Figure 2.1 posits that gene transfer events occur between different species of colonic bacteria. What is the evidence that such transfers can occur and that if they do occur, they are common? A first attempt to answer this question was made in a 2001 publication by Shoemaker et al. . In this study, two sets of human colonic Bacteroides strains were screened. One set had been isolated prior to 1970 and was obtained from the culture collection of the now defunct Virginia Polytechnic Institute Anaerobe Laboratory (Blacksburg, Virginia, U.S.A.). The second set included isolates obtained after 1990. The two sets of strains were further divided into clinical and community isolates. The community isolates were derived from healthy people. The clinical isolates were obtained from patients with Bacteroides infections. The reason for looking at these two groups separately was that if the reservoir hypothesis is correct, both sets should follow the same pattern of gene acquisition, rather than clinical isolates exhibiting a different ecology as might be expected if events happened primarily in a clinical setting.
The patterns of antibiotic resistance genes seen in the clinical and community isolates were indeed similar. A striking difference was apparent, however, when the pre-1970 and post-1990 strains were compared. The older strains had a much lower rate of carriage of tetQ and the erm genes than the strains isolated after 1990. So, something had happened in the two-decade period that separated the two sets of strains, a period characterized by extensive use of antibiotics, such as tetracycline and the macrolides . It is also surprising how high the carriage rate was in the isolates obtained prior to 1970, before the onset of intensive use of antibiotics in the treatment of human disease. This type of anomaly has been seen in other cases, such as detection of antibiotic-resistant bacteria in "pristine" environments [31,32]. This raises the question of whether antibiotics are the only force selecting for antibiotic-resistant bacteria, a still-unanswered question to which we will return at the end of this chapter.
The high number of strains in the post-1990 period that carry tetQ, even in the community isolates obtained from people who were not taking antibiotics, indicates that once acquired, tetQ is maintained very stably. Since, as already indicated, tetQ is found almost exclusively on a type of CTn exemplified by CTnDOT, a human Bacteroides CTn, this indicates that the CTn itself is also maintained very stably. It is interesting to note another characteristic of CTnDOT: its excision and transfer are stimulated 100- to 1000-fold by exposure of the bacteria to tetracycline [33-35]. Tetracycline is used not only to treat acute human infections, but also in dermatology and agriculture. In the treatment of acne, tetracycline is administered orally in relatively low doses over a period that can extend from months to years [36,37]. In agriculture, tetracycline has been used to stimulate growth of some animals . Thus, long dosage regimens for tetracycline have been widespread and could have been responsible for the increased carriage of tetQ between 1970 and 1990.
The tetQ gene is not the only gene whose carriage has increased over the past few decades. Carriage of some of the erm genes, principally ermB, ermF, and ermG, increased dramatically between the pre-1970 and post-1990 period. A particularly interesting aspect of this increase in carriage by human colonic Bacteroides strains is that ermB and ermG were previously thought to be "Gram-positive" resistance genes, because they were found primarily in Gram-positive bacteria. These genes seem to have entered Bacteroides spp. only very recently . Could they be coming in from Gram-positive bacteria? The largest population of bacteria in the human colon is that of the Gram-positive anaerobes, a little studied and poorly understood group of bacteria [39,40]. Similarly, Gram-positive bacteria are the predominant population of bacteria in the human mouth and in the intestines of farm animals [41,42].
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