Diversity among the gram-positive acetyltransferases inactivating streptogramin A and structurally related compounds and characterization of a new staphylococcal determinant, vatB

Metagenomics uncovers microbiome and resistome in soil and reindeer faeces from Ny-Ålesund (Svalbard, High Arctic)

Research on the microbiome and resistome in polar environments, such as the Arctic, is crucial for understanding the emergence and spread of antibiotic resistance genes (ARGs) in the environment. In this study, soil and reindeer faeces samples collected from Ny-Ålesund (Svalbard, High Arctic) were examined to analyze the microbiome, ARGs, and biocide/metal resistance genes (BMRGs). The dominant phyla in both soil and faeces were Pseudomonadota, Actinomycetota, and Bacteroidota. A total of 2618 predicted Open Reading Frames (ORFs) containing antibiotic resistance genes (ARGs) were detected. These ARGs belong to 162 different genes across 17 antibiotic classes, with rifamycin and multidrug resistance genes being the most prevalent. We focused on investigating antibiotic resistance mechanisms in the Ny-Ålesund environment by analyzing the resistance genes and their biological pathways. Procrustes analysis demonstrated a significant correlation between bacterial communities and ARG/BMRG profiles in soil and faeces samples. Correlation analysis revealed that Pseudomonadota contributed most to multidrug and triclosan resistance, while Actinomycetota were predominant contributors to rifamycin and aminoglycoside resistance. The geochemical factors, SiO42- and NH4+, were found to significantly influence the microbial composition and ARG distribution in the soil samples. Analysis of ARGs, BMRGs, virulence factors (VFs), and pathogens identified potential health risks associated with certain bacteria, such as Cryobacterium and Pseudomonas, due to the presence of different genetic elements. This study provided valuable insights into the molecular mechanisms and geochemical factors contributing to antibiotic resistance and enhanced our understanding of the evolution of antibiotic resistance genes in the environment.

Vancomycin Resistance in Enterococcus and Staphylococcus aureus

Enterococcus faecalis, Enterococcus faecium and Staphylococcus aureus are both common commensals and major opportunistic human pathogens. In recent decades, these bacteria have acquired broad resistance to several major classes of antibiotics, including commonly employed glycopeptides. Exemplified by resistance to vancomycin, glycopeptide resistance is mediated through intrinsic gene mutations, and/or transferrable van resistance gene cassette-carrying mobile genetic elements. Here, this review will discuss the epidemiology of vancomycin-resistant Enterococcus and S. aureus in healthcare, community, and agricultural settings, explore vancomycin resistance in the context of van and non-van mediated resistance development and provide insights into alternative therapeutic approaches aimed at treating drug-resistant Enterococcus and S. aureus infections.

Molecular Mechanisms of Drug Resistance in Staphylococcus aureus

This paper discusses the mechanisms of S. aureus drug resistance including: (1) introduction. (2) resistance to beta-lactam antibiotics, with particular emphasis on the mec genes found in the Staphylococcaceae family, the structure and occurrence of SCCmec cassettes, as well as differences in the presence of some virulence genes and its expression in major epidemiological types and clones of HA-MRSA, CA-MRSA, and LA-MRSA strains. Other mechanisms of resistance to beta-lactam antibiotics will also be discussed, such as mutations in the gdpP gene, BORSA or MODSA phenotypes, as well as resistance to ceftobiprole and ceftaroline. (3) Resistance to glycopeptides (VRSA, VISA, hVISA strains, vancomycin tolerance). (4) Resistance to oxazolidinones (mutational and enzymatic resistance to linezolid). (5) Resistance to MLS-B (macrolides, lincosamides, ketolides, and streptogramin B). (6) Aminoglycosides and spectinomicin, including resistance genes, their regulation and localization (plasmids, transposons, class I integrons, SCCmec), and types and spectrum of enzymes that inactivate aminoglycosides. (7). Fluoroquinolones (8) Tetracyclines, including the mechanisms of active protection of the drug target site and active efflux of the drug from the bacterial cell. (9) Mupirocin. (10) Fusidic acid. (11) Daptomycin. (12) Resistance to other antibiotics and chemioterapeutics (e.g., streptogramins A, quinupristin/dalfopristin, chloramphenicol, rifampicin, fosfomycin, trimethoprim) (13) Molecular epidemiology of MRSA.

Madumycin II inhibits peptide bond formation by forcing the peptidyl transferase center into an inactive state

The emergence of multi-drug resistant bacteria is limiting the effectiveness of commonly used antibiotics, which spurs a renewed interest in revisiting older and poorly studied drugs. Streptogramins A is a class of protein synthesis inhibitors that target the peptidyl transferase center (PTC) on the large subunit of the ribosome. In this work, we have revealed the mode of action of the PTC inhibitor madumycin II, an alanine-containing streptogramin A antibiotic, in the context of a functional 70S ribosome containing tRNA substrates. Madumycin II inhibits the ribosome prior to the first cycle of peptide bond formation. It allows binding of the tRNAs to the ribosomal A and P sites, but prevents correct positioning of their CCA-ends into the PTC thus making peptide bond formation impossible. We also revealed a previously unseen drug-induced rearrangement of nucleotides U2506 and U2585 of the 23S rRNA resulting in the formation of the U2506•G2583 wobble pair that was attributed to a catalytically inactive state of the PTC. The structural and biochemical data reported here expand our knowledge on the fundamental mechanisms by which peptidyl transferase inhibitors modulate the catalytic activity of the ribosome.

Lincosamides, Streptogramins, Phenicols, and Pleuromutilins: Mode of Action and Mechanisms of Resistance

Lincosamides, streptogramins, phenicols, and pleuromutilins (LSPPs) represent four structurally different classes of antimicrobial agents that inhibit bacterial protein synthesis by binding to particular sites on the 50S ribosomal subunit of the ribosomes. Members of all four classes are used for different purposes in human and veterinary medicine in various countries worldwide. Bacteria have developed ways and means to escape the inhibitory effects of LSPP antimicrobial agents by enzymatic inactivation, active export, or modification of the target sites of the agents. This review provides a comprehensive overview of the mode of action of LSPP antimicrobial agents as well as of the mutations and resistance genes known to confer resistance to these agents in various bacteria of human and animal origin.

