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In mating experiments, the erythromycin resistance methylase gene erm(A) was successfully transferred from erm(A)-positive clinical isolates of Streptococcus pyogenes to macrolide-susceptible recipients of S. pyogenes, Enterococcus faecalis and Listeria innocua. Compared with the SmaI macrorestriction pattern of the S. pyogenes recipient, the patterns of S. pyogenes transconjugants shared the lack of a fragment and the appearance of a new, larger fragment. This is the first experimental evidence that the erm(A) gene can be transferred from erythromycin-resistant S. pyogenes to macrolide-susceptible S. pyogenes as well as to other Gram-positive recipients. [ABSTRACT FROM AUTHOR]

Two ketolides, telithromycin and HMR 3004, were evaluated for their in vitro activity against erythromycin-susceptible and -resistant strains of Streptococcus pyogenes and Streptococcus pneumoniae. On the basis of their resistance to macrolide, lincosamide and streptogramin (MLS) antibiotics, erythromycin-resistant test strains were assigned to the constitutive resistance (cMLS) phenotype, the inducible resistance (iMLS) phenotype or the M phenotype. iMLS S. pyogenes strains were further subdivided into the three recently described subtypes iMLS-A, -B and -C. Telithromycin and HMR 3004 were uniformly and highly active against pneumococci (regardless of their susceptibility or resistance to erythromycin and/or penicillin), erythromycin-susceptible S. pyogenes and erythromycin-resistant S. pyogenes strains of the M phenotype (in which resistance is mediated by an efflux system) or iMLS-B or -C phenotype (in which resistance is mediated by a methylase encoded by the ermTR gene). Both ketolides were less active against erythromycin-resistant S. pyogenes strains with the cMLS phenotype or the iMLS-A subtype (where resistance is mediated by a methylase encoded by the ermAM gene), these strains ranging in phenotype from the upper limits of susceptibility to low-level resistant. [ABSTRACT FROM AUTHOR]

Forty-two clinical isolates belonging to ten species of staphylococci were studied for their ability to develop single-step resistance, in vitro, to glycopeptide antibiotics. Selection was attempted through separate exposure to four glycopeptides (vancomycin, teicoplanin, and two investigational semisynthetic derivatives of the latter, TD-A3 and CTA-A1) on agar containing 10 mg/l of the test drug. No survivors from any test strain were recovered after exposure to TD-A3 or CTA-A1. After exposure to vancomycin or teicoplanin, surviving clones were only recovered from strains of three species, Staphylococcus aureus, S. epidermidis, and S. haemolyticus. Emergence of resistant clones was easier to observe from strains of S. haemolyticus exposed to teicoplanin. When tested for susceptibility, many survivors exhibited vancomycin and teicoplanin MICs below the drug concentration used for in-vitro selection, probably due to an inoculum effect in the plating procedure. In particular, the vancomycin MICs did not exceed 8 mg/l for S. aureus and S. epidermidis clones, and reached 16 mg/l for some clones from a S. haemolyticus strain. Teicoplanin MICs did not exceed 8 mg/l for S. aureus clones, but reached 64 mg/l for some clones of S. epidermidis, and were particularly high (64 to greater than or equal to 128 mg/l) for most clones of S. haemolyticus. In contrast, against all clones selected from all three species, the MICs of TD-A3 and CTA-A1 did not exceed 2 and 4 mg/l, respectively. Morphological investigations indicated that the colonies of a highly resistant S. haemolyticus clone were smaller and more butyraceous in consistency than those of the parent strain. In transmission and scanning electron microscopy studies, this same S. haemolyticus clone showed a more irregular cell wall than the parent strain. [ABSTRACT FROM AUTHOR]