Ap, ampicillin; Kf, cephalotin; Atm, aztreonam; Caz, ceftazidime; Cro, ceftriaxone; Ctx, cefotaxime; Cm, chloramphenicol; Gm, gentamicin; Sm, streptomycin; Su, sulfonamides; Tp, trimethoprim; Tc, tetracycline; Nal, nalidixic acid; RDNC, reaction did not conform; NT, not typable.
^aThe strains were screened for resistance to ampicillin (10 µg), cephalotin (30 µg), cefotaxime (30 µg), chloramphenicol (30 µg), ciprofloxacin (5 µg), gentamicin (10 µg), nalidixic acid (30 µg), streptomycin (10 µg), sulfonamides (300 µg), tetracycline (30 µg), and trimethoprim (5 µg). Strains resistant to cefotaxime were subsequently tested for susceptibility to aztreonam (30 µg), ceftazidime (30 µg), and ceftriaxone (30 µg). Resistance was determined by disk diffusion (4). The double-disk synergy test was performed (4) on strains presumed to produce extended-spectrum ß-lactamase (ESBL). Plasmid DNA was extracted by an alkaline lysis method (5). Electrophoresis on 0.7% agarose gels was performed on samples of plasmid DNA. The approximate molecular weight of plasmids was estimated by comparison with plasmids of known molecular size extracted from Escherichia coli. Conjugation experiments were carried out in Luria-Bertani broth. Transconjugant colonies of E. coli were selected after growth on MacConkey agar containing rifampin (300 µg/ml) and ampicillin (50 µg/ml), streptomycin (30 µg/ml), chloramphenicol (30 µg/ml), or tetracycline (30 µg/ml). All resistant isolates were screened for class I integrons by a strict protocol with oligonucleotide primers specific for the sequence of the 5'-CS and 3'-CS regions adjacent to the site-specific recombinational insertion sequence (6). Primer sequences were 5'-CS, GGCATCCAAGCAGCAAG and 3'-CS,A AGCAGACTTGACCTGA (5).
^bSource in outbreak.
^cNumbers in bold indicate the approximate molecular size of resistance plasmids.