Mode of action of macrolides

Novel Antibacterial Targets in Protein Biogenesis Pathways

Antibiotic resistance has emerged as a global threat due to the ability of bacteria to quickly evolve in response to the selection pressure induced by anti-infective drugs. Thus, there is an urgent need to develop new antibiotics against resistant bacteria. In this review, we discuss pathways involving bacterial protein biogenesis as attractive antibacterial targets since many of them are essential for bacterial survival and virulence. We discuss the structural understanding of various components associated with bacterial protein biogenesis, which in turn can be utilized for rational antibiotic design. We highlight efforts made towards developing inhibitors of these pathways with insights into future possibilities and challenges. We also briefly discuss other potential targets related to protein biogenesis.

Polyketide synthase thioesterases catalyze rapid hydrolysis of peptidyl thioesters

Polyketide synthase (PKS) thioesterases (TEs) catalyze the macrocyclization of linear acyl chains into macrolactones. Herein we show that peptide based substrates are processed by PKS TEs with greater catalytic efficiency than more native like acyl substrates. This result strengths the link between PKS and non-ribosomal peptide synthetase systems and provides a new tool for studying PKS TEs.

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.

Role of protonated and neutral forms of macrolides in binding to ribosomes from gram-positive and gram-negative bacteria

Erythromycin binds to a single site on the bacterial 50S ribosomal subunit and perturbs protein synthesis. However, erythromycin contains desosamine and thus exists in both protonated (greater than 96%) and neutral (less than 4%) forms at physiological pH because of the pKa of the dimethylamino group. We therefore examined the relative roles of both forms in binding to ribosomes isolated from two species each of gram-positive and gram-negative bacteria. We developed a system to directly measure the forward (association) rate constant of formation of the macrolide-ribosome complex, and we have measured both the forward and reverse (dissociation) rate constants as a function of pH. Forward rate constants and binding affinity did not correlate with pH when the interaction of erythromycin with ribosomes from both gram-positive and gram-negative bacteria was examined, demonstrating that the protonated form of this macrolide binds to ribosomes. Conversely, the neutral form of macrolide cannot be the sole binding species and appears to bind with the same kinetics as the protonated form. Forward rate constants were 3- to 4-fold greater at physiological pH, and binding affinity calculated from rate constants was 5- to 10-fold greater than previously estimated. Similar results were obtained with azithromycin, a novel 15-membered macrolide that contains an additional tertiary amine in the macrolide ring. Ribosome- and macrolide-specific kinetic parameters were demonstrated at neutral pH and may be related to the potency of the two macrolides against gram-positive and gram-negative bacteria.

Macrolide accumulation by Bacteroides fragilis ATCC 25285

The accumulation of macrolide antibiotics in Bacteroides fragilis ATCC 25285 was increased in the order erythromycin, josamycin, and rokitamycin, depending on hydrophobicity. The half-times of efflux were also prolonged in the same order. Furthermore, MICs of the antibiotics were correlated with the extent of hydrophobicity. These findings suggest that the macrolide antibiotics are accumulated in B. fragilis by means of their hydrophobic properties, and the efficient accumulation of the drugs may explain the susceptibility of this gram-negative bacterium to macrolides.

Carbomycin, a macrolide antibiotic

Carbomycin is produced by Streptomyces halstedii. It was produced in a medium containing the following ingredients (g/l): soybean meal, 30.0; glucose, 22.0; NaCl, 1.0; CaCO3, 5.0; CoCl2 . 6 H2O, 0.005; and lard oil, 4.0. Influence of trace elements on the biosynthesis of carbomycin was recorded. Methods of extraction and purification were given in the review article. Chemical and physical properties of carbomycin were also described. A microbiological assay method for carbomycin determination was described. Biosynthesis of carbomycin was reported. Mechanism of action of carbomycin on micro-organisms was also given in the review article.

Susceptibility of some plaque microorganisms to chemotherapeutic agents

Investigators have used chemotherapeutic agents topically for plaque control without knowing the drug concentration necessary to inhibit the growth of odontopathic microorganisms. S mutans, S sanguis, A viscosus and A naeslundii are important components of the plaque flora. The minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of niddamycin, vancomycin, bacitracin, and kanamycin were determined for each organism in liquid culture. These antibiotics were selected because of their low absorption from the gastrointestinal tract. Niddamycin, vancomycin, and bacitracin had the lowest MIC, from 0.2 to 10 units/ml. Kanamycin was inhibitory only at much higher concentrations (130 to 500 units/ml). The corresponding MBC was generally higher than the MIC. A viscosus was the most resistant organism tested. These data are important in designing controlled release devices for delivering a suitable antibiotic on a continuous basis intraorally.

