Resistance of enterococci to aminoglycosides and glycopeptides

High-level resistance to aminoglycosides in enterococci often is mediated by aminoglycoside-modifying enzymes, and the corresponding genes generally are located on self-transferable plasmids. These enzymes are similar to those in staphylococci but differ from the modifying enzymes of gram-negative bacteria. Three classes of enzymes are distinguished, depending upon the reaction catalyzed. All but amikacin and netilmicin confer high-level resistance to the antibiotics that are modified in vitro. However, the synergistic activity of these last two antibiotics in combination with beta-lactam agents can be suppressed, as has always been found in relation to high-level resistance to the aminoglycosides. Acquisition of glycopeptide resistance by enterococci recently was reported. Strains of two phenotypes have been distinguished: those that are resistant to high levels of vancomycin and teicoplanin and those that are inducibly resistant to low levels of vancomycin and susceptible to teicoplanin. In strains of Enterococcus faecium highly resistant to glycopeptides, we have characterized plasmids ranging from 34 to 40 kilobases that are often self-transferable to other gram-positive organisms. The resistance gene vanA has been cloned, and its nucleotide sequence has been determined. Hybridization experiments showed that this resistance determinant is present in all of our enterococcal strains that are highly resistant to glycopeptides. The vanA gene is part of a cluster of plasmid genes responsible for synthesis of peptidoglycan precursors containing a depsipeptide instead of the usual D-alanyl-D-alanine terminus. Reduced affinity of glycopeptides to these precursors confers resistance to the antibiotics.

Characterization of a Mycobacterium tuberculosis insertion sequence, IS6110, and its application in diagnosis

An insertion sequence-like element, IS6110, was isolated from a Mycobacterium tuberculosis cosmid library as a repetitive sequence. IS6110 shows similarities with elements of the IS3 family. This insertion sequence was found to be specific to mycobacteria belonging to the M. tuberculosis complex. For detection and identification of M. tuberculosis bacilli in uncultured specimens, oligonucleotides derived from the IS6110 sequence were used as primers and probes in polymerase chain reaction studies. The results obtained were consistent with results of classical identification procedures, bacteriological data, and clinical criteria.

Cloning and heterospecific expression of the resistance determinant vanA encoding high-level resistance to glycopeptides in Enterococcus faecium BM4147

Fragments of plasmid pIP816, which confers high-level glycopeptide resistance in Enterococcus faecium BM4147, were cloned into a conjugative gram-negative-gram-positive shuttle vector. The resulting hybrids were transferred by conjugation from Escherichia coli to Enterococcus faecalis and Bacillus thuringiensis. A 4-kilobase EcoRI fragment from pIP816 was found to confer vancomycin resistance in these hosts but not in E. coli or Bacillus subtilis.

Rapid diagnosis of tuberculosis by amplification of mycobacterial DNA in clinical samples

A method based on DNA amplification and hybridisation for the rapid detection of Mycobacterium tuberculosis was used to test 35 clinical specimens (sputum, gastric aspirate, abscess aspirate, biopsy sample) from 34 patients in whom tuberculosis was suspected. M tuberculosis was detected in 15 specimens, 2 of which were negative by standard microbiological criteria (microscopy and/or culture). 20 specimens, negative by standard methods, were also negative by the amplification method. M tuberculosis was also detected in peripheral blood samples of 2 of 4 patients with AIDS from whom the organism had been isolated.

