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.     

Resistance of Shigella to antibiotics

Antibiotic sensitivity of 104 Shigella clinical strains and 104 Escherichia coli strains isolated from patients with acute dysentery not treated with antibiotics in 1986-1987 was studied. It was shown that 100 per cent of the dysentery pathogens and colon bacilli were antibiotic resistant. Strains resistant simultaneously to chloramphenicol, ampicillin, streptomycin, tetracycline, monomycin and kanamycin were the most frequent among the dysentery pathogens. Colon bacilli and dysentery pathogens isolated from the same patient had specific sets of antibiotic resistance markers. Pathogenetic therapy of dysentery and exclusion of antibiotic use for several years did not result in lower numbers of Shigella antibiotic resistant strains.     

Detection of an integrated tetracycline resistance plasmid in the chromosome of methicillin-resistant Staphylococcus aureus

The majority of multiresistant Staphylococcus aureus strains isolated in Australian hospitals since 1970 carry a chromosomally-encoded minocycline and tetracycline resistance determinant. By using DNA-DNA hybridization, some of these multiresistant strains were shown to possess also a tetracycline resistance plasmid, equivalent to pT181, integrated into a unique site in the chromosome. By relating the hybridization data to the map of pT181, the site of integration on this plasmid was established to be between the genes for replication and tetracycline resistance.     

Multicopy Tn10 tet plasmids confer sensitivity to induction of tet gene expression.

We inserted the Tn10 tetracycline resistance determinant (tet) into the multicopy plasmid pACYC177, and we examined the phenotype of Escherichia coli K-12 strains harboring these plasmids. In agreement with others, we find that Tn10 tet exhibits a negative gene dosage effect. Strains carrying multicopy Tn10 tet plasmids are 4- to 12-fold less resistant to tetracycline than are strains with a single copy of Tn10 in the bacterial chromosome. In addition, we find that multicopy tet strains are 30- to 100-fold less resistant to the tetracycline derivative 5a,6-anhydrotetracycline than are single-copy tet strains. Multicopy tet strains are, in fact, 10- to 25-fold more sensitive to anhydrotetracycline than are strains that lack tet altogether. The hypersensitivity of multi-copy strains to anhydrotetracycline is correlated with the effectiveness of anhydrotetracycline as an inducer of tet gene expression, rather than its effectiveness as an inhibitor of protein synthesis. Anhydrotetracycline is 50- to 100-fold more effective than tetracycline as an inducer of tetracycline resistance and as an inducer of beta-galactosidase in strains that harbor tet-lac gene fusions. In contrast, anhydrotetracycline appears to be two- to fourfold less effective than tetracycline as an inhibitor of protein synthesis. Both anhydrotetracycline and tetracycline induce synthesis of tet polypeptides in minicells harboring multicopy tet plasmids. Differences between E. coli K-12 backgrounds influence the tetracycline and anhydrotetracycline sensitivity of multicopy strains; ZnCl2 enhances the tetracycline and anhydrotetracycline sensitivity of these strains two- to threefold. We propose that the overexpression of one or more Tn10 tet gene products inhibits the growth of multicopy tet strains and accounts for their relative sensitivity to inducers of tet gene expression.     

Tn5381, a conjugative transposon identifiable as a circular form in Enterococcus faecalis.

We have identified two 19-kb conjugative transposons (Tn5381 and Tn5383) in separate strains of multiply resistant Enterococcus faecalis. These transposons confer resistance to tetracycline and minocycline via a tetM gene, are capable of both chromosomal and plasmid integration in a Rec- environment, and transfer between strains in the absence of detectable plasmid DNA at frequencies ranging from < 1 x 10(-9) to 2 x 10(-5) per donor CFU, depending on the donor strain and the growth conditions. Hybridization studies indicate that these transposons are closely related to Tn916. We have identified bands of ca. 19 kb on agarose gel separations of alkaline lysis preparations from E. faecalis strains containing chromosomal copies of Tn5381, which we have confirmed to be a circularized form of this transposon. This phenomenon has previously been observed only when Tn916 has been cloned in Escherichia coli. Overnight growth of donor strains in the presence of subinhibitory concentrations of tetracycline results in an approximately 10-fold increase in transfer frequency of Tn5381 into enterococcal recipients and an increase in the amount of the circular form of Tn5381 as detectable by hybridization. These results suggest that Tn5381 is a Tn916-related conjugative transposon for which the appearance of a circular form and the conjugative-transfer frequency are regulated by a mechanism(s) affected by the presence of tetracycline in the growth medium.     

