Heterogeneity of tetracycline resistance determinants in Streptococcus

Genotypic and phenotypic tetracycline-based properties of Trueperella pyogenes isolates from bovine samples

The purpose of this study was to investigate the tetracycline resistance in Trueperella pyogenes isolates from bovine samples in Burdur, Turkiye, and assess 16 tetracycline-resistance genes distribution among the isolates. Forty-nine T. pyogenes isolates were phenotypically characterized for anti-microbial resistance to doxycycline, oxytetracycline and tetracycline by disc diffusion method. Presence of tetracycline genes of T. pyogenes was investigated by multiplex and singleplex polymerase chain reaction. Our results indicated that 87.80% and 42.86% of the isolates were resistant to tetracycline and oxytetracycline, respectively, and the rate of resistance to doxycycline was 6.12%. Total of 21 (42.85%) were carrying tetracycline-resistance genes and tet(A) was present in 12 (24.49%) isolates; whereas, the tet(W) gene was identified in 9 (18.37%) and 2 (4.08%) of the isolates carried both tet(A) and tet(W), respectively. The study indicated antibiotic resistance patterns of tetracycline agents and links to the tet-genes among T. pyogenes were detected. It makes it worthwhile that this is the first report for detection of tet(A) gene in T. pyogenes.

Characterization of pTS14, an IncF2:A1:B1 Plasmid Carrying tet(M) in a Salmonella enterica Isolate

The objective of this study was to explore the genetic and biological features of the tet(M)-harboring plasmid pTS14 in Salmonella enterica strain S14 isolated from a chicken fecal sample. Plasmid pTS14 was identified by conjugation, S1-pulsed-field gel electrophoresis (PFGE), Southern hybridization, and plasmid sequencing. The biological characteristics of pTS14 were assessed via stability, growth kinetics, and starvation survival experiments. Strain S14, belonging to ST3007, harbored a 119-kb tet(M)-bearing IncF2:A1:B1 conjugative plasmid pTS14. The plasmid pTS14 contained a novel transposon Tn6709 with the genetic structure IS26-tnpA1-tnpA2-Δorf13-LP-tet(M)-tnpX-ΔtnpR-IS26, and the resistance genes tet(B), tet(D), strAB, sul2, and bla_TEM–1b. In addition, pTS14 was found to be highly stable in the recipient strain E. coli J53. The transconjugant TS14 exhibited a higher survival ratio than E. coli J53 under permanent starvation-induced stress. The tet(M)-bearing IncF2 epidemic plasmid lineage may accelerate the dissemination of tet(M) and other genes by coselection, which could constitute a potentially serious threat to clinical treatment regimens.

Microbial Diversity and Antimicrobial Resistance Profile in Microbiota From Soils of Conventional and Organic Farming Systems

Soil is one of the biggest reservoirs of microbial diversity, yet the processes that define the community dynamics are not fully understood. Apart from soil management being vital for agricultural purposes, it is also considered a favorable environment for the evolution and development of antimicrobial resistance, which is due to its high complexity and ongoing competition between the microorganisms. Different approaches to agricultural production might have specific outcomes for soil microbial community composition and antibiotic resistance phenotype. Therefore in this study we aimed to compare the soil microbiota and its resistome in conventional and organic farming systems that are continually influenced by the different treatment (inorganic fertilizers and pesticides vs. organic manure and no chemical pest management). The comparison of the soil microbial communities revealed no major differences among the main phyla of bacteria between the two farming styles with similar soil structure and pH. Only small differences between the lower taxa could be observed indicating that the soil community is stable, with minor shifts in composition being able to handle the different styles of treatment and fertilization. It is still unclear what level of intensity can change microbial composition but current conventional farming in Central Europe demonstrates acceptable level of intensity for soil bacterial communities. When the resistome of the soils was assessed by screening the total soil DNA for clinically relevant and soil-derived antibiotic resistance genes, a low variety of resistance determinants was detected (resistance to β-lactams, aminoglycosides, tetracycline, erythromycin, and rifampicin) with no clear preference for the soil farming type. The same soil samples were also used to isolate antibiotic resistant cultivable bacteria, which were predominated by highly resistant isolates of Pseudomonas, Stenotrophomonas, Sphingobacterium and Chryseobacterium genera. The resistance of these isolates was largely dependent on the efflux mechanisms, the soil Pseudomonas spp. relying mostly on RND, while Stenotrophomonas spp. and Chryseobacterium spp. on RND and ABC transporters.

Antimicrobial Resistance in Streptococcus spp

The genus Streptococcus includes Gram-positive organisms shaped in cocci and organized in chains. They are commensals, pathogens, and opportunistic pathogens for humans and animals. Most Streptococcus species of veterinary relevance have a specific ecological niche, such as S. uberis, which is almost exclusively an environmental pathogen causing bovine mastitis. In contrast, S. suis can be considered as a true zoonotic pathogen, causing specific diseases in humans after contact with infected animals or derived food products. Finally, Streptococcus species such as S. agalactiae can be sporadically zoonotic, even though they are pathogens of both humans and animals independently. For clarification, a short taxonomical overview will be given here to highlight the diversity of streptococci that infect animals. Several families of antibiotics are used to treat animals for streptococcal infections. First-line treatments are penicillins (alone or in combination with aminoglycosides), macrolides and lincosamides, fluoroquinolones, and tetracyclines. Because of the selecting role of antibiotics, resistance phenotypes have been reported in streptococci isolated from animals worldwide. Globally, the dynamic of resistance acquisition in streptococci is slower than what is experienced in Enterobacteriaceae, probably due to the much more limited horizontal spread of resistance genes. Nonetheless, transposons or integrative and conjugative elements can disseminate resistance determinants among streptococci. Besides providing key elements on the prevalence of resistance in streptococci from animals, this article will also largely consider the mechanisms and molecular epidemiology of the major types of resistance to antimicrobials encountered in the most important streptococcal species in veterinary medicine.

