Tetracycline resistance genes from Bacillus plasmid pAB124 confer decreased accumulation of the antibiotic in Bacillus subtilis but not in Escherichia coli

Antibacterial spectra of drugs used for chemotherapy of mycobacterial infections

The mechanism of action of many antimycobacterial agents is poorly understood. To obtain preliminary information on whether the targets for some of these drugs might also occur in other bacteria, the in vitro activities of selected agents against Escherichia coli, Bacillus subtilis and Staphylococcus aureus were determined. Dapsone, p-aminosalicylic acid and thiacetazone failed to inhibit the above organisms (MIC values > 100 micrograms/ml) that may therefore lack targets for these drugs. Capreomycin, viomycin and clofazimine demonstrated activity against some of the organisms (MIC values < 100 micrograms/ml) suggesting that the targets of these drugs may not be restricted to mycobacterial species. The agents were all potent inhibitors of Mycobacterium bovis bacille Calmette-Guérin (MIC values 0.08-0.5 microgram/ml).

Chromosomal tetA(L) gene of Bacillus subtilis: regulation of expression and physiology of a tetA(L) deletion strain

Deletion of the tetA(L) chromosomal region of Bacillus subtilis in a strain designated JC112 increased the strain's sensitivity to low tetracycline concentrations. It also resulted in phenotypic changes that correlate with the previously found role of TetA(L) in mediating electrogenic NA+/H+ antiport. Growth of JC112 was impaired relative to that of the wild type at both pH 7.0 and 8.3; Na(+)- and K(+)-dependent pH homeostases were impaired at alkaline pH. The phenotype of JC112 was complemented by plasmid-borne tetA(L) and related tet(K) genes; the antiport activity conferred by the tet(K) gene had an apparently higher preference for K+ over Na+ than that conferred by tetA(L). The data were consistent with TetA(L) being the major Na+(K+)/H+ antiporter involved in pH homeostasis in B. subtilis as well as a significant Na+ extrusion system. The phenotype of JC112 was much more pronounced than that of an earlier transposition mutant, JC111, with a disruption in the putative tetA(L) promoter region. Northern (RNA) blot analysis of tetA(L) RNA from wild-type and JC111 strains revealed the same patterns. That JC111 nevertheless exhibited some Na+ and alkali sensitivity may be accounted for by disruption of regulatory features that, in the wild type, allow increased tetA(L) expression under specific conditions of pH and monovalent cation concentration. Evidence for several different regulatory effects emerged from studies of lacZ expression from the transposon of JC111 and from a tetA(L)-lacZ translational fusion introduced into the amyE locus of wild-type and JC112 strains.

Expression of the tetK gene from Staphylococcus aureus in Escherichia coli: comparison of substrate specificities of TetA(B), TetA(C), and TetK efflux proteins

The tetK gene, which encodes a tetracycline efflux pump from Staphylococcus aureus, was expressed in Escherichia coli by using an inducible, low-level expression system. The tetK gene, as well as the tetA(B) gene from the transposon Tn10 and the tetA(C) gene from plasmid pBR322, was subjected to the regulatory control of the lac repressor, and resistance to tetracycline was measured as a function of the isopropyl-beta-D-thiogalactopyranoside concentration. The maximum resistance of the E. coli strain containing the tetK construct was comparable to the maximum resistance of the strain containing the tetA(C) construct but was less than the resistance of the strain containing the tetA(B) construct. Overexpression of the tetK, tetA(B), or tetA(C) genes was toxic. When expression was regulated so that resistance to tetracycline was comparable, then the TetA(B) and TetA(C) proteins conferred very similar levels of resistance to a variety of tetracycline derivatives. In contrast, the TetK protein was less capable of conferring resistance to the tetracycline derivatives minocycline, 6-deoxy-6-demethyltetracycline, and doxycycline. The implications for the recognition of various tetracycline substituents by the TetK protein are discussed.

Characterization of chromosomal DNA amplifications with associated tetracycline resistance in Bacillus subtilis

