Hilary-Kay Young

Antibiotic resistance in the tropics 1. The genetics of bacterial ampicillin resistance in tropical areas

Ampicillin and its derivatives are the most widely used beta-lactam antibiotics throughout the world. Ampicillin resistance in Gram-negative bacteria is usually manifested by plasmid-encoded beta-lactamases, which hydrolyse the beta-lactam ring of the antibiotic. There are at least 30 different plasmid-encoded beta-lactamases but almost all of them are found very infrequently. The one exception is the TEM-1 beta-lactamase which is found wherever transferable ampicillin resistance emerges and accounts for over 50% of all plasmid encoded ampicillin resistance. In India, the incidence of ampicillin resistance is high (82%) and, amongst Escherichia coli, a significant proportion of the plasmid-encoded beta-lactamases are different from those found in the United Kingdom. Although many Gram-negative species are able to accept the TEM-1 beta-lactamase, certain species have a pre-disposition to their own plasmid beta-lactamase types.

Detection in the UK of trimethoprim-resistantEscherichia coli encoding the type V dihydrofolate reductase

Seven major types of trimethoprim-resistant dihydrofolate reductases (DHFRs) coded for by plasmids in gram-negative bacteria have now been described, with several of these major groups being further divided into related subtypes (1). Of the seven major types, the type V DHFR was originally identified in clinical isolates of enterobacteria from Sri Lanka (2) and has not been reported to occur in Europe. We have previously described a DNA probe suitable for monitoring the spread of plasmids mediating this form of trimethoprim resistance (3). This probe was used in a retrospective study to examine 14 trimethoprim R plasmids originally isolated during the period 1980-82 as part of an investigation into the distribution of the DHFR type I gene in the Nottingham area of the UK (4). The 14 trimethoprim R plasmids selected (Table 1) were those that had previously been found not to show any hybridisation with the DNA probe for the predominant type I gene (4).

Detection of novel trimethoprim resistance determinants in the United Kingdom using biotin-labelled DNA probes.

Two collections of trimethoprim R plasmids, isolated from strains of Escherichia coli during 1978-83 and 1987-8 respectively, were retrospectively screened with specific biotinylated DNA probes for the presence of genes encoding particular DHFR enzymes. The results confirmed that the type I DHFR gene was the predominant plasmid-encoded gene conferring trimethoprim resistance in strains of E. coli from the Nottingham area of the UK, but indicated that genes encoding the more recently recognized types of DHFR enzymes had appeared in the bacterial gene pool and could be recognized with increased frequency in the latter plasmid collection. This was particularly true of the type IIIa and type VII enzymes which together accounted for 27% of the trimethoprim R plasmids examined in 1987-8.