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

Acquired inducible antimicrobial resistance in Gram-positive bacteria

A major contributor to the emergence of antibiotic resistance in Gram-positive bacterial pathogens is the expansion of acquired, inducible genetic elements. Although acquired, inducible antibiotic resistance is not new, the interest in its molecular basis has been accelerated by the widening distribution and often 'silent' spread of the elements responsible, the diagnostic challenges of such resistance and the mounting limitations of available agents to treat Gram-positive infections. Acquired, inducible antibiotic resistance elements belong to the accessory genome of a species and are horizontally acquired by transformation/recombination or through the transfer of mobile DNA elements. The two key, but mechanistically very different, induction mechanisms are: ribosome-sensed induction, characteristic of the macrolide-lincosamide-streptogramin B antibiotics and tetracycline resistance, leading to ribosomal modifications or efflux pump activation; and resistance by cell surface-associated sensing of β-lactams (e.g., oxacillin), glycopeptides (e.g., vancomycin) and the polypeptide bacitracin, leading to drug inactivation or resistance due to cell wall alterations.

Induction of efflux-mediated macrolide resistance in Streptococcus pneumoniae

The antimicrobial efflux system encoded by the operon mef(E)-mel on the mobile genetic element MEGA in Streptococcus pneumoniae and other Gram-positive bacteria is inducible by macrolide antibiotics and antimicrobial peptides. Induction may affect the clinical response to the use of macrolides. We developed mef(E) reporter constructs and a disk diffusion induction and resistance assay to determine the kinetics and basis of mef(E)-mel induction. Induction occurred rapidly, with a >15-fold increase in transcription within 1 h of exposure to subinhibitory concentrations of erythromycin. A spectrum of environmental conditions, including competence and nonmacrolide antibiotics with distinct cellular targets, did not induce mef(E). Using 16 different structurally defined macrolides, induction was correlated with the amino sugar attached to C-5 of the macrolide lactone ring, not with the size (e.g., 14-, 15- or 16-member) of the ring or with the presence of the neutral sugar cladinose at C-3. Macrolides with a monosaccharide attached to C-5, known to block exit of the nascent peptide from the ribosome after the incorporation of up to eight amino acids, induced mef(E) expression. Macrolides with a C-5 disaccharide, which extends the macrolide into the ribosomal exit tunnel, disrupting peptidyl transferase activity, did not induce it. The induction of mef(E) did not require macrolide efflux, but the affinity of macrolides for the ribosome determined the availability for efflux and pneumococcal susceptibility. The induction of mef(E)-mel expression by inducing macrolides appears to be based on specific interactions of the macrolide C-5 saccharide with the ribosome that alleviate transcriptional attenuation of mef(E)-mel.

Efflux pumps in bacteria: overview, clinical relevance, and potential pharmaceutical target

Trends in microbial resistance suggest a dramatic increase in the frequency of reports of multi-drug efflux pumps in bacteria and fungi. Although it is difficult to determine whether this increase is due to the increased attention given to this resistance mechanism, or an increase in frequency, efflux pumps are becoming an important consideration in resistance emergence. These efflux pumps comprise at least four different classes in Gram-positive and Gram-negative bacteria, as well as in Streptomyces and fungi. As more efflux pumps are characterised and studied, both biochemically and structurally, the opportunity for intervention may arise.

In vitro activities of new ketolide, telithromycin, and eight other macrolide antibiotics against Streptococcus pneumoniae having mefA and ermB genes that mediate macrolide resistance

The comparative in vitro activity of a new ketolide, telithromycin (TEL), and eight other macrolide-lincosamide antibiotics (MLS) against 215 strains, of Streptococcus pneumoniae including penicillin-resistant isolates (PRSP), was determined by the agar dilution method. These strains were isolated from patients with pneumonia, otitis media, and purulent meningitis between 1995 and 1997. Two genes, mefA and ermB, that encode MLS resistance in the strains were identified by polymerase chain reaction (PCR). Of the strains, 30.2% (n = 65) had the mefA gene, 37.7% (n = 81) had the ermB gene, and 1.4% (n = 3) had both resistant genes. The minimum inhibitory concentration (MIC90s) of TEL and 16-membered ring MLS for strains having the mefA gene were 0.063-0.25 microg/ml, which were the same level as those for MLS-susceptible strains. On the other hand, the strains with the mefA gene showed low-level resistance to 14- and 15-membered ring MLS, with MIC90s ranging from 1 to 4 microg/ml. Only the MIC90 of TEL at 0.5 microg/ml, for strains having the ermB gene was superior to that of the 14-, 15-, and 16-membered ring MLS (MIC90, > or =64 microg/ml). TEL also showed excellent activity against PRSP having abnormal pbp1a, pbp2x, and pbp2b genes. Most strains having the mefA and ermB genes were serotyped to 3, 6, 14, 19, and 23. These results suggest that TEL may be a useful chemotherapeutic agent for respiratory tract infections caused by S. pneumoniae.

