Mechanism of Tet(O)-mediated tetracycline resistance

Mechanism of action of the novel aminomethylcycline antibiotic omadacycline

Omadacycline is a novel first-in-class aminomethylcycline with potent activity against important skin and pneumonia pathogens, including community-acquired methicillin-resistant Staphylococcus aureus (MRSA), β-hemolytic streptococci, penicillin-resistant Streptococcus pneumoniae, Haemophilus influenzae, and Legionella. In this work, the mechanism of action for omadacycline was further elucidated using a variety of models. Functional assays demonstrated that omadacycline is active against strains expressing the two main forms of tetracycline resistance (efflux and ribosomal protection). Macromolecular synthesis experiments confirmed that the primary effect of omadacycline is on bacterial protein synthesis, inhibiting protein synthesis with a potency greater than that of tetracycline. Biophysical studies with isolated ribosomes confirmed that the binding site for omadacycline is similar to that for tetracycline. In addition, unlike tetracycline, omadacycline is active in vitro in the presence of the ribosomal protection protein Tet(O).

Antimicrobial resistance mechanisms among Campylobacter

Campylobacter jejuni and Campylobacter coli are recognized as the most common causative agents of bacterial gastroenteritis in the world. Humans most often become infected by ingesting contaminated food, especially undercooked chicken, but also other sources of bacteria have been described. Campylobacteriosis is normally a self-limiting disease. Antimicrobial treatment is needed only in patients with more severe disease and in those who are immunologically compromised. The most common antimicrobial agents used in the treatment of Campylobacter infections are macrolides, such as erythromycin, and fluoroquinolones, such as ciprofloxacin. Tetracyclines have been suggested as an alternative choice in the treatment of clinical campylobacteriosis but in practice are not often used. However, during the past few decades an increasing number of resistant Campylobacter isolates have developed resistance to fluoroquinolones and other antimicrobials such as macrolides, aminoglycosides, and beta-lactams. Trends in antimicrobial resistance have shown a clear correlation between use of antibiotics in the veterinary medicine and animal production and resistant isolates of Campylobacter in humans. In this review, the patterns of emerging resistance to the antimicrobial agents useful in treatment of the disease are presented and the mechanisms of resistance to these drugs in Campylobacter are discussed.

Mechanism of tetracycline resistance by ribosomal protection protein Tet(O)

Tetracycline resistance protein Tet(O), which protects the bacterial ribosome from binding the antibiotic tetracycline, is a translational GTPase with significant similarity in both sequence and structure to the elongation factor EF-G. Here, we present an atomic model of the Tet(O)-bound 70S ribosome based on our cryo-electron microscopic reconstruction at 9.6 Å resolution. This atomic model allowed us to identify the Tet(O)-ribosome binding sites, which involve three characteristic loops in domain 4 of Tet(O). Replacements of the three-amino acid tips of these loops by a single glycine residue result in loss of Tet(O)-mediated tetracycline resistance. On the basis of these findings, the mechanism of Tet(O)-mediated tetracycline resistance can be explained in molecular detail.

Structural basis for TetM-mediated tetracycline resistance

Ribosome protection proteins (RPPs) confer tetracycline resistance by binding to the ribosome and chasing the drug from its binding site. The current model for the mechanism of action of RPPs proposes that drug release is indirect and achieved via conformational changes within the drug-binding site induced upon binding of the RPP to the ribosome. Here we report a cryo-EM structure of the RPP TetM in complex with the 70S ribosome at 7.2-Å resolution. The structure reveals the contacts of TetM with the ribosome, including interaction between the conserved and functionally critical C-terminal extension of TetM and the decoding center of the small subunit. Moreover, we observe direct interaction between domain IV of TetM and the tetracycline binding site and identify residues critical for conferring tetracycline resistance. A model is presented whereby TetM directly dislodges tetracycline to confer resistance.

Molecular basis for different levels of tet(M) expression in Streptococcus pneumoniae clinical isolates

Seventy-four unrelated clinical isolates of Streptococcus pneumoniae harboring the tet(M) gene were studied. Seven strains with low tetracycline (Tc) MICs (0.25 to 0.5 μg/ml) were found to harbor truncated tet(M) alleles that were inactivated by different frameshift mutations. In contrast, five strains bore deletions in the tet(M) promoter region, among which four displayed increased Tc MICs (16 to 64 μg/ml). The same promoter mutations were detected in Tc-resistant mutants selected in vitro from various susceptible strains. Sequence analysis revealed that these deletions might impede the formation of the transcriptional attenuator located immediately upstream of tet(M). Expression in Enterococcus faecalis of a tet(M) reporter gene transcribed from these promoter mutants conferred a level of Tc resistance similar to that observed in the parental S. pneumoniae strains. These results show that different levels of Tc susceptibility found in clinical isolates of S. pneumoniae can be explained by frameshift mutations within tet(M) and by alterations of the upstream transcriptional attenuator.

