ANTIMICROBIAL EFFECTIVENESS O F ICANAMYCIN, TOBRAMYCIN COMBINED WITH CARBENICILLIN

M A N Y severe infections in hospital patients, especially those with altered host defences against infection, are caused by Gram-negative bacilli. Bacteria isolated from patients with disseminated tumours, leukaemias, haematosarcomas and other debilitating diseases are often resistant to many commonly used antibioitics. No single drug regimen has yet been found effective enough to be used for the initial treatment of infections thought to be due to Gramnegative bacilli under these conditions ; a combination of antibiotics is therefore often given empirically (Schimpff et al., 1971 ; Tattersall, Spiers and Darrell, 1972). Another reason for using combinations of antimicrobial agents in the treatment of severe bacterial infections is to take advantage of possible synergistic interaction between them. It has been shown that infections treated with combinations that are synergistic in vitro usually have a better clinical outcome than those treated with combinations that are antagonistic or additive (McCabe and Jackson, 1965; Klastersky, Cappel and Daneau, 1972). The investigation of combinations of drugs in vitro may be important, therefore, both to assess the antibacterial spectrum of various combinations and to investigate the frequency with which these combinations are synergistic. The present study explores these two aspects of carbenicillin and cephalothin in combination with different aminoglycosides. Until recently the most commonly used parenteral aminoglycoside antibiotics were kanamycin and gentamicin. Other potentially useful parenteral aminoglycoside antibiotics have now been introduced for investigational purposes. These are tobramycin, sisomicin, BB-K8 and aminosidin. Tobramy sin and sisomicin are similar to gentamicin in their antibacterial action and pharmacologically. Comparative studies in vitro have shown slight differences in activity especially with respect to Pseudomonas aeruginosa (Waitz et al., 1972). BB-K8 and aminosidin are kanamycin-like antibiotics of which the former has much higher activity against this organism.