The stoichiometry of erythromycin binding to ribosomal particles of Staphylococcus aureus

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.

Elongating ribosomes in vivo are refractory to erythromycin

We have studied the kinetics of erythromycin inhibition of translation in growing bacteria. In order to simplify the interpretation of our data, we have used a mutant (envA), known to have an increased permeability to several antibiotics, including erythromycin. The data clearly show that an initial stage of translation is sensitive to erythromycin, but that the elongating ribosome is insensitive to the antibiotic.

Binding parameters and microbiological activity of macrolides, lincosamides and streptogramins against Staphylococcus aureus

Parameters of erythromycin binding to Staphylococcus aureus were measured in-vitro using an equilibrium method with [3H]erythromycin. The dissociation constant of the complex, erythromycin-S. aureus sensitive strain, was KD = 0.11 microM. The maximal binding, representing the density of binding sites was 14,847 molecules/cell. No binding was detectable on the constitutive resistant strain. Macrolides, streptogramins and lincosamides displaced bound [3H]erythromycin by a competitive process indicating that these compounds share common binding sites on the bacteria, i.e. 50 S ribosomal subunits. A good correlation (r = 0.99) was demonstrated between the corresponding inhibition constants (Ki) and the minimal inhibitory concentration. It is proposed that knowledge of the binding parameters provides a good indication of bacterial susceptibility and may serve as a useful adjunct in developing new compounds.

Binding of [14C]erythromycin to Escherichia coli ribosomes

Erythromycin binding to Escherichia coli ribosomes required K(+) and Mg(2+). Under optimal conditions, the dissociation constant for erythromycin binding to E. coli ribosomes was found to be 1.0 x 10(-8) M and 1.4 x 10(-8) M at 24 C and 5 C, respectively. One molecule of [(14)C]erythromycin was bound to each 70S ribosome at equilibrium. Binding of erythromycin to ribosomes was rapid and reversible. The specific rate constants for the forward and reverse reactions were 1.7 x 10(7) liters per mol per min and 0.15 per min, respectively.

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

STUDIES ON ERYTHROMYCIN RESISTANCE IN STRAINS OF GROUP A STREPTOCOCCI INDICATED THAT THEY WERE COMPRISED OF TWO TYPES: (i) an inducible, resistant type (IR strains) was seen, which manifested immediate logarithmic growth in media containing high concentrations of the drug only after brief previous exposure (induction period) of the organisms to subinhibitory concentrations of erythromycin, and (ii) a constitutive, resistant type (CR strains) which demonstrated, without prior drug exposure, continued logarithmic growth in media containing high concentrations of erythromycin. Subinhibitory concentrations of either chloramphenicol or puromycin, when added to IR strains prior to induction, interfered with their induction by erythromycin. Exposure of CR strains to chloramphenicol did not visibly affect the subsequent growth curve of these strains in media containing high concentrations of erythromycin. In IR strains, resistance to other macrolide antibiotics (oleandomycin, spiramycin, carbomycin, magnamycin) and to lincomycin also was inducible in nature. There was cross-inducibility between erythromycin, other macrolide antibiotics, and lincomycin. CR strains were constitutively resistant to these antibiotics.

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.

Erythromycin-resistant mutant of Escherichia coli with altered ribosomal protein component

Erythromycin combines with 50S ribosomal subunit of an erythromycin-sensitive Escherichia coli (strain Q13), while ribosomes from an erythromycin-resistant mutant from this strain have little affinity for the antibiotic. A protein component of the 50S subunit of the mutant strain is distinct from that of the parent Q13 strain.

Accumulation in gram-postive and gram-negative bacteria as a mechanism of resistance to erythromycin

Erythromycin was recovered in high yield after incubation with gram-negative bacteria. The cell-free protein-synthesizing preparation from gram-negative bacteria is equally as susceptible to the antibiotic as is that from gram-positive bacteria. Thus, neither destruction of erythromycin nor the absence of the step susceptible to the antibiotic plays an important role in the resistance mechanism of gram-negative bacteria. A 100-fold difference in accumulation of erythromycin between gram-positive and gram-negative bacteria was observed. This alone explains the resistance of gram-negative bacteria to erythromycin. Furthermore, data showed that the inhibition of growth is closely related to the accumulation of erythromycin. The concentration of intracellular erythromycin in gram-positive bacteria was found to be 44- to 90-fold greater than that of the extracellular medium. However, the antibiotic did not accumulate on the cell walls, nor was the accumulation energy-dependent. It is proposed that it takes place by the binding of erythromycin to the bacterial ribosomes, forming a very stable complex. The dissociation constants of erythromycin-Staphylococcus aureus complex and erythromycin-Bacillus subtilis complex were determined to be 1.1 x 10(-7) and 3.4 x 11(-7)m, respectively.