Prevalence of mefE, erm and tet(M) genes in Streptococcus pneumoniae strains from Central Italy

Detection of tetracycline and macrolide resistance determinants in Enterococci of animal and environmental origin using multiplex PCR

An occurrence of resistance to tetracycline (TET) and erythromycin (ERY) was ascertained in 82 isolates of Enterococcus spp. of animal and environmental origin. Using E test, 33 isolates were resistant to TET and three isolates to ERY. Using polymerase chain reaction (PCR; single and multiplex), the TET determinants tet(M) and tet(L) were detected in 35 and 13 isolates, respectively. Twelve isolates carried both tet(M) and tet(L) genes. Eight isolates possessed ermB gene associated with ERY resistance. Multiplex PCR was shown to be a suitable method for simultaneous determination of all three resistance determinants that occurred most frequently in bacteria isolated from poultry. This study also demonstrates that gastrointestinal tract of broilers may be a reservoir of enterococci with acquired resistance to both TET and ERY that can be transferred to humans via food chain.

Antimicrobial susceptibility of Streptococcus pyogenes and Streptococcus pneumoniae: surveillance from 1993 to 2004 in Central Italy

The susceptibility of 1870 Streptococcus pyogenes and 1595 Streptococcus pneumoniae to macrolides and lincosamides has been monitored from 1993 to 2004 in Central Italy. Among S. pyogenes, 30.2% were erythromycin resistant; 18.5% were also resistant to josamycin and clindamycin (MLS phenotype). After an increasing erythromycin resistance rate in 1993-1997 (maximum 53.16%), a definite decrease was observed since 2001 with resistance rates always less than 30%. Thirty six percent of pneumococcal isolates were erythromycin-resistant, with minor temporal fluctuations; the MLS phenotype was the most prevalent overall (32.6%) and in individual years. S. pneumoniae strains were also tested for susceptibility to beta-lactams and other antimicrobial agents: 11.2% were penicillin non-susceptible, with a gradually increasing prevalence after 2001 (maximum rate 17.3% in 2004), 31.15% were resistant to tetracycline, 4.9% to chloramphenicol, 0.74% to rifampin. All pneumococcal isolates were susceptible to teicoplanin and 99.9% to ceftriaxone and ofloxacin.

Comparison and Transferability of theerm(B) Genes between Human and Farm Animal Streptococci

To obtain better insights into the possible exchange of resistance genes between human and animal streptococci, the sequences of the erm (B) genes of streptococcal isolates from humans, pigs, pork carcasses, chickens, and calves were compared. Identical erm (B) gene sequences were present in strains from humans, pigs, pork carcasses, and calves. During in vitro mating experiments, the erm (B) gene was exchanged between porcine Streptococcus suis and human S. pneumoniae, S. pyogenes, and S. oralis strains. The presence of different tetracycline resistance genes and the int Tn 1545 gene was determined in animal streptococci carrying the erm (B) gene. Although tet(M) and int Tn 1545 genes were detected in 24% of the porcine and pork carcass streptococcal strains, the tet(O) gene was the predominant tetracycline resistance gene in these strains (81%). The latter gene was co-transferred with the erm (B) gene from porcine S. suis strains to human streptococci in the mating experiments. These results show that, identical erm (B) gene sequences were present in animal and human streptococci and that transfer of the erm (B) gene from porcine S. suis to human streptococci and vice versa is possible, but probably occurs at a low frequency.

Distribution of theerm(B) Gene,tetracycline Resistance Genes, and Tn1545-like Transposons in Macrolide- and Lincosamide-Resistant Enterococci from Pigs and Humans

The distribution of the erm (B) and the tetracycline resistance genes tet(K), tet(L), tet(M), tet(O), and tet(S) was investigated among macrolide- and lincosamide-resistant enterococci originating from humans, pigs, and pork carcasses. The presence of transposons of the Tn916/Tn1545 family was also traced in these isolates. Furthermore, the porcine strains were tested for the presence of glycopeptide resistance genes vanA and vanB. The erm(B) gene was found in 85% of the porcine and in all human isolates. Ninety-eight percent of the porcine and 89% of the human erm(B)-positive enterococci carried the tet(M) gene. Seventy-seven percent and 70%, respectively, of these strains harbored a Tn1545-like element. Tet(L) was observed in 68% of the porcine and in 65% of the human enterococci. The other tetracycline resistance genes were very rare and the glycopeptide resistance genes vanA and vanB were not detected among the porcine isolates. The similar frequencies of resistance genes and the highly mobile Tn1545-like transposon among porcine and human enterococci might indicate exchange of resistant strains or their resistance genes between humans and pigs or the existence of a common reservoir.

