R-plasmid transfer in vivo in the absence of antibiotic selection pressure

Transfer of antibiotic resistance between commensal and pathogenic members of the Enterobacteriaceae under ileal conditions

AIM

To determine the rate of antibiotic resistance transmission between commensal and pathogenic representatives of the Enterobacteriaceae.

METHODS AND RESULTS

Through the use of a validated in vitro simulation of the porcine ileum, the transmission of antibiotic resistance was detected between commensal Escherichia coli, E. coli O157 and Salmonella spp. Countable transconjugant populations arose readily and, in one example, proved capable of indefinite persistence.

CONCLUSIONS

Genetic material conferring antibiotic resistance is readily transmissible between members of the Enterobacteriaceae under ileal conditions. Recipient phenotype influences the persistence of multi-resistant transconjugants.

SIGNIFICANCE AND IMPACT OF THE STUDY

The observation that the conjugal transmission of antibiotic resistance is commonplace under ileal conditions impacts primarily on the risk of food contamination by multi-resistant bacteria. The establishment of a multi-resistant transconjugant population as a dominant member of the microflora maintains a genetic reservoir of antimicrobial resistance.

Plasmid Frequency Fluctuations in Bacterial Populations from Chemically Stressed Soil Communities

The frequency of plasmids in chemically stressed bacterial populations was investigated by individually adding various concentration of kanamycin, ampicillin, and mercuric chloride to soil samples. Viable bacterial populations were enumerated, soil respiration was monitored for up to 6 weeks as an indicator of physiological stress, and bacterial isolates from stressed and control soils were screened for the presence of plasmids. Low levels of the chemical stress factors did not for the most part significantly alter population viability, soil respiration, or plasmid frequency. Exposure to high stress levels of mercury and ampicillin, however, resulted in altered numbers of viable organisms, soil respiration, and plasmid frequency. Plasmid frequency increased in response to ampicillin exposure but was not significantly changed after exposure to kanamycin. In mercuric chloride-stressed soils, there was a decrease in plasmid frequency despite an increase in overall mercury resistance of the isolates, suggesting that mercury resistance in these populations is largely, if not completely, chromosome encoded. Chemical stress did not cause an increase in plasmid-mediated multiple resistance. A genetic response (change in plasmid frequency) was not found unless a physiological (phenotypic) response (change in viable cells and respiratory activity) was also observed. The results indicate that a change in plasmid frequency is dependent on both the amount and type of chemical stress.

Characterization of Tn3926, a new mercury-resistance transposon from Yersinia enterocolitica

A new transposon coding for mercury resistance (HgR), Tn3926, has been found in a strain of Yersinia enterocolitica, YE138A14. The element has a size of 7.8 kb and transposes to conjugative plasmids belonging to different incompatibility groups. A restriction map has been established. DNA-DNA hybridization indicates that Tn3926 displays homology with both Tn501 and Tn21; the greatest homology is shown with the regions of these transposons that encode HgR. Weaker homology is observed between Tn3926 sequences and those regions of Tn501 and Tn21 that encode transposition functions. Complementation experiments indicate that the Tn3926 transposase mediates transposition of Tn21, albeit somewhat inefficiently, but not of Tn501, while the resolvase mediates resolution of transposition cointegrates formed via Tn21, Tn501, or Tn1721.

Antibiotic resistance and R-plasmids in food chain Salmonella: evidence of plasmid relatedness

A large number of strains (1,783) belonging to 15 Salmonella serovars isolated, in Canada, from the three major links of the human food chain were screened for multiple antibiotic resistance and the presence of R-plasmids. Multiresistant strains occurred among animal feed, livestock, and human isolates at frequencies of 4, 22, and 14%, respectively. Conjugation analysis revealed that 58% of the isolates from feeds, 87% of those from livestock, and 89% of the human strains carried all or part of their resistance determinants extrachromosomally on R-plasmids. Conjugative plasmids representing nine different incompatibility groups were detected, with the Inc I alpha group being predominant. Within the limits of the parameters measured, certain of these plasmids show a degree of relatedness suggestive of a common ancestry.