Macrolide resistance mechanisms in Gram-positive cocci

Two principal mechanisms of resistance to macrolides have been identified in Gram-positive bacteria. Erythromycin-resistant methylase is encoded by erm genes. Resultant structural changes to rRNA prevent macrolide binding and allow synthesis of bacterial proteins to continue. Presence of the erm gene results in high-level resistance. Modification of the mechanism whereby antibiotics are eliminated from the bacteria also brings about resistance. Bacteria carrying the gene encoding macrolide efflux (i.e. the mefE gene) display relatively low-level resistance. Azithromycin, because of its ability to achieve concentrations at sites of infections, is capable of eradicating mefE-carrying strains. Other resistance mechanisms, involving stimulation of enzymatic degradation, appear not to be clinically significant.

The effect of antibiotics on the production of superantigen from Staphylococcus aureus isolated from atopic dermatitis

Staphylococcus aureus (S. aureus) often colonizes on the skin of patients with atopic dermatitis. It is known that superantigens which are staphylococcal enterotoxins can activate T cells without processing by antigen presenting cells. It has been suggested that activated T cells release various cytokines which may exacerbate or prolong the cutaneous inflammation associated with atopic dermatitis. Reduction of bacterial colonization from skin lesions has been reported to be effective in the treatment of atopic dermatitis. Therefore, antimicrobial therapy using antibiotics may be a treatment option for atopic dermatitis in selected patients. We examined the effect of antibiotics on the production of superantigen from S. aureus in vitro to determine the action mechanism of antibiotics in the treatment of atopic dermatitis. It was found that antibiotics with inhibitory effect on protein synthesis can suppress the production of superantigen. On the other hand, the superantigen production was not suppressed by antibiotics having either the inhibitory effect on cell wall synthesis or on nucleic acid synthesis. Levels of the suppressive effect on superantigen production by S. aureus varied with strains tested in this study. Moreover, we demonstrated that replication of DNA coding of superantigen produced by S. aureus was suppressed only by roxithromycin (ROX), which is a new macrolide. This finding suggests that ROX may have an effect at the gene level. These results suggested that the suppressive effects of antimicrobial agents that act as inhibitors of protein synthesis on superantigen production from S. aureus may be useful in the treatment of atopic dermatitis.

Mechanisms of bacterial resistance to macrolide antibiotics

Macrolides have been used in the treatment of infectious diseases since the late 1950s. Since that time, a finding of antagonistic action between erythromycin and spiramycin in clinical isolates1 led to evidence of the biochemical mechanism and to the current understanding of inducible or constitutive resistance to macrolides mediated by erm genes containing, respectively, the functional regulation mechanism or constitutively mutated regulatory region. These resistant mechanisms to macrolides are recognized in clinically isolated bacteria. (1) A methylase encoded by the erm gene can transform an adenine residue at 2058 (Escherichia coli equivalent) position of 23S rRNA into an 6N, 6N-dimethyladenine. Position 2058 is known to reside either in peptidyltransferase or in the vicinity of the enzyme region of domain V. Dimethylation renders the ribosome resistant to macrolides (MLS). Moreover, another finding adduced as evidence is that a mutation in the domain plays an important role in MLS resistance: one of several mutations (transition and transversion) such as A2058G, A2058C or U, and A2059G, is usually associated with MLS resistance in a few genera of bacteria. (2) M (macrolide antibiotics)- and MS (macrolide and streptogramin type B antibiotics)- or PMS (partial macrolide and streptogramin type B antibiotics)-phenotype resistant bacteria cause decreased accumulation of macrolides, occasionally including streptogramin type B antibiotics. The decreased accumulation, probably via enhanced efflux, is usually inferred from two findings: (i) the extent of the accumulated drug in a resistant cell increases as much as that in a susceptible cell in the presence of an uncoupling agent such as carbonylcyanide-m-chlorophenylhydrazone (CCCP), 2,4-dinitrophenol (DNP), and arsenate; (ii) transporter proteins, in M-type resistants, have mutual similarity to the 12-transmembrane domain present in efflux protein driven by proton-motive force, and in MS- or PMS-type resistants, transporter proteins have mutual homology to one or two ATP-binding segments in efflux protein driven by ATP. (3) Two major macrolide mechanisms based on antibiotic inactivation are dealt with here: degradation due to hydrolysis of the macrolide lactone ring by an esterase encoded by the ere gene; and modification due to macrolide phosphorylation and lincosamide nucleotidylation mediated by the mph and lin genes, respectively. But enzymatic mechanisms that hydrolyze or modify macrolide and lincosamide antibiotics appear to be relatively rare in clinically isolated bacteria at present. (4) Important developments in macrolide antibiotics are briefly featured. On the basis of information obtained from extensive references and studies of resistance mechanisms to macrolide antibiotics, the mode of action of the drugs, as effectors, and a hypothetical explanation of the regulation of the mechanism with regard to induction of macrolide resistance are discussed.

