evidence suggests the ketolides are primarily metabolised in the liver and that elimination is by a combination of biliary, hepatic and urinary excretion. Pharmacodynamically, ketolides display an element of concentration dependent killing unlike macrolides which are considered time dependent killers. Clinical trial data are only available for telithromycin and have focused on respiratory infections including community-acquired pneumonia, acute exacerbations of chronic bronchitis, sinusitis and streptococcal pharyngitis. Bacteriological and clinical cure rates have been similar to comparators. Limited data suggest very good eradication of macrolide-resistant and penicillin-resistant S. pneumoniae. As a class, the macrolides are well tolerated and can be used safely. Limited clinical trial data suggest that ketolides have similar safety profiles to the newer macrolides. Telithromycin interacts with the cytochrome P450 enzyme system (specifically CYP 3A4) in a reversible fashion and limited clinically significant drug interactions occur. In summary, clinical trials support the clinical efficacy of the ketolides in upper and lower respiratory tract infections caused by typical and atypical pathogens including strains resistant to penicillins and macrolides. Considerations such as local epidemiology, patterns of resistance and ketolide adverse effects, drug interactions and cost relative to existing agents will define the role of these agents. The addition of the ketolides in the era of antibacterial resistance provides clinicians with more options in the treatment of respiratory infections.

In vitro development of resistance to enrofloxacin, erythromycin, tylosin, tiamulin and oxytetracycline in Mycoplasma gallisepticum, Mycoplasma iowae and Mycoplasma synoviae

The in vitro emergence of resistance to enrofloxacin, erythromycin, tylosin, tiamulin, and oxytetracycline in three avian Mycoplasma species, Mycoplasma gallisepticum, Mycoplasma synoviae and Mycoplasma iowae was studied. Mutants were selected stepwise and their MICs were determined after 10 passages in subinhibitory concentrations of antibiotic. High-level resistance to erythromycin and tylosin developed within 2-6 passages in the three Mycoplasma species. Resistance to enrofloxacin developed more gradually. No resistance to tiamulin or oxytetracycline could be evidenced in M. gallisepticum or M. synoviae after 10 passages whereas, resistant mutants were obtained with M. iowae. Cross-sensitivity tests performed on mutants demonstrated that mycoplasmas made resistant to tylosin were also resistant to erythromycin, whereas mutants made resistant to erythromycin were not always resistant to tylosin. Some M. iowae tiamulin-resistant mutants were also resistant to both macrolide antibiotics. Enrofloxacin and oxytetracycline did not induce any cross-resistance to the other antibiotics tested. These results show that Mycoplasma resistance to macrolides can be quickly selected in vitro, and thus, providing that similar results could be obtained under field conditions, that development of resistance to these antibiotics in vivo might also be a relatively frequent event.

Activity of the ketolide telithromycin is refractory to Erm monomethylation of bacterial rRNA

Methylation of specific nucleotides in rRNA is one of the means by which bacteria achieve resistance to macrolides-lincosamides-streptogramin B (MLS(B)) and ketolide antibiotics. The degree of resistance is determined by how effectively the rRNA is methylated. We have implemented a bacterial system in which the rRNA methylations are defined, and in this study we investigate what effect Erm mono- and dimethylation of the rRNA has on the activity of representative MLS(B) and ketolide antibiotics. In the test system, >80% of the rRNA molecules are monomethylated by ErmN (TlrD) or dimethylated by ErmE. ErmE dimethylation confers high resistance to all the MLS(B) and ketolide drugs. ErmN monomethylation predictably confers high resistance to the lincosamides clindamycin and lincomycin, intermediate resistance to the macrolides clarithromycin and erythromycin, and low resistance to the streptogramin B pristinamycin IA. In contrast to the macrolides, monomethylation only mildly affects the antimicrobial activities of the ketolides HMR 3647 (telithromycin) and HMR 3004, and these drugs remain 16 to 250 times as potent as clarithromycin and erythromycin. These differences in the macrolide and ketolide activities could explain the recent reports of variation in the MICs of telithromycin for streptococcal strains that have constitutive erm MLS(B) resistance and are highly resistant to erythromycin.