Target- and resistance-based mechanistic studies with TP-434, a novel fluorocycline antibiotic

TP-434 is a novel, broad-spectrum fluorocycline antibiotic with activity against bacteria expressing major antibiotic resistance mechanisms, including tetracycline-specific efflux and ribosomal protection. The mechanism of action of TP-434 was assessed using both cell-based and in vitro assays. In Escherichia coli cells expressing recombinant tetracycline resistance genes, the MIC of TP-434 (0.063 μg/ml) was unaffected by tet(M), tet(K), and tet(B) and increased to 0.25 and 4 μg/ml in the presence of tet(A) and tet(X), respectively. Tetracycline, in contrast, was significantly less potent (MIC ≥ 128 μg/ml) against E. coli cells when any of these resistance mechanisms were present. TP-434 showed potent inhibition in E. coli in vitro transcription/translation (50% inhibitory concentration [IC(50)] = 0.29 ± 0.09 μg/ml) and [(3)H]tetracycline ribosome-binding competition (IC(50) = 0.22 ± 0.07 μM) assays. The antibacterial potencies of TP-434 and all other tetracycline class antibiotics tested were reduced by 4- to 16-fold, compared to that of the wild-type control strain, against Propionibacterium acnes strains carrying a 16S rRNA mutation, G1058C, a modification that changes the conformation of the primary binding site of tetracycline in the ribosome. Taken together, the findings support the idea that TP-434, like other tetracyclines, binds the ribosome and inhibits protein synthesis and that this activity is largely unaffected by the common tetracycline resistance mechanisms.

Differential effects of thiopeptide and orthosomycin antibiotics on translational GTPases

The ribosome is a major target in the bacterial cell for antibiotics. Here, we dissect the effects that the thiopeptide antibiotics thiostrepton (ThS) and micrococcin (MiC) as well as the orthosomycin antibiotic evernimicin (Evn) have on translational GTPases. We demonstrate that, like ThS, MiC is a translocation inhibitor, and that the activation by MiC of the ribosome-dependent GTPase activity of EF-G is dependent on the presence of the ribosomal proteins L7/L12 as well as the G' subdomain of EF-G. In contrast, Evn does not inhibit translocation but is a potent inhibitor of back-translocation as well as IF2-dependent 70S-initiation complex formation. Collectively, these results shed insight not only into fundamental aspects of translation but also into the unappreciated specificities of these classes of translational inhibitors.

A computational study of elongation factor G (EFG) duplicated genes: diverged nature underlying the innovation on the same structural template

Background

Elongation factor G (EFG) is a core translational protein that catalyzes the elongation and recycling phases of translation. A more complex picture of EFG's evolution and function than previously accepted is emerging from analyzes of heterogeneous EFG family members. Whereas the gene duplication is postulated to be a prominent factor creating functional novelty, the striking divergence between EFG paralogs can be interpreted in terms of innovation in gene function.

Methodology/Principal Findings

We present a computational study of the EFG protein family to cover the role of gene duplication in the evolution of protein function. Using phylogenetic methods, genome context conservation and insertion/deletion (indel) analysis we demonstrate that the EFG gene copies form four subfamilies: EFG I, spdEFG1, spdEFG2, and EFG II. These ancient gene families differ by their indispensability, degree of divergence and number of indels. We show the distribution of EFG subfamilies and describe evidences for lateral gene transfer and recent duplications. Extended studies of the EFG II subfamily concern its diverged nature. Remarkably, EFG II appears to be a widely distributed and a much-diversified subfamily whose subdivisions correlate with phylum or class borders. The EFG II subfamily specific characteristics are low conservation of the GTPase domain, domains II and III; absence of the trGTPase specific G2 consensus motif “RGITI”; and twelve conserved positions common to the whole subfamily. The EFG II specific functional changes could be related to changes in the properties of nucleotide binding and hydrolysis and strengthened ionic interactions between EFG II and the ribosome, particularly between parts of the decoding site and loop I of domain IV.