Phenotypic and molecular characterization of tetracycline- and erythromycin-resistant strains of Streptococcus pneumoniae

Sixty-five clinical isolates of Streptococcus pneumoniae, all collected in Italy between 1999 and 2002 and resistant to both tetracycline (MIC, >or=8 microg/ml) and erythromycin (MIC, >or=1 microg/ml), were investigated. Of these strains, 11% were penicillin resistant and 23% were penicillin intermediate. With the use of the erythromycin-clindamycin-rokitamycin triple-disk test, 14 strains were assigned to the constitutive (cMLS) phenotype of macrolide resistance, 44 were assigned to the partially inducible (iMcLS) phenotype, 1 was assigned to the inducible (iMLS) phenotype, and 6 were assigned to the efflux-mediated (M) phenotype. In PCR assays, 64 of the 65 strains were positive for the tetracycline resistance gene tet(M), the exception being the one M isolate susceptible to kanamycin, whereas tet(K), tet(L), and tet(O) were never found. All cMLS, iMcLS, and iMLS isolates had the erythromycin resistance gene erm(B), and all M phenotype isolates had the mef(A) or mef(E) gene. No isolate had the erm(A) gene. The int-Tn gene, encoding the integrase of the Tn916-Tn1545 family of conjugative transposons, was detected in 62 of the 65 test strains. Typing assays showed the strains to be to a great extent unrelated. Of 16 different serotypes detected, the most numerous were 23F (n = 13), 19A (n = 10), 19F (n = 9), 6B (n = 8), and 14 (n = 6). Of 49 different pulsed-field gel electrophoresis types identified, the majority (n = 39) were represented by a single isolate, while the most numerous type included five isolates. By high-resolution restriction analysis of PCR amplicons with four endonucleases, the tet(M) loci from the 64 tet(M)-positive pneumococci were classified into seven distinct restriction types. Overall, a Tn1545-like transposon could reasonably account for tetracycline and erythromycin resistance in the vast majority of the pneumococci of cMLS, iMcLS, and iMLS phenotypes, whereas a Tn916-like transposon could account for tetracycline resistance in most M phenotype strains.

Phenotypes and genotypes of erythromycin-resistant pneumococci in Italy

Of 120 erythromycin-resistant pneumococci isolated in Italian hospitals, 39 (32.5%) were M-type isolates, carrying the mef gene alone. The mef gene was also detected, together with erm(AM), in one constitutively resistant isolate and in five isolates of the partially inducible phenotype. Among the 45 mef-positive isolates, 25 (55.6%) carried mef(A) and 20 (44.4%) carried mef(E) as observed from PCR-restriction fragment length polymorphism analysis of a 1,743-bp amplicon. The same result was obtained by a similar method applied to a more common 348-bp amplicon.

Macrolide resistance mechanisms in Gram-positive cocci

Two principal mechanisms of resistance to macrolides have been identified in Gram-positive bacteria. Erythromycin-resistant methylase is encoded by erm genes. Resultant structural changes to rRNA prevent macrolide binding and allow synthesis of bacterial proteins to continue. Presence of the erm gene results in high-level resistance. Modification of the mechanism whereby antibiotics are eliminated from the bacteria also brings about resistance. Bacteria carrying the gene encoding macrolide efflux (i.e. the mefE gene) display relatively low-level resistance. Azithromycin, because of its ability to achieve concentrations at sites of infections, is capable of eradicating mefE-carrying strains. Other resistance mechanisms, involving stimulation of enzymatic degradation, appear not to be clinically significant.

Clinical relevance of macrolide-resistant Streptococcus pneumoniae for community-acquired pneumonia

Macrolides are often the first choice for empirical treatment of community-acquired pneumonia. However, macrolide resistance among Streptococcus pneumoniae has escalated at alarming rates in North America and worldwide. Macrolide resistance among pneumococci is primarily due to genetic mutations affecting the ribosomal target site (ermAM) or active drug efflux (mefE). Prior antibiotic exposure is the major risk factor for amplification and perpetuation of resistance. Clonal spread facilitates dissemination of drug-resistant strains. Data assessing the impact of macrolide resistance on clinical outcomes are spare. Many experts believe that the clinical impact is limited. Ribosomal mutations confer high-grade resistance, whereas efflux mutations can likely be overridden in vivo. Favorable pharmacokinetics and pharmacodynamics, high concentrations at sites of infections, and additional properties of macrolides may enhance their efficacy. In this article, we discuss the prevalence of macrolide resistance among S. pneumoniae, risk factors and mechanisms responsible for resistance, therapeutic strategies, and implications for the future.

Molecular surveillance of macrolide, tetracycline and quinolone resistance mechanisms in 1191 clinical European Streptococcus pneumoniae isolates

Streptococcus pneumoniae isolates (n=1191) were collected during a 1997-1999 European surveillance study. In addition to susceptibility data, a molecular epidemiological survey of their mechanisms of resistance to macrolides, tetracyclines, and quinolones was provided. Of the isolates tested, 72.6% were penicillin-susceptible, 19.9% penicillin-intermediate and 7.5% penicillin-resistant. There was an obvious relationship between resistance to penicillin and resistance to erythromycin (19% of all isolates), clindamycin (14%) and tetracycline (23%). Only one isolate was resistant to levofloxacin. Seventy-three percent of the European S. pneumoniae isolates resistant to erythromycin (n=229) carried the erm(B) gene, while the remaining 27% possessed the mef(A) gene. No mutations were detected in 23S rRNA or in ribosomal proteins L4 and L22. All tetracycline-resistant isolates (n=277) carried the tet(M) gene; none carried the tet(O) gene. Classical mutations in gyrA (Ser 81-Phe or Tyr) and parC (Ser 79-Phe and Asp 83-Asn) and efflux contributed to the decreased quinolone susceptibility. This study of recent European S. pneumoniae isolates can be used to recognize any changes in susceptibility patterns and resistance mechanisms that may occur in the future.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Two new mechanisms of macrolide resistance in clinical strains of Streptococcus pneumoniae from Eastern Europe and North America