Streptococcus pneumoniae and Streptococcus pyogenes resistant to macrolides but sensitive to clindamycin: a common resistance pattern mediated by an efflux system

Macrolide-resistant Streptococcus pyogenes isolates from Finland, Australia, and the United Kingdom and, more recently, Streptococcus pneumoniae and S. pyogenes strains from the United States were shown to have an unusual resistance pattern to macrolides, lincosamides, and streptogramin B antibiotics. This pattern, referred to as M resistance, consists of susceptibility to clindamycin and streptogramin B antibiotics but resistance to 14- and 15-membered macrolides. An evaluation of the macrolide-lincosamide-streptogramin B resistance phenotypes among our streptococcal strains collected from 1993 to 1995 suggested that this unusual resistance pattern is not rare. Eighty-five percent (n = 66) of the S. pneumoniae and 75% (n = 28) of the S. pyogenes strains in our collection had an M phenotype. The mechanism of M resistance was not mediated by target modification, as isolated ribosomes from a pneumococcal strain bearing the M phenotype were fully sensitive to erythromycin. Further, the presence of an erm methylase was excluded with primers specific for an erm consensus sequence. However, results of studies that determined the uptake and incorporation of radiolabeled erythromycin into cells were consistent with the presence of a macrolide efflux determinant. The putative efflux determinant in streptococci seems to be distinct from the multicomponent macrolide efflux system in coagulase-negative staphylococci. The recognition of the prevalence of the M phenotype in streptococci has implications for sensitivity testing and may have an impact on the choice of antibiotic therapy in clinical practice.

Clinical strain of Staphylococcus aureus inactivates and causes efflux of macrolides

Searching through a collection of 124 Staphylococcus aureus clinical strains, we found one isolate, strain 01A1032, that inactivates 14- and 16-membered macrolides. The products of inactivation were purified from supernatant fluids of cultures exposed to erythromycin for 3 h and were found to be identical to products of inactivation from Escherichia coli strains that encode either an EreA or EreB esterase. Further, strain 01A1032 was shown to be resistant to azithromycin, a 15-membered macrolide, by an alternate mechanism, efflux. Thus, strain 01A1032 harbors determinants encoding an esterase activity that hydrolyzes 14- and 16-membered macrolides and a macrolide efflux system.

Semi-synthetic derivatives of erythromycin

Semi-synthetic derivatives of erythromycin have played an important role in antimicrobial chemotherapy. First generation derivatives such as 2'-esters and acid-addition salts significantly improved the chemical stability and oral bioavailability of erythromycin. A second generation of erythronolide-modified derivatives: roxithromycin, clarithromycin, azithromycin, dirithromycin and flurithromycin, have been synthesized and have exhibited significant improvements in pharmacokinetic and/or microbiological features. In addition, erythromycin itself has expanded its utility as an effective antibiotic against a variety of newly emerged pathogens. As a result of these developments, macrolide antibiotics have enjoyed a resurgence in clinical interest and use during the past half-dozen years, and semi-synthetic derivatives of erythromycin should continue to be important contributors to this macrolide renaissance. Despite these recent successes, other useful niches for macrolide antibiotics will remain unfilled. Consequently, the search for new semi-synthetic derivatives of erythromycin possessing even better antimicrobial properties should be pursued.

Treatment of subclinical mastitis

Topics addressed in this article include applied pharmacology of the bovine mammary gland, principles of antibiotic sensitivity testing, mechanisms of antimicrobial resistance, causes of treatment failures, diagnostic considerations, and therapy of specific subclinical mastitis syndromes. Recent research concerning systemic therapy of subclinical mastitis is highlighted and critically reviewed. Limitations of antibiotic sensitivity testing are discussed. The lack of proven, efficacious therapy for many subclinical mastitis syndromes is emphasized.