Conclusions/Significance

Our work, for the first time, comprehensively identifies and describes EFG subfamilies and improves our understanding of the function and evolution of EFG duplicated genes.

Prevalence, antibiograms, and transferable tet(O) plasmid of Campylobacter jejuni and Campylobacter coli isolated from raw chicken, pork, and human clinical cases in Korea

The antibiotic resistance patterns and prevalence of the transferable tet(O) plasmid were investigated in Campylobacter jejuni and Campylobacter coli isolates from raw chicken, pork, and humans with clinical campylobacteriosis. A total of 180 C. jejuni and C. coli isolates were identified, and the prevalence rates of C. jejuni and C. coli in raw chicken samples were 83% (83 of 100) and 73% (73 of 100), respectively. Twelve percent (6 of 50) and 10% (5 of 50) of pork samples were contaminated with C. jejuni and C. coli, respectively. Disk diffusion susceptibility testing revealed that the most frequently detected resistance was to tetracycline (92.2%), followed by nalidixic acid (75.6%), ciprofloxacin (65.0%), azithromycin (41.5%), ampicillin (33.3%), and streptomycin (26.1%). Of the C. jejuni and C. coli isolates, 65.7% (n=109) contained plasmids carrying the tet(O) gene. Six C. jejuni isolates and two C. coli isolates with high-level resistance to tetracycline (MIC=256 microg/ml) harbored the tet(O) plasmid, which is transferable to other C. jejuni and C. coli isolates. These results demonstrate the presence of an interspecies transferable plasmid containing the tet(O) gene and a high prevalence of antibiotic resistance in Korean Campylobacter isolates and provide an understanding of the antibiotic resistance distribution among Campylobacter species in Korea.

Regulation of antibiotic resistance in Staphylococcus aureus

Staphylococcus aureus has a formidable ability to adapt to varying environmental conditions and an extraordinary capacity to rapidly become resistant to virtually all antibiotics. Resistance develops either through mutations and rearrangements within the staphylococcal genome, or by the acquisition of resistance determinants. Antibiotic resistances often impose a fitness burden on the host. Such biological costs can be reduced by tight regulation and antibiotic-inducible expression of resistance genes, or by compensatory mutations. Resistance induction by antibiotics can be mediated by dedicated, antibiotic-recognizing signal transducers or by mechanisms relieving translational attenuation. Antibiotic tolerance and the expression of resistance phenotypes can also be strongly influenced by the genetic backgrounds of strains and several other factors. Modification and indirect regulation of resistance levels can occur by mutations that alter gene expression or substrate specificity of genes contributing to resistance. Insertion elements can alter resistance profiles by turning relevant genes on or off. Environmental conditions and stress response mechanisms triggered by perturbation of the cell envelope, DNA damage, or faulty intermediary metabolism can also have an impact on resistance development and expression. Clinically relevant resistance is often built up through multiple steps, each of which contributes to an increase in resistance. The driving force behind resistance formation is antibiotic stress, and under clinical conditions selection for resistance is continuously competing with selection for bacterial fitness.

Fate and transport of antibiotic residues and antibiotic resistance genes following land application of manure waste

Antibiotics are used in animal livestock production for therapeutic treatment of disease and at subtherapeutic levels for growth promotion and improvement of feed efficiency. It is estimated that approximately 75% of antibiotics are not absorbed by animals and are excreted in waste. Antibiotic resistance selection occurs among gastrointestinal bacteria, which are also excreted in manure and stored in waste holding systems. Land application of animal waste is a common disposal method used in the United States and is a means for environmental entry of both antibiotics and genetic resistance determinants. Concerns for bacterial resistance gene selection and dissemination of resistance genes have prompted interest about the concentrations and biological activity of drug residues and break-down metabolites, and their fate and transport. Fecal bacteria can survive for weeks to months in the environment, depending on species and temperature, however, genetic elements can persist regardless of cell viability. Phylogenetic analyses indicate antibiotic resistance genes have evolved, although some genes have been maintained in bacteria before the modern antibiotic era. Quantitative measurements of drug residues and levels of resistance genes are needed, in addition to understanding the environmental mechanisms of genetic selection, gene acquisition, and the spatiotemporal dynamics of these resistance genes and their bacterial hosts. This review article discusses an accumulation of findings that address aspects of the fate, transport, and persistence of antibiotics and antibiotic resistance genes in natural environments, with emphasis on mechanisms pertaining to soil environments following land application of animal waste effluent.

