The oxazolidinones (linezolid and tedizolid) are last-resort antimicrobial agents used for the treatment of severe infections in humans caused by MDR Gram-positive bacteria. They bind to the peptidyl transferase centre of the bacterial ribosome inhibiting protein synthesis. Even if the majority of Gram-positive bacteria remain susceptible to oxazolidinones, resistant isolates have been reported worldwide. Apart from mutations, affecting mostly the 23S rDNA genes and selected ribosomal proteins, acquisition of resistance genes (cfr and cfr-like, optrA and poxtA), often associated with mobile genetic elements [such as non-conjugative and conjugative plasmids, transposons, integrative and conjugative elements (ICEs), prophages and translocatable units], plays a critical role in oxazolidinone resistance. In this review, we briefly summarize the current knowledge on oxazolidinone resistance mechanisms and provide an overview on the diversity of the mobile genetic elements carrying oxazolidinone resistance genes in Gram-positive and Gram-negative bacteria.

Sixty-three recent Italian clinical isolates of Streptococcus pyogenes resistant to both erythromycin (MICs ≥ 1 μg/ml) and tetracycline (MICs ≥ 8 μg/ml) were genotyped for macrolide and tetracycline resistance genes. We found 19 isolates carrying the mef (A) and the tet (O) genes; 25 isolates carrying the erm (A) and tet (O) genes; and 2 isolates carrying the erm (A), tet (M), and tet (O) genes. The resistance of all erm (A)-containing isolates was inducible, but the isolates could be divided into two groups on the basis of erythromycin MICs of either >128 or 1 to 4 μg/ml. The remaining 17 isolates included 15 isolates carrying the erm (B) gene and 2 isolates carrying both the erm (B) and the mef (A) genes, with all 17 carrying the tet (M) gene. Of these, 12 carried Tn 916 -Tn 1545 -like conjugative transposons. Conjugal transfer experiments demonstrated that the tet (O) gene moved with and without the erm (A) gene and with the mef (A) gene. These studies, together with the results of pulsed-field gel electrophoresis experiments and hybridization assays with DNA probes specific for the tet (O), erm (A), and mef (A) genes, suggested a linkage of tet (O) with either erm (A) or mef (A) in erythromycin- and tetracycline-resistant S. pyogenes isolates. By amplification and sequencing experiments, we detected the tet (O) gene ca. 5.5 kb upstream from the mef (A) gene. This is the first report demonstrating the presence of the tet (O) gene in S. pyogenes and showing that it may be linked with another gene and can be moved by conjugation from one chromosome to another.

This study was directed at characterizing the genetic elements carrying the methylase gene erm (B), encoding ribosome modification-mediated resistance to macrolide, lincosamide, and streptogramin B (MLS) antibiotics, in Streptococcus pyogenes . In this species, erm (B) is responsible for MLS resistance in constitutively resistant isolates (cMLS phenotype) and in a subset (iMLS-A) of inducibly resistant isolates. A total of 125 erm (B)-positive strains were investigated, 81 iMLS-A (uniformly tetracycline susceptible) and 44 cMLS (29 tetracycline resistant and 15 tetracycline susceptible). Whereas all tetracycline-resistant isolates carried the tet (M) gene, tet (M) sequences were also detected in most tetracycline-susceptible isolates (81/81 iMLS-A and 7/15 cMLS). In 2 of the 8 tet (M)-negative cMLS isolates, erm (B) was carried by a plasmid-located Tn 917 -like transposon. erm (B)- and tet (M)-positive isolates were tested by PCR for the presence of genes int (integrase), xis (excisase), and tndX (resolvase), associated with conjugative transposons of the Tn 916 family. In mating experiments using representatives of different combinations of phenotypic and genotypic characteristics as donors, erm (B) and tet (M) were consistently cotransferred, suggesting their linkage in individual genetic elements. The linkage was confirmed by pulsed-field gel electrophoresis and hybridization studies, and different elements, variably associated with the different phenotypes/genotypes, were detected and characterized by amplification and sequencing experiments. A previously unreported genetic organization, observed in all iMLS-A and some cMLS isolates, featured an erm (B)-containing DNA insertion into the tet (M) gene of a defective Tn 5397 , a Tn 916 -related transposon. This new element was designated Tn 1116 . Genetic elements not previously described in S. pyogenes also included Tn 6002 , an unpublished transposon whose complete sequence is available in GenBank, and Tn 3872 , a composite element resulting from the insertion of the Tn 917 transposon into Tn 916 [associated with a tet (M) gene expressed in some cMLS isolates and silent in others]. The high frequency of association between a tetracycline-susceptible phenotype and tet (M) genes suggests that transposons of the Tn 916 family, so far typically associated solely with a tetracycline-resistant phenotype, may be more widespread in S. pyogenes than currently believed.

Phim46.1, the recognized representative of the most common variant of mobile, prophage-associated genetic elements carrying resistance genes mef(A) (which confers efflux-mediated erythromycin resistance) and tet(O) (which confers tetracycline resistance) in Streptococcus pyogenes, was fully characterized. Sequencing of the Phim46.1 genome (55,172 bp) demonstrated a modular organization typical of tailed bacteriophages. Electron microscopic analysis of mitomycin-induced Phim46.1 revealed phage particles with the distinctive icosahedral head and tail morphology of the Siphoviridae family. The chromosome integration site was within a 23S rRNA uracil methyltransferase gene. BLASTP analysis revealed that the proteins of Phim46.1 had high levels of amino acid sequence similarity to the amino acid sequences of proteins from other prophages, especially Phi10394.4 of S. pyogenes and lambdaSa04 of S. agalactiae. Phage DNA was present in the host cell both as a prophage and as free circular DNA. The lysogeny module appears to have been split due to the insertion of a segment containing tet(O) (from integrated conjugative element 2096-RD.2) and mef(A) (from a Tn1207.1-like transposon) into the unintegrated phage DNA. The phage attachment sequence lies in the region between tet(O) and mef(A) in the unintegrated form. Thus, whereas in this form tet(O) is approximately 5.5 kb upstream of mef(A), in the integrated form, tet(O), which lies close to the right end of the prophage, is approximately 46.3 kb downstream of mef(A), which lies close to the left end of the prophage.

Skin and chronic wound infections are an increasing and urgent health problem worldwide. Their management is difficult and the development of antibiotic resistance by both planktonic and biofilm-associated bacteria necessitates the use of alternative treatments. The purpose of this study was to compare the antimicrobial activity of four honeys from different floral and geographical origins: Melipona beecheii honey (Cuba) and three Apis mellifera honeys [Manuka honey (New Zealand), Apis mellifera honey (Cuba), and African honey (Kenya)]. The physicochemical parameters were within the ranges reported for these honeys and M. beecheii honey stood out due to its acidic character. An agar incorporation technique was used to determine the minimum active dilution of each honey against 52 clinical isolates (34 Gram-positive, 17 Gram-negative, and 1 Candida albicans). The antibiofilm activity of honeys was tested by assessing their ability to inhibit biofilm formation and to disrupt preformed biofilms. Overall, M. beecheii honey had the highest antimicrobial and antibiofilm activity, although a marked disruption in preformed biofilms was shared by all tested honeys. Structural changes induced by M. beecheii honey on Staphylococcus aureus and Pseudomonas aeruginosa cells were observed by transmission electron microscopy suggesting that this honey has a potent antimicrobial action and may be an excellent candidate for the development of topical preparations for the treatment of infected wounds.