Novel antibiotic resistance transfer in Bacteroides

Abstract

Resistance to tetracycline and lincosamide antibiotics was transferred en bloc from a strain of Bacteroidesfragilis (V503) to a plasmidless strain of Bacteroides uniformis (V528) during in vitro filter matings. Resistance transfer was detected at frequencies of 1i-5 to 10-6 drug-resistant progeny per input donor cell and was dependent on cell-to-cell contact of donors and recipients. Transfer was insensitive to DNase and was not mediated by chloroform-or filter-sterilized donor broth cultures. A determinant for resistance to cefoxitin in V503 was not transferred in this system. V503 contained a 3.7 x 106-dalton plasmid (pVA503). Drug-resistant progeny of V503 x V528 matings usually contained pVA503, but up to 20o of the total progeny of such crosses were plasmid free. Filter blot DNA hybridization studies (Southern method) confirmed that pVA503 was not integrated into the host chromosome of the plasmidless progeny. Drug-resistant progeny from V503 x V528 matings (with or without pVA503) conjugally transferred clindamycin resistance and tetracycline resistance to a suitable recipient strain. None of the drug resistance determinants of V503 were affected by treatment with standard plasmid curing regimens, and methods designed to detect very large plasmid molecules failed to suggest the involvement of extrachromosomal DNA in this resistance transfer system. The well-characterized Bacteroides R plasmid, pBF4 (conferring clindamycin resistance), was found to share hybridizing sequences with bulk cellular V503 DNA when examined by filter blot hybridization. Similarly sized sequences were found in drug-resistant progeny recovered from matings. Neither of the two pBF4 derivatives carrying deletions that abolished clindamycin resistance hybridized with V503 DNA.In the last 3 years there have been several reports describing self-transferable resistance (R) plasmids isolated from anaerobic bacteria.Brefort et al. (2)

The region of a Bacteroides conjugal chromosomal tetracycline resistance element which is responsible for production of plasmidlike forms from unlinked chromosomal DNA might also be involved in transfer of the element

“…2A (Table 2). One of them, ERL, resembles the DOT element in that it exhibits tetracycline-regulated transfer, whereas transfer of the 12256 element is not affected by pregrowth on tetracycline (6,22). Tcr ERL, like Tcr Emr DOT, exhibited tetracycline-enhanced Plf activity (Fig.…”

“…The mechanism by which tetracycline enhances growth is not known but is presumed to be some kind of regulation (6,8,9). Previously, it was noted that unless cells containing Tcr elements are preexposed to low levels of tetracycline before inoculation into media with high levels of tetracycline, growth is dramatically suspended after one or two divisions.…”

“…This element exhibits constitutive transfer and mobilization. All Tcr elements, including Tcr Emr 12256, have a Tcr gene that is regulated by tetracycline (6).…”

The Bacteroides mobilizable transposon Tn4555 integrates by a site-specific recombination mechanism similar to that of the gram-positive bacterial element Tn916

“…Conjugative transposons also have been discovered in the phylogenetically ancient anaerobes, the gram-negative Bacteroides spp. The genus Bacteroides is host to a family of conjugative elements called tetracycline resistance elements (Tc r elements) (20,31). These large transposons (Ϸ70 kb) encode all functions necessary for their own conjugation; nearly all members of this family encode resistance to tetracycline, but some do not encode known antibiotic resistance phenotypes (32).…”

Genetic and Structural Analysis of the Bacteroides Conjugative Transposon CTn341

“…The Bacteroides CTns were among the first CTns discovered, and they have since been shown to play a central role in the dissemination of antibiotic resistance genes in this genus and related genera (26,31,33,37). This role stems from two novel features of the Bacteroides CTns.…”