Plasmid-Mediated Antimicrobial Resistance in Staphylococci and Other Firmicutes

In staphylococci and other Firmicutes, resistance to numerous classes of antimicrobial agents, which are commonly used in human and veterinary medicine, is mediated by genes that are associated with mobile genetic elements. The gene products of some of these antimicrobial resistance genes confer resistance to only specific members of a certain class of antimicrobial agents, whereas others confer resistance to the entire class or even to members of different classes of antimicrobial agents. The resistance mechanisms specified by the resistance genes fall into any of three major categories: active efflux, enzymatic inactivation, and modification/replacement/protection of the target sites of the antimicrobial agents. Among the mobile genetic elements that carry such resistance genes, plasmids play an important role as carriers of primarily plasmid-borne resistance genes, but also as vectors for nonconjugative and conjugative transposons that harbor resistance genes. Plasmids can be exchanged by horizontal gene transfer between members of the same species but also between bacteria belonging to different species and genera. Plasmids are highly flexible elements, and various mechanisms exist by which plasmids can recombine, form cointegrates, or become integrated in part or in toto into the chromosomal DNA or into other plasmids. As such, plasmids play a key role in the dissemination of antimicrobial resistance genes within the gene pool to which staphylococci and other Firmicutes have access. This chapter is intended to provide an overview of the current knowledge of plasmid-mediated antimicrobial resistance in staphylococci and other Firmicutes.

Genetic mechanisms of antimicrobial resistance identified in Salmonella enterica, Escherichia coli, and Enteroccocus spp. isolated from U.S. food animals

The prevalence of antimicrobial resistance (AR) in bacteria isolated from U.S. food animals has increased over the last several decades as have concerns of AR foodborne zoonotic human infections. Resistance mechanisms identified in U.S. animal isolates of Salmonella enterica included resistance to aminoglycosides (e.g., alleles of aacC, aadA, aadB, ant, aphA, and StrAB), β-lactams (e.g., bla_{CMY−2, TEM−1, PSE−1}), chloramphenicol (e.g., floR, cmlA, cat1, cat2), folate pathway inhibitors (e.g., alleles of sul and dfr), and tetracycline [e.g., alleles of tet(A), (B), (C), (D), (G), and tetR]. In the U.S., multi-drug resistance (MDR) mechanisms in Salmonella animal isolates were associated with integrons, or mobile genetic elements (MGEs) such as IncA/C plasmids which can be transferred among bacteria. It is thought that AR Salmonella originates in food animals and is transmitted through food to humans. However, some AR Salmonella isolated from humans in the U.S. have different AR elements than those isolated from food animals, suggesting a different etiology for some AR human infections. The AR mechanisms identified in isolates from outside the U.S. are also predominantly different. For example the extended spectrum β-lactamases (ESBLs) are found in human and animal isolates globally; however, in the U.S., ESBLs thus far have only been found in human and not food animal isolates. Commensal bacteria in animals including Escherichia coli and Enterococcus spp. may be reservoirs for AR mechanisms. Many of the AR genes and MGEs found in E. coli isolated from U.S. animals are similar to those found in Salmonella. Enterococcus spp. isolated from animals frequently carry MGEs with AR genes, including resistances to aminoglycosides (e.g., alleles of aac, ant, and aph), macrolides [e.g., erm(A), erm(B), and msrC], and tetracyclines [e.g., tet(K), (L), (M), (O), (S)]. Continuing investigations are required to help understand and mitigate the impact of AR bacteria on human and animal health.

Complete genome sequence of the fish pathogen Flavobacterium branchiophilum

Members of the genus Flavobacterium occur in a variety of ecological niches and represent an interesting diversity of lifestyles. Flavobacterium branchiophilum is the main causative agent of bacterial gill disease, a severe condition affecting various cultured freshwater fish species worldwide, in particular salmonids in Canada and Japan. We report here the complete genome sequence of strain FL-15 isolated from a diseased sheatfish (Silurus glanis) in Hungary. The analysis of the F. branchiophilum genome revealed putative mechanisms of pathogenicity strikingly different from those of the other, closely related fish pathogen Flavobacterium psychrophilum, including the first cholera-like toxin in a non-Proteobacteria and a wealth of adhesins. The comparison with available genomes of other Flavobacterium species revealed a small genome size, large differences in chromosome organization, and fewer rRNA and tRNA genes, in line with its more fastidious growth. In addition, horizontal gene transfer shaped the evolution of F. branchiophilum, as evidenced by its virulence factors, genomic islands, and CRISPR (clustered regularly interspaced short palindromic repeats) systems. Further functional analysis should help in the understanding of host-pathogen interactions and in the development of rational diagnostic tools and control strategies in fish farms.

Pathogens resistant to antibacterial agents

Resistance to antimicrobial drugs is increasing at an alarming rate among both gram-positive and gram-negative bacteria. Traditionally, bacteria resistant to multiple antimicrobial agents have been restricted to the nosocomial environment. A disturbing trend has been the recent emergence and spread of resistant pathogens in nursing homes, in the community, and in the hospital. This article reviews the epidemiology, molecular mechanisms of resistance, and treatment options for pathogens resistant to antimicrobial drugs.

Characterization of two newly identified genes, vgaD and vatH, [corrected] conferring resistance to streptogramin A in Enterococcus faecium

We characterized two new streptogramin A resistance genes from quinupristin-dalfopristin-resistant Enterococcus faecium JS79, which was selected from 79 E. faecium isolates lacking known genes encoding streptogramin A acetyltransferase. A 5,650-bp fragment of HindIII-digested plasmid DNA from E. faecium JS79 was cloned and sequenced. The fragment contained two open reading frames carrying resistance genes related to streptogramin A, namely, genes for an acetyltransferase and an ATP efflux pump. The first open reading frame comprised 648 bp encoding 216 amino acids with a predicted left-handed parallel β-helix domain structure; this new gene was designated vatH. [corrected] The second open reading frame consisted of 1,575 bp encoding 525 amino acids with two predicted ATPase binding cassette transporters comprised of Walker A, Walker B, and LSSG motifs; this gene was designated vgaD. vgaD is located 65 bp upstream from vatH, [corrected] was detected together with vatH [corrected] in 12 of 179 quinupristin-dalfopristin-resistant E. faecium isolates, and was located on the same plasmid. Also, the 5.6-kb HindIII-digested fragment which was observed in JS79 was detected in nine vgaD- and vatH-containing [corrected] E. faecium isolates by Southern hybridization. Therefore, it was expected that these two genes were strongly correlated with each other and that they may be composed of a transposon. Importantly, vgaD is the first identified ABC transporter conferring resistance to streptogramin A in E. faecium. Pulsed-field gel electrophoresis patterns and sequence types of vgaD- and vatH-containing [corrected] E. faecium isolates differed for isolates from humans and nonhumans.

Activity of quinupristin/dalfopristin against extracellular and intracellular Staphylococcus aureus with various resistance phenotypes

OBJECTIVES

Treatment of chronic or recurrent Staphylococcus aureus infections may require using antibiotics with activity against intracellular multiresistant organisms. Quinupristin/dalfopristin (3:7) has been examined in this context.

