Strains of
Pseudomonas aeruginosa
resistant to either gentamicin or carbenicillin have been noted since their introduction into clinical use. During a 6-month period, twice-weekly cultures were obtained from all patients treated with either gentamicin or carbenicillin and from all patients with a positive culture for
P. aeruginosa
. Susceptibility testing to gentamicin and carbenicillin and pyocine typing were performed on all isolates. Organisms with a minimal inhibitory concentration greater than 12.5 μg of gentamicin per ml or greater than 100 μg of carbenicillin per ml were defined as resistant.
P. aeruginosa
was cultured from 238 patients. One patient was initially infected with a gentamicin-resistant isolate. In 11 other patients, serial cultures revealed the emergence of resistance to gentamicin. All but one of these resistant isolates occurred in patients treated with gentamicin. In eight instances the pyocine and/or serological types before and after the change in sensitivity pattern were the same. Gentamicin-resistant
P. aeruginosa
emerged significantly more often in patients treated with gentamicin than in those who did not receive gentamicin. Carbenicillin-resistant
P. aeruginosa
emerged in four of 14 patients treated with carbenicillin. Seventeen of the 238 patients were infected de novo with carbenicillin-resistant
P. aeruginosa
. Carbenicillin-resistant
P. aeruginosa
emerged significantly more often in patients treated with carbenicillin than in those who did not receive carbenicillin. No evidence was found for in-hospital spread of resistant
P. aeruginosa
.
A nine-test system using multiple-inoculation agar plates for biotyping of Escherichia coli is described. Testing of 959 strains resulted in 78 biotypes. On repeated testing, 96% of 182 strains had identical biotypes or differed by only one test. This system provides satisfactory differentiation among strains and is reproducible. Precise standardization of inoculum size is not required. Multiple inoculation allows time and cost-efficient testing of large numbers of strains.
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