Intrinsic and unusual resistance to macrolide, lincosamide, and streptogramin antibiotics in bacteria

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Role of an energy-dependent efflux pump in plasmid pNE24-mediated resistance to 14- and 15-membered macrolides in Staphylococcus epidermidis

We have elucidated a new mechanism for bacterial resistance to the 14-membered macrolides oleandomycin and erythromycin and the 15-membered macrolide azithromycin. Plasmid pNE24, previously isolated from a clinical specimen of Staphylococcus epidermidis, was characterized as causing resistance to 14-membered but not 16-membered macrolides by a mechanism suggested to involve reduced antibiotic permeation of bacterial cells (B. C. Lampson, W. von David, and J. T. Parisi, Antimicrob. Agents Chemother. 30:653-658, 1986). Our recent investigations have demonstrated that S. epidermidis 958-2 containing plasmid pNE24 also contains an energy-dependent macrolide efflux pump which maintains intracellular antibiotic concentrations below those required for binding to ribosomes. Thus, when strain 958-2 was pretreated with the inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP), macrolide accumulated at the same rate and to the same extent as in CCCP-treated or untreated control cells lacking plasmid pNE24 (strain 958-1). In contrast, macrolide did not accumulate in energy-competent strain 958-2 but did accumulate to levels equal to those of ribosomes immediately following CCCP addition. Furthermore, intracellular macrolide was excreted and bacteria resumed growth when CCCP but not macrolide was removed from the growth medium. As expected, the 16-membered macrolide niddamycin accumulated to the same level in energy-competent strains 958-1 and 958-2 at the same rapid rate. Macrolide incubated with lysates prepared from both strains or recovered from cells of strain 958-2 was unmodified and bound to ribosomes from strains 958-1 and 958-2 with identical affinities and kinetics, thus precluding a role for ribosome or drug alteration in the resistance mechanism. We conclude that the presence of plasmid pNE24 results in specific energy-dependent efflux of 14- and 15-membered macrolides.

Characterization of plasmids that confer inducible resistance to 14-membered macrolides and streptogramin type B antibiotics in Staphylococcus aureus

During a period from 1978 to 1989, 413 Staphylococcus aureus strains were isolated at 27 different geographical regions in Hungary; they exhibited an inducible resistance to the 14-membered macrolides and streptogramin type B antibiotics, but not to the 16-membered macrolides and lincosamides: this resistance is referred to as PMS resistance phenotype. The isolates were mostly associated with patients suffering from staphylococcal diseases and with hygienic screenings in hospitals and closed communities. They were rarely isolated from food-poisoning cases, food hygienic screenings, or animal sources. Strains with PMS resistance phenotype were resistant to penicillin (99.0%), tetracycline (78.7%), and chloramphenicol (63.0%); however, they were susceptible to oxacillin. Most of them (94.2%) belonged to the phage type 52-complex. The determinant for PMS phenotype was located on plasmids, which also encoded beta-lactamase production and cadmium ion resistance, but not arsenate resistance. Three types of plasmid with molecular size of 50 kilobases (kb), 23.8 kb, and 16.8 kb, were found among the strains with PMS resistance phenotype, and the 50 kb and 23.8 kb plasmids also encoded mercury resistance. The 16.8 kb and 23.8 kb plasmids belonged to incompatibility group 1.

Erythromycin and azithromycin transport into Haemophilus influenzae ATCC 19418 under conditions of depressed proton motive force (delta mu H)

The effect of collapsing the electrochemical proton gradient (delta mu H) on [3H]erythromycin and [14C]azithromycin transport in Haemophilus influenzae ATCC 19418 was studied. The proton gradient and membrane potential were determined from the distribution of [2-14C]dimethadione and rubidium-86, respectively. delta mu H was reduced from 124 to 3 mV in EDTA-valinomycin-treated cells at 22 degrees C with 150 mM KCl and 0.1 mM carbonyl cyanide m-chlorophenylhydrazone. During the collapse of delta mu H, macrolide uptake increased. Erythromycin efflux studies strongly suggested that this increase was not due to an energy-dependent efflux pump but was likely due to increased outer membrane permeability. These data indicated that macrolide entry was not a delta mu H-driven active transport process but rather a passive diffusion process.

