faecal specimens were collected from the healthy black population in the Transvaal Province of South Africa. Each specimen was examined for the prevalence of antimicrobial resistance in commensal bacteria. Volunteers, from both rural and urban dwellings, were divided into four age groups. The overall carriage rate of resistance varied from 88-6 % for ampicillin, 74-2 % for trimethoprim, 52-6 % for chloramphenicol, 10-2 % for nalidixic acid to 7-5 % for gentamicin. The carriage of resistance found to each individual antimicrobial agent was slightly higher in the rural population rather than the urban population but there was no correlation between the prevalence of antimicrobial resistance and the age group.

Summary. The complete sequence of the type VI dihydrofolate reductase (DHFR) gene from plasmid pUK672 was determined. The structural gene coded for a polypeptide of 157 amino acids which had a deduced mol. wt of 17 424. Comparison with amino-acid sequences of the type I, type V and Escherichia coli K12 chromosomal DHFRs showed that there was 63%, 61% and 3 1% homology respectively. Putative RNA polymerase and ribosomal binding sites were identified proximal to the initiation codon and a feature consistent with transcription termination was present distal to the coding region. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed that the enzyme had a subunit mol. wt of 17 500.

A Proteus mirabilis strain, isolated in South Africa, carried a 79kb transferable plasmid (pUK672) which encoded high-level resistance to trimethoprim (minimum inhibitory concentration greater than 2048 mg/l). The trimethoprim resistance was mediated by a dihydrofolate reductase which had completely different properties from any plasmid-encoded dihydrofolate reductase previously described. This enzyme has been designated type VI. The type VI enzyme is small (10,000 daltons) and is unstable in vitro. It is highly resistant to inhibition by both trimethoprim and methotrexate.