METHODS

Quinupristin and dalfopristin were used separately or mixed. Strains used were: (i) methicillin-susceptible and -resistant S. aureus (MSSA and MRSA); (ii) one vat(B) MSSA and msr(A/B) MRSA; (iii) erm(A)+ [MSSA, MRSA, vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA)]; and (iv) one erm(A/B)+ cfr+ MRSA resistant to quinupristin, dalfopristin and their combination. Assessment of activity was determined by: (i) MICs (CLSI method); and (ii) concentration-response curves in broth and after phagocytosis by THP-1 macrophages, with descriptors of the model (Emin) and the pharmacodynamic response [maximal relative efficacy (Emax), relative potency (EC50) and apparent static concentration (Cstatic)].

RESULTS

erm(A)-positive strains were all susceptible to quinupristin/dalfopristin (except strain CM05), with MICs not adversely influenced by acid pH or by the MRSA, VISA or VRSA character of the strain. In concentration-response experiments, quinupristin/dalfopristin showed similar patterns for all strains (except strain CM05), with a >3 log10 cfu decrease in broth and a 1.3 [erm(A) strain] to 2.6 [fully susceptible, vat(B) and msr(A/B) strains] log10 cfu decrease for intracellular bacteria at the maximal extracellular concentration tested (25 mg/L). Maximal extracellular and intracellular activity was obtained for a quinupristin/dalfopristin ratio of 3:7. For strain CM05, quinupristin/dalfopristin was static in all conditions.

CONCLUSIONS

Based on historical comparisons with rifampicin, fluoroquinolones, lipoglycopeptides and other antistaphylococcal drugs with a large accumulation in eukaryotic cells, quinupristin/dalfopristin appears to be one of the most active antibiotics against intracellular S. aureus studied in this model so far, largely irrespective of its resistance phenotype.

Rickettsia phylogenomics: unwinding the intricacies of obligate intracellular life

Background

Completed genome sequences are rapidly increasing for Rickettsia, obligate intracellular α-proteobacteria responsible for various human diseases, including epidemic typhus and Rocky Mountain spotted fever. In light of phylogeny, the establishment of orthologous groups (OGs) of open reading frames (ORFs) will distinguish the core rickettsial genes and other group specific genes (class 1 OGs or C1OGs) from those distributed indiscriminately throughout the rickettsial tree (class 2 OG or C2OGs).

Methodology/Principal Findings

We present 1823 representative (no gene duplications) and 259 non-representative (at least one gene duplication) rickettsial OGs. While the highly reductive (∼1.2 MB) Rickettsia genomes range in predicted ORFs from 872 to 1512, a core of 752 OGs was identified, depicting the essential Rickettsia genes. Unsurprisingly, this core lacks many metabolic genes, reflecting the dependence on host resources for growth and survival. Additionally, we bolster our recent reclassification of Rickettsia by identifying OGs that define the AG (ancestral group), TG (typhus group), TRG (transitional group), and SFG (spotted fever group) rickettsiae. OGs for insect-associated species, tick-associated species and species that harbor plasmids were also predicted. Through superimposition of all OGs over robust phylogeny estimation, we discern between C1OGs and C2OGs, the latter depicting genes either decaying from the conserved C1OGs or acquired laterally. Finally, scrutiny of non-representative OGs revealed high levels of split genes versus gene duplications, with both phenomena confounding gene orthology assignment. Interestingly, non-representative OGs, as well as OGs comprised of several gene families typically involved in microbial pathogenicity and/or the acquisition of virulence factors, fall predominantly within C2OG distributions.

Conclusion/Significance

Collectively, we determined the relative conservation and distribution of 14354 predicted ORFs from 10 rickettsial genomes across robust phylogeny estimation. The data, available at PATRIC (PathoSystems Resource Integration Center), provide novel information for unwinding the intricacies associated with Rickettsia pathogenesis, expanding the range of potential diagnostic, vaccine and therapeutic targets.

Contribution of exogenous genetic elements to the group A Streptococcus metagenome

Variation in gene content among strains of a bacterial species contributes to biomedically relevant differences in phenotypes such as virulence and antimicrobial resistance. Group A Streptococcus (GAS) causes a diverse array of human infections and sequelae, and exhibits a complex pathogenic behavior. To enhance our understanding of genotype-phenotype relationships in this important pathogen, we determined the complete genome sequences of four GAS strains expressing M protein serotypes (M2, M4, and 2 M12) that commonly cause noninvasive and invasive infections. These sequences were compared with eight previously determined GAS genomes and regions of variably present gene content were assessed. Consistent with the previously determined genomes, each of the new genomes is ∼1.9 Mb in size, with ∼10% of the gene content of each encoded on variably present exogenous genetic elements. Like the other GAS genomes, these four genomes are polylysogenic and prophage encode the majority of the variably present gene content of each. In contrast to most of the previously determined genomes, multiple exogenous integrated conjugative elements (ICEs) with characteristics of conjugative transposons and plasmids are present in these new genomes. Cumulatively, 242 new GAS metagenome genes were identified that were not present in the previously sequenced genomes. Importantly, ICEs accounted for 41% of the new GAS metagenome gene content identified in these four genomes. Two large ICEs, designated 2096-RD.2 (63 kb) and 10750-RD.2 (49 kb), have multiple genes encoding resistance to antimicrobial agents, including tetracycline and erythromycin, respectively. Also resident on these ICEs are three genes encoding inferred extracellular proteins of unknown function, including a predicted cell surface protein that is only present in the genome of the serotype M12 strain cultured from a patient with acute poststreptococcal glomerulonephritis. The data provide new information about the GAS metagenome and will assist studies of pathogenesis, antimicrobial resistance, and population genomics.

Quinupristin-dalfopristin nonsusceptibility in pneumococci from sickle cell disease patients

Sickle cell disease (SCD) is a risk factor for fatal pneumococcal infection. Nonsusceptibilty to quinupristin-dalfopristin (Q-D) was absent from 105 non-SCD-associated pneumococcal isolates but was present in 33/148 (22%) SCD-associated isolates. One-third of the isolates harbored a known resistance mechanism. Q-D is not optimal for use for the treatment of pneumococcal infection in SCD patients.

Correlation of agar dilution and VITEK2 system for detection of resistance to macrolides, lincosamides and pristinamycin among Staphylococcus aureus and Staphylococcus epidermidis: association with genotypes

The performance of the VITEK2 system was evaluated against the agar dilution reference procedure for testing susceptibility of Staphylococcus aureus and Staphylococcus epidermidis to macrolides, lincosamides and streptogramins (MLS). Eighty clinical isolates were selected according to their resistance phenotype and genotype. Results for erythromycin and clindamycin showed 100% agreement; results for lincomycin showed agreement of 78%, with one very major error and 17 minor errors; and results for pristinamycin showed agreement of 46%, with one major error and 43 minor errors. Most isolates resistant to lincomycin and streptogramin A (L SgAr phenotype) were falsely susceptible to lincomycin, and intermediately-resistant or resistant to pristinamycin, with the VITEK2 system. No resistance gene was detected. Most (80%) isolates resistant constitutively to MLS (MLS(r)BC phenotype) were falsely intermediately-resistant to pristinamycin with the VITEK2 system. The erm(A) gene was more common than erm(C) in MLS(r)BC strains. Resistance to pristinamycin alone (SgA SgB PTr phenotype), or associated with either lincomycin resistance (L SgA SgB PTr phenotype) or constitutive MLS(B) resistance (MLS(BC) SgA PTr phenotype), was well-characterised without discordant results. Resistance to pristinamycin was always associated with resistance to streptogramin A, encoded by the vga(A), vga(B), vgb(A) and vat(A) genes in association with the erm(A) or erm(C) genes.