R-plasmid transfer frequencies from environmental isolates of Escherichia coli to laboratory and fecal strains

Multiple-drug-resistant strains of Escherichia coli were isolated from the water at an estuarine site. They represented about 8.3% of the total E. coli population. Fifty-five strains, representing each of the 32 resistance patterns identified, were mated with an E. coli K-12 F- strain. Matings were performed on membrane filters, and the cells were washed to remove any colicins produced by the donors. Thirty-one strains, about 5% of the mean E. coli density in the samples, transferred drug resistance and, hence, posessed conjugative R plasmids. Of these, 80% transferred drug resistance at a frequency of about 10(-4) or less. Nine environmental R+ strains were mated with three fecal recipients. The R-plasmid transfer frequencies to the fecal strains from the environmental donors correlated well with those from a derepressed K-12 R+ laboratory donor. The R+ X K-12 F- lac- transconjugants from 16 environmental strains were "backcrossed" to a lac+ K-12 F- strain. All transfer frequencies were higher in the backcrosses than in the original matings from the environmental donor. Furthermore, 7 of 13 different transconjugants, which accepted plasmids at repressed frequencies of less than 10(-3), donated them at frequencies greater than 10(-2). This suggests that these were derepressed plasmids in a repressed host.

New translocation sequence mediating tetracycline resistance found in Escherichia coli pathogenic for piglets

A discrete piece of deoxyribonucleic acid coding for tetracycline (Tc) resistance was found to move from one R plasmid to another in an Escherichia coli strain which is pathogenic for piglets. Since this phenomenon took place also in rec strains, the Tc segment was classified as a transposon and called Tn804. Restriction enzyme analysis with EcoRI, BglII, and HindIII indicated that Tn804 is related to Tn10, a well-known transposon that codes for resistance to tetracycline. Hybridization between plasmids carrying the two transposons provided proof of homology between Tn10 and prt of Tn804. Electron microscopic studies showed a transposon-like structure composed of one loop-stem structure with inverted repetitions of approximately 0.9 megadaltons inserted into the loop of a second loop-stem structure. It is suggested that Tn804 is composed of Tn10 plus another transposable sequence.

R-plasmic transfer from Serratia liquefaciens to Escherichia coli in vitro and in vivo in the digestive tract of gnotobiotic mice associated with human fecal flora

It was shown that a strain of Serratia liquefaciens harbors a conjugative R-plasmid responsible for reistance to the following 14 antibiotics: ampicillin, carbenicillin, cephalothin, butirosin, neomycin, paramomycin, kanamycin, lividomycin, gentamicin, tobramycin, streptomycin, tetracycline, sulfonamide, and chloramphenicol, which belong to five families, the beta-lactamines, the aminoglycosides, the tetracyclines, the sulfonamides, and the phenicols. Resistance to th 14 antibiotics was cotransferred by in vitro conjugation between S. liquefaciens and strains of Escherichia coli. Mating between S. liquefaciens and E. coli also occurred in vivo, in the digestive tract of axenic mice and gnotobiotic mice associated with the whole human fecal flora. It was also shown that mating between these two strains occurred even when the donor S. liquefaciens strain was only transient in the digestive tract of the gnotobiotic host animals. A dense population of Bacteroides (10(10) viable cells per g of fresh feces) did not hinder this mating. All the matings occurred in the absence of an antibiotic selection pressure, and the resulting transferred strain of E. coli did not have the same colonizing capacity as the recipient parental strain. However, during antibiotic administration to mice, and even after the end of the drug intake, the transconjugant became established in the dominant population and replaced the parental recipient strain.

Parameters controlling interbacterial plasmid spreading in a gnotoxenic chicken gut system: influence of plasmid and bacterial mutations

Conjugative transfer of R plasmids R64 and R64drd-11 has been compared in vitro and in vivo without selective pressure by antibiotics in a simplified experimental system; the ecosystem was the bowel of germfree chickens, with the host bacteria almost isogenic, and the plasmids differing only in their conjugative transfer frequency. The spread of repressed and derepressed (drd) R plasmids in recipient bacterial populations was very extensive. The repressed phenotype had only a transient effect during the first 4 h. The level of implantation of the donor bacterial population seems to be of minor importance. Only with a poor recipient (con strain) could the spread of R plasmids be reduced and a steady state with a predominantly sensitive bacterial population be established. It is suggested that this steady state results from an equilibrium between the frequencies of R plasmid transfer and loss.