Tetracycline-Inactivating Enzymes

Tetracyclines have been foundational antibacterial agents for more than 70 years. Renewed interest in tetracycline antibiotics is being driven by advancements in tetracycline synthesis and strategic scaffold modifications designed to overcome established clinical resistance mechanisms including efflux and ribosome protection. Emerging new resistance mechanisms, including enzymatic antibiotic inactivation, threaten recent progress on bringing these next-generation tetracyclines to the clinic. Here we review the current state of knowledge on the structure, mechanism, and inhibition of tetracycline-inactivating enzymes.

Evolutionary origins of the emergent ST796 clone of vancomycin resistant Enterococcus faecium

From early 2012, a novel clone of vancomycin resistant Enterococcus faecium (assigned the multi locus sequence type ST796) was simultaneously isolated from geographically separate hospitals in south eastern Australia and New Zealand. Here we describe the complete genome sequence of Ef_aus0233, a representative ST796 E. faecium isolate. We used PacBio single molecule real-time sequencing to establish a high quality, fully assembled genome comprising a circular chromosome of 2,888,087 bp and five plasmids. Comparison of Ef_aus0233 to other E. faecium genomes shows Ef_aus0233 is a member of the epidemic hospital-adapted lineage and has evolved from an ST555-like ancestral progenitor by the accumulation or modification of five mosaic plasmids and five putative prophage, acquisition of two cryptic genomic islands, accrued chromosomal single nucleotide polymorphisms and a 80 kb region of recombination, also gaining Tn1549 and Tn916, transposons conferring resistance to vancomycin and tetracycline respectively. The genomic dissection of this new clone presented here underscores the propensity of the hospital E. faecium lineage to change, presumably in response to the specific conditions of hospital and healthcare environments.

Using whole genome sequencing to identify resistance determinants and predict antimicrobial resistance phenotypes for year 2015 invasive pneumococcal disease isolates recovered in the United States

Our whole genome sequence (WGS) pipeline was assessed for accurate prediction of antimicrobial phenotypes. For 2316 invasive pneumococcal isolates recovered during 2015 we compared WGS pipeline data to broth dilution testing (BDT) for 18 antimicrobials. For 11 antimicrobials categorical discrepancies were assigned when WGS-predicted MICs and BDT MICs predicted different categorizations for susceptibility, intermediate resistance or resistance, ranging from 0.9% (tetracycline) to 2.9% (amoxicillin). For β-lactam antibiotics, the occurrence of at least four-fold differences in MIC ranged from 0.2% (meropenem) to 1.0% (penicillin), although phenotypic retesting resolved 25%-78% of these discrepancies. Non-susceptibility to penicillin, predicted by penicillin-binding protein types, was 2.7% (non-meningitis criteria) and 23.8% (meningitis criteria). Other common resistance determinants included mef (475 isolates), ermB (191 isolates), ermB + mef (48 isolates), tetM (261 isolates) and cat (51 isolates). Additional accessory resistance genes (tetS, tet32, aphA-3, sat4) were rarely detected (one to three isolates). Rare core genome mutations conferring erythromycin-resistance included a two-codon rplD insertion (rplD69-KG-70) and the 23S rRNA A2061G substitution (six isolates). Intermediate cotrimoxazole-resistance was associated with one or two codon insertions within folP (238 isolates) or the folA I100L substitution (38 isolates), whereas full cotrimoxazole-resistance was attributed to alterations in both genes (172 isolates). The two levofloxacin-resistant isolates contained parC and/or gyrA mutations. Of 11 remaining isolates with moderately elevated MICs to both ciprofloxacin and levofloxacin, seven contained parC or gyrA mutations. The two rifampin-resistant isolates contained rpoB mutations. WGS-based antimicrobial phenotype prediction was an informative alternative to BDT for invasive pneumococci.

Biological and Epidemiological Features of Antibiotic-Resistant Streptococcus pneumoniae in Pre- and Post-Conjugate Vaccine Eras: a United States Perspective

Streptococcus pneumoniae inflicts a huge disease burden as the leading cause of community-acquired pneumonia and meningitis. Soon after mainstream antibiotic usage, multiresistant pneumococcal clones emerged and disseminated worldwide. Resistant clones are generated through adaptation to antibiotic pressures imposed while naturally residing within the human upper respiratory tract. Here, a huge array of related commensal streptococcal strains transfers core genomic and accessory resistance determinants to the highly transformable pneumococcus. β-Lactam resistance is the hallmark of pneumococcal adaptability, requiring multiple independent recombination events that are traceable to nonpneumococcal origins and stably perpetuated in multiresistant clonal complexes. Pneumococcal strains with elevated MICs of β-lactams are most often resistant to additional antibiotics. Basic underlying mechanisms of most pneumococcal resistances have been identified, although new insights that increase our understanding are continually provided. Although all pneumococcal infections can be successfully treated with antibiotics, the available choices are limited for some strains. Invasive pneumococcal disease data compiled during 1998 to 2013 through the population-based Active Bacterial Core surveillance program (U.S. population base of 30,600,000) demonstrate that targeting prevalent capsular serotypes with conjugate vaccines (7-valent and 13-valent vaccines implemented in 2000 and 2010, respectively) is extremely effective in reducing resistant infections. Nonetheless, resistant non-vaccine-serotype clones continue to emerge and expand.

Enterococcal endocarditis revisited

The Enterococcus species is the third main cause of infective endocarditis (IE) worldwide, and it is gaining relevance, especially among healthcare-associated cases. Patients with enterococcal IE are older and have more comorbidities than other types of IE. Classical treatment options are limited due to the emergence of high-level aminoglycosides resistance (HLAR), vancomycin resistance and multidrug resistance in some cases. Besides, few new antimicrobial alternatives have shown real efficacy, despite some of them being recommended by major guidelines (including linezolid and daptomycin). Ampicillin plus ceftriaxone 2 g iv./12 h is a good option for Enterococcus faecalis IE caused by HLAR strains, but randomized clinical trials are essential to demonstrate its efficacy for non-HLAR EFIE and to compare it with ampicillin plus short-course gentamicin. The main mechanisms of resistance and treatment options are also reviewed for other enterococcal species.

