The problems of drug-resistant pathogenic bacteria. Macrolide resistance in staphylococci

Translational attenuation: the regulation of bacterial resistance to the macrolide-lincosamide-streptogramin B antibiotics

The regulation of ermC is described in detail as an example of regulation on the level of translation. ermC specifies a ribosomal RNA methylase which confers resistance to the macrolide-lincosamide-streptogramin B group of antibiotics. Synthesis of the ermC gene product is induced by erythromycin, a macrolide antibiotic. Stimulation of methylase synthesis is mediated by binding of erythromycin to an unmethylated ribosome. The translational attenuation model, supported by sequencing data and by mutational analysis, proposes that binding of erythromycin causes stalling of a ribosome during translation of a "leader peptide", resulting in isomerization of the ermC transcript from an inactive to an active conformer. The ermC system is analogous to the transcriptional attenuation systems described for certain biosynthetic operons. ermC is unique in that interaction with a small molecule inducer mediates regulation on the translational level. However, it is but one example of nontranscriptional -level control of protein synthesis. Other systems are discussed in which control is also exerted through alterations of RNA conformation and an attempt is made to understand ermC in this more general context. Finally, other positive examples of translational attenuation are presented.

Macrolide resistance in Staphylococcus aureus: induction of macrolide-resistant protein synthesis

Induction of resistance to macrolide-, lincosamide-, and streptogramin B-type antibiotics in Staphylococcus aureus was studied by monitoring the appearance of erythromycin A (EM)-resistant [(14)C]leucine incorporation. Examination of the induction process revealed saturation kinetics and a time course much like that reported for penicillinase in gram-positive bacteria. Induction kinetics in exponentially growing cells were sigmoidal and appeared to reach a maximum and constant rate when growth reached stationary phase. Since the induction of EM-resistant colony-forming ability was complete within 60 min, ribosome modification cannot be limited to a fraction of the population and must occur in essentially every cell. However, EM-resistant growth was expressed in cells where less than half the [(14)C]leucine-incorporating activity was resistant to EM. This suggests that resistance requires that only a threshold level of ribosome modification be exceeded and that, once exceeded, resistance is dominant to sensitivity.

Genetics of resistance to macrolide antibiotics and lincomycin in natural isolates of Streptococcus pyogenes

Of 5 clinically isolated strains of Streptococcus pyogenes, 3 showed high-level resistance to erythromycin and lincomycin that was inducible by subinhibitory concentrations of these drugs (IR strains) while 2 strains exhibited constitutive erythromycin and lincomycin resistance (CR strains) which was expressed without prior exposure to low drug concentrations. The CR strain 15346 showed spontaneous loss of resistance whereas resistance in the other strains was quite stable even under curing conditions. The IR strain 13234 was found to be polylysogenic for at least 4 different phages designated P13234ma, mi, mu, and mo. Phage mo, antigenically distinct from the other three, was shown to mediate the transfer of the resistance determinant ERL1 of strain 13234. ERL1 if borne by appropriate strains was also transducible by the virulent phage A25. ERL1 behaved as a discrete genetic unit in transduction experiments, was not linked to either of two chromosomal regions governing resistance to antibiotics that affect the ribosome, could be transferred to recombination deficient hosts, represented a relatively large UV inactivation target, and showed no stimulation of transduction by low UV doses. These findings suggest that resistance to erythromycin and lincomycin in certain natural isolates of S. pyogenes is specified by, or under the control of, a plasmid.

Mutant of Staphylococcus aureus with lincomycin- and carbomycin-inducible resistance to erythromycin

A mutant of Staphylococcus aureus was isolated in which lincomycin and carbomycin (but not erythromycin) induced resistance to erythromycin. This pattern displayed a reversal of the usual specificity of induction seen in the erythromycin-inducible parent strain from which the mutant was selected.

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.