AERUGZNOSA ISOLATED IN AUCKLAND

PSEUDOMONAS AERUGINOSA causes severe infections, particularly in debilitated or immunosuppressed patients who are difficult to treat because the organism is resistant to many antibiotics. Gentamicin has been the most effective anti-biotic for pseudomonas infections and intially the majority of strains were sensitive to it. However, recent reports show that the incidence of gentamicin-resistant strains of P. aeruginosa is increasing (Meyer et al., 1976), that this resistance is frequently plasmid-borne (Witchitz and Chabbert, 1971; Knothe, Krcmery and Stietzen, 1973; Bryan, Shahrabadi and Van den Elzen, 1974; Jacoby, 1974a) and that it is often associated with resistance to other amino-glycoside antibiotics. This paper describes the results of a survey of the antibiotic-resistance patterns of clinical isolates of P. aeruginosa in Auckland and their ability to transfer this resistance by conjugation. MATERIALS AND METHODS Organisms. All strains of P. aeruginosa isolated at Auckland Hospital from clinical speci-mens during 1975 were examined for their susceptibility to gentamicin and all resistant strains were included in the investigation. The characteristics and source of the recipient strains used in tests for conjugal transfer of antibiotic resistance are shown in table I. Culture media. Trypticase Soy Broth (Difco) and trypticase soy broth solidified with 1.2% New Zealand Agar (Davis Gelatine Ltd) were used for most experiments. Minimal agar was that specified by Clowes and Hayes (1 968) with Difco agar used to solidify the minimal salts solution. MacConkey agar was Oxoid Code No. CM3 agar. Selective plates for genetically distinct strains were prepared from minimal agar supplemented with the appropriate amino acids at a final concentration of 20 pg/ml for the growth requirements of the selected strain. Antibiotics. Freshly prepared solutions of antibiotics were added to the selective media to give the following final concentrations (pglml) rifamycin 50, gentamicin 5, tobramycin 5, amikacin 25. Determination of antibiotic resistance. Antibiotic susceptibility was determined by the Stoke