Among 95 strains of trimethoprim-resistant Gram-negative bacteria isolated from clinical material in various centres in the UK, one was a thymine-requiring variant of and 10 carried transmissible trimethoprim R factors. The trimethoprim R factors belonged to three different compatibility groups. All gave rise to high levels of trimethoprim resistance. Eight belonged to group W, conferred resistance to trimethoprim and sulphonamides and appeared to be identical with one another and with those found in some London hospitals a year earlier. One belonged to a new group, Iδ, conferred resistance to trimethoprim and streptomycin and determined I pili. One belonged to group P and conferred resistance to trimethoprim alone. The W plasmid, carried by a particular klebsiella strain of capsular type 9, appeared to be established in several hospitals in London, but it also occurred sporadically and in other bacterial hosts. The other trimethoprim R factors were identified only in single bacterial strains.


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