Localization of a determinant mediating partial macrolide resistance in Staphylococcus aureus

Four out of more than 8,200 Staphylococcus aureus strains isolated in Japan between 1961 and 1980 were constitutively resistant to a variety of macrolide antibiotics except tylosin and rokitamycin, but susceptible to lincosamide and streptogramin type B antibiotics (PM). The data obtained by agarose gel electrophoresis, CsCl-ethidium bromide density gradient analysis, diagnosis with ATP-dependent deoxyribonuclease, and a test transducing into a rec- mutant with phage 80L2 propagated on PM-resistant S. aureus all suggested that the determinant for the PM-resistance is located in chromosome.

Binding of novel macrolide structures to macrolides-lincosamides-streptogramin B-resistant ribosomes inhibits protein synthesis and bacterial growth

Dimethylation of adenine 2058 in 23S rRNA renders bacteria resistant to macrolides, lincosamides, and streptogramin B (MLS resistance), because the antibiotic binding site on the altered 50S ribosomal subunit is no longer accessible. We now report that certain 6-O-methyl-11,12-cyclic carbamate derivatives of erythromycin are able to bind to dimethylated MLS-resistant 50S ribosomal subunits, thus inhibiting protein synthesis and cell growth. One of these novel structures, an 11-deoxy-11-(carboxyamino)-6-O-methylerythromycin A 11,12-(cyclic ester) derivative, structure 1a, was studied in detail. It inhibited in vitro protein synthesis in extracts prepared from both susceptible and MLS-resistant Bacillus subtilis with 50% inhibitory concentrations of 0.4 and 20 microM, respectively. The derivative bound specifically to a single site on the 50S subunit of MLS-resistant ribosomes prepared from B. subtilis and Staphylococcus aureus, and no binding to 30S subunits was observed. The association rate constant of derivative 1a with sensitive and resistant ribosomes was 100- and 500-fold slower, respectively, than that of the parent compound, erythromycin, with sensitive ribosomes. The dissociation rate constant of 1a from sensitive and resistant ribosomes was 50- to 100-fold slower than the rate of erythromycin dissociation from sensitive ribosomes. Furthermore, 1a binding to sensitive 50S subunits led to induction of ermC and ermD, while binding to resistant 50S subunits did not, showing that perturbation of sensitive and resistant 50S subunit function by 1a differs. These data demonstrated that 1a is unique in its interaction with MLS-resistant ribosomes and that this interaction causes a novel allosteric perturbation of ribosome function.

Mechanisms of macrolide resistance in Ureaplasma urealyticum: a study on collection and clinical strains

Ureaplasma urealyticum is considered as a species which is intrinsically sensitive to macrolides (MIC less than 1 microgram/ml). Nevertheless, some of the strains recently isolated in our laboratories showed moderate to high levels of resistance (MICs ranging from 2 micrograms/ml to 100 micrograms/ml). In particular, a strain (CT28) isolated from a patient with nongonococcal urethritis long treated with erythromycin revealed a MIC greater than 100 micrograms/ml for this antibiotic. In order to investigate the mechanisms of resistance, strain CT28 and ten clinical and laboratory U. urealyticum strains were compared for the sensitivity to six antibiotics including three macrolides. Moreover the amount of macrolide uptake and the specific antibiotic binding to ribosomes were studied. Strain CT28 was resistant to josamycin, erythromycin, roxithromycin, lincomycin and clindamycin but sensitive to minocycline. When compared to a sensitive strain, strain CT28 showed a six-fold reduction in intracellular macrolide influx and accumulation and a reduction in antibiotic binding to ribosomes. The mechanisms implicated in these differences may be important for macrolide resistance in U. urealyticum.

Use of plasmid profiles in epidemiologic surveillance of disease outbreaks and in tracing the transmission of antibiotic resistance

Plasmids are circular deoxyribonucleic acid molecules that exist in bacteria, usually independent of the chromosome. The study of plasmids is important to medical microbiology because plasmids can encode genes for antibiotic resistance or virulence factors. Plasmids can also serve as markers of various bacterial strains when a typing system referred to as plasmid profiling, or plasmid fingerprinting is used. In these methods partially purified plasma deoxyribonucleic acid species are separated according to molecular size by agarose gel electrophoresis. In a second procedure, plasmid deoxyribonucleic acid which has been cleaved by restriction endonucleases can be separated by agarose gel electrophoresis and the resulting pattern of fragments can be used to verify the identity of bacterial isolates. Because many species of bacteria contain plasmids, plasmid profile typing has been used to investigate outbreaks of many bacterial diseases and to trace inter- and intra-species spread of antibiotic resistance.