Plasmodium berghei: Development of resistance to clindamycin and minocycline in mice

Strains of Plasmodium berghei resistant to clindamycin or minocycline were selected by a procedure in which groups of infected mice were treated with increasing doses of drug during each of a series of subpassages. Groups of five mice, each infected by intravenous inoculation with 10 million parasitized erythrocytes, were treated orally with different doses of drug for four consecutive days beginning on the day of infection. Subpassages were routinely made by Day 7, using donor mice from the group that had been treated with the highest dose of drug that allowed for some development of parasitemia during the preceding passage. Drug doses were increased in each passage as dictated by the development of parasitemia during the previous treated passage.The rate of development of resistance to clindamycin or minocycline was much slower than to conventional antimalarials such as chloroquine, quinine, or pyrimethamine. P. berghei developed total resistance to the latter compounds in nine to 12 treated passages in mice over a period of 60 to 85 days. In contrast, development of total resistance to clindamycin required 42 treated passages over a period of 300 days. Total resistance to minocycline was not attained during 86 successive minocycline-treated passages in mice over a period of 600 days, but a sixfold increase in resistance to minocycline was observed.The clindamycin-resistant strain was normally sensitive to minocycline, chloroquine, quinine, and pyrimethamine. The strain partially resistant to minocycline was normally sensitive to clindamycin, chloroquine, quinine, and pyrimethamine. Resistance to clindamycin was stable during 51 drug-free passages in mice over a period of 1 year. Resistance to minocycline was unstable. During 16 drug-free passages in mice the strain reverted towards normal sensitivity to minocycline. Strains resistant to clindamycin or minocycline showed no difference in rate of development in mice as compared to the parent strain. Likewise, only minor morphological modifications were seen in Giemsa-stained blood smears between the two resistant strains and the parent strain.These results suggest that other species of malaria may develop resistance to clindamycin or minocycline. Should resistance to one of these compounds appear, however, it should not invalidate the use of the other in the treatment of malaria.     

Antimicrobial Susceptibility of Clostridium difficile from Different Sources

A total of 79 Clostridium difficile strains from healthy young and elderly adults, elderly patients without gastrointestinal disease, elderly patients receiving antibiotics without gastrointestinal complications, and elderly patients with antibiotic-associated diarrhea or pseudomembranous colitis were tested for their susceptibilities to 24 antimicrobial agents. All of the 79 strains were inhibited by low concentrations of rifampicin, metronidazole, fusidic acid, vancomycin, ampicillin, and penicillin G. The strains were highly resistant to aminoglycosides, trimethoprim, sulfamethoxazole, nalidixic acid, and cycloserine and often resistant to neomycin, cefoxitin, and cefalexin. Wide variations in the susceptibility of C. difficile strains to erythromycin, clindamycin, lincomycin, chloramphenicol, and tetracycline were found. Strains resistant to erythromycin, clindamycin, and lincomycin were more frequently found among strains isolated from elderly adults than those isolated from young adults, with particularly high frequency among strains isolated from elderly patients receiving antibiotics. None of the 23 strains isolated from healthy young adults was resistant to chloramphenicol. All of the 14 strains resistant to erythromycin, clindamycin, lincomycin, and chloramphenicol were sensitive to tetracycline and all of the 15 strains resistant to erythromycin, clindamycin, lincomycin, and tetracycline were sensitive to chloramphenicol. Only one out of 19 tetracycline-resistant strains was highly toxigenic, whereas 42 (70%) of 60 sensitive strains were highly toxigenic.     

Comparison of therapeutic effects of antibiotics of the tetracycline group in the treatment of anthrax caused by a strain inheriting tet-gene of plasmid pBC16]

In vivo and in vitro efficacy of tetracyclines was studied with respect to anthracic infection induced by a tetracycline-resistant resistant strain containing plasmid pBC16. The plasmid-containing strain was resistant to tetracycline, doxycycline and minocycline, the MICs exceeding those for the initial strain 500, 640 and 80 times, respectively. There was no therapeutic effect of tetracycline and doxycycline in the treatment and urgent prophylaxis of anthracic infection caused by the tetracycline-resistant strain of Bacillus anthracis. High therapeutic efficacy of minocycline in the average therapeutic concentrations was shown irrespective of the contaminating doses and strains. Minocycline was recommended for treatment and urgent prophylaxis of anthracic infection caused by tetracycline-resistant B. anthracis strains.     