Inactivation of lincosaminide antibiotics in Staphylococcus. Identification of lincosaminide O-nucleotidyltransferases and comparison of the corresponding resistance genes

Resistance to lincomycin by inactivation has been detected in numerous clinical isolates of Staphylococcus; in crude extracts of Staphylococcus haemolyticus BM4610 and Staphylococcus aureus BM4611, inactivation of lincomycin and clindamycin requires the presence of a nucleoside 5'-triphosphate (ATP, GTP, CTP, or UTP) as nucleotidyl donor and Mg2+ as cofactor. The biochemical mechanism of lincosaminide inactivation was elucidated by determination of the structure of inactivated lincomycin and clindamycin by physicochemical techniques, including UV absorption spectrophotometry, 31P and 1H nuclear magnetic resonance, and periodate oxidation. In the two strains, inactivation of lincomycin gave rise to lincomycin 3-(5'-adenylate), whereas clindamycin was inactivated through its conversion to clindamycin 4-(5'-adenylate). The gene linA' encoding the 3-lincomycin, 4-clindamycin O-nucleotidyltransferase in S. aureus BM4611 has been sequenced and displays 93% homology with the gene linA encoding the 3-lincomycin, 4-clindamycin O-nucleotidyltransferase found in S. haemolyticus BM4610. The two enzymes are 161 amino acids long and differ by 14 amino acid substitutions.

Mechanism of action of spiramycin and other macrolides

Macrolide antibiotics constitute a group of 12 to 16-membered lactone rings substituted with one or more sugar residues, some of which may be amino sugars. They inhibit bacterial protein synthesis both in vivo and in vitro with varying potencies. Macrolides are generally bacteriostatic, although some of these drugs may be bactericidal at very high concentrations. The mechanism of action of macrolides has been a matter of controversy for some time. Spiramycin, a 16-membered macrolide, inhibits translocation by binding to bacterial 50S ribosomal subunits with an apparent 1:1 stoichiometry. This antibiotic is a potent inhibitor of the binding to the ribosome of both donor and acceptor substrates. Spiramycin induces rapid breakdown of polyribosomes, an effect which has formerly been interpreted as occurring by normal ribosomal run-off followed by an antibiotic-induced block at or shortly after initiation of a new peptide. However, there is now convincing evidence that spiramycin, and probably all macrolides, act primarily by stimulating the dissociation of peptidyl-tRNA from ribosomes during translocation. Although the ribosomes of both Gram-positive and Gram-negative organisms are susceptible to macrolides, these antibiotics are mainly used against Gram-positive bacteria since they are unable to enter the porins of Gram-negative bacteria. Resistance to macrolides in clinical isolates is most frequently due to post-transcriptional methylation of an adenine residue of 23S ribosomal RNA, which leads to co-resistance to macrolides, lincosamides and streptogramins type B (the so-called MLSB phenotype). Other mechanisms of resistance involving cell impermeability or drug inactivation have been detected in Staphylococcus spp. and Escherichia coli. These strains are resistant to 14-membered macrolides (erythromycin and oleandomycin) but remain susceptible to spiramycin.

Evidence for natural gene transfer from gram-positive cocci to Escherichia coli

High-level resistance to macrolide-lincosamide-streptogramin type B (MLS) antibiotics in Escherichia coli BM2570 is due to the presence on the conjugative plasmid pIP1527 of the MLS resistance determinant ermBC, which is almost identical to the erm genes previously described in plasmid pAM77 from Streptococcus sanguis (ermAM) and in transposon Tn917 from Enterococcus faecalis (ermB). This gene and its regulatory region are located downstream from the insertion sequence IS1. The 23S rRNA methylase encoded by pIP1527 differs by three and six amino acids from those encoded by Tn917 and pAM77, respectively. Unlike the streptococcal elements which confer the inducible MLS phenotype, the ermBC gene is expressed constitutively in E. coli and Bacillus subtilis, due to several mutations in the regulatory region. Transcription of the ermBC gene starts from three different sites following three overlapping promoters which function in both E. coli and B. subtilis. Promoters P2 and P3 are located within the region homologous to pAM77 and Tn917, and P1 is a hybrid promoter constituted by -35 and -10 sequences located at the end of IS15 and in the streptococcal region, respectively. These results constitute evidence for the recent in vivo transfer from Streptococcus spp. to E. coli. This transfer could have been mediated by transposons such as Tn917 or Tn1545 from Streptococcus pneumoniae, which also bears an MLS determinant that is homologous to ermB. We speculate that the insertion sequences IS15 and IS1 could have played a role in the expression and dissemination of ermBC, which has been found in numerous strains of enterobacteria.

