Effect of Oral Tetracycline on the Occurrence of Tetracycline-Resistant Strains of Escherichia coli in the Intestinal Tract of Humans

Abstract

Serial samples of feces from normal human subjects were analyzed for tetracycline-resistant strains of Escherichia coli. Subjects were found to be excreting tetracycline-resistant strains in amounts fluctuating with time. Ingestion of tetracycline increased both the number of individuals excreting tetracycline-resistant strains of E. coli and the proportion of the total E. coli isolates resistant to the antibiotic.Studies show that tetracycline taken orally will increase the number of tetracycline-resistant bacteria excreted from the intestinal tract of humans (2-4). Little is known, however, concerning the change with time in the number of tetracycline-resistant bacteria in the same individual after oral ingestion of a therapeutic regimen of tetracycline. Therefore, it was the purpose of this study to determine the changes that occur in the proportion of tetracyclineresistant Escherichia coli cells excreted by normal human volunteers after a therapeutic regimen of tetracycline.Beginning 5 weeks prior to administration of tetracycline, fecal samples from 11 normal human subjects were analyzed for tetracyclineresistant strains of E. coli. Serial dilutions of the samples were plated on MacConkey agar and MacConkey agar containing 25 ug of tetracycline/ml. The subjects, who professed having no known immediate contact with an antimicrobial agent, ranged in age from 21 to 25 years. Tetracycline was administered at an oral dose of 1,000 mg per day (250 mg four times a day) for 9 days. Fecal samples were analyzed for tetracycline-resistant E. coli cells 4, 9, 16, 23, and 37 days after the subjects commenced ingesting the antibiotic.The results of the analysis of the fecal samples prior to tetracycline are shown in Table 1. Over the 5-week period, between 0 and 45% of the subjects at any one time were found to be excreting tetracycline-resistant strains of E. coli. Of those individuals shedding tetracyclineresistant strains of E. coli, 0.001 to 100% of the total number of E. coli cells shed in the feces were found to be tetracycline-resistant.All of the subjects shown to be shedding tetracycline-resistant E. coli cells prior to ingestion of the antibiotic, except one (no. 5), were shedding resistant E. coli cells during the time tetracycline was being ingested. Subject 3 cannot be evaluated during this interval because no samples were submitted. Of the four subjects not excreting tetracycline-resistant E. coli cells prior to tetracycline ingestion, only two were found to be shedding resistant E. coli cells during this time. During the 9 days of tetracycline administration, the proportion of the total E. coli cells found to be resistant increased in the feces of those subjects excreting tetracycline-resistant E. coli cells compared to that found the week prior to ingestion of the antibiotic.In the 2 weeks after cessation of the antibiotic, the ratio of tetracycline-resistant to total E. coli cells decreased in most subjects. Exceptions were no. 5 and 14, who did not excrete tetracycline-resistant E. coli cells during adm...

Potential for In Vivo Acquisition of R Plasmids by One Strain of Vibrio cholerae Biotype El Tor

“…After treatment with tetracycline (500 mg orally four times a day for 6 days), although there was no significant change in the total numbers of coliforms present more than 50% of these were in each case resistant to tetracycline. This finding agrees with previous similar studies (8,15,18). No V. cholerae were isolated from the post-treatment specimens.…”

“…In the majority of cases such resistance has been found to be mediated by conjugative R plasmids. Oral administration of antibiotics, particularly tetracycline, may be followed by a marked increase in the numbers of organisms in the bowel flora which are resistant to the administered antibiotic (8,15). In studies by Datta (8), most of these organisms were also found to be resistant to other antibiotics, and again in most cases the resistance was mediated by R plasmids.…”

Comparative Effect of Tetracycline and Doxycycline on the Occurrence of Resistant Escherichia coli in the Fecal Flora

“…The relative proportion of resistant strains, however, increased with tetracycline administration and this effect was significantly greater than with doxycycline. Similar changes after oral tetracycline have been previously reported by Hirsh et al (7). These investigators noted that seven of eight subjects shed tetracycline-resistant E. coli during ingestion of this antimicrobial, and that five subjects continued to harbor large populations of resistant strains at least 28 days after therapy was discontinued.…”

Antibiotic residues and drug resistance in human intestinal flora

“…2). These factors may explain the reason that when a large number of people are checked once for the bacterial resistance levels in feces, the data obtained display a wide scatter (16,19). The result is that in humans it is almost impossible to detect any minor change in the level of resistance due to the presence of antimicrobial residues.…”