The ClpXP protease is responsible for the degradation of the Epsilon antidote to the Zeta toxin of the streptococcal pSM19035 plasmid

Most bacterial genomes contain different types of toxin-antitoxin (TA) systems. The ω-ε-ζ proteinaceous type II TA cassette from the streptococcal pSM19035 plasmid is a member of the ε/ζ family, which is commonly found in multiresistance plasmids and chromosomes of various human pathogens. Regulation of type II TA systems relies on the proteolysis of antitoxin proteins. Under normal conditions, the Epsilon antidote neutralizes the Zeta toxin through the formation of a tight complex. In this study, we show, using both in vivo and in vitro analyses, that the ClpXP protease is responsible for Epsilon antitoxin degradation. Using in vivo studies, we examined the stability of the plasmids with active or inactive ω-ε-ζ TA cassettes in B. subtilis mutants that were defective for different proteases. Using in vitro assays, the degradation of purified His6-Epsilon by the His6-LonBs, ClpPBs, and ClpXBs proteases from B. subtilis was analyzed. Additionally, we showed that purified Zeta toxin protects the Epsilon protein from rapid ClpXP-catalyzed degradation.

Complete genome sequence of Enterococcus faecium strain TX16 and comparative genomic analysis of Enterococcus faecium genomes

BACKGROUND

Enterococci are among the leading causes of hospital-acquired infections in the United States and Europe, with Enterococcus faecalis and Enterococcus faecium being the two most common species isolated from enterococcal infections. In the last decade, the proportion of enterococcal infections caused by E. faecium has steadily increased compared to other Enterococcus species. Although the underlying mechanism for the gradual replacement of E. faecalis by E. faecium in the hospital environment is not yet understood, many studies using genotyping and phylogenetic analysis have shown the emergence of a globally dispersed polyclonal subcluster of E. faecium strains in clinical environments. Systematic study of the molecular epidemiology and pathogenesis of E. faecium has been hindered by the lack of closed, complete E. faecium genomes that can be used as references.

RESULTS

In this study, we report the complete genome sequence of the E. faecium strain TX16, also known as DO, which belongs to multilocus sequence type (ST) 18, and was the first E. faecium strain ever sequenced. Whole genome comparison of the TX16 genome with 21 E. faecium draft genomes confirmed that most clinical, outbreak, and hospital-associated (HA) strains (including STs 16, 17, 18, and 78), in addition to strains of non-hospital origin, group in the same clade (referred to as the HA clade) and are evolutionally considerably more closely related to each other by phylogenetic and gene content similarity analyses than to isolates in the community-associated (CA) clade with approximately a 3-4% average nucleotide sequence difference between the two clades at the core genome level. Our study also revealed that many genomic loci in the TX16 genome are unique to the HA clade. 380 ORFs in TX16 are HA-clade specific and antibiotic resistance genes are enriched in HA-clade strains. Mobile elements such as IS16 and transposons were also found almost exclusively in HA strains, as previously reported.

CONCLUSIONS

Our findings along with other studies show that HA clonal lineages harbor specific genetic elements as well as sequence differences in the core genome which may confer selection advantages over the more heterogeneous CA E. faecium isolates. Which of these differences are important for the success of specific E. faecium lineages in the hospital environment remain(s) to be determined.

Conjugative plasmids of Neisseria gonorrhoeae

Many clinical isolates of the human pathogen Neisseria gonorrhoeae contain conjugative plasmids. The host range of these plasmids is limited to Neisseria species, but presence of a tetracycline (tetM) determinant inserted in several of these plasmids is an important cause of the rapid spread of tetracycline resistance. Previously plasmids with different backbones (Dutch and American type backbones) and with and without different tetM determinants (Dutch and American type tetM determinants) have been identified. Within the isolates tested, all plasmids with American or Dutch type tetM determinants contained a Dutch type plasmid backbone. This demonstrated that tetM determinants should not be used to differentiate between conjugal plasmid backbones. The nucleotide sequences of conjugative plasmids with Dutch type plasmid backbones either not containing the tetM determinant (pEP5233) or containing Dutch (pEP5289) or American (pEP5050) type tetM determinants were determined. Analysis of the backbone sequences showed that they belong to a novel IncP1 subfamily divergent from the IncP1α, β, γ, δ and ε subfamilies. The tetM determinants were inserted in a genetic load region found in all these plasmids. Insertion was accompanied by the insertion of a gene with an unknown function, and rearrangement of a toxin/antitoxin gene cluster. The genetic load region contains two toxin/antitoxins of the Zeta/Epsilon toxin/antitoxin family previously only found in Gram positive organisms and the virulence associated protein D of the VapD/VapX toxin/antitoxin family. Remarkably, presence of VapX of pJD1, a small cryptic neisserial plasmid, in the acceptor strain strongly increased the conjugation efficiency, suggesting that it functions as an antitoxin for the conjugative plasmid. The presence of the toxin and antitoxin on different plasmids might explain why the host range of this IncP1 plasmid is limited to Neisseria species. The isolated plasmids conjugated efficiently between N. gonorrhoeae strains, but did not enhance transfer of a genetic marker.

Phenotyping and genotyping of antibiotic-resistant Escherichia coli isolated from a natural river basin

Scientists have become increasingly concerned about the occurrence of antibacterial resistance in the environment. In this study, Escherichia coli resistant to one or more antibiotics among nine antibiotics was screened from Wenyu River Basin in Beijing, China, with mean frequency of 48.7 +/- 8.7% of 388 isolates in summer and 47 +/- 6% of 236 isolates in winter. The mean multiantibiotic resistance (MAR) index in summer was 0.11 +/- 0.03, slightly lower than that (0.14 +/- 0.04) in winter. Most frequent resistance appeared for sulfonamides, tetracycline, and ampicillin. The distribution of 20 tetracycline, three sulfonamide, and three beta-lactam resistance genes was assessed in the resistant isolates. While 97% of the ampicillin (AMP) resistant mechanism could be explained by the resistance gene TEM, 90% of the tetracycline (TC) and 96% of the sulfonamide (SXT) resistances could be explained by tet(A), tet(B), tet(M), and their combinations and sul(I), sul(II), sul(III), and their combinations, respectively. tet(M), a tetracycline-resistant gene originally detected in Gram-positive bacteria, and its combinations with tet(A) or tet(B) were first detected in E. coli isolated from a natural river basin, suggesting that tet(M) in E. coli might have been transferred from other bacterial species through horizontal gene transfer, which was supported by the fact that no tet(M) was detected in the isolates of human and chicken sources, except for only one isolate from swine. The source of sulfonamide-resistant E. coli in the river was supposed to be mainly from humans, based on a comparison of the sulfonamide resistance genotypes in animals and humans.