Phenotypic expression and genetic heterogeneity of lincosamide inactivation in Staphylococcus spp

We examined the resistance phenotype and the genetic basis of lincosamide modification in 25 clinical isolates of Staphylococcus spp. inactivating lincomycin and clindamycin. The strains were resistant to high levels of lincomycin but remained susceptible to clindamycin. However, MBCs and inoculum effects showed that the activity of clindamycin was impaired. The distribution in these strains of nucleotide sequences related to linA and linA', the genes encoding lincosamide nucleotidylation in Staphylococcus haemolyticus BM4610 and S. aureus BM4611, respectively, was studied by dot blot hybridization. The genes responsible for lincosamide inactivation in Staphylococcus spp. were found to constitute a family of related sequences which are not species specific.

Origin and evolution of genes specifying resistance to macrolide, lincosamide and streptogramin antibiotics: data and hypotheses

Resistance to macrolide, lincosamide and streptogramin antibiotics is due to alteration of the target site or detoxification of the antibiotic. Postranscriptional methylation of 23S ribosomal rRNA confers resistance to macrolide (M), lincosamide (L) and streptogramin (S) B-type antibiotics, the so-called MLSB phenotype. Several classes of rRNA methylases conferring resistance to MLSB antibiotics have been characterized in Gram-positive cocci, in Bacillus spp, and in strains of actinomycetes producing erythromycin. The enzymes catalyze N6-dimethylation of an adenine residue situated in a highly conserved region of prokaryotic 23S rRNA. In this review, we compare the amino acid sequences of the rRNA methylases and analyze the codon usage in the corresponding erm (erythromycin resistance methylase) genes. The homology detected at the protein level is consistent with the notion that an ancestor of the erm genes was implicated in erythromycin resistance in a producing strain. However, the rRNA methylases of producers and non-producers present substantial sequence diversity. In Gram-positive bacteria the preferential codon usage in the erm genes reflects the guanosine plus cytosine content of the chromosome of the host. These observations suggest that the presence of erm genes in these micro-organisms is ancient. By contrast, it would appear that enterobacteria have acquired only recently an rRNA methylase gene of the ermB class from a Gram-positive coccus since the genes isolated in Escherichia coli and in Gram-positive cocci are highly homologous (homology greater than 98%) and present a codon usage typical of the latter micro-organisms. As opposed to the MLSB phenotype which results from a single biochemical mechanism, inactivation of structurally related antibiotics of the MLS group involves synthesis of various other enzymes. In enterobacteria, resistance to erythromycin and oleandomycin is due to production of erythromycin esterases which hydrolyze the lactone ring of the 14-membered macrolides. We recently reported the nucleotide sequence of ereA and ereB (erythromycin resistance esterase) genes which encode erythromycin esterases type I and II, respectively. The amino acid sequences of the two isozymes do not exhibit statistically significant homology. Analysis of codon usage in both genes suggests that esterase type I is indigenous to E. coli, whereas the type II enzyme was acquired by E. coli from a phylogenetically remote micro-organism. Inactivation of lincosamides, first reported in staphylococci and lactobacilli of animal origin, was also recently detected in Gram-positive cocci isolated from humans.(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleotide sequence of gene linA encoding resistance to lincosamides in Staphylococcus haemolyticus

We have determined the nucleotide sequence of gene linA in plasmid pIP855, which confers resistance to lincomycin by inactivating it in Staphylococcus haemolyticus. The gene was defined by start and stop codons and an open reading frame of 483 bp corresponding to a product with an Mr of 19020. The apparent size of the resistance-conferring protein was 21 kDa, when a minicell extract was electrophoresed in the sodium dodecyl sulfate-polyacrylamide gels. Full expression of linA was obtained both in Escherichia coli and Bacillus subtilis.