Resistance transfer to the resident intestinal Escherichia coli of rats

Natural transfer of sulphonamide and ampicillin resistance between Escherichia coli residing in the human intestine

OBJECTIVES

The aim of this study was to investigate whether the sulphonamide resistance gene sul2 could be transferred between Escherichia coli in the human gut.

METHODS

Nine volunteers ingested a 10(9) cfu suspension of sulphonamide-susceptible, rifampicin-resistant E. coli recipients of human origin. Three hours later, they ingested a 10(7) cfu suspension of a sulphonamide-resistant (MIC>1024 mg/L) E. coli donor of pig origin. Stool samples were collected 24 h prior to ingestion, daily for 7 days and at days 14 and 35. Samples were plated on selective plates and monitored for the acquisition of sulphonamide-resistance by the recipient from the indigenous or administrated donor E. coli. Possible transconjugants were typed by PFGE and tested for the presence of plasmids containing the sul2 gene, which was also sequenced.

RESULTS

Concentrations of the human and animal E. coli reached a maximum of 7.5x10(6) cfu/g faeces and colonized for more than 7 days, and 2x10(8) cfu/g for more than 14 days, respectively. On day 2, a transconjugant was detected in one volunteer. This volunteer was colonized with sulphonamide-resistant E. coli at day 0. The transconjugant was sul2-positive, had an MIC>1024 mg/L for sulfamethoxazole and the same PFGE profile as the recipient. The resident E. coli transferred a plasmid (>63 kb), containing the sul2 gene, to the recipient. The sul2 sequence of the transconjugant was identical to that of the volunteer's own E. coli from day 0, but differed from the animal strain. Co-transfer of ampicillin resistance was also demonstrated.

CONCLUSIONS

Transfer of sul2 was observed between E. coli bacteria in the human intestine. The transconjugant's sul2 gene came from the volunteer's own flora. The origin of the E. coli donor is unknown.

Subtherapeutic tetracycline effects on recovery patterns of calves after Salmonella typhimurium challenge

Holstein calves were maintained on a subtherapeutic dose of chlortetracycline to determine if an oxytetracycline therapy, given after a Salmonella typhimurium challenge, would be compromised by the previous subtherapy. Two of the four groups of seven calves were maintained on a subtherapeutic amount of chlortetracycline. All calves then were challenged with Salmonella typhimurium, and with the onset of clinical symptoms one group with and one group without subtherapy were given a therapeutic dose of oxytetracycline. The two groups receiving a therapeutic dosage of oxytetracycline had the most quickly declining body temperatures and the highest average body weights post-challenge. Two calves died in the group receiving no antibiotic treatments, and one calf died in the group receiving only the subtherapeutic treatment. There were no differences in postchallenge body temperatures or body weight changes between subtherapeutic and nonsubtherapeutic groups of calves. The conclusion was that the subtherapeutic dosing of chlortetracycline did not affect the therapeutic treatment effects of oxytetracycline after a Salmonella typhimurium challenge.

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.

Effect of dietary oxytetracycline on microorganisms in turkey feces

Thirty-six two-day old male turkey poults were divided into three groups and fed three levels of oxytetracycline; 0, 110 or 440 mg. per kg. of ration, respectively, for nine weeks. The poults were given an oral spectinomycin plus vitamin treatment at the hatchery but were not fed before the start of the experiment. Defecated feces from each group were sampled at weekly intervals and analyzed for bacteriological content, especially for members of the Enterobacteriaceae. Total aerobic counts of 3 x 10(8) and total anaerobic counts of 2 x 10(9) were obtained. The majority of the Enterobacteriaceae were Escherichia coli but some Klebsiella sp., the Bethesda-Ballerup group of paracolon bacteria and Arizona sp. were also isolated. Antibiograms of the enteric isolates indicated that an increase in multiple antibiotic resistance occurred when increased amounts of antibiotic were fed.

Factors modifying host resistance to viral infection. I. Effect of immunosuppressive drugs on experimental infection of mice with encephalomyocarditis virus

Mature beagles were fed a ground-meal diet containing 0, 2, or 10 μg of oxytetracycline per g for 44 days. The 10-μg/g diet resulted in a shift from a predominantly drug-susceptible population of enteric lactose-fermenting organisms to a multiply antibiotic-resistant population which peaked at 78% resistant organisms. Since a shift to drug-resistant organisms did not occur in the group fed 2 μg/g, the level of oxytetracycline that results in increased incidence of antibiotic resistance lies between 2 and 10 μg/g in this dog model. Rats and hamsters fed diets containing oxytetracycline (10 μg/g or greater) or dihydrostreptomycin (10 μg/g), and provided suspensions of drug-susceptible Escherichia coli, did not develop a population of antibiotic-resistant organisms.