Antibiotic resistance in Campylobacter: emergence, transmission and persistence

Campylobacter is a leading foodborne bacterial pathogen, which causes gastroenteritis in humans. This pathogenic organism is increasingly resistant to antibiotics, especially fluoroquinolones and macrolides, which are the most frequently used antimicrobials for the treatment of campylobacteriosis when clinical therapy is warranted. As a zoonotic pathogen, Campylobacter has a broad animal reservoir and infects humans via contaminated food, water or milk. Antibiotic usage in both animal agriculture and human medicine, can influence the development of antibiotic-resistant Campylobacter. This review will describe the trend in fluoroquinolone and macrolide resistance in Campylobacter, summarize the mechanisms underlying the resistance to various antibiotics and discuss the unique features associated with the emergence, transmission and persistence of antibiotic-resistant Campylobacter. Special attention will be given to recent findings and emphasis will be placed on Campylobacter resistance to fluoroquinolones and macrolides. A future perspective on antibiotic resistance and potential approaches for the control of antibiotic-resistant Campylobacter, will also be discussed.

Chimeras of bacterial translation factors Tet(O) and EF-G

Ribosomal protection proteins (RPPs) confer bacterial resistance to tetracycline by releasing this antibiotic from ribosomes stalled in protein synthesis. RPPs share structural similarity to elongation factor G (EF-G), which promotes ribosomal translocation during normal protein synthesis. We constructed and functionally characterized chimeric proteins of Campylobacter jejuni Tet(O), the best characterized RPP, and Escherichia coli EF-G. A distinctly conserved loop sequence at the tip of domain 4 is required for both factor-specific functions. Domains 3-5: (i) are necessary, but not sufficient, for functional specificity; and (ii) modulate GTP hydrolysis by EF-G, while minimally affecting Tet(O), under substrate turnover conditions.

The weird and wonderful world of bacterial ribosome regulation

In every organism, translation of the genetic information into functional proteins is performed on the ribosome. In Escherichia coli up to 40% of the cell's total energy turnover is channelled toward the ribosome and protein synthesis. Thus, elaborate networks of translation regulation pathways have evolved to modulate gene expression in response to growth rate and external factors, ranging from nutrient deprivation, to chemical (pH, ionic strength) and physical (temperature) fluctuations. Since the fundamental players involved in regulation of the different phases of translation have already been extensively reviewed elsewhere, this review focuses on lesser known and characterized factors that regulate the ribosome, ranging from processing, modification and assembly factors, unusual initiation and elongation factors, to a variety of stress response proteins.

Investigation of transcription repression and small-molecule responsiveness by TetR-like transcription factors using a heterologous Escherichia coli-based assay

The SCO7222 protein and ActR are two of approximately 150 TetR-like transcription factors encoded in the Streptomyces coelicolor genome. Using bioluminescence as a readout, we have developed Escherichia coli-based biosensors that accurately report the regulatory activity of these proteins and used it to investigate their interactions with DNA and small-molecule ligands. We found that the SCO7222 protein and ActR repress the expression of their putative target genes, SCO7223 and actII-ORF2 (actA), respectively, by interacting with operator sequence in the promoters. The operators recognized by the two proteins are related such that O(7223) (an operator for SCO7223) could be bound by both the SCO7222 protein and ActR with similar affinities. In contrast, O(act) (an operator for actII-ORF2) was bound tightly by ActR and more weakly by the SCO7222 protein. We demonstrated ligand specificity of these proteins by showing that while TetR (but not ActR or the SCO7222 protein) interacts with tetracyclines, ActR (but not TetR or the SCO7222 protein) interacts with actinorhodin and related molecules. Through operator-targeted mutagenesis, we found that at least two nucleotide changes in O(7223) were required to disrupt its interaction with SCO7222 protein, while ActR was more sensitive to changes on O(act). Most importantly, we found that the interaction of each protein with wild-type and mutant operator sequences in vivo and in vitro correlated perfectly. Our data suggest that E. coli-based biosensors of this type should be broadly applicable to TetR-like transcription factors.

