Antibiotics in Laboratory Medicine by Amsterdam Daniel

Author:Amsterdam, Daniel
Language: eng
Format: epub
ISBN: 9781451176759
Publisher: LWW
Published: 2014-08-07T16:00:00+00:00

High-level resistance to macrolides is most often mediated by the enzyme-catalyzed methylation of the 23S rRNA by Erm (erythromycin ribosome methylase) methyltransferases encoded by erm genes that confer constitutive or inducible macrolide-lincosamide-streptogramin B (MLSB) resistance phenotypes (122). The number of described erm genes has already passed 40 (http://faculty.washington.edu/marilynr/). Four major erm classes are detected in pathogenic bacteria: erm(A), erm(B), erm(C), and erm(F) (118,122). Although erm(A) and erm(C) are typically detected in staphylococci, erm(B) class genes are mostly observed in streptococci and enterococci, whereas erm(F) are mostly described in Bacteroides and other anaerobes. In addition to the erm(B) genes, the ermTR genes (a subset of the erm[A] class) are widely distributed in β-hemolytic streptococci (118). The notion that each erm class may have a relatively specific distribution, but not strictly confined to a bacterial genus, reflects their association to mobile genetic elements and various mechanisms for horizontal gene transfer (118,123).

Low-level macrolide resistance is mainly associated with streptococci and linked to the production of an efflux pump (M phenotype) conferring resistance to erythromycin but not to clindamycin and/or streptogramins (122). A macrolide efflux system in streptococci was firmly established in 1996 (124). This system was phenotypically recognized and characterized to confer low-level resistance to 14- and 15-membered macrolides only. Several mef determinants have been described and the number varies dependent on criteria for definition of new elements (118,120).

A number of reports has shown marked differences between mef(A) and mef(E) (118). For instance, the different genetic elements carrying mef(A) or mef(E) and their contexts have been studied (125). Moreover, the two genes have disseminated markedly different in an ever growing number of species (120).

Molecular Detection Macrolide, Lincosamide, and Streptogramin Resistance Determinants

It is not much of a surprise to find that PCR is by far the most established method for the detection of MLS resistance genes and their genetic support. The high degree of similarity between major groups of resistance determinants does not allow a reliable discrimination to be made by using DNA hybridization experiments. Recommended primers for detection of MLS genes are available at http://faculty.washington.edu/marilynr/. Moreover, a diversity of different PCR primer combinations for amplification of the mef gene have been reported creating amplification products ranging from 202 to 1,759 bp (Table 3 in [120]). However, when using PCR to target resistance determinants with nucleotide diversity, one must take into account mismatches and in particular those located at the ultimate 3′-end of the primer sequence or multiple mismatches that may be present along the sequence of the primer(s). The use of such primers may result in an inefficient PCR or preferential amplification of mef gene subpopulations without the mismatch. A straightforward method for discrimination between mef(A) and mef(E) is based on the differential presence of restriction enzyme recognition sites in the two genes. Restriction enzyme digest resolved by agarose gel electrophoresis is sufficient to establish the difference (120).

The complexity of MLS-related resistance phenotypes are increasing. Several additional phenotypes affecting L or S antibiotics only (L phenotype) or in combination (LS phenotype) and their respective resistance determinants have been described lately (126–128).

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.