Identification of a Spiroplasma citri hydrophilic protein associated with insect transmissibility

With the aim of identifyingSpiroplasma citriproteins involved in transmission by the leafhopperCirculifer haematoceps, protein maps of four transmissible and four non-transmissible strains were compared. Total cell lysates of strains were analysed by two-dimensional gel electrophoresis using commercially available immobilized pH gradients (IPGs) covering a pH range of 4–7. Approximately 530 protein spots were visualized by silver staining and the resulting protein spot patterns for the eight strains were found to be highly similar. However, comparison using PDQuest 2-D analysis software revealed two trains of protein spots that were present only in the four transmissible strains. Using MALDI-TOF (matrix-assisted laser desorption/ionization time-of-flight) mass spectrometry and a nearly completeS. citriprotein database, established during the still-ongoingS. citriGII-3-3X genome project, the sequences of both proteins were deduced. One of these proteins was identified in the general databases as adhesion-related protein (P89) involved in the attachment ofS. citrito gut cells of the insect vector. The second protein, with an apparent molecular mass of 32 kDa deduced from the electrophoretic mobility, could not be assigned to a known protein and was named P32. The P32-encoding gene (714 bp) was carried by a large plasmid of 35·3 kbp present in transmissible strains and missing in non-transmissible strains. PCR products with primers designed from thep32gene were obtained only with genomic DNA isolated from transmissible strains. Therefore, P32 has a putative role in the transmission process and it could be considered as a marker forS. citrileafhopper transmissibility. Functional complementation of a non-transmissible strain with thep32gene did not restore the transmissible phenotype, despite the expression of P32 in the complemented strain. Electron microscopic observations of salivary glands of leafhoppers infected with the complemented strain revealed a close contact between spiroplasmas and the plasmalemma of the insect cells. This further suggests that P32 protein contributes to the association ofS. citriwith host membranes.

The toxin-antitoxin system of the streptococcal plasmid pSM19035

pSM19035 of the pathogenic bacterium Streptococcus pyogenes is a low-copy-number plasmid carrying erythromycin resistance, stably maintained in a broad range of gram-positive bacteria. We show here that the omega-epsilon-zeta operon of this plasmid constitutes a novel proteic plasmid addiction system in which the epsilon and zeta genes encode an antitoxin and toxin, respectively, while omega plays an autoregulatory function. Expression of toxin Zeta is bactericidal for the gram-positive Bacillus subtilis and bacteriostatic for the gram-negative Escherichia coli. The toxic effects of zeta gene expression in both bacterial species are counteracted by proper expression of epsilon. The epsilon-zeta toxin-antitoxin cassette stabilizes plasmids in E. coli less efficiently than in B. subtilis.

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.

Mechanisms of resistance among respiratory tract pathogens

Antimicrobial resistance among respiratory tract pathogens represents a significant health care threat. Identifying the antimicrobial agents that remain effective in the presence of resistance, and knowing why, requires a thorough understanding of the mechanisms of action of the various agents as well as the mechanisms of resistance demonstrated among respiratory tract pathogens. The primary goal of antimicrobial therapy is to eradicate the pathogen, via killing or inhibiting bacteria, from the site of infection; the defenses of the body are required for killing any remaining bacteria. Targeting a cellular process or function specific to bacteria and not to the host limits the toxicity to patients. Currently, there are four general cellular targets to which antimicrobials are targeted: cell wall formation and maintenance, protein synthesis, DNA replication, and folic acid metabolism. Resistance mechanisms among respiratory tract pathogens have been demonstrated for all four targets. In general, the mechanisms of resistance used by these pathogens fall into one of three categories: enzymatic inactivation of the antimicrobial, prevention of intracellular accumulation, and modification of the target site to which agents bind to exert an antimicrobial effect. Resistance to some agents can be overcome by modifying the dosage regimens (e.g., using high-dose therapy) or inhibiting the resistance mechanism (e.g., b-lactamase inhibitors), whereas other mechanisms of resistance can only be overcome by using an agent from a different class. Understanding the mechanisms of action of the various agents and the mechanisms of resistance used by respiratory tract pathogens can help clinicians identify the agents that will increase the likelihood of achieving optimal outcomes.

Xis protein of the conjugative transposon Tn916 plays dual opposing roles in transposon excision

The binding of Tn916 Xis protein to its specific sites at the left and right ends of the transposon was compared using gel mobility shift assays. Xis formed two complexes with different electrophoretic mobilities with both right and left transposon ends. Complex II, with a reduced mobility, formed at higher concentrations of Xis and appeared at an eightfold lower Xis concentration with a DNA fragment from the left end of the transposon rather than with a DNA fragment from the right end of the transposon, indicating that Xis has a higher affinity for the left end of the transposon. Methylation interference was used to identify two G residues that were essential for binding of Xis to the right end of Tn916. Mutations in these residues reduced binding of Xis. In an in vivo assay, these mutations increased the frequency of excision of a minitransposon from a plasmid, indicating that binding of Xis at the right end of Tn916 inhibits transposon excision. A similar mutation in the specific binding site for Xis at the left end of the transposon did not reduce the affinity of Xis for the site but did perturb binding sufficiently to alter the pattern of protection by Xis from nuclease cleavage. This mutation reduced the level of transposon excision, indicating that binding of Xis to the left end of Tn916 is required for transposon excision. Thus, Xis is required for transposon excision and, at elevated concentrations, can also regulate this process.