Role of the plasmid-encoded tet(O) gene in tetracycline-resistant clinical isolates of Campylobacter jejuni and Campylobacter coli

The prevalence of tetracycline resistance, tetracycline MICs and tet(O) gene localization were investigated in 83 Campylobacter isolates from patients suffering from acute gastroenteritis in Germany. Combined biochemical and molecular markers identified 74 isolates (89 %) as Campylobacter jejuni, including seven atypical isolates that failed to hydrolyse hippurate, and nine isolates (11 %) as Campylobacter coli. Tetracycline resistance was detected in six out of nine Campylobacter coli isolates (67 %) and 13 out of 74 C. jejuni isolates (18 %). Low-level tetracycline resistance was observed for C. coli (MIC 16 microg ml(-1) for all strains), whereas C. jejuni showed high-level resistance (MIC >256 microg ml(-1) for all strains). Both low- and high-level tetracycline resistance was associated with the presence of the tet(O) gene. In C. jejuni, tet(O) was plasmid-encoded in 54 % of tetracycline-resistant isolates, whereas in C. coli, tet(O) appeared to be located on the chromosome.

Evolution and ecology of antibiotic resistance genes

A new perspective on the topic of antibiotic resistance is beginning to emerge based on a broader evolutionary and ecological understanding rather than from the traditional boundaries of clinical research of antibiotic-resistant bacterial pathogens. Phylogenetic insights into the evolution and diversity of several antibiotic resistance genes suggest that at least some of these genes have a long evolutionary history of diversification that began well before the 'antibiotic era'. Besides, there is no indication that lateral gene transfer from antibiotic-producing bacteria has played any significant role in shaping the pool of antibiotic resistance genes in clinically relevant and commensal bacteria. Most likely, the primary antibiotic resistance gene pool originated and diversified within the environmental bacterial communities, from which the genes were mobilized and penetrated into taxonomically and ecologically distant bacterial populations, including pathogens. Dissemination and penetration of antibiotic resistance genes from antibiotic producers were less significant and essentially limited to other high G+C bacteria. Besides direct selection by antibiotics, there is a number of other factors that may contribute to dissemination and maintenance of antibiotic resistance genes in bacterial populations.

An improved procedure for expression and purification of ribosomal protection protein Tet(O) for high-resolution structural studies

Tetracycline (Tc) is a broad spectrum antibiotic that binds to the A site of the bacterial ribosome inhibiting delivery of aminoacyl-tRNA to the A site for productive protein biosynthesis. Tet(O) is in a class of the ribosomal protection proteins (RPPs) found in many pathogenic bacteria, that dislodges Tc from the A site of 70S ribosome to restore polypeptide elongation and confer Tc resistance to the bacteria. Considerable difficulty has been encountered in overexpressing and purifying Tet(O) from various Escherichia coli strains using lambdaPI, tac or T7 promoters. Here we report molecular cloning, overexpression of His-tagged Tet(O) in E. coli, an improved purification procedure and initial biochemical and biophysical characterization of His-tagged Tet(O).

Phylogenetic distribution of translational GTPases in bacteria

Background

Translational GTPases are a family of proteins in which GTPase activity is stimulated by the large ribosomal subunit. Conserved sequence features allow members of this family to be identified.

Results

To achieve accurate protein identification and grouping we have developed a method combining searches with Hidden Markov Model profiles and tree based grouping. We found all the genes for translational GTPases in 191 fully sequenced bacterial genomes. The protein sequences were grouped into nine subfamilies.

Analysis of the results shows that three translational GTPases, the translation factors EF-Tu, EF-G and IF2, are present in all organisms examined. In addition, several copies of the genes encoding EF-Tu and EF-G are present in some genomes. In the case of multiple genes for EF-Tu, the gene copies are nearly identical; in the case of multiple EF-G genes, the gene copies have been considerably diverged. The fourth translational GTPase, LepA, the function of which is currently unknown, is also nearly universally conserved in bacteria, being absent from only one organism out of the 191 analyzed. The translation regulator, TypA, is also present in most of the organisms examined, being absent only from bacteria with small genomes.

Surprisingly, some of the well studied translational GTPases are present only in a very small number of bacteria. The translation termination factor RF3 is absent from many groups of bacteria with both small and large genomes. The specialized translation factor for selenocysteine incorporation – SelB – was found in only 39 organisms. Similarly, the tetracycline resistance proteins (Tet) are present only in a small number of species.

Proteins of the CysN/NodQ subfamily have acquired functions in sulfur metabolism and production of signaling molecules. The genes coding for CysN/NodQ proteins were found in 74 genomes. This protein subfamily is not confined to Proteobacteria, as suggested previously but present also in many other groups of bacteria.