Resistance to macrolides in pneumococci is generally mediated by methylation of 23S rRNA via erm(B) methylase which can confer a macrolide (M)-, lincosamide (L)-, and streptogramin B (S(B))-resistant (MLS(B)) phenotype or by drug efflux via mef(A) which confers resistance to 14- and 15-membered macrolides only. We studied 20 strains with unusual ML or MS(B) phenotypes which did not harbor erm(B) or mef(A). The strains had been isolated from patients in Eastern Europe and North America from 1992 to 1998. These isolates were found to contain mutations in genes for either 23S rRNA or ribosomal proteins. Three strains from the United States with an ML phenotype, each representing a different clone, were characterized as having an A2059G (Escherichia coli numbering) change in three of the four 23S rRNA alleles. Susceptibility to macrolides and lincosamides decreased as the number of alleles in isogenic strains containing A2059G increased. Sixteen MS(B) strains from Eastern Europe were found to contain a 3-amino-acid substitution ((69)GTG(71) to TPS) in a highly conserved region of the ribosomal protein L4 ((63)KPWRQKGTGRAR(74)). These strains formed several distinct clonal types. The single MS(B) strain from Canada contained a 6-amino-acid L4 insertion ((69)GTGREKGTGRAR), which impacted growth rate and also conferred a 500-fold increase in MIC on the ketolide telithromycin. These macrolide resistance mechanisms from clinical isolates are similar to those recently described for laboratory-derived mutants.

Antimicrobial use and colonization with erythromycin-resistant Streptococcus pneumoniae in Greece during the first 2 years of life

We evaluated nasopharyngeal colonization with erythromycin-resistant Streptococcus pneumoniae during the first 2 years of life in central and southern Greece. Of 2448 children studied from February 1997 to February 1999, 766 (31%) carried 781 pneumococcal isolates. Ninety-five (3.9%) of the children attended day care centers. Eighteen percent of the pneumococci were resistant to erythromycin (minimal inhibitory concentration 1 to >128 microg/mL), with 67.9% of them carrying the erm(B) gene and 29.2% mef(A) gene products. Four strains possessed neither the erm(B) nor the mef(A) gene. Multidrug resistance occurred in 97% and 40% of isolates carrying the erm(B) and mef(A) gene, respectively. An association was found between the erm(B) gene and serotypes 6B and 23F and between the mef(A) gene and serotypes 14 and 19F. A significant relationship existed between carriage of erythromycin-resistant pneumococci and use of macrolides or beta-lactams in the previous 3 months; the association was strongest when macrolide therapy was administered during the last month (odds ratio, 5.92; P=.0001). The findings indicate the necessity of a judicious use of both macrolides and beta-lactams in young children to reduce the colonization with erythromycin-resistant pneumococci and the subsequent spread of such strains to the community.

Mutations in 23S rRNA and ribosomal protein L4 account for resistance in pneumococcal strains selected in vitro by macrolide passage

The mechanisms responsible for macrolide resistance in Streptococcus pneumoniae mutants, selected from susceptible strains by serial passage in azithromycin, were investigated. These mutants were resistant to 14- and 15-membered macrolides, but resistance could not be explained by any clinically relevant resistance determinant [mef(A), erm(A), erm(B), erm(C), erm(TR), msr(A), mph(A), mph(B), mph(C), ere(A), ere(B)]. An investigation into the sequences of 23S rRNAs in the mutant and parental strains revealed individual changes of C2611A, C2611G, A2058G, and A2059G (Escherichia coli numbering) in four mutants. Mutations at these residues in domain V of 23S rRNA have been noted to confer erythromycin resistance in other species. Not all four 23S rRNA alleles have to contain the mutation to confer resistance. Some of the mutations also confer coresistance to streptogramin B (C2611A, C2611G, and A2058G), 16-membered macrolides (all changes), and clindamycin (A2058G and A2059G). Interestingly, none of these mutations confer high-level resistance to telithromycin (HMR-3647). Further, two of the mutants which had no changes in their 23S rRNA sequences had changes in a highly conserved stretch of amino acids ((63)KPWRQKGTGRAR(74)) in ribosomal protein L4. One mutant contained a single amino acid change (G69C), while the other mutant had a 6-base insert, resulting in two amino acids (S and Q) being inserted between amino acids Q67 and K68. To our knowledge, this is the first description of mutations in 23S rRNA genes or ribosomal proteins in macrolide-resistant S. pneumoniae strains.