R plasmids coding for supra-levels of gentamicin, tobramycin and sisomicin resistance in Proteus morganii and P. mirabilis: high-level resistant strains from two hospitals.

Transfer of high-level gentamycin-tobramycin-sisomycin resistance could be easily demonstrated in strains of P. morganii and P. mirabilis which emerged, in two hospitals, at the end of 1976. First such strains were demonstrated in a patient of a urological ward who died, in September 1976, from generalised sepsis caused by a high-level gentamycin-tobramycin-sisomycin-resistant P. morganii. Since that event, at least nine such strains were isolated in 1976, and the presence of transferable resistant to the antibiotics listed plus other antibacterial substances including carbenicillin and more classical antibiotics could be demonstrated either by a high-frequence direct transfer to suitable recipient strains of Gentamycin or Tobramycin resistance, or by indirect selection, i.e. by analysis of exconjugants selected with kanamycin, streptomycin or carbenicillin. Further numerous strains of P. morganii highly resistant to gentamycin, tobramycin and sisomycin (M.I.C. over 128 mcg/ml) still emerge from wards in the two hospitals monitored and their transferability is under experimental study. It is stressed that, in order to demonstrate a transfer of gentamycin or tobramycin resistance in strains resistant to these substances, it is inevitable to examine properly also exconjugnants showing direct transfer to other, more classical antibiotics. We could not demonstrate, in our strains, any prodromal signs of resistance to netilmycin or amikacin.