Chromosomal and Plasmid-Mediated Transfer of Clindamycin Resistance in Bacteroides fragilis
The characteristics of the clindamycin-erythromycin (clinr) resistance transfer factor from Bacteroides fragilis TMP 10 are presented. Transfer ability and the determinant for clinr are found on a 15.6 kilobase plasmid named pBFTM 10. Recent clindamycin and tetracycline resistant strains of Bacteroides fragilis have been isolated in Chicago. The Chicago tetr isolate, TMP 230, transfers both clinr and tetr, but appears to be plasmid free when tested by standard methods. Homology between the clinr transfer factor pBFTM 10 and the chromosome of the TMP 230 could be demonstrated by the Southern hybridization technique. The location of the clinr determinant on the chromosome and mode of transfer are under invistigation.
Anaerobic bacteria are prominent members of the normal flora of man; in the colon anaerobic organisms including Bacteroides, Clostridia and non-sporing, gram-positive bacilli outnumber facultative bacteria such as E. coli and Streptococcus fecaelis by about 1000:1 (1). Over the past 20–25 years anaerobic bacteria 1have been increasingly recognized as important pathogens in human suppurative infections (2). Bacteroides fragilis emerges as the most important anaerobic bacterium in abdominal, surgical and gynecological infections because it most frequently invades the bloodstream in this setting. Most B. fragilis strains are resistant to intermediate levels of penicillin G and cephalosporins; they are uniformly resistant to the aminoglycoside antibiotics, and in the 19601s it was noted that there was the emergence of widespread resistance to tetracycline (3,4). This latter resistance was important because tetracycline was the agent of choice in treating infections involving B. fragilis in the 1950’s. More recently there have been reports of the increasing incidence of high-level penicillin resistance and scattered reports of resistance to chloramphenicol, clindamycin, cefoxitin, and metronidazole (5–9). Clindamycin resistance is important because this drug has currently been the prime agent for treating bacteroides infections.
Because of the widespread resistance in Bacteroides fragilis and closely related species, numerous attempts have been made to transfer the penicillin or tetracycline resistance (tetr) determinants both within B. fragilis and from B. fragilis to B.coli (10,11). Until the late 1970’s the only documented successful transfer of tetracycline resistance was from B. fragilis to E.coli by an undescribed mechanism by Mancini and Behme (12). There is one report of the transfer of ampicillin resistance from E. coli to B fragilis and a fusobacterium, but the ampicillin resistance was unstable (13). Transformation of E coli to ampicillin resistance was reported with DNA from B fragilis; however, the plasmid used to transform could not be visualized in the E. coli (15). In 1979, three laboratories concurrently reported the transfer of clindamycin resistance determinants within the genus Bacteroides, Privitera et al. at the Pasteur Institute, Welch and Macrina in Richmond, Virginia, and our own studies (15,16,17).
Investigations at the Pasteur Institute disclosed the transfer of both clindamycin (clinr) and tetracycline resistance (tetr) from a strain of B fragilis isolated in France to another B. fragilis strain. They showed that erythromycin and streptogramin resistance were transferred with the clinr, and these resistances were spontaneously curable. Further work by the French group demonstrated that transfer of tetr in B. fragilis could be induced to a higher frequency by pretreatment of the donor culture with subinhibitory levels of tetracycline (18). Welch and Macrina working with the isolate from the Pasteur Institute demonstrated that the transfer of clindamycin, erythromycin and streptogramin resistance was associated with a 27 megadalton plasmid (16). Our laboratory was working with a different strain of Bacteroides fragilis, isolated in California, that was highly resistant to clindamycin and erythromycin and possessed a different plasmid associated with the transfer of the clindamycin resistance. This paper describes the characterization of our clindamycin resistance transfer factor.
Standard anaerobic techniques in an anaerobic glovebox were employed, and the matings were carried out utilizing Nalgene filters (17). DNA was analyzed by agarose gel electrophoresis, and cells were lysed by a number of different procedures. DNA-DNA hybridization studies were carried out by a modification of the Southern technique (19,20,21,22,23).
KeywordsTetracycline Resistance Pasteur Institute Ampicillin Resistance Bacteroides Fragilis Widespread Resistance
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