Multiplex PCR for the detection of tetracycline resistant genes

Specific primer pairs were selected for the PCR amplification of 14 tetracycline resistant genes commonly found in Gram positive and Gram negative organisms. Combinations of primer pairs were used in multiplex PCR reactions to detect specific groups of tet genes as follows; Group I tet (B), tet (C), tet (D); Group II tet (A), tet (E), tet (G); Group III tet (K), tet (L), tet (M), tet (O), tet (S); Group IV tetA (P), tet (Q), tet (X). To test the multiplex PCR, Groups I and II were used on 25 clinical isolates of Salmonella enterica serovar Typhimurium DT104. Group III primers were used to investigate 19 clinical isolates of methicillin-resistant Staphylococcus aureus. Multiplex PCR should result in significant savings in terms of labour and cost in analysis of a large number of strains when compared with using an individual PCR for targeting each gene. It may also be a useful method to differentiate the types of tetracycline resistance when used as an additional marker for the purpose of outbreak investigation and surveillance.

Mutations in the interdomain loop region of the tetA(A) tetracycline resistance gene increase efflux of minocycline and glycylcyclines

A novel class of tetracyclines, the glycylcyclines, have been shown to be active against bacterial strains harboring genes encoding tetracycline efflux pumps. However, two veterinary Salmonella isolates that carried tetracycline resistance determinants of the tetA(A) class were found to have reduced susceptibility to glycylcyclines, especially two early investigational glycylcyclines, DMG-MINO and DMG-DMDOT. These isolates were also quite resistant to tetracycline and minocycline. The isolates, one a strain of S. cholerasuis and the other, S. typhimurium, both carried the same novel tetA(A) variant, based on DNA sequencing, with one determinant plasmid encoded and the other located on the chromosome. This tetA(A) variant was cloned and shown to provide reduced susceptibility to the glycylcycline class although GAR-936, a glycylcycline currently in clinical development, was the least affected. The novel tetA(A) gene carries two mutations in the largest cytoplasmic loop of the efflux pump, which causes a double frameshift in codons 201, 202, and 203. This "interdomain region" of the efflux pump has generally been regarded as having no functional role in the efflux of tetracycline but the double frameshift is most likely responsible for the enhanced resistance observed and points to an interaction that was previously unrecognized. Mutants of the tetA(B) class with decreased susceptibility to the glycylcyclines were also generated in vitro. These all carried mutations in the portion of the tetA(B) gene encoding a transmembrane spanning region of the efflux pump. The laboratory-generated mutants point to the tight constraints in substrate recognition of the transmembrane-spanning region and may suggest that it will be the interdomain region of the pump that is likely to be the locus of future glycylcycline resistance mutations as these compounds enter clinical use.

Mechanisms of antibiotic resistance and tolerance in Streptococcus pneumoniae

Streptococcus pneumoniae is a major pathogen causing potentially life-threatening community-acquired diseases in both the developed and developing world. Since 1967, there has been a dramatic increase in the incidence of penicillin-resistant and multiply antibiotic-resistant pneumococci worldwide. Prevention of access of the antibiotic to the target, inactivation of the antibiotic and alteration of the target are mechanisms that S. pneumoniae has developed to resist antibiotics. Recent studies on antibiotic-tolerant pneumococcal mutants permitted development of a novel model for the control of bacterial cell death.

Genetic diversity of the tet(M) gene in tetracycline-resistant clonal lineages of Streptococcus pneumoniae

The aim of the present study was to examine the stability and evolution of tet(M)-mediated resistance to tetracyclines among members of different clonal lineages of Streptococcus pneumoniae. Thirty-two tetracycline-resistant isolates representing three national (Spanish serotype 14, Spanish serotype 15, and Polish serotype 23F) and one international (Spanish serotype 23F) multidrug-resistant epidemic clones were all found to be tet(M) positive and tet(O), tet(K), and tet(L) negative. These isolates all carried the integrase gene, int, which is associated with the Tn1545-Tn916 family of conjugative transposons. High-resolution restriction analysis of tet(M) products identified six alleles, tet(M)1 to tet(M)6: tet(M)1 to tet(M)3 and tet(M)5 in isolates of the Spanish serotype 14 clone, tet(M)4 in both the Spanish serotype 15 and 23F clones, and tet(M)6, the most divergent allele, in the Polish 23F clone. This indicates that tet(M) variation can occur at the inter- and intraclone levels in pneumococci. Two alleles of int were identified, with int1 being found in all isolates apart from members of the international Spanish 23F clone, which carried int2. Susceptibility to tetracycline, doxycycline, and minocycline was evaluated for all isolates with or without preincubation in the presence of subinhibitory concentrations of tetracyclines. Resistance to tetracyclines was found to be inducible in isolates of all clones; however, the strongest induction was observed in the Spanish serotype 15 and 23F clones carrying tet(M)4. Tetracycline was found to be the strongest inducer of resistance, and minocycline was found to be the weakest inducer of resistance.

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.

The molecular mechanisms of tetracycline resistance in the pneumococcus

Tetracycline resistance in the pneumococcus is a result of the acquisition of one of two resistance determinants, tet(M) or tet(O). These genes encode ribosomal protection proteins that have homology to the elongation factors G and Tu. Tet(M) and Tet(O) both have GTPase activity that appears to be important in the displacement of tetracycline from the ribosome. Modification of tRNA may also be important for tetracycline resistance. Transcription of tet(M) is thought to be regulated by transcriptional attenuation. Transcription of tet(O) is constitutive, however, upstream of the gene are sequences that also appear to be involved in transcriptional attenuation. tet(M) is transferred on the conjugative transposons, Tn1545 and Tn5151. It is not yet known whether tet(O) is transported on transposons or plasmids, or whether it is chromosomally integrated, in pneumococci.

Ribosomal protection from tetracycline mediated by Tet(O): Tet(O) interaction with ribosomes is GTP-dependent

Tet(O) mediates tetracycline resistance by protecting the ribosome from inhibition. A recombinant Tet(O) protein with a histidine tag was purified and its activity in protein synthesis characterized. Tetracycline inhibited the rate of poly(Phe) synthesis, producing short peptide chains. Tet(O)-His was able to restore the elongation rate and processivity. 70S ribosomes bound tetracycline with high affinity. Tet(O)-His in the presence of GTP, but not GDP or GMP, reduced the affinity of the ribosomes for tetracycline. Non-hydrolyzable GTP analogs in the presence of the factor were also able to interfere with tetracycline binding. Ribosomes increased the affinity of Tet(O)-His for GTPgammaS. Tet(O), 70S ribosomes and GTPgammaS formed a complex that could be isolated by gel filtration. The GTP conformer is the active form of Tet(O) that interacts with the ribosome. GTP binding is necessary for Tet(O) activity.