Correlation between erythromycin and acid phosphatase in mouse liver

1. Whole liver homogenates obtained from mice 1h after an intraperitoneal injection of erythromycin lactobionate (343 mg/kg, 1/3 LD 50) were fractionated into nuclear (7000 times g min), mitochondrial (33000 times g min) and lysosomr-rich (250 000 times g min) fractions. 2. The resulting fractions, as well as the final supernatant, were analyzed for erythromycin, acid phosphatase and protein. 3. The highest relative specific activities (per cent total protein) of erythromycin and of acid phosphatase were exhibited by the lysosome-rich fraction. 4. It was of interest, therefore, to examine the effects of erythromycin upon the free activity of acid phosphatase in soluble form and on its in vitro and in vivo release from liver lysosomes. 5. Concentrations of 1.7 - 10-4 M, 3.4 - 10-4 M, 6.8 - 10-4 M and 13. 6 - 10-4 M of erythromycin lactobionate had no significant effect upon the free activity of acid phosphatase in soluble form but retarded the release of this enzyme from the liver lysosome-rich preparation. This effect of erythromycin lactobionate was dose- and time-dependent. 6. Treatment of mice with erythromycin lactobionate (343 mg/kg, 1/3 LD 50) iwtraperitoneally for 7 days significantly decreased the unsedimentable acid phosphatase activity expressed as per cent of total activity in whole liver homogenates. This indicated an in vivo diminished release of acid phosphatase from liver lysosomes by erythromycin. 7. Since erythromycin lactobionate is ionisable it could be possible that erythromycin basis as many other cationic molecules accumulates in lysosomes. 8. The in vitro and in vivo diminished release of acid phosphatase may suggest that erythromycin decreases permeability of lysosomal membrane.

Reversal by syngeneic spleen cells of inhibitory effects of drugs and irradiation on Friend virus

The dissociation constants for binding to ribosomes from Escherichia coli and concentrations at which 50% inhibition of [¹⁴C]erythromycin binding to ribosomes occurred were determined for 45 erythromycin analogues. These values were correlated with their antibacterial activities against Bacillus subtilis. Compounds which bound to ribosomes best showed the greatest activities; those which were poorly bound to ribosomes showed little or no antibacterial activity. The ribosomal binding assays therefore reflected the general antibacterial potential of the erythromycin analogues.

Mutation to erythromycin dependence in Escherichia coli K-12

A nitrosoguanidine-induced mutant of Escherichia coli K-12 strain JC12 was absolutely dependent on erythromycin or related macrolide antibiotics for growth. The only other drugs which permitted growth (lincomycin and chloramphenicol) are, like the macrolides, inhibitors of the 50S ribosome. The order of relative effectiveness of these drugs was macrolides > lincomycin > chloramphenicol. Rates of growth with all drugs were concentration dependent. Erythromycin starvation was followed by normal rates of increase in cell mass and macromolecular synthesis for approximately one mass-doubling time, after which macromolecular synthesis abruptly ceased and cell lysis and death occurred. The dependent mutant gave rise spontaneously to revertants to independence with very high frequency (10(-4)). The gene (mac) for macrolide dependence is located near minute 25 on the E. coli chromosome; it does not result in increased resistance to these drugs. A separate gene for erythromycin resistance (eryA) is located in the cluster of ribosomal structural genes near spc, close to minute 63. Dependence on macrolides was most clearly evident in strains carrying mutations at both eryA and mac.

Mutation of a cytoplasmic gene in Chlamydomonas alters chlorplast ribosome function

A mutation, car, determining resistance to several macrolide antibiotics, including carbomycin, has been identified in the alga Chlamydomonas as cytoplasmic, and mapped in the known cytoplasmic linkage group close to genes determining resistance to other antibiotics, including streptomycin, erythromycin, and spectinomycin. The effect of the car mutation on chloroplast ribosome function was demonstrated with an in vitro system incorporating amino acids especially developed to assess activity of 70S chloroplast ribosomes. In an S-30 extract containing both 70S chloroplast and 80S cytoplasmic ribosomes, low concentrations of Mg(++) and spermidine favored 80S ribosome activity, and high concentrations activated 70S ribosomes and reversibly inactivated the 80S component. Under conditions favoring chloroplast ribosome activity, carbomycin inhibited incorporation by an S-30 extract, and by purified 70S ribosomes from wild-type but not from car cells. These results show that cytoplasmic genes are directly involved in chloroplast ribosome function and they suggest that the car gene product is a ribosomal protein; the results further strengthen the evidence that the cytoplasmic linkage group is located in chloroplast DNA.

Peptidyl puromycin synthesis; effect of several antibiotics which act on 50 S ribosomal subunits

The effects of several antibiotics which are know to bind with 50 S ribosomal subunits, on the formation of several di- and tri-peptidyl puromycins have been examined. Tylosin and spiramycin inhibited the formation of phenylalanyl-(14)C-phenylalanyl-puromycin, glycyl-(14)C-phenyllalanyl-puromycin, leucyl-(14)C-phenylalanyl-puromycin, N(epsilon)-carbobenzoxylysyl-(14)C-phenylalanyl-puromycin, and valyl-glycyl-(14)C-phenylalanyl-puromycin as well as N-acetyl-(14)C-phenylalanyl-puromycin. Of these compounds, erythromycin and oleandomycin selectively inhibited the formation of phenylalanyl-(14)C-phenylalanyl-puromycin. Although chloramphenicol and lincomycin inhibited the formation of most of these peptidyl puromycins, the formation of phenylalanyl-(14)C-phenylalanyl-puromycin and leucyl-(14)C-phenylalanyl-puromycin was found to be resistant to these antibiotics. So far, no significant effect of siomycin has been observed on pepetidyl puromycin formation in the absence of G factor.

Antibacterial activity of 2'-esters of erythromycin

The effect of esterification at the 2'-position of desosamine on the antibacterial activity of erythromycin was investigated by determining the bacteriostatic and bactericidal activities of erythromycin and a number of its 2'-esters on S. aureus and relating these activities to the hydrolysis rates of the esters. These studies, together with comparison of the inhibition of protein synthesis in a cell-free system isolated from S. aureus, lead to the conclusion that 2'-esters of erythromycin are inactive until hydrolyzed. Loss of activity appears to result from inability of erythromycin esters to bind to bacterial ribosomes and thus inhibit synthesis of protein.