Molecular analysis of resistance to streptogramin A compounds conferred by the Vga proteins of staphylococci

The Vga and Msr resistance determinants, encoded by mobile genetic elements in various staphylococcal strains, belong to a family of ATP-binding cassette (ABC) proteins whose functions and structures are ill defined. Their amino acid sequences are similar to those of proteins involved in the immunity of streptomycetes to the macrolide-lincosamide-streptogramin antibiotics that they produce. Sequence analysis of the genomes of the gram-positive bacteria with low G+C contents revealed that Lmo0919 from Listeria monocytogenes is more closely related to Vga variants than to Msr variants. In the present study we compared the antibiotic resistance profiles conferred by the Vga-like proteins in two staphylococcal hosts. It was shown that Vga(A), the Vga(A) variant [Vga(A)v], and Lmo0919 can confer resistance to lincosamides and streptogramin A compounds, while only Vga(B) is able to increase the level of resistance to pristinamycin, a mixture of streptogramin A and streptogramin B compounds. By using polyclonal antibodies, we found that the Vga(A) protein colocalized with the beta subunit of the F(1)-F(0) ATPase in the membrane fractions of staphylococcal cells. In order to identify functional units in these atypical ABC proteins, such as regions that might be involved in substrate specificity and/or membrane targeting, we analyzed the resistance phenotypes conferred by various plasmids carrying parts or modified versions of the vga(A) gene and we determined the subcellular localization of the gene products. Only polypeptides composed of two ABC domains were detected in the cell membranes. No region of drug specificity was identified. Resistance properties were dependent on the integrities of both Walker B motifs.

Molecular basis of bacterial resistance to chloramphenicol and florfenicol

Chloramphenicol (Cm) and its fluorinated derivative florfenicol (Ff) represent highly potent inhibitors of bacterial protein biosynthesis. As a consequence of the use of Cm in human and veterinary medicine, bacterial pathogens of various species and genera have developed and/or acquired Cm resistance. Ff is solely used in veterinary medicine and has been introduced into clinical use in the mid-1990s. Of the Cm resistance genes known to date, only a small number also mediates resistance to Ff. In this review, we present an overview of the different mechanisms responsible for resistance to Cm and Ff with particular focus on the two different types of chloramphenicol acetyltransferases (CATs), specific exporters and multidrug transporters. Phylogenetic trees of the different CAT proteins and exporter proteins were constructed on the basis of a multisequence alignment. Moreover, information is provided on the mobile genetic elements carrying Cm or Cm/Ff resistance genes to provide a basis for the understanding of the distribution and the spread of Cm resistance--even in the absence of a selective pressure imposed by the use of Cm or Ff.

Biological counterstrike: antibiotic resistance mechanisms of Gram-positive cocci

The development of antibiotic resistance by bacteria is an evolutionary inevitability, a convincing demonstration of their ability to adapt to adverse environmental conditions. Since the emergence of penicillinase-producing Staphylococcus aureus in the 1940s, staphylococci, enterococci and streptococci have proved themselves adept at developing or acquiring mechanisms that confer resistance to all clinically available antibacterial classes. The increasing problems of methicillin-resistant S. aureus and coagulase-negative staphylococci (MRSA and MRCoNS), glycopeptide-resistant enterococci and penicillin-resistant pneumococci in the 1980s, and recognition of glycopeptide-intermediate S. aureus in the 1990s and, most recently, of fully vancomycin-resistant isolates of S. aureus have emphasised our need for new anti-Gram-positive agents. Antibiotic resistance is one of the major public health concerns for the beginning of the 21st century. The pharmaceutical industry has responded with the development of oxazolidinones, lipopeptides, injectable streptogramins, ketolides, glycylcyclines, second-generation glycopeptides and novel fluoroquinolones. However, clinical use of these novel agents will cause new selective pressures and will continue to drive the development of resistance. This review describes the various antibiotic resistance mechanisms identified in isolates of staphylococci, enterococci and streptococci, including mechanisms of resistance to recently introduced anti-Gram-positive agents.

A new phenotype of resistance to lincosamide and streptogramin A-type antibiotics in Streptococcus agalactiae in New Zealand

OBJECTIVES

To characterize a new type of resistance to clindamycin in Streptococcus agalactiae.

METHODS

Nineteen erythromycin-susceptible, clindamycin-resistant S. agalactiae isolates from New Zealand were studied. MICs of macrolide, lincosamide and streptogramin antibiotics were determined. Clindamycin and streptogramin resistance genes were searched for by PCR. Isolates were compared by serotyping and by DNA macrorestriction patterns determined by PFGE. Conjugative transfer of resistance traits to recipient strains of S. agalactiae and Enterococcus faecium was assayed.

RESULTS

The 19 S. agalactiae isolates were intermediate or resistant to clindamycin (MIC range: 0.5-2 mg/L) and lincomycin (MIC range: 1-8 mg/L) and had high MICs of dalfopristin (4-32 mg/L), a streptogramin A-type antibiotic, compared with controls. By contrast, the strains were susceptible to macrolides and quinupristin, a streptogramin B-type antibiotic. This new phenotype was called LSA (lincosamide-streptogramin A). Clindamycin resistance could not be transferred to recipient strains. Thirteen isolates belonged to serotype III and to a single PFGE genotype A, and five isolates belonged to serotype I and to genotype B. One isolate was non-typeable and belonged to a distinct genotype C.

CONCLUSIONS

We have characterized a new LSA phenotype in S. agalactiae. Analysis of restriction patterns of S. agalactiae chromosomal DNA showed that the resistance was spread in a minimum of three bacterial clones. The genetic and biochemical basis for the resistance remains unknown.

Quinupristin‐Dalfopristin Resistance in Gram‐Positive Bacteria: Mechanism of Resistance and Epidemiology

Antimicrobial resistance in gram-positive bacteria is a continuing problem resulting in significant morbidity, mortality, and cost. Because of this resistance, new antimicrobial agents have been needed. Quinupristin-dalfopristin is a recently approved agent for treatment of these infections. Shortly after its introduction into clinical medicine, resistance was reported. Resistance can occur by one or more of several mechanisms, including enzymatic modification, active transport of efflux mediated by an adenosine triphosphate-binding protein, and alteration of the target site. Resistance is rare in isolates of staphylococci and Enterococcus faecium from humans. Resistance is common in isolates recovered from food animals and is related to the use of virginiamicin as a feed additive. Considering the effect antimicrobial resistance has on human health, as well as its economic impact, measures to preserve the usefulness of these agents and delay the development of resistance are urgently needed.

