Aminoglycoside cross-resistance patterns of gentamicin-resistant bacteria

Ten strains each of Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa were habituated to gentamicin by serial passage in antibiotic containing medium. Complete cross-resistance to streptomycin, neomycin, kanamycin, and tobramycin in a linear proportional fashion was demonstrated at all stages of habituation. Most strains of Staph. aureus and Ps. aeruginosa showed a greater increase in resistance to gentamicin than to the other three aminoglycosides. E. coli required more transfers to reach the same degree of resistance than did the other two species. Reversion to greater susceptibility to gentamicin took place after serial passage on antibiotic-free media. 'Wild' gentamicin-resistant strains showed no such proportionality of resistance to kanamycin, neomycin, or streptomycin. But many of these strains showed a proportional increase in resistance to tobramycin.