Host mutations (miaA and rpsL) reduce tetracycline resistance mediated by Tet(O) and Tet(M)

The effects of mutations in host genes on tetracycline resistance mediated by the Tet(O) and Tet(M) ribosomal protection proteins, which originated in Campylobacter spp. and Streptococcus spp., respectively, were investigated by using mutants of Salmonella typhimurium and Escherichia coli. The miaA, miaB, and miaAB double mutants of S. typhimurium specify enzymes for tRNA modification at the adenosine at position 37, adjacent to the anticodon in tRNA. In S. typhimurium, this involves biosynthesis of N6-(4-hydroxyisopentenyl)-2-methylthio-adenosine (ms2io6A). The miaA mutation reduced the level of tetracycline resistance mediated by both Tet(O) and Tet(M), but the latter showed a greater effect, which was ascribed to the isopentenyl (i6) group or to a combination of the methylthioadenosine (ms2) and i6 groups but not to the ms2 group alone (specified by miaB). In addition, mutations in E. coli rpsL genes, generating both streptomycin-resistant and streptomycin-dependent strains, were also shown to reduce the level of tetracycline resistance mediated by Tet(O) and Tet(M). The single-site amino acid substitutions present in the rpsL mutations were pleiotropic in their effects on tetracycline MICs. These mutants affect translational accuracy and kinetics and suggest that Tet(O) and Tet(M) binding to the ribosome may be reduced or slowed in the E. coli rpsL mutants in which the S12 protein is altered. Data from both the miaA and rpsL mutant studies indicate a possible link between stability of the aminoacyl-tRNA in the ribosomal acceptor site and tetracycline resistance mediated by the ribosomal protection proteins.

Antibiotic resistance among enterococci isolated from clinical specimens between 1953 and 1954

Two hundred twenty group D streptococci isolated from 1953 to 1954 from patients in the Washington, D.C., area were characterized. All were susceptible to ampicillin, vancomycin, and gentamicin; none produced beta-lactamase activity. High-level resistance to streptomycin was expressed by 117 strains, and 2 strains were resistant to >8 microg of chloramphenicol per ml. Three isolates were resistant to both erythromycin and lincomycin, and DNA from these hybridized to an ermAM probe. Of 118 strains resistant to tetracycline and minocycline, genomic DNA from 63 was examined for homology to tet(M), tet(O), and tet(S). DNA from 20 strains hybridized to tet(M), DNA from 37 strains hybridized to tet(S), and DNA from none of the strains hybridized to tet(O).

Identification of the tetracycline resistance gene, tet(O), in Streptococcus pneumoniae

Five isolates of Streptococcus pneumoniae resistant to tetracycline but lacking tet(M) were studied. The tetracycline resistance gene, tet(O), was detected for the first time in the pneumococcus. The gene was amplified and sequenced and found to share 99% nucleotide sequence identity and 99, 99, and 98% deduced amino acid sequence identity with the tet(O) resistance genes of Streptococcus mutans, Campylobacter coli, and Campylobacter jejuni, respectively.

Genetic organization and distribution of tetracycline resistance determinants in Clostridium perfringens

The Tet P determinant from the conjugative Clostridium perfringens R plasmid pCW3 two functional overlapping tetracycline resistance genes, tetA(P) and tetB(P). The tetA(P) gene encodes a putative 46-kDa transmembrane protein which mediates active efflux of tetracycline from the cell, while tetB(P) encodes a putative 72.6-kDa protein which has significant similarity to Tet M-like tetracycline resistance proteins (J. Sloan, L.M. McMurry, D. Lyras, S. B. Levy, and J. I. Rood, Mol. Microbiol. 11:403-415, 1994). In the present study, hybridization and PCR analysis of 81 tetracycline-resistant isolates of C. perfringens showed that they all carried the tetA(P) gene. Most of these isolates (93%) carried a second tetracycline resistance gene, with 53% carrying tetB(P) and 40% carrying a tet(M)-like gene. Despite the wide distribution of the tetB(P) and tet(M) genes, no isolate which carried both of these determinants was detected. In isolates that carried both tetA(P) and tetB(P) these genes overlapped, as in pCW3. Isolates carrying this combination of genes originated from diverse geographical locations and environmental sources. The single Clostridium paraputrificum isolate examined carried tetA(P), indicating that this gene is not confined to C.perfringens. However, neither tetA(P) nor tetB(P) was detected in the nine Clostridium difficile isolates tested. Nucleotide sequence analysis of isolates lacking tetB(P) revealed that they contained the tetA408(P) gene, which lacked the codons for the 12 carboxy-terminal amino acids of the TetA(P) protein.

Three subtypes of the tet(M) gene identified in bacterial isolates from periodontal pockets

The tet(M) genes were characterized from 84 isolates of 10 different bacterial species isolated from the periodontal pockets of 16 patients with periodontal disease. A 740 bp polymerase chain reaction product from the hypervariable region of the tet(M) structural gene was cleaved with the restriction enzymes AluI and HinfI. Three different restriction patterns were identified for each of the two enzymes. By DNA sequencing, using a direct solid-phase automated sequencing method, the isolates could be grouped into 3 different clusters of tet(M) subtypes. The internal DNA homology within each subtype was 98-100%; the homology between clusters was 89-94%. Two different subtypes were identified in 9 of 10 bacterial species, and the remaining species had 3 different subtypes. One of the subtypes (M3) was seen mainly in the anaerobic isolates. This subtype was different from all earlier sequenced structural tet(M) genes present in the Genbank. Most patients had two different subtypes of tet(M), and a third subtype was seen in the 3 patients who exhibited the greatest variety of tetracycline-resistant bacterial species. It appears that the presence of one subtype of the tet(M) gene within a patient or bacterial species does not prevent the acquisition of another subtype of the same gene. This study identified a new subtype of the tet(M) gene and grouped it into 3 distinct yet highly homologous genetic subtypes.