Endogenous chromosomal DNA amplifications with associated tetracycline resistance (Tcr) in Bacillus subtilis were first described by C. R. Wilson and A. E. Morgan (J. Bacteriol. 163:445-453, 1985). We have confirmed and extended their results, and we show that fusion of protoplasts from Tcs B. subtilis 168 trpC2 with polyethylene glycol and regeneration on medium containing 20 micrograms of tetracycline per ml induces Tcr regenerants that contain amplified DNA. This phenomenon appeared to be recE dependent and requires the addition of polyethylene glycol. Along with three regenerants kindly provided by Wilson and Morgan (RAD1, RAD6, and RAD7), we characterized three strains (CLI20, CLI22, CLI30) isolated in this laboratory. All six contain an amplified region of DNA which was independently cloned on plasmid pCIS7. Integration of pCIS7 into the wild-type (Tcs) B. subtilis chromosome and amplification of the plasmid sequences generated a Tcr phenotype, even though the DNA on pCIS7 was cloned from Tcs B. subtilis KS162 (Ives and Bott, J. Bacteriol. 171:1801-1810, 1989). The amplified DNA also showed homology (through hybridization analysis) with pAM alpha 1 delta 1, a gram-positive Tcr plasmid, indicating that B. subtilis normally contains a silent integrated copy of the gene whose amplification confers Tcr. The amplifications were determined to lie between purA and gyrB on the B. subtilis chromosome, and the endpoints were mapped. RAD6 and CLI30 may share the same left-hand endpoint, but the other endpoints are different in each isolate. The amplified DNAs of RAD1, RAD6, CLI20, and CLI30 end near known DNA membrane binding sites. The number of amplified units of DNA was determined through dot blot analysis to do approximately 80 to 100 copies per cell, with corresponding increases in transcription of RAD1, RAD6, CLI20, CLI22, and CLI30.

Cloned Bacillus subtilis chromosomal DNA mediates tetracycline resistance when present in multiple copies

Plasmid pCIS7, containing 11.5 kilobases (kb) of Bacillus subtilis DNA, was isolated from a Tn917 transposon insertion in tetracycline-sensitive B. subtilis KS162. When integrated into the chromosome of B. subtilis 168, this plasmid conferred tetracycline resistance upon reiteration of the plasmid DNA sequences in the chromosome. Deletions and subclones of pCIS7 were constructed and introduced into an Escherichia coli in vitro transcription-translation system. A 72-kilodalton protein was localized to a 3.1-kb PstI-EcoRI fragment of the plasmid. Amplification of the 3.1-kb PstI-EcoRI fragment was required for expression of tetracycline resistance in B. subtilis 168. By hybridization to previously characterized clones, the 11.5-kb fragment was localized to the origin region of the chromosome. Through contour-clamped homogeneous electric field electrophoresis, this cluster of clones was shown to reside on a 200-kb NotI fragment bridging SfiI fragments of 150 and 250 kb and was oriented with respect to the purA and guaA loci, developing an accurate physical map of the region surrounding the origin of replication.

Characterization of the tetracycline resistance gene of plasmid pT181 of Staphylococcus aureus

Some genetic and biochemical properties of the tetracycline resistance element of the Staphylococcus aureus plasmid pT181 have been studied. Resequencing of a portion of the tetracycline resistance gene (tet) showed the presence of a single open reading frame of 1,299 nucleotides capable of encoding a polypeptide of 433 amino acids. Analysis of BAL 31 nuclease-generated deletion mutants of the tet gene showed the presence of two complementation groups within this region. Northern blot hybridizations demonstrated that the tet gene encodes a single mRNA, and its initiation site has been mapped by S1 nuclease protection experiments. We also identified an approximately 52,000-dalton tetracycline-inducible polypeptide in Bacillus subtilis minicells carrying pT181. Induction of the tet gene by tetracycline resulted in a 4-fold increase in the levels of TET mRNA and at least a 15-fold increase in the amount of TET protein in B. subtilis minicells.

Streptococcal tetracycline resistance mediated at the level of protein synthesis

The mechanism of tetracycline resistance was examined in strains containing each of the three previously identified resistance determinants in Streptococcus spp. Uptake of tetracycline was measured in tetracycline-sensitive cells as well as in cells containing each of the three resistance determinants. In cells containing tetL, uptake was not observed. However, in sensitive cells and cells containing either tetM or tetN, tetracycline was accumulated approximately 25-fold against a concentration gradient. Furthermore, there was no evidence for modification of intracellular tetracycline recovered from sensitive, tetM, or tetN cells. Protein synthesis in extracts derived from organisms containing tetM or tetN was resistant to tetracycline. In contrast, extracts of sensitive and tetL cells were sensitive to tetracycline.

Cloning and analysis of the Clostridium perfringens tetracycline resistance plasmid, pCW3

Clostridium perfringens strain CW92 carries pCW3, a conjugative 47-kb plasmid that confers inducible resistance to tetracycline. The plasmid was examined by restriction endonuclease analysis and by cloning each of the five ClaI fragments of pCW3 in Escherichia coli, using pBR322. Analysis of the recombinant plasmids allowed the deduction of a detailed restriction map of pCW3. The tetracycline resistance determinant of pCW3 was mapped by examining the phenotype of recombinant E. coli clones derived from the cloning, into pUC vector plasmids, of EcoRI fragments from pCW3. The C. perfringens tetracycline resistance determinant was expressed in E. coli and was shown to be located on two juxtaposed EcoRI fragments which together encompass a 4-kb region of pCW3. Deletion experiments showed that the tetracycline resistance gene, and/or its control regions, contained internal EcoRI and SphI sites. E. coli strains that carried recombinant plasmids with only the 4-kb region were found to express tetracycline resistance constitutively. In contrast, recombinant plasmids harboring a 10.5-kb ClaI fragment of pCW3, that included the 4-kb region, coded for an inducible tetracycline resistance phenotype. The existence of a negatively regulated resistance gene, similar to that proposed for several other bacteria is postulated.