Hybridization analysis of the class P tetracycline resistance determinant from the Clostridium perfringens R-plasmid, pCW3

The tetracycline resistance determinant from pCW3, a conjugative plasmid from Clostridium perfringens, has been identified and the structural gene localized to within a 1.4-kb region. Hybridization analysis, which utilized an internal 0.8-kb specific gene probe, showed that eight nonconjugative tetracycline resistant C. perfringens strains all carried homologous resistance determinants. No homology was detected in DNA prepared from tetracycline resistant isolates of Clostridium difficile or Clostridium sporogenes. However, the one strain of Clostridium paraputrificum that was tested did contain an homologous determinant. No homology was found to any of the recognized classes of tetracycline resistance determinants. The C. perfringens tetracycline resistance determinant represents a new hybridization group, Class P.     

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.     

Mutations in the 16S rRNA genes of Helicobacter pylori mediate resistance to tetracycline

Low-cost and rescue treatments for Helicobacter pylori infections involve combinations of several drugs including tetracycline. Resistance to tetracycline has recently emerged in H. pylori. The 16S rRNA gene sequences of two tetracycline-resistant clinical isolates (MIC = 64 microg/ml) were determined and compared to the consensus H. pylori 16S rRNA sequence. One isolate had four nucleotide substitutions, and the other had four substitutions and two deletions. Natural transformation with the 16S rRNA genes from the resistant organisms conferred tetracycline resistance on susceptible strains. 16S rRNA genes containing the individual mutations were constructed and tested for the ability to confer resistance. Only the 16S rRNA gene containing the triple mutation, AGA965-967TTC, was able to confer tetracycline resistance on H. pylori 26695. The MICs of tetracycline for the transformed strains were equivalent to those for the original clinical isolates. The two original isolates were also metronidazole resistant, but this trait was not linked to the tetracycline resistance phenotype. Serial passage of several H. pylori strains on increasing concentrations of tetracycline yielded mutants with only a very modest increase in tetracycline resistance to a MIC of 4 to 8 microg/ml. These mutants all had a deletion of G942 in the 16S rRNA genes. The mutations in the 16S rRNA are clearly responsible for tetracycline resistance in H. pylori.     

Sensitivity of Neisseria gonorrhoeae to antibiotics]

Since 1976 strains of Neisseria gonorrhoeae producing beta-lactamase have been isolated in many countries. Strains with high level resistance to tetracycline have been also described. The appearance of resistant strains implies a constant surveillance of the isolated gonococcus and the study of their antibiotics sensitivity.     

A multiresistant clone of Shiga toxin-producing Escherichia coli O118:[H16] is spread in cattle and humans over different European countries

Multiresistant Shiga toxin-producing Escherichia coli (STEC) O118:H16 and O118 nonmotile strains (designated O118:[H16]) were detected by examination of 171 STEC isolates for their antimicrobial sensitivity. Of 48 STEC O118:[H16] strains, 98% were resistant to sulfonamide, 96% were resistant to streptomycin, 79% were resistant to kanamycin, 75% were resistant to tetracycline, 67% were resistant to ampicillin, 60% were resistant to chloramphenicol, 48% were resistant to trimethoprim, and 10% each were resistant to gentamicin and nalidixic acid. Nalidixic acid resistance and reduced susceptibility to ciprofloxacin were associated with the mutation gyrA(LEU-83). The STEC O118:[H16] strains were found to belong to a single genetic clone as investigated by multilocus enzyme electrophoresis and by multilocus sequence analysis of E. coli housekeeping genes. The STEC O118:[H16] strains originated from humans and cattle and were isolated in seven different countries of Europe between 1986 and 1999. Strains showing multiresistance to up to eight different antimicrobials predominated among the more recent STEC O118:[H16] strains. The genes in parentheses were associated with resistance to kanamycin (aphA1-Ia), chloramphenicol (catA1), tetracycline [tet(A)], and ampicillin (bla(TEM-1)). Class 1 integrons containing sulI (sulfonamide resistance), aadA1a (streptomycin resistance), or dfrA1 (trimethoprim resistance)-aadA1a gene cassettes were detected in 28 strains. The bla(TEM-1b) gene was present in 18 of 21 strains that were examined by nucleotide sequencing. Class 1 integrons and bla(TEM) genes were localized on plasmids and/or on the chromosome in different STEC O118:[H16] strains. Hybridization of XbaI-digested chromosomal DNA separated by pulsed-field gel electrophoresis revealed that bla(TEM) genes were integrated at different positions in the chromosome of STEC O118:[H16] strains that could have occurred by Tn2 insertion. Our data suggest that strains belonging to the STEC O118:[H16] clonal group have a characteristic propensity for acquisition and maintenance of resistance determinants, thus contrasting to STEC belonging to other serotypes.     