Conclusion

Four of the translational GTPase subfamilies (IF2, EF-Tu, EF-G and LepA) are represented by at least one member in each bacterium studied, with one exception in LepA. This defines the set of translational GTPases essential for basic cell functions.

The highly conserved LepA is a ribosomal elongation factor that back-translocates the ribosome

The ribosomal elongation cycle describes a series of reactions prolonging the nascent polypeptide chain by one amino acid and driven by two universal elongation factors termed EF-Tu and EF-G in bacteria. Here we demonstrate that the extremely conserved LepA protein, present in all bacteria and mitochondria, is a third elongation factor required for accurate and efficient protein synthesis. LepA has the unique function of back-translocating posttranslocational ribosomes, and the results suggest that it recognizes ribosomes after a defective translocation reaction and induces a back-translocation, thus giving EF-G a second chance to translocate the tRNAs correctly. We suggest renaming LepA as elongation factor 4 (EF4).

A role for the tet(O) plasmid in maintaining Campylobacter plasticity

Genomic sequencing projects are beginning to reveal regions of extensive DNA homology between bacterial genera. Public fears of the spread of genetically modified organisms into the food chain and the increasing prevalence of multi-drug resistant disease in humans highlight the implications of horizontal gene transfer. The striking DNA sequence similarity between the two uniquely identified tetracycline resistant (Tc(R)) Campylobacter plasmids, pCC31 and pTet, suggests their conserved acquisition and maintenance within Campylobacter [Batchelor, R.A., Pearson, B.M., Friis, L.M., Guerry, P., Wells, J.M. 2004. Nucleotide sequences and comparison of two large conjugative plasmids from different Campylobacter species. Microbiology 150, 3507-3517]. It is thus likely that these and other conjugative plasmids are highly prevalent and broadly distributed across several continents. Microarray technology is now enabling fast and extensive genomic comparisons to be made and allows us to investigate intra- and inter-genetic conservation and variability. This study details the development of a microarray specific for genes from Campylobacter plasmids pCC31, pTet and pVir and its application to the analysis of Campylobacter plasmid gene presence and preservation throughout environmental and clinical isolates. Application of the iterative algorithm GENCOM (freely available at ) is used as a rapid and effective way of comparing the content and conservation of plasmids in bacteria and provides details of the Campylobacter flexible gene pool and its contribution to genomic plasticity.

Genetic mechanisms contributing to reduced tetracycline susceptibility of Campylobacter isolated from organic and conventional dairy farms in the midwestern and northeastern United States

Campylobacter is one of the most common causes of gastroenteritis and can be acquired through contact with farm animals or the consumption of raw milk. Because of concerns over the role of food-producing animals in the dissemination of antimicrobial resistance to humans, we evaluated the prevalence of antimicrobial resistance in Campylobacter isolates from dairy farms and the genetic mechanism conferring the observed resistance. Evaluation of antimicrobial resistance was completed on 912 isolates from conventional and 304 isolates from organic dairy farms to eight drugs (azithromycin, chloramphenicol, ciprofloxacin, clindamycin, erythromycin, gentamicin, nalidixic acid, and tetracycline) with microbroth dilution. Resistance to seven of eight drugs was very low and did not differ by farm type. However, tetracycline resistance was common in Campylobacter isolated from both organic and conventional dairy farms, with 48 and 58% of isolates affected, respectively. By multiplex PCR, we determined that tetracycline resistance was highly associated with the carriage of tetO in Campylobacter isolates (X2 = 124, P < 0.01, kappa = 0.86).

pVir and bloody diarrhea in Campylobacter jejuni enteritis

The plasmid pVir may play a role in the virulence of Campylobacter jejuni, a leading cause of bacterial gastroenteritis. The pVir plasmid was identified in 17% of 104 C. jejuni clinical isolates studied and was significantly associated with the occurrence of blood in patient stool, a marker of invasive infection. The pVir plasmid was not associated with greater occurrence of diarrhea, fever, pain, vomiting, or need for patient hospitalization. Isolates containing pVir were also associated with the presence of a tetracycline-resistance plasmid, but pVir did not transfer with tetracycline-resistance plasmids to recipient strains of C. jejuni. The association of pVir and bloody stool suggests that pVir may be clinically relevant in C. jejuni infections.

