Enzymatic modification of aminoglycoside antibiotics by Branhamella catarrhalis carrying an R factor

Fifteen out of 89 clinical strains of Branhamella catarrhalis isolated from patients at the University Hospital of Zaragoza were resistant to aminoglycosides and other antimicrobials. In two strains, B. catarrhalis 220 and B. catarrhalis 115, the resistance to aminoglycosides was associated with synthesis of aminoglycoside-modifying enzymes, namely 3"-O-phosphotransferase [APH(3")] and 3'-O-phosphotransferase [APH(3')]. B. catarrhalis 115 was resistant to ampicillin, streptomycin, kanamycin, neomycin, butirosin, lividomycin, ribostamycin, paromomycin and trimethoprim-sulfamethoxazole and harboured a 32 megadalton (Md) plasmid. The resistance determinants of the latter were transferred to Neisseria subflava by conjugation and to Escherichia coli by transformation. The transconjugant strain presented an antibiotic resistance pattern similar to the donor strain and carried the same plasmid. The transformant strain acquired the 32 Md plasmid but presented, besides the resistance pattern already mentioned, resistance to tetracycline, gentamicin and tobramycin. Resistance to gentamicin and tobramycin was mediated by the synthesis of a 3-N-acetyltransferase. This resistance and the related enzyme were expressed neither in the donor B. catarrhalis strain nor in the transconjugant N. subflava strain.