A mode of resistance to macrolide antibiotics in Staphylococcus aureus

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.

Erythromycin-inducible resistance in Staphylococcus aureus: requirements for induction

At least two functionally different types of ribosomes are found in strains of Staphylococcus aureus which display "dissociated" resistance to erythromycin. One type of ribosome is found under conditions of growth in ordinary nutrient broth, and the second is formed during growth in the presence of erythromycin. In these strains, erythromycin acts as an inducer of resistance to three different classes of inhibitors of the 50S ribosomal subunit-the macrolides, lincosamides, and streptogramin B-type antibiotics. The optimal inducing concentration of erythromycin is between 10(-8) and 10(-7)m. Concentrations as low as 10(-9)m can produce a 10-fold increase in resistant cells over the uninduced, background level, whereas concentrations greater than 10(-7)m block induction owing to inhibition of protein synthesis. Resistant cells begin to appear within 5 to 10 min after addition of erythromycin (to 10(-7)m), and within 40 min (i.e., about one generation) more than 90% of the entire culture is resistant to erythromycin as well as to lincomycin and vernamycin B(alpha). A resistant culture becomes sensitive if grown for 90 min in the absence of erythromycin. The process of induction is inhibited by chloramphenicol and streptovaricin, which inhibit protein and ribonucleic acid synthesis, respectively, but not by novobiocin, which inhibits deoxyribonucleic acid synthesis. Resistant cells produced in this manner fail to concentrate (14)C-erythromycin and (14)C-lincomycin, but not (14)C-chloramphenicol. Constitutively erythromycin-resistant strains which do not require the presence of erythromycin for expression of resistance can be selected on media containing antibiotics which belong to any one of the three classes. Two patterns of constitutive resistance have been found. These are (i) generalized constitutive resistance-which involves resistance in the absence of erythromycin to all members of each of the three cited classes of 50S subunit inhibitors which were tested, and (ii) partial constitutive resistance-which involves different degrees of resistance, in the absence of erythromycin, to various members of the three classes. Several different patterns of variable constitutivity are possible. 50S ribosomal subunits isolated from induced or constitutively resistant cells show decreased ability to bind erythromycin and lincomycin, and possible enzymatic inactivation of these antibiotics has been rigorously excluded. The induced change, therefore involves modification of ribosome structure rather than modification of the antibiotic.

Erythromycin-inducible resistance in Staphylococcus aureus: survey of antibiotic classes involved

Certain erythromycin-resistant strains of Staphylococcus aureus remain sensitive to other macrolide antibiotics. If these strains are exposed to low levels of erythromycin, resistance to other antibiotics is induced. The antibiotics to which resistance is induced by erythromycin include: other macrolides as well as lincosaminide, streptogramin (group B) antibiotics but not chloramphenicol, amicetin, streptogramin (group A) antibiotics, tetracyclines, and aminoglycosides. Hence erythromycin induces resistance exclusively towards inhibitors of 50S ribosomal subunit function and, thus far, only with respect to three of six known classes of inhibitors which act on this subunit. In the four strains tested, erythromycin did not induce resistance to pactamycin or bottromycin, to fusidic acid (which inhibits a function involving both subunits), or to other antibiotics which do not inhibit ribosomal function. Thus, by inducing resistance erythromycin could antagonize the action of other antibiotics, and a consistent pattern of antagonism was observed to each antibiotic class in all of the strains in which this could be tested, as well as to other antibiotic members of the same chemical class in each bacterial strain.