Translational control of tetracycline resistance and conjugation in the Bacteroides conjugative transposon CTnDOT

The tetQ-rteA-rteB operon of the Bacteroides conjugative transposon CTnDOT is responsible for tetracycline control of the excision and transfer of CTnDOT. Previous studies revealed that tetracycline control of this operon occurred at the translational level and involved a hairpin structure located within the 130-base leader sequence that lies between the promoter of tetQ and the start codon of the gene. This hairpin structure is formed by two sequences, designated Hp1 and Hp8. Hp8 contains the ribosome binding site for tetQ. Examination of the leader region sequence revealed three sequences that might encode a leader peptide. One was only 3 amino acids long. The other two were 16 amino acids long. By introducing stop codons into the peptide coding regions, we have now shown that the 3-amino-acid peptide is the one that is essential for tetracycline control. Between Hp1 and Hp8 lies an 85-bp region that contains other possible RNA hairpin structures. Deletion analysis of this intervening DNA segment has now identified a sequence, designated Hp2, which is essential for tetracycline regulation. This sequence could form a short hairpin structure with Hp1. Mutations that made the Hp1-Hp2 structure more stable caused nearly constitutively high expression of the operon. Thus, stalling of ribosomes on the 3-amino-acid leader peptide could favor formation of the Hp1-Hp2 structure and thus preclude formation of the Hp1-Hp8 structure, releasing the ribosome binding site of tetQ. Finally, comparison of the CTnDOT tetQ leader regions with upstream regions of five tetQ genes found in other elements reveals that the sequences are virtually identical, suggesting that translational attenuation is responsible for control of tetracycline resistance in these other cases as well.

Characterisation of viridans group streptococci with different levels of Tet(M)-mediated tetracycline resistance

Streptococcus oralis 264-3, Streptococcus mitis 254-1 and S. mitis 264-1, isolated from the oral cavities of two children were each found to carry the tet(M) gene but exhibited different degrees of reduced susceptibility to tetracycline (tetracycline MICs of 2, 8 and 64 mg/L, respectively). The aim of this study was to determine the molecular basis for the different levels of tetracycline resistance (Tc(R)) observed. Escherichia coli HB101 carrying the cloned tet(M) genes exhibited similar levels of tetracycline susceptibility to those observed in the parental streptococcal strains (MICs of 1, 16, and 64 mg/L for tet(M) genes from S. oralis 264-3, S. mitis 254-1 and S. mitis 264-1, respectively). DNA sequencing revealed that S. oralis 264-3 had a tet(M) gene highly homologous to tet(M) carried by Tn916 from Enterococcus faecalis (99.6% identity), while the intermediate- and high-level Tc(R) strains had tet(M) sequences that resembled the tet(M) gene of Tn5251 from Streptococcus pneumoniae (99.3% and 99.4% identity, respectively). No differences were observed in the upstream attenuator structure for each of the strains and differences in reduced tetracycline susceptibilities could be attributed to changes in the deduced amino acid sequences of the Tet(M) proteins.

Incidence of antibiotic resistance in Campylobacter jejuni isolated in Alberta, Canada, from 1999 to 2002, with special reference to tet(O)-mediated tetracycline resistance

Of 203 human clinical isolates of Campylobacter jejuni from Alberta, Canada (1999 to 2002), 101 isolates (50%) were resistant to at least 64 microg of tetracycline/ml, with four isolates exhibiting higher levels of tetracycline resistance (512 microg/ml). In total, the MICs for 37% of tetracycline-resistant isolates (256 to 512 microg/ml) were higher than those previously reported in C. jejuni (64 to 128 microg/ml). In the tetracycline-resistant clinical isolates, 67% contained plasmids and all contained the tet(O) gene. Four isolates resistant to high levels of tetracycline (MIC = 512 microg/ml) contained plasmids carrying the tet(O) gene, which could be transferred to other isolates of C. jejuni. The tetracycline MICs for transconjugants were comparable to those of the donors. Cloning of tet(O) from the four high-level tetracycline-resistant isolates conferred an MIC of 32 microg/ml for Escherichia coli DH5alpha. In contrast, transfer to a strain of C. jejuni by using mobilization conferred an MIC of 128 microg/ml. DNA sequence analysis determined that the tet(O) genes encoding lower MICs (64 to 128 microg/ml) were identical to one other, although the tet(O) genes encoding a 512-microg/ml MIC demonstrated several nucleotide substitutions. The quinolone resistance determining region of four ciprofloxacin-resistant isolates (2%) was analyzed, and resistance was associated with a chromosomal mutation in the gyrA gene resulting in a Thr-86-Ile substitution. In addition, six kanamycin-resistant isolates contained large plasmids that carry the aphA-3 marker coding for 3'-aminoglycoside phosphotransferase. Resistance to erythromycin was not detected in 203 isolates. In general, resistance to most antibiotics in C. jejuni remains low, except for resistance to tetracycline, which has increased from about 8 to 50% over the past 20 years.