Characterization and expression of a cloned tetracycline resistance determinant from the chromosome of Streptococcus mutans

A chromosomal tetracycline resistance (Tcr) determinant previously cloned from Streptococcus mutans into Streptococcus sanguis (Tobian and Macrina, J. Bacteriol. 152:215-222, 1982) was characterized by using restriction endonuclease mapping, deletion analysis, and Southern blot hybridization. Deletion analysis allowed localization of the Tcr determinant to a 2.8-kilobase region of the originally cloned 10.4-kilobase sequence. This cloned determinant hybridized to a representative of the tetM class of streptococcal Tcr determinants but not to representatives of the tetL and tetN classes and, like other tetM determinants, mediated high-level resistance to tetracycline and low-level resistance to minocycline. A portion (approximately 3 kilobases) of the isolated streptococcal fragment was subcloned into Escherichia coli, where it conferred resistance to tetracycline and minocycline. Two proteins with apparent molecular weights of 33,000 and 35,000, encoded by the S. mutans DNA, were synthesized in E. coli minicells. Insertion of DNA into a unique SstI site of the cloned S. mutans fragment resulted in inactivation of Tcr expression in E. coli and S. sanguis, as well as loss of production of both the 33,000- and 35,000-dalton proteins in E. coli minicells. Incubation of minicells in subinhibitory concentrations of tetracycline did not result in changes in the levels of synthesis of either protein. Our data suggest that at least one of these proteins is involved in the expression of Tcr.

Transposon Tn10-like tetracycline resistance determinants in Haemophilus parainfluenzae

Tetracycline resistance (Tcr) determinants from three different strains of Haemophilus parainfluenzae expressed 10-fold higher levels of resistance when mated into Escherichia coli. No plasmid was found in any of the E. coli recipients, even in matings in which a plasmid was identified in the donor Haemophilus sp. The Tcr determinant from Haemophilus sp. caused instability of resident plasmids in the recipient E. coli: all plasmids were lost within 30 generations in antibiotic-free media. However, by serial subculture in antibiotics, stable resident plasmids were obtained which carried the Tcr determinant from Haemophilus sp. and were transferable by conjugation and transformation among E. coli strains. All Haemophilus determinants hybridized with a probe for the Tcr determinant on Tn10, which bears inducible Tcr. However, Haemophilus determinants were constitutively resistant to tetracycline in the Haemophilus donors and in the E. coli recipients. This constitutive expression was recessive to wild-type Tn10 in the same cell, indicating that the constitutive phenotype resulted from the absence of an active repressor. Restrictive enzyme analysis of various E. coli plasmid derivatives bearing a Tcr determinant from Haemophilus sp. demonstrated that the inserted DNA was of similar size (8.95 to 9.35 kilobases), close to that of Tn10. Heteroduplex analysis and DNA:DNA hybridization confirmed that the Tcr determinant from Haemophilus sp. had greater than 90% homology with the Tn10 determinant, including the DNA sequence for the repressor.

Regeneration of insertionally inactivated streptococcal DNA fragments after excision of transposon Tn916 in Escherichia coli: strategy for targeting and cloning of genes from gram-positive bacteria

The conjugative transposon Tn916 (15 kilobases), originally identified in Streptococcus faecalis DS16, has been cloned as an intact element on the pBR322-derived vector pGL101 in Escherichia coli. The EcoRI F' (EcoRI F::Tn916) fragment of pAM211 (pAD1::Tn916) was cloned into the single EcoRI site of pGL101 to form the chimera, pAM120, by selecting for the expression of Tn916-encoded tetracycline resistance (Tcr). Interestingly, in the absence of continued selection for Tcr, Tn916 excised from pAM120 at high frequency. This excision event resulted in a plasmid species consisting of the pGL101 vector and a 2.7-kilobase restriction fragment comigrating with the EcoRI F fragment of pAD1 during agarose gel electrophoresis. Filter blot hybridization experiments showed the 2.7-kilobase fragment generated as a result of Tn916 excision to be homologous with the EcoRI F fragment of pAD1. Analogous results were obtained with another chimera, pAM170, generated by ligating the EcoRI D' (EcoRI D::Tn916) fragment of pAM210 (pAD1::Tn916) to EcoRI-digested pGL101. Comparison of the AluI and RsaI cleavage patterns of the EcoRI F fragment isolated after Tn916 excision with those from an EcoRI F fragment derived from pAD1 failed to detect any difference in the two fragments: data in support of a precise Tn916 excision event in E. coli. Subcloning experiments showed that an intact transposon was required for Tn916 excision and located the Tcr determinant near the single HindIII site on Tn916. Although excision occurred with high frequency in E. coli, Tn916 insertion into the E. coli chromosome was a much rarer event. Tcr transformants were not obtained when pAM120 DNA was used to transform a polA1 strain, E. coli C2368.