Transposon Tn5 encodes streptomycin resistance in nonenteric bacteria.

Strains of Caulobacter crescentus, Pseudomonas putida, Acinetobacter calcoaceticus, Rhizobium meliloti, and Rhodopseudomonas sphaeroides carrying the kanamycin resistance-encoding transposon Tn5 were 15 to 500 times more resistant to streptomycin than transposon-free strains. The streptomycin resistance determinant, which is separable from the kanamycin resistance determinant of Tn5, was not expressed in Escherichia coli or Klebsiella aerogenes.     

Antibiotic sensitivity and the characteristics of transmissive R plasmids in Shigella belonging to various species]

Sensitivity of 241 Shigella strains isolated from patients at various regions of the USSR in 1975--1978 was tested with respect to 14 antibiotics by the method of serial dilutions. 90.5 per cent of the isolates proved to be resistant to the antibacterial drugs and the greater part of 75.9 per cent of them had multiple resistance. The resistance of the Shigella was most pronounced and frequent with respect to tetracycline, streptomycin, levomycetin, as well as ampicillin and carbenicillin. Gentamicin, cephaloridin, polymyxin M, kanamycin, monomycin, neomycin and rifampicin were highly active against the Shigella. More than 50 per cent of the isolates were sensitive to levomycetin, ampicillin and carbenicillin. Differences in the frequency of the resistant strains and the spectrum of the antibiotic resistance of different Shigella subgroups (species) were observed. The study of 173 multiple resistant Shigella strains showed that about 67 per cent of the strains had a capacity for transduction of the resistance markers into the recipient cells of E. coli. The conjugative R-plasmids were most frequent in Sh. boydii and Sh. sonnei (95 and 95 per cent respectively), less frequent in Sh. flexneri and Sh. newcastle (68 and 53 per cent respectively) and least frequent in the mannitol negative Shigella (25 per cent). The capacity for transduction of R-plasmids in the strains carrying the determinants of resistance to 2 or 3 antibiotics was higher than in the strains carrying the determinant of resistance to one antibiotic. The clinical Shigella strains tested mainly had transmissive R-plasmids of fi--character (79 per cent).     

Phenotypic and genotypic characterization of tetracycline and minocycline resistance in Clostridium perfringens

The aim of this study was to determine the incidence of tetracycline resistance and the prevalence of tetracycline-resistance genes in strains of Clostridium perfringens isolated from different sources between 1994 and 2005. Susceptibility to tetracycline and minocycline in strains from humans (35 isolates), chickens (15 isolates), food (21 isolates), soil (16 isolates) and veterinary sources (6 isolates) was determined, and tetracycline-resistance genes were detected. Resistance was most common in strains isolated from chickens, followed by those from soils, clinical samples and foods. The most highly resistant strains were found among clinical and food isolates. tetA(P) was the most common resistance gene, and along with tetB(P) was found in all resistant strains and some sensitive strains. One tetracycline-resistant food isolate had an intact tet(M) gene. However, PCR fragments of 0.4 or 0.8 kb with high degrees of identity to parts of the tet(M) sequences of other bacteria were found, mainly in clinical isolates, and often in isolates with tetB(P). No correlation between level of sensitivity to tetracycline or minocycline and the presence of tetA(P), tetB(P) or part of tet(M) was found. The presence of part of tet(M) in some strains of C. perfringens containing tetB(P) may have occurred by recent gene transfer.     