Three genes encoding for putative methyl- and acetyltransferases map adjacent to the wzm and wzt genes and are essential for O-antigen biosynthesis in Rhizobium etli CE3

The elucidation of the structure of the O-antigen of Rhizobium etli CE3 predicts that the R. etli CE3 genome must contain genes encoding acetyl- and methyltransferases to confer the corresponding modifications to the O-antigen. We identified three open reading frames (ORFs) upstream of wzm, encoding the membrane component of the O-antigen transporter and located in the lps alpha-region of R. etli CE3. The ORFs encode two putative acetyltransferases with similarity to the CysE-LacA-LpxA-NodL family of acetyltransferases and one putative methyltransferase with sequence motifs common to a wide range of S-adenosyl-L-methionine-dependent methyltransferases. Mutational analysis of the ORFs encoding the putative acetyltransferases and methyltransferase revealed that the acetyl and methyl decorations mediated by these specific enzymes are essential for O-antigen synthesis. Composition analysis and high performance anion exchange chromatography analysis of the lipopolysaccharides (LPSs) of the mutants show that all of these LPSs contain an intact core region and lack the O-antigen polysaccharide. The possible role of these transferases in the decoration of the O-antigen of R. etli is discussed.

Genetics of capsule O-acetylation in serogroup C, W-135 and Y meningococci

Capsular polysaccharides of serogroup C, W-135 and Y meningococci were previously reported to be O-acetylated at the sialic acid residues. There is evidence that O-acetylation affects the immunogenicity of polysaccharide vaccines. We identified genes indispensable for O-acetylation of serogroup C, W-135 and Y meningococci downstream of the capsule synthesis genes siaA-D. The genes were co-transcribed with the sia operon as shown by reverse transcription polymerase chain reaction analysis. The putative capsular polysaccharide O-acetyltransferases were designated OatC and OatWY. The protein OatWY of serogroups W-135 and Y showed sequence homologies to members of the NodL-LacA-CysE family of bacterial acetyltransferases, whereas no sequence homology with any known protein in the different databases was found for the serogroup C protein OatC. In serogroup W-135 and Y meningococci, several clonal lineages either lacked OatWY or OatWY was inactivated by insertion of IS1301. For serogroup C meningococci, we observed in vitro phase variation of O-acetylation, which resulted from slipped-strand mispairing in homopolymeric tracts. This finding explains the observation of naturally occurring de-O-acetylated serogroup C meningococci. Our report is the first description of sequences of sialic acid O-acetyltransferase genes that have not been cloned from either other bacterial or mammalian organisms.

Multiplex PCR assay for simultaneous detection of nine clinically relevant antibiotic resistance genes in Staphylococcus aureus

In this study we describe a multiplex PCR assay for the detection of nine clinically relevant antibiotic resistance genes of Staphylococcus aureus. Conditions were optimized to amplify fragments of mecA (encoding methicillin resistance), aacA-aphD (aminoglycoside resistance), tetK, tetM (tetracycline resistance), erm(A), erm(C) (macrolide-lincosamide-streptogramin B resistance), vat(A), vat(B), and vat(C) (streptogramin A resistance) simultaneously in one PCR amplification. An additional primer pair for the amplification of a fragment of the staphylococcal 16S rDNA was included as a positive control. The multiplex PCR assay was evaluated on 30 different S. aureus isolates, and the PCR results correlated with the phenotypic antibiotic resistance data obtained by the broth microdilution assay. The multiplex PCR assay offers a rapid, simple, and accurate identification of antibiotic resistance profiles and could be used in clinical diagnosis as well as for the surveillance of the spread of antibiotic resistance determinants in epidemiological studies.

Antimicrobial growth promoters used in animal feed: effects of less well known antibiotics on gram-positive bacteria

There are not many data available on antibiotics used solely in animals and almost exclusively for growth promotion. These products include bambermycin, avilamycin, efrotomycin, and the ionophore antibiotics (monensin, salinomycin, narasin, and lasalocid). Information is also scarce for bacitracin used only marginally in human and veterinary medicine and for streptogramin antibiotics. The mechanisms of action of and resistance mechanisms against these antibiotics are described. Special emphasis is given to the prevalence of resistance among gram-positive bacteria isolated from animals and humans. Since no susceptibility breakpoints are available for most of the antibiotics discussed, an alternative approach to the interpretation of MICs is presented. Also, some pharmacokinetic data and information on the influence of these products on the intestinal flora are presented.

Clonal diversity among streptogramin A-resistant Staphylococcus aureus isolates collected in French hospitals

We analyzed 62 clinical isolates of streptogramin A-resistant (SGA(r)) Staphylococcus aureus collected between 1981 and 2001 in 14 hospitals located in seven French cities. These isolates, including five with decreased susceptibility to glycopeptides, were distributed into 45 antibiotypes and 38 SmaI genotypes. Each of these genotypes included between 1 and 11 isolates, the SmaI patterns of which differed by no more than three bands. Although numerous clones were identified, we observed the spread of monoclonal isolates either within the same hospital or within hospitals in distinct cities and at large time intervals. Hybridization with probes directed against 10 SGA(r) genes (vatA, vatB, vatC, vatD, vatE, vgaA, vgaB, vgaAv, vgbA, and vgbB) revealed six patterns: vgaAv (21 isolates), vatA-vgbA (24 isolates), vgaAv-vatB-vgaB (14 isolates), vgaAv-vatA-vgbA (1 isolate), vgaAv-vatA-vgbA-vatB-vgaB (1 isolate), and vgaA (1 isolate). We detected at least one SGA(r) determinant in all of the tested isolates. vgaAv, which is part of the recently characterized transposon Tn5406, was found in 59.7% of the tested isolates. Of the 16 streptogramin B-susceptible isolates, 14 carried vgaAv alone and were susceptible to the mixtures of streptogramins, whereas the 2 isolates carrying vgaAv-vatB-vgaB were resistant to these mixtures. vatA-vgbA was found on plasmids of the same apparent size in 26 (42%) of the tested clinical isolates from 18 unrelated SmaI genotypes. The possible dissemination of some of the multiple clones characterized in the present study with an expected increased selective pressure of streptogramins following the recent licensing of Synercid (quinupristin-dalfopristin) must be carefully monitored.