Distribution of tet(H) among Pasteurella isolates from the United States and Canada

Tetracycline-resistant isolates of Pasteurella multocida and Pasteurella haemolytica obtained from various locations in the United States and Canada were studied to determine the distribution of the tet(H) gene. Of the 31 isolates examined, 25 were found to contain the tet(H) gene. Chromosomal or plasmid DNA obtained from those that did not contain the tet(H) gene did not hybridize with probes specific for classes A through G, though chromosomal DNA from one isolate lacking tet(H) hybridized with a probe specific for class M. The tet(H) gene was found on plasmid as well as on chromosomal DNA, suggesting that it is carried on a transposable element.

Plasmid-borne high-level resistance to gentamicin in Enterococcus hirae, Enterococcus avium, and Enterococcus raffinosus

Enterococcus hirae, E. avium, and E. raffinosus isolated in Romania, Tunisia, and Portugal harbored plasmids pICC8, pIP1700, and pIP1701, respectively, encoding resistance to high levels of gentamicin (Gmr). The Gmr marker was carried on pIP1700 by a Tn4001-like element and on pICC8 and pIP1701 by Tn4001-truncated structures. pICC8 carried, in addition to Gmr, chloramphenicol, erythromycin, and tetracycline-minocycline (TetM) resistance determinants. The gene tetM of pICC8 was carried on a Tn916-like element.

Tetracycline resistance determinants among streptococci of serological group G and L

In the present study 56 streptococci of serological group G and L isolated from various animal species and from humans were investigated for tetracycline and minocycline resistance and for the presence of genes conferring this combined resistance. Among the 45 group G streptococci, 2 isolates from dogs, 3 from cattle and 2 from humans, respectively, as well as all 11 group L streptococci isolated from cattle, pigs or poultry were resistant to tetracycline and simultaneously to minocycline. The restriction endonuclease digested and blotted DNA-preparation of the tetracycline-and minocycline resistant group G streptococci from dogs and humans hybridized with the tet (M) gene probe, those from bovines with the tet (O) gene probe. Six group L streptococci carried the gene tet (M), whereas 5 isolates harboured the gene tet (O). The tet (M)-and tet (O) gene probes recognized complementary sequences on EcoRI-fragments of various sizes.

Characterization and heterologous expression of the tetL gene and identification of iso-ISS1 elements from Enterococcus faecalis plasmid pJH1

The tetracycline-resistance (TcR) determinant of the Enterococcus faecalis plasmid pJH1 has been identified and located on a 2.2-kb RsaI-EcoRI fragment. The fragment was cloned in Escherichia coli, and specified TcR in this host. The nucleotide (nt) sequence of the cloned fragment showed the presence of an open reading frame (ORF) of 1374 bp, designated tetL. The nt sequence of tetL from pJH1 was identical to that of the tetL present on pLS1 from Streptococcus agalactiae. Upstream of the pJH1 tetL, part of another ORF was found that, except for two single-nt substitutions, was identical to an iso-ISS1 element from Lactococcus lactis. Hybridization studies indicated the presence of several ISS1-like elements in plasmid pJH1, but not on the En. faecalis chromosome. To study its usefulness as a marker in Gram+ organisms, the pJH1 tetL was cloned on the broad-host-range plasmid pNZ124, resulting in pNZ280, that was found to give resistance to 40 micrograms Tc/ml in Lc. lactis and Bacillus subtilis.

Detection of tet(M) and tet(O) using the polymerase chain reaction in bacteria isolated from patients with periodontal disease

The polymerase chain reaction was used to examine 114 tetracycline-resistant anaerobic and facultative anaerobic bacterial isolates from patients with periodontal disease for the tet(M) and tet(O) genes. A 740-base-pair fragment of the tet(M) gene was amplified from 84 of 114 isolates, and a 519-base-pair fragment of the tet(O) gene was amplified from 13 streptococcal isolates. Six of 7 tetracycline-resistant isolates of Veillonella spp. and tetracycline-resistant isolates of Eubacterium spp. (n = 3), Eubacterium saburreum (n = 1), Streptococcus intermedius (n = 5) and Gemella morbillorum (n = 2) all harbored the tet(M) gene. The tet(M) and tet(O) negative as well as selected positive isolates were tested for the tet(K) and tet(L) genes using DNA probes. All isolates of Staphylococcus spp. (n = 11) hybridized with the tet(K) probe. None of the isolates tested hybridized with the probe for tet(L). This is the first report of the tet(M) gene in the facultative bacterium G. morbillorum and in E. saburreum.

Presence of the Listeria tetracycline resistance gene tet(S) in Enterococcus faecalis

Two hundred thirty-eight tetracycline- and minocycline-resistant clinical isolates of Enterococcus and Streptococcus spp. were investigated by dot blot hybridization for the presence of nucleotide sequences related to tet(S) (first detected in Listeria monocytogenes BM4210), tet(K), tet(L), tet(M), tet(O), tet(P), and tet(Q) genes. The tet(S) determinant was found in 22 strains of Enterococcus faecalis, associated with tet(M) in 9 of these isolates and further associated with tet(L) in 3 of these strains. tet(M) was detected in all strains of Streptococcus spp. and in all but 10 isolates of Enterococcus spp.; tet(L) was found in 93 enterococci and tet(O) was found in single isolates of E. faecalis and Streptococcus milleri. No hybridization with the tet(K), tet(P), and tet(Q) probes was observed. Transfer of tet(S) by conjugation to E. faecalis or to E. faecalis and L. monocytogenes was obtained from 8 of the 10 E. faecalis strains harboring only this tet gene. Hybridization experiments with DNAs of four donors and of the corresponding transconjugants suggested that tet(S) was located in the chromosome. These results indicate that the genetic support of tet(S) in E. faecalis is different from that in L. monocytogenes, where it is carried by self-transferable plasmids, and confirm the notion of exchange of genetic information between Enterococcus and Listeria spp. in nature.