Resistance mechanisms of gram-positive bacteria

The introduction and increasing use of antibiotics for antibacterial therapy has initiated a rapid development and expansion of antibiotic resistance in microorganisms, particularly in human pathogens. Additionally, a shift to an increase in number and severity of Gram-positive infections has been observed the last decades. Common to these pathogens is their tendency to accumulate multiple resistances under antibiotic pressure and selection. Methicillin-resistant Staphylococcus aureus (MRSA), that have acquired multiresistance to all classes of antibiotics, have become a serious nosocomial problem. Recently, the emergence of the first MRSA with reduced vancomycin susceptibility evoked the specter of a totally resistant S. aureus. Problems with multiresistance expand also to penicillin-resistant Streptococcus pneumoniae that are partially or totally resistant to multiple antibiotics, and to vancomycin-resistant Enterococcus ssp., completely resistant to all commonly used antibiotics. The rapid development of resistance is due to mutational events and/or gene transfer and acquisition of resistance determinants, allowing strains to survive antibiotic treatment.

[Interpretive reading of the antibiogram: Intellectual exercise or clinical need?]

Clinical categorisation of susceptibility testing results according to criteria established by different committees is daily performed in clinical microbiology laboratories. By this process clinicians can predict the therapeutic success of antimicrobial treatment in patients infected with susceptible microorganisms. In addition, microbiology laboratories that include a suitable number of antimicrobial agents in susceptibility tests can perform interpretive reading of the antibiogram. With this approach, resistance phenotypes are recognized and allow microbiologist: a) detection of mechanisms of resistance, including low levels of expression; b) modification of clinical classifications that are inconsistent with the inferred resistance mechanism; and c) inference of susceptibility values for antimicrobials that are not included in the antibiogram. In the laboratory, this approach facilitates quality control and validation of susceptibility results. Moreover, it increases the value of the results obtained because new mechanisms of resistance can be characterized and the epidemiology of resistance can be established. From the clinical point of view, this approach contributes to improving the adequacy of treatment (since it is useful for predicting therapeutic failure with the use of antimicrobials in patients with infections due to resistant microorganisms) and to controlling and defining antimicrobial policies. Despite the growing complexity of resistance mechanisms, which makes interpretative reading of the antibiogram difficult, this process should be incorporated into routine practice in microbiology laboratories. Interpretive reading of antibiograms is clinically necessary and not simply a intellectual exercise.

Ketolides in the treatment of respiratory infections

The ketolides are a new class of macrolides specifically designed to combat respiratory tract pathogens that have acquired resistance to macrolides. The ketolides are semi-synthetic derivatives of the 14-membered macrolide erythromycin A. There are currently two ketolides in the late stages of clinical development in the US (telithromycin [HMR-364, Kelek; Aventis] and ABT-773 [Abbot Laboratories]), as well as newer compounds in earlier stages of testing. Ketolides have a mechanism of action very similar to that of erythromycin A. They potently inhibit protein synthesis by interacting close to the peptidyl transferase site of the bacterial 50S ribosomal subunit. Ketolides bind to ribosomes with higher affinity than macrolides. The ketolides exhibit good activity against Gram-positive and some Gram-negative aerobes and have are active against macrolide-resistant Streptococcus species, including most mef A and erm B strains of Streptococcus pneumoniae. Ketolides have pharmacokinetics which allow once-daily dosing and extensive tissue distribution with very high uptake into respiratory tissues and fluids relative to serum. Evidence suggests the ketolides are primarily metabolised by the cytochrome P450 (CYP) enzyme system in the liver and that elimination is a combination of biliary, hepatic and urinary excretion. Clinical trial data are only available for telithromycin and have focused on respiratory tract infections (RTIs) including community-acquired pneumonia (CAP), acute exacerbations of chronic bronchitis (AECB), sinusitis and streptococcal pharyngitis. Bacteriological and clinical cure rates have been similar to comparators. Ketolides have similar safety profiles to the newer macrolides. In summary, early clinical trials support the clinical efficacy of the ketolides in common RTIs, including activity against macrolide-resistant pathogens.