Tn5406, a new staphylococcal transposon conferring resistance to streptogramin a and related compounds including dalfopristin

We characterized a new transposon, Tn5406 (5,467 bp), in a clinical isolate of Staphylococcus aureus (BM3327). It carries a variant of vgaA, which encodes a putative ABC protein conferring resistance to streptogramin A but not to mixtures of streptogramins A and B. It also carries three putative genes, the products of which exhibit significant similarities (61 to 73% amino acid identity) to the three transposases of the staphylococcal transposon Tn554. Like Tn554, Tn5406 failed to generate target repeats. In BM3327, the single copy of Tn5406 was inserted into the chromosomal att554 site, which is the preferential insertion site of Tn554. In three other independent S. aureus clinical isolates, Tn5406 was either present as a single plasmid copy (BM3318), as two chromosomal copies (BM3252), or both in the chromosome and on a plasmid (BM3385). The Tn5406-carrying plasmids also contain two other genes, vgaB and vatB. The insertion sites of Tn5406 in BM3252 were studied: one copy was in att554, and one copy was in the additional SCCmec element. Amplification experiments revealed circular forms of Tn5406, indicating that this transposon might be active. To our knowledge, a transposon conferring resistance to streptogramin A and related compounds has not been previously described.

Molecular analysis of streptogramin resistance in enterococci

The new semi-synthetic streptogramin antibiotic combination quinupristin/dalfopristin (Synercid) is a promising alternative for a treatment of infections with multiple resistant gram-positive pathogens, e.g. glycopeptide- and multi-resistant Enterococcus faecium. Streptogramins consist of two unrelated compounds, a streptogramin A and B, which act synergistically when given in combination. Mechanisms conferring resistance against both components are essential for resistance against the combination in E. faecium. In this species resistance to streptogramin A compounds is mediated via related acetyltransferases VatD and VatE. Resistance against streptogramins B is either encoded by the widespread ermB gene cluster conferring resistance to macrolide-lincosamide-streptogramin B antibiotics or via expression of the vgbA gene, which encodes a staphylococcal-type lactonase. E. faecalis is intrinsically resistant to streptogramins. Due to a wide use of streptogramins (virginiamycins S/M) in commercial animal farming a reservoir of streptogramin-resistant E. faecium isolates had already been selected. Determinants for streptogramin resistance are localized on plasmids that can be transferred into an E. faecium recipient both in vitro in filter-matings and in vivo in the digestive tracts of rats. Hybridization and sequencing experiments revealed a linkage of resistance determinants for streptogramins A and B on definite plasmid fragments.

Resistance to quinupristin-dalfopristin due to mutation of L22 ribosomal protein in Staphylococcus aureus

The mechanism of resistance to the streptogramin antibiotics quinupristin and dalfopristin was studied in a Staphylococcus aureus clinical isolate selected under quinupristin-dalfopristin therapy, in four derivatives of S. aureus RN4220 selected in vitro, and in a mutant selected in a model of rabbit aortic endocarditis. For all strains the MICs of erythromycin, quinupristin, and quinupristin-dalfopristin were higher than those for the parental strains but the MICs of dalfopristin and lincomycin were similar. Portions of genes for domains II and V of 23S rRNA and the genes for ribosomal proteins L4 and L22 were amplified and sequenced. All mutants contained insertions or deletions in a protruding beta hairpin that is part of the conserved C terminus of the L22 protein and that interacts with 23S rRNA. Susceptible S. aureus RN4220 was transformed with plasmid DNA encoding the L22 alteration, resulting in transformants that were erythromycin and quinupristin resistant. Synergistic ribosomal binding of streptogramins A and B, studied by analyzing the fluorescence kinetics of pristinamycin I(A)-ribosome complexes, was abolished in the mutant strain, providing an explanation for quinupristin-dalfopristin resistance.

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.

Methicillin-resistant, quinupristin-dalfopristin-resistant Staphylococcus aureus with reduced sensitivity to glycopeptides

Of 3,052 Staphylococcus aureus strains collected by the European SENTRY surveillance study, 35 were found to be nonsusceptible to quinupristin-dalfopristin (MIC of > or =2 mg/liter). These isolates originated from four hospitals in France and one in Spain. In isolates from two Parisian hospitals exhibiting the same SmaI macrorestriction pattern, streptogramin resistance was based on vatA and vgbA. One isolate from a hospital in Lyon and 22 from a hospital in Lille were of the vatB vgaB streptogramin A resistance genotype and possessed ermA and/or ermC. As deduced from the loss of either streptogramin A or streptogramin B resistance determinants in particular isolates, resistance to quinupristin-dalfopristin requires mechanisms conferring resistance to both compounds. The SmaI macrorestriction patterns of strains from hospitals in Lille and Lyon were different; however, similarity analysis suggested a relatedness of 20 methicillin-resistant S. aureus strains from the Lille hospital, a finding confirmed by PCR typing based on three different genomic polymorphisms. These groups of isolates were found to be hetero-glycopeptide-intermediate susceptible S. aureus. Information about the failure of glycopeptide chemotherapy has not been available.

Streptogramin resistance and shared pulsed-field gel electrophoresis patterns in vanA-containing Enterococcus faecium and Enterococcus hirae isolated from humans and animals in Spain

The present study was performed to determine if any of the 45 vanA-containing Enterococcus faecium or 18 vanA-containing E. hirae strains were shared by chickens (32 E. faecium/l7 E. hirae) and humans (13 E. faecium/1 E. hirae) using pulsed field gel electrophoresis (PFGE) and to study quinupristin-dalfopristin (Q-D) resistance. Seven of the 45 E. faecium isolates (from 2 outpatients and from 5 poultry products) were resistant to Q-D (MIC > or = 16 microg/ml); one strain was shown to have satA by PCR and sequencing and, in the other six isolates, the recently described satG gene was demonstrated. Six different PFGE patterns were detected among the 7 Q-D E. faecium-resistant isolates. None of the E. hirae isolates showed Q-D resistance. Among the 45 vanA -containing E. faecium strains, 25 unrelated clones were found by PFGE with highly diverse patterns and an indistinguishable PFGE pattern was observed in vanA-containing E. faecium strains from two humans and two poultry products. A single PFGE pattern was detected in 17 of 18 vanA-containing E. hirae isolates, obtained from one human and 16 chicken samples. Based on the presence of indistinguishable PFGE patterns among VR E. faecium and E. hirae from humans and chickens, we conclude that horizontal transfer of these strains could occur between both groups.

Novel streptogramin antibiotics

Streptogramins represent a unique class of antibiotics remarkable for their antibacterial activity and their unique mechanism of action. These antibiotics are produced naturally as secondary metabolites by a number of Streptomyces species and have been classified into two main groups. They consist of at least two structurally unrelated compounds, group A or M (macrolactones) and group B or S (cyclic hexadepsipeptides). Both groups bind bacterial ribosomes and inhibit protein synthesis at the elongation step and they act synergistically in vitro against many microorganisms. Streptogramins A and B act synergistically in vivo; the mixture of the two compounds is more powerful than the individual components and their combined action is irreversible. The pharmacokinetic parameters of group A and B streptogramins in blood are similar. The major gap, limiting the therapeutic use of the natural compounds, was represented by the lack dissolution in water. The synthesis of water-soluble derivatives of pristinamycin I(A) and II(B) has allowed the development of injectable, first represented by quinupristin/dalfopristin (Synercid) and oral formulations, represented by RPR-106972, streptogramins with fixed compositions. Streptogramins have demonstrated activity against Gram-positive microorganisms in vitro and in vivo, including those with multi-drug resistance. Moreover, the absence of cross-resistance to macrolides in many of these microorganisms and the rarity of cross-resistance between the two groups of antibiotics associated with the rapid bacterial killing are the principal features of the streptogramins, offering the possibility for treating the rising number of infections that are caused by multi-resistant Gram-positive bacteria.