A dedicated translation factor controls the synthesis of the global regulator Fis

BipA is a highly conserved protein with global regulatory properties in Escherichia coli. We show here that it functions as a translation factor that is required specifically for the expression of the transcriptional modulator Fis. BipA binds to ribosomes at a site that coincides with that of elongation factor G and has a GTPase activity that is sensitive to high GDP:GTP ratios and stimulated by 70S ribosomes programmed with mRNA and aminoacylated tRNAs. The growth rate-dependent induction of BipA allows the efficient expression of Fis, thereby modulating a range of downstream processes, including DNA metabolism and type III secretion. We propose a model in which BipA destabilizes unusually strong interactions between the 5' untranslated region of fis mRNA and the ribosome. Since BipA spans phylogenetic domains, transcript-selective translational control for the 'fast-track' expression of specific mRNAs may have wider significance.

Regulation of a Bacteroides operon that controls excision and transfer of the conjugative transposon CTnDOT

CTnDOT is a conjugative transposon (CTn) that is found in many Bacteroides strains. Transfer of CTnDOT is stimulated 100- to 1,000-fold if the cells are first exposed to tetracycline (TET). Both excision and transfer of CTnDOT are stimulated by TET. An operon that contains a TET resistance gene, tetQ, and two regulatory genes, rteA and rteB, is essential for control of excision and transfer functions. At first, it appeared that RteA and RteB, which are members of a two-component regulatory system, might be directly responsible for the TET effect. We show here, however, that neither RteA nor RteB affected expression of the operon. TetQ, a ribosome protection type of TET resistance protein, actually reduced operon expression, possibly by interacting with ribosomes that are translating the tetQ message. Fusions of tetQ with a reporter gene, uidA, were only expressed at a high level when the fusion was cloned in frame with the first six codons of tetQ. However, out of frame fusions or fusions ending at the other five codons of tetQ showed much lower expression of the uidA gene. Moreover, reverse transcription-PCR amplification of tetQ mRNA revealed that despite the fact that the uidA gene product, beta-glucuronidase (GUS), was produced only when the cells were exposed to TET, tetQ mRNA was produced in both the presence and absence of TET. Computer analysis of the region upstream of the tetQ start codon predicted that the mRNA in this region could form a complex RNA hairpin structure that would prevent access of ribosomes to the ribosome binding site. Mutations that abolished base pairing in the stem that formed the base of this putative hairpin structure made GUS production as high in the absence of TET as in TET-stimulated cells. Compensatory mutations that restored the hairpin structure led to a return of regulated production of GUS. Thus, the tetQ-rteA-rteB operon appears to be regulated by a translational attenuation mechanism.

Xanthine nucleotide-specific G-protein alpha-subunits: a novel approach for the analysis of G-protein-mediated signal transduction

Pro- and eukaryotic cells express multiple GTP-binding proteins that play crucial roles in signal transduction. GTP-binding proteins possess a highly conserved NKX D motif critically involved in guanine binding. In order to selectively activate a defined GTP-binding protein, base-specificity can be switched from guanine to xanthine by mutating the conserved aspartate into asparagine (D/N-mutation). This approach was very successful at elucidating the function of structurally diverse GTP-binding proteins in complex systems. However, attempts to generate functional xanthine nucleotide-specific alpha-subunits of heterotrimeric GTP-binding proteins (G-proteins) met more difficulties. Recent studies have shown that a sufficiently high GDP-affinity is critical for functional expression of xanthine nucleotide-selective G-protein mutants. Moreover, xanthosine 5'-[gamma-thio]triphosphate and xanthosine 5'-[gamma, beta-imido]triphosphate are not functionally equivalent activators of D/N-G-protein mutants. We are now in the position to exploit xanthine nucleotide-specific G-proteins to dissect signaling pathways activated by a given G-protein in complex systems.