[show abstract][hide abstract] ABSTRACT: Seventy-four unrelated clinical isolates of Streptococcus pneumoniae harboring the tet(M) gene were studied. Seven strains with low tetracycline (Tc) MICs (0.25 to 0.5 μg/ml) were found to harbor truncated tet(M) alleles that were inactivated by different frameshift mutations. In contrast, five strains bore deletions in the tet(M) promoter region, among which four displayed increased Tc MICs (16 to 64 μg/ml). The same promoter mutations were detected in Tc-resistant mutants selected in vitro from various susceptible strains. Sequence analysis revealed that these deletions might impede the formation of the transcriptional attenuator located immediately upstream of tet(M). Expression in Enterococcus faecalis of a tet(M) reporter gene transcribed from these promoter mutants conferred a level of Tc resistance similar to that observed in the parental S. pneumoniae strains. These results show that different levels of Tc susceptibility found in clinical isolates of S. pneumoniae can be explained by frameshift mutations within tet(M) and by alterations of the upstream transcriptional attenuator.
Antimicrobial Agents and Chemotherapy 07/2012; 56(10):5040-5. · 4.57 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Sirscan2000automatic MIC determination (SIR-MD) system is a new system for MIC determination based on the automatic detection of growth of bacteria spotted onto agar medium using a camera scan. To evaluate its accuracy, 3608 Streptococcus pneumoniae clinical isolates yielding 18,165 MICs were tested in parallel with the SIR-MD and a standard interpretive antibiogram procedure. The overall percent agreement between the 2 methods within 1 log(2) dilution was 86.9%. After exclusion of the 11.8% noninterpretable results, errors in the deduction of susceptibilities were very major in 0.03%, major in 0.2%, and minor in 1.3%.
[show abstract][hide abstract] ABSTRACT: With respect to pneumococci, there is a need to detect first-step mutants with reduced fluoroquinolone (FQ) susceptibility from which second-step, resistant mutants are likely to be selected in the presence of antipneumococcal FQs. Here, we describe an interpretative disk diffusion test, of which three options are presented, that allows the distinction between first- and second-step mutants. Using five FQ disks (pefloxacin, norfloxacin, levofloxacin, ciprofloxacin, and sparfloxacin, option 1), all known mechanisms of altered FQ susceptibility found in first-step mutants (ParC, ParE, GyrA, or efflux) and in second-step mutants (ParC and GyrA or ParE and GyrA) can be accurately detected, making this option a useful epidemiological tool. Using three FQ disks (pefloxacin, norfloxacin, and levofloxacin, option 2), the most prevalent FQ-resistant mutants, but not the first-step GyrA mutants, can be detected. With only two FQ disks (norfloxacin and levofloxacin) in the third and simplest option, first-step mutants can be distinguished from second-step mutants, however, without differentiation of ParC, ParE, or efflux alterations.
Antimicrobial Agents and Chemotherapy 03/2006; 50(2):572-9. · 4.57 Impact Factor