Quinupristin/dalfopristin-resistant enterococci of the satA (vatD) and satG (vatE) genotypes from different ecological origins in Germany

The semisynthetic streptogramin combination quinupristin/dalfopristin (Synercid) is a promising alternative for treatment of infections due to multiply resistant gram-positive bacteria including vancomycin-resistant Enterococcus faecium. Resistance is mediated by acetyltransferases SatA (VatD) or SatG (VatE). Recent papers have indicated a possible link between the use of the streptogramin virginiamycin S/M as a feed additive in commercial animal husbandry and a selection of quinupristin/dalfopristin-resistant E. faecium (QDRE). We screened manure samples from two different turkey farms and from six different pig farms (using virginiamycin), samples from a sewage water treatment plant, 24 broiler carcasses, 10 pork samples, and 200 stool samples of nonhospitalized humans for QDRE. Our strain culture collection of hospital E. faecium isolates from the last 2 years was also reviewed for QDRE. All manure and sewage samples were positive for QDRE, as well as 11 from broiler carcasses (46%), 1 from pork (10%), and 28 from human stool specimens (14%). Thirty-six hospital isolates of E. faecium exhibited resistance to quinupristin/dalfopristin. In 141 QDRE of different origin satA (vatD) and satG (vatE) genes were detected (seven isolates from humans with an unknown resistance mechanism). Streptogramin resistance determinants were tansferable in filtermating experiments for 5 of 10 satA (vatD) and 9 of 22 satG (vatE) isolates. Different EcoRI patterns of satG (vatE) plasmids and corresponding hybridizations of the satG (vatE) gene indicated nonhomologous resistance plasmids in isolates of different origin. The results of this study indicate a common gene pool for streptogramin resistance in E. faecium of different ecological origin. A selection of QDRE using the streptogramin virginiamycin S/M as a feed additive and a spread of the resistance via the food chain to humans is probable.

Linkage of determinants for streptogramin A, macrolide-lincosamide-streptogramin B, and chloramphenicol resistance on a conjugative plasmid in Enterococcus faecium and dissemination of this cluster among streptogramin-resistant enterococci

A new streptogramin A resistance gene, satG (= vatE), has been recently identified in Enterococcus faecium UW1965 (Werner and Witte 1999. Antimicrob. Agents Chemother. 43: 1813-1814). Further sequence analysis of this plasmid revealed that vatE is in a cluster together with other resistance genes. The identified ORFs were nearly identical with the already known genes ermB and cat. The ermB fragment exhibited more than 99% identity with a resistance region from the streptococcal plasmid pIP501, whereas the cat fragment also contained a truncated rep gene homologue with more than 99% identity to sequences in small staphylococcal plasmids. The cat-rep and the ermB-vatE segments were linked by an IS1216V insertion sequence widely distributed among enterococci. PCR analysis of additional 76 streptogramin-resistant isolates possessing vatE and ermB revealed a linkage of both genes in 45 isolates (59%); 15 of them with a gene arrangement, cat-repU-IS1216V-ermB-vatE, identical to the reference strain UW1965. An identical linkage of IS1216V-ermB-vatE was found among isolates from poultry manure, poultry meat, stool samples of humans, and hospital patients indicating a possible spread of the resistance gene cluster via the food chain to humans.

Characterization of a variant of vga(A) conferring resistance to streptogramin A and related compounds

A variant of the vga(A) gene (1,575 bp), encoding an ATP-binding cassette protein conferring resistance to streptogramin A and related antibiotics, was cloned from the chromosome of a Staphylococcus aureus clinical isolate and sequenced. The sequence of the variant was similar to that of the vga(A) gene (83.2% identity). However, the G+C content of the variant (35.6%) was higher than that of vga(A) (29%) and there was no cross hybridization between vga(A) and the variant at high stringency (> or =60 degrees C), the highest temperature at which a signal was detected being 55 degrees C. Unlike previous reports for vga(A) and vga(B), the variant of vga(A) may be present in multiple copies in the genome. These copies are chromosomal in some isolates and both chromosomal and plasmid-borne in others. Nucleotide sequences hybridizing at 65 degrees C with the vga(A) variant were found in all the staphylococcal strains harboring plasmids carrying both vga(B) and vat(B), which also encode resistance to streptogramin A.

Antimicrobial susceptibility and presence of resistance genes in staphylococci from poultry

The species distribution, susceptibility to 19 antimicrobial agents and presence of selected genes encoding resistance to macrolides, streptogramins and tetracyclines were examined among 118 staphylococcal isolates from infections of poultry in Denmark. Isolates were identified using a combination of conventional biochemical testing and 16S rDNA sequencing. The most common species were Staphylococcus aureus (83), Staphylococcus hyicus (11), Staphylococcus xylosus (9) and Staphylococcus cohnii (6). The isolates were susceptible to most antimicrobials tested. A high frequency of S. aureus (30%) was resistant to ciprofloxacin. Only six (7%) S. aureus isolates and one Staphylococcus saprophyticus were penicillin resistant. Resistance to sulphamethoxazole was observed among 16 (19%) of S. aureus isolates and two coagulase negative staphylococci (CNS). Twenty (24%) of the S. aureus isolates were resistant to erythromycin and 19 of these isolates contained the ermA gene, whereas the remaining isolate contained the ermC gene. Eleven (48%) of the novobiocin resistant CNS were resistant to erythromycin and all these isolates contained the ermA gene. Two isolates identified as S. xylosus, were found to be resistant to streptogramins and both contained the vatB- and the vgaB-genes. Thirty-nine (47%) of the S. aureus isolates, three of nine S. hyicus and eight of the 23 novobiocin resistant CNS were tetracycline resistant and all contained the tet(K) gene. A single S. aureus isolate also contained the tet(M) gene. The present study showed a frequent occurrence of resistance to fluoroquinolones, tetracycline and macrolides among staphylococci isolated from broilers in Denmark, whereas the occurrence of resistance to other antimicrobial agents remains low. Similar genes, encoding resistance to erythromycin, tetracycline and streptogramins to those previously observed, were detected.