Prevalence of Antimicrobial Resistance and Transfer of Tetracycline Resistance Genes in Escherichia coli Isolates from Beef Cattle

The aim of this study was to investigate the prevalence and transferability of resistance in tetracycline-resistant Escherichia coli isolates recovered from beef cattle in South Korea. A total of 155 E. coli isolates were collected from feces in South Korea, and 146 were confirmed to be resistant to tetracycline. The tetracycline resistance gene tet(A) (46.5%) was the most prevalent, followed by tet(B) (45.1%) and tet(C) (5.8%). Strains carrying tet(A) plus tet(B) and tet(B) plus tet(C) were detected in two isolates each. In terms of phylogenetic grouping, 101 (65.2%) isolates were classified as phylogenetic group B1, followed in decreasing order by D (17.4%), A (14.2%), and B2 (3.2%). Ninety-one (62.3%) isolates were determined to be multidrug resistant by the disk diffusion method. MIC testing using the principal tetracyclines, namely, tetracycline, chlortetracycline, oxytetracycline, doxycycline, and minocycline, revealed that isolates carrying tet(B) had higher MIC values than isolates carrying tet(A). Conjugation assays showed that 121 (82.9%) isolates could transfer a tetracycline resistance gene to a recipient via the IncFIB replicon (65.1%). This study suggests that the high prevalence of tetracycline-resistant E. coli isolates in beef cattle is due to the transferability of tetracycline resistance genes between E. coli populations which have survived the selective pressure caused by the use of antimicrobial agents.     

Tetracycline resistance in Staphylococcus intermedius

Staphylococcus intermedius cultures commonly exhibited combined resistance to tetracycline and to minocycline. The respective tetracycline resistance genes could not be demonstrated to be plasmidborne as known from other staphylococcal species. They were found to be located in the chromosomal DNA. Southern blot hybridization with specific gene probes identified the tetracycline resistance genes of Staph. intermedius to belong to the class M of bacterial tet genes .     

Analysis of antibiotic susceptibility and extrachromosomal DNA content of Ruminococcus albus and Ruminococcus flavefaciens

Seventeen Ruminococcus albus and Ruminococcus flavefaciens strains have been screened for naturally occurring antibiotic resistance, as determined by zones of inhibition from antibiotic disks. These strains were also examined for extrachromosomal DNA content. All strains screened are resistant to low levels (10-200 micrograms/mL) of streptomycin. In contrast to the previously reported data, we have found that R. flavefaciens C-94 is now susceptible to both kanamycin and tetracycline. However, R. flavefaciens FD-1 is not susceptible to kanamycin (minimum inhibitory concentration (MIC) = 40 micrograms/mL). Furthermore, R. albus 8 is resistant to tetracycline (MIC = 40 micrograms/mL), and erythromycin (MIC = 100 micrograms/mL). Six freshly isolated strains showed resistance to tetracycline (35-70 micrograms/mL), and all tetracycline-resistant strains also showed resistance to minocycline. None of these Ruminococcus determinants share homology with the streptococcal tetL, tetM, or tetN determinants. All 17 strains were screened for extrachromosomal DNA content. Nine different techniques for the detection and isolation of extrachromosomal DNA were tested. However, owing to difficulties in demonstrating or isolating plasmid DNA, it has not been possible to determine if these antibiotic resistance genes are plasmid borne. Evidence is presented to suggest that the presence of oxygen may affect the quality of the DNA obtained from Ruminococcus.     

The sensitivity to 12 antibiotics of 125 strains of Streptococcus group B isolated in a maternity home

Sensitivity of 125 strains of group B streptococci isolated from newborns, their mothers and personnel in a maternity home was studied with respect to 12 antibiotics: benzylpenicillin, ampicillin, methicillin, cephalotin, erythromycin, lincomycin, levomycetin (chloramphenicol), oxacillin, tetracycline, streptomycin, gentamicin and ristomycin. The method of serial dilutions in a solid medium was applied. All the strains were sensitive to ristomycin and erythromycin. The predominating number of the strains were sensitive to lincomycin, levomycetin and the beta-lactam antibiotics. Strains resistant or moderately resistant to benzylpenicillin, ampicillin, oxacillin, methicillin and cephalotin were detected. The majority of the strains were resistant to streptomycin, tetracycline and gentamicin. Multiple antibiotic resistance with 2-7 determinants was revealed in 11.2 per cent of the strains. The antibiotic sensitivity of the strains isolated from the newborns, their mothers and the personnel in the maternity home was on the whole similar or insignificantly differed.