Biochemical and genetic characterization of the tet determinant of Bacillus plasmid pAB124

A fragment of Bacillus plasmid pAB124 carrying the genes encoding tetracycline resistance was previously cloned into Escherichia coli plasmid pSF2124 (S.J. Eccles, A. Docherty, I. Chopra, S. Shales, and P. Ball, J. Bacteriol. 145:1417-1420, 1981). The cloned pAB124 tet fragment conferred low-level resistance in E. coli, but exposure of this strain to a subinhibitory level of tetracycline led to selection of a mutant plasmid in which high-level resistance, associated with decreased drug accumulation, was expressed constitutively. In this plasmid, the Bacillus tet determinant appeared to be transcribed from a promoter on the vector. Construction of tetracycline-sensitive derivatives of this plasmid by transposon insertion mutagenesis allowed identification of a 32,000-dalton membrane-located protein, which apparently promoted decreased accumulation of tetracycline. This protein was also synthesized as a 32,000-dalton polypeptide in a coupled, in vitro transcription-translation system directed by plasmid DNA. The pAB124 tet determinant differed from the tetA through tetD determinants found in gram-negative bacteria in DNA-DNA hybridization and in the ability to prevent accumulation of different tetracycline derivatives, but was closely related to the tet determinant of another plasmid isolated from Bacillus species, pBC16.

Translational attenuation: the regulation of bacterial resistance to the macrolide-lincosamide-streptogramin B antibiotics

The regulation of ermC is described in detail as an example of regulation on the level of translation. ermC specifies a ribosomal RNA methylase which confers resistance to the macrolide-lincosamide-streptogramin B group of antibiotics. Synthesis of the ermC gene product is induced by erythromycin, a macrolide antibiotic. Stimulation of methylase synthesis is mediated by binding of erythromycin to an unmethylated ribosome. The translational attenuation model, supported by sequencing data and by mutational analysis, proposes that binding of erythromycin causes stalling of a ribosome during translation of a "leader peptide", resulting in isomerization of the ermC transcript from an inactive to an active conformer. The ermC system is analogous to the transcriptional attenuation systems described for certain biosynthetic operons. ermC is unique in that interaction with a small molecule inducer mediates regulation on the translational level. However, it is but one example of nontranscriptional -level control of protein synthesis. Other systems are discussed in which control is also exerted through alterations of RNA conformation and an attempt is made to understand ermC in this more general context. Finally, other positive examples of translational attenuation are presented.

Tetracycline-resistant L-forms isolated from an antibiotic-susceptible strain of Listeria monocytogenes

A tetracycline-susceptible strain of Listeria monocytogenes type 4b was converted to stable L-forms by penicillin. L-form variants resistant to tetracycline were then selected from a predominantly tetracycline-susceptible L-form population on plates containing penicillin and increasing concentrations of tetracycline. The origin of tetracycline-resistant L-forms from the parent Listeria strain was confirmed biochemically, by immunofluorescence, and by polyacrylamide gel electrophoresis. Scanning and transmission electron microscopy confirmed the typical L-form structure and the complete lack of cell wall in both L-form strains. The level of [3H]tetracycline uptake was lower in tetracycline-resistant than in susceptible cells.

Heterogeneity of tetracycline resistance determinants in Streptococcus

We found that naturally occurring tetracycline resistance in streptococci is encoded by more than one genetic determinant. Two of these distinct determinants were cloned, and the regions that are necessary and sufficient for expression of tetracycline resistance were defined by deletion analysis. These cloned determinants were further characterized by DNA-DNA hybridization experiments which also identified a third genetically unrelated tetracycline resistance determinant. Some of these genetic differences appear to represent mechanistic differences. The tetL determinant was associated with small nonconjugative plasmids and mediated resistance to tetracycline. The tetM determinant was most often "nonplasmid" associated and mediated resistance to minocycline as well as tetracycline. The tetN determinant was represented on a large conjugative plasmid and was genetically distinct from tetL and tetM, although phenotypically it resembled tetM.