Activity of linezolid against Gram-positive cocci possessing genes conferring resistance to protein synthesis inhibitors

Linezolid belongs to a new class of antimicrobials, the oxazolidinones, that act by inhibiting protein synthesis. To detect cross-resistance with other inhibitors of protein synthesis (chloramphenicol, macrolides, lincosamides, streptogramins, aminoglycosides and tetracyclines), the in vitro activity of linezolid was determined against isolates harbouring known genes conferring resistance to these antimicrobials. Neither the presence of modifying enzymes (LinA, LinA', LinB, Vgb, Vat, SatA, ANT(4') (4")-I, AAC(6')-APH(2"), APHA-3 and Cat), nor the presence of an efflux mechanism (MsrA, MefE, MefA, MreA, Vga, TetK and TeL), nor the modification or protection of antimicrobial target (because of ribosomal methylases or TetM and TetO) affected linezolid activity as demonstrated by similar in vitro activity against resistant isolates and sensitive control isolates.

Influence of resistance to streptogramin A type antibiotics on the activity of quinupristin-dalfopristin in vitro and in experimental endocarditis due to Staphylococcus aureus

We evaluated the activity of quinupristin-dalfopristin (Q-D) against three clinical strains of Staphylococcus aureus susceptible to Q (MIC, 8 microg/ml) and Q-D (MICs, 0.5 to 1 microg/ml) but displaying various levels of susceptibility to D. D was active against S. aureus HM 1054 (MIC, 4 microg/ml) and had reduced activity against S. aureus RP 13 and S. aureus N 95 (MICs, 32 and 64 microg/ml, respectively). In vitro, Q-D at a concentration two times the MIC (2xMIC) produced reductions of 4.3, 3.9, and 5.8 log(10) CFU/ml after 24 h of incubation for HM 1054, RP 13, and N 95, respectively. Comparable killing was obtained at 8xMIC. Q-D-resistant mutants were selected in vitro at a frequency of 2 x 10(-8) to 2 x 10(-7) for the three strains on agar containing 2xMIC of Q-D; no resistant bacteria were detected at 4xMIC. Rabbits with aortic endocarditis were treated for 4 days with Q-D at 30 mg/kg of body weight intramuscularly (i.m.) three times a day (t.i.d.) or vancomycin at 50 mg/kg i.m. t.i.d. In vivo, Q-D and vancomycin were similarly active and bactericidal against the three tested strains compared to the results for control animals (P < 0.01). Among animals infected with RP 13 and treated with Q-D, one rabbit retained Q-D-resistant mutants that were resistant to Q and to high levels of D (MICs, 64, >256, and 8 microg/ml for Q, D, and Q-D, respectively). We conclude that the bactericidal activity of Q-D against strains with reduced susceptibility to D and susceptible to Q-D is retained and is comparable to that of vancomycin. Acquisition of resistance to both Q and D is necessary to select resistance to Q-D.

Identification of a streptogramin A acetyltransferase gene in the chromosome of Yersinia enterocolitica

Streptogramins are polypeptide antibiotics inhibiting protein synthesis by the prokaryotic ribosome. Gram-positive organisms are susceptible to streptogramins, while most gram-negative bacteria are intrinsically resistant. We have found a genomic fragment from a Yersinia enterocolitica isolate with an open reading frame coding for a polypeptide similar to the virginiamycin acetyltransferases found in various plasmids from gram-positive bacteria. The susceptible Escherichia coli strain DB10 was transformed to resistance to the type A streptogramins and to mixed (A + B) streptogramins upon introduction of a plasmid containing that gene. In addition, we showed streptogramin acetylating activity in vitro dependent on the presence of the Y. enterocolitica sat gene. Southern blot hybridization experiments showed that the sat gene was present in all the Y. enterocolitica isolates examined. These data together show that the gene in the Y. enterocolitica chromosome encoded an active streptogramin acetyltransferase. The deduced sequence of the Y. enterocolitica Sat protein was close to those of sat gene products found in gram-positive bacteria and cyanobacteria, suggesting a common evolutionary origin.

Mechanisms of resistance to quinupristin-dalfopristin among isolates of Enterococcus faecium from animals, raw meat, and hospital patients in Western Europe

Twenty-eight quinupristin-dalfopristin-resistant isolates of Enterococcus faecium from hospital patients and nonhuman sources in European countries were studied. High-level resistance (MICs, >/=32 microg/ml) was associated with the presence of vat(E) (satG) (14 isolates ¿50%) or vat(D) (satA) (6 isolates ¿21%). These genes were not detected in eight (29%) isolates with lower levels of quinupristin-dalfopristin resistance (MICs, 4 to 16 microg/ml). This suggests the presence of further mechanisms of resistance to quinupristin-dalfopristin in E. faecium.

satG, conferring resistance to streptogramin A, is widely distributed in Enterococcus faecium strains but not in staphylococci

A gene almost identical to satG was isolated from an Enterococcus faecium strain. This gene was transferred to a Staphylococcus aureus recipient strain where it conferred resistance to streptogramin A. satG was found to be widely distributed among E. faecium strains but not detected among staphylococci.

Effects of genes encoding resistance to streptogramins A and B on the activity of quinupristin-dalfopristin against Enterococcus faecium

Quinupristin-dalfopristin is a streptogramin combination active against multiply resistant Enterococcus faecium. Among 45 E. faecium isolated from patients in various French hospitals, only two strains were intermediate (MIC = 2 microgram/ml) and one, E. faecium HM1032, was resistant (MIC = 16 microgram/ml) to quinupristin-dalfopristin, according to British Society for Antimicrobial Chemotherapy and National Committee for Clinical Laboratory Standards approved breakpoints. The latter strain contained the vgb and satA genes responsible for hydrolysis or acetylation of quinupristin and dalfopristin, respectively, and an ermB gene (also previously referred to as ermAM) encoding a ribosomal methylase. The two intermediate strains had an LS(A) phenotype characterized by resistance to lincomycin (L), increased MICs (>/=8 microgram/ml) of dalfopristin (streptogramin A [S(A)]), and susceptibility to erythromycin and quinupristin. This phenotype was also detected in eight other strains susceptible to quinupristin-dalfopristin. No genes already known and conferring resistance to dalfopristin by acetylation or active efflux were detected in these LS(A) strains. Nineteen other strains resistant to erythromycin but susceptible to the quinupristin-dalfopristin combination displayed elevated MICs of quinupristin after induction (from 16 to >128 microgram/ml) and contained ermB genes. The effects of ermB, vgb, and satA genes on the activity of the streptogramin combination were tested by cloning these genes individually or in various combinations in recipient strains susceptible to quinupristin-dalfopristin, E. faecium HM1070 and Staphylococcus aureus RN4220. The presence of both the satA and vgb genes (regardless of the presence of an ermB gene) was necessary to confer full quinupristin-dalfopristin resistance to the host. The same genetic constructs were introduced into E. faecium BM4107 which displays a LS(A) phenotype. Addition of the satA or vgb gene to this LS(A) background conferred resistance to quinupristin-dalfopristin.