Persistence of plasmid-carrying tetracycline-resistant Escherichia coli in a married couple, one of whom was receiving antibiotics

This paper describes an extension to an earlier account of the coliform flora carried by a married couple, one of whom was taking tetracycline for prolonged periods. The latter phase of this study was notable for the following: first, certain tetracycline-resistant Escherichia coli O antigen types persisted in one of the participants for several weeks after tetracycline was withdrawn; second, a course of ampicillin led to replacement of the tetracycline-resistant flora by one that was ampicillin resistant, but the end of the ampicillin course led to the reappearance of the tetracycline-resistant line, even though no tetracycline was being taken; and third, the tetracycline-sensitive O75 E. coli, which appeared toward the end of the survey, had not lost their plasmid but carried a derivative in which the tetracycline resistance gene(s) had been inactivated by the insertion of an extra piece of deoxyribonucleic acid with a molecular weight of about 1 megadalton.

Regional preference of insertion of Tn501 and Tn802 into RP1 and its derivatives

The sites of insertion of Tn501 into RP1 and into derivatives of this plasmid that either lack the Tn801 (TnA) element or contain it in a different location have been determined. Similarly, the sites of insertion of Tn802 into a derivative of RP1 that lacks the Tn801 element and into recombinants of this plasmid with Tn501 were determined. 'Hot spots' for insertion were observed with both transposons; but it is clear that a particular DNA sequence is not sufficient to define a 'hot spot', since a particular region does contain many insertions when present in one plasmid but does not do so when part of another.

Chloramphenicol resistance plasmids in Escherichia coli isolated from diseased piglets

The plasmids in 19 chloramphenicol-resistant Escherichia coli strains of three pig pathogenic antigen types were studied in conjugation and transduction experiments. The plasmids had identical resistance patterns: streptomycin, spectinomycin, sulfonamides, and chloramphenicol (Sm, Sp, Su, Cm) and belonged to IncFII. One plasmid carried ampicillin resistance in addition. Restriction enzyme analysis of the deoxyribonucleic acid from five of the plasmids originating from the same herd showed that their digestion patterns with EcoRI were indistinguishable. EcoRI cleaved the deoxyribonucleic acid of a sixth plasmid from the same herd and displayed nine of the ten bands of the other five plasmids plus an additional six. It appears that the five plasmids with identical restriction patterns have a common origin and may be copies of the same plasmid from which the sixth may have developed. Four strains carried two plasmids each. In two of these strains, a plasmid with a tetracycline marker (Tc), or possibly the tetracycline marker alone, recombined frequently with the Sm Sp Su Cm plasmid without destroying any known function of the latter. The possibility that Tc is carried on a translocation sequence is discussed.

Mapping of functions in the R-plasmid R388 by examination of deletion mutants generated in vitro

Mutant plasmids in which large segments of R388 DNA are deleted were constructed in vitro from two R388::TnA (Tn801) plasmids, using the BamHI site of TnA and the BamHI and BglII sites of R388. These deletion mutants permitted mapping of genetic functions into the restriction map of R388.

Spread of a single plasmid clone to an untreated individual from a person receiving prolonged tetracycline therapy

The aerobic gram-negative intestinal flora of two individuals living in close proximity was followed for 17 months. One of these persons was receiving a prolonged tetracycline treatment for acne vulgaris and was colonized by tetracycline-resistant Escherichia coli strains throughout the survey. The other person studied received no antibiotics during the period in question, but was frequently found to excrete a number of E. coli strains harboring tetracycline-resistant plasmids. The two E. coli strains (O75.H7 and O23.H16) excreted from both persons most frequently were indistinguishable, and so were the R-plasmids they carried. This suggests that R-plasmid-carrying E. coli may spread from individuals under treatment to close relatives that have not been treated.

Transposition of TnA does not generate deletions

We have examined the incidence of loss of the TnA unit, Tn801, from RP1 under conditions where transposition of Tn801 to another replicon. R388, was readily detected. We found that the frequency of transposition of Tn801 from RP1 to R388 exceeded, by at least a factor of one hundred, the frequency at which it was deleted from RP1. We conclude that, in general, transposition of Tn801 does not generate derivatives of the donor plasmid which specifically lack Tn801. The relevance of these findings to the mechanism of transposition is discussed.

Inhibition of TnA translocation by TnA

Plasmids already containing TnA showed decreased susceptibility to the translocation of a further TnA unit when compared with related plasmids that did not contain TnA. The translocation immunity imposed by TnA is exerted only on the plasmid of which it is part. It is suggested that this desensitization by a translocation unit is a general phenomenon that reduces the mutational effects of translocation.