Cloning and sequencing of the replication origin (oriC) of the Spiroplasma citri chromosome and construction of autonomously replicating artificial plasmids

A 5.6-kbp fragment of Spiroplasma citri DNA containing the dnaA gene has been cloned and sequenced. Nucleotide sequence analysis shows that this fragment harbors the genes for the replication initiator protein (dnaA), the beta subunit of DNA polymerase III (dnaN), and the DNA gyrase subunits A and B (gyrA and gyrB). The arrangement of these genes, dnaA-dnaN-gyrB-gyrA, is similar to that found in all Gram-positive bacterial genomes studied so far, except that no recF gene was found between dnaN and gyrB. Several DnaA-box consensus sequences were found upstream of dnaA and in the dnaA-dnaN intergenic region. The dnaA region with the flanking DnaA-boxes and the tetracycline resistance determinant, tetM, were linked into a circular recombinant DNA. This DNA was able to replicate autonomously when introduced by electroporation into S. citri cells. These experiments show that the dnaA region with the DnaA-boxes is the origin of replication of S. citri and can be used to construct gene vectors.

Inhibition of protein synthesis occurring on tetracycline-resistant, TetM-protected ribosomes by a novel class of tetracyclines, the glycylcyclines

One of the two major mechanisms of tetracycline resistance is ribosomal protection. Of this resistance type, tet(M) is the best characterized. Although the mechanism of tet(M) resistance has not yet been fully elucidated, it has been demonstrated that ribosomes isolated from a tet(M) strain are resistant to inhibition of protein synthesis by tetracycline. A new generation of tetracycline compounds, the glycylcyclines, that are able to inhibit protein synthesis occurring on tetracycline-resistant, TetM-protected ribosomes, as well as wild-type, tetracycline-sensitive ribosomes, have been identified.

Characterization of antibiotic resistance in Streptococcus suis

Twenty one isolates of Streptococcus suis were screened for antibiotic resistance by growth on antibiotic-containing media, by measuring minimum inhibitory concentrations, by hybridization to specific DNA and oligonucleotide probes for antibiotic resistance genes, and by PCR. The isolates were from a slaughter house survey of respiratory pathogens in Norwegian pigs in 1986. Fifteen isolates were resistant to tetracycline, with MICs ranging from 4-128 micrograms/ml. Genes coding for the Tet O and Tet M determinants were detected in eight and five isolates, respectively. Genes coding for other Gram positive Tet determinants, Tet K, Tet L, and Tet P, were not detected. One isolate was constitutive resistant to erythromycin with MIC of 128 micrograms/ml. Five other isolates carried inducible erythromycin resistance. All these isolates, and five others, were positive in a PCR assay for erythromycin resistance, and hybridized with the Erm C and/or Erm B probes. No resistance against chloramphenicol (5 micrograms/ml) or rifampin (10 micrograms/ml) could be could be detected, but five isolates were resistant to streptomycin (250 micrograms/ml), four isolates were resistant to kanamycin (10 micrograms/ml), and one isolate was resistant to fusicic acid (10 micrograms/ml). In mating experiments with Enterococcus faecalis JH2-2 as recipient, tetracycline, erythromycin, and kanamycin genes were transferred separately to the recipient strain at a rate of 10(-7) transconjugants/recipient cell.

Macrolide-lincosamide resistance determinants in streptococcal species isolated from the bovine mammary gland

Seventy one streptococci isolated from dairy cows with clinical mastitis were tested for erythromycin and lincomycin susceptibility. Ten isolates (7.1%) were resistant to erythromycin and/or lincomycin and seven were constitutive and three were inducibly resistant. Nine of the isolates hybridized with one or more of the Erm probes tested and eight isolates gave PCR products with rRNA methylase primers. The Erm determinants were transferable at frequency of 10(-5) to 10(-6) per recipient.

Integrative Cloning, Expression, and Stability of the cryIA(c) Gene from Bacillus thuringiensis subsp. kurstaki in a Recombinant Strain of Clavibacter xyli subsp. cynodontis

A bacterial endophyte was engineered for insecticidal activity against the European corn borer. The cryIA(c) gene from Bacillus thuringiensis subsp. kurstaki was introduced into the chromosome of Clavibacter xyli subsp. cynodontis by using an integrative plasmid vector. The integration vectors pCG740 and pCG741 included the replicon pGEM5Zf(+), which is maintained in Escherichia coli but not in C. xyli subsp. cynodontis; tetM as a marker for selection in C. xyli subsp. cynodontis ; and a chromosomal fragment of C. xyli subsp. cynodontis to allow for homologous recombination between the vector and the bacterial chromosome. Insertion of vector DNA into the chromosome was demonstrated by DNA hybridization. Recombinant strains MDR1.583 and MDR1.586 containing the cryIA(c) gene were shown to produce the 133,000-kDa protoxin and several smaller immunoreactive proteins. Both strains were equally toxic to insect larvae in bioassays. Significant insecticidal activity was demonstrated in planta. The cryIA(c) gene and the tetM gene introduced into strain MDR1.586 were shown to be deleted from some cells, thereby giving rise to a noninsecticidal segregant population. In DNA hybridization experiments and insect bioassays, these segregants were indistinguishable from the wild-type strain. Overall, these results demonstrate the plausibility of genetically engineered bacterial endophytes for insect control.

Effect of mutational alteration of Asn-128 in the putative GTP-binding domain of tetracycline resistance determinant Tet(O) from Campylobacter jejuni

The deduced amino acid sequence of Campylobacter jejuni Tet(O), cloned in Escherichia coli, has shown that it contains the five highly conserved sequences of the GTP-binding domain found in other GTPases. Asn-128 belongs to the G4 motif of such a domain and is involved in hydrogen bonding with the guanine ring of the nucleotide. Substitution of Asn-128 by 11 other amino acids resulted in a decrease in tetracycline resistance, indicating that tetracycline resistance conferred by Tet(O) is related to GTP binding. The effect of the mutations on the GTP-binding domain is discussed with the EF-Tu-GDP complex as a model.