Determination of antimicrobial susceptibility of Pseudomonas aeruginosa by disk diffusion and microdilution methods

Disk diffusion susceptibility tests were performed on 180 recent clinical isolates of Pseudomonas aeruginosa. Minimal inhibitory concentration values were determined at the same time by a broth microdilution method. All isolates were sensitive to colistin (< 4 migrogram/ml), but resistant to ampicillin (greater than or equal to 16 microgram/ml), cephalothin (greater than or equal to 64 microgram/ml), and nitrofurantoin (> microgram/ml). More than 90% of the isolates were sensitive to gentamicin (median, less than or equal to 0.25 microgram/ml), tobramycin (median, less than or equal to 0.25 microgram/ml), and amikacin (median, less than or equal to 1.0 microgram/ml) and more than 70% were sensitive to carbenicillin (median, 64 microgram/ml). When the resistant and intermediate categories were combined, over 90% of the isolates were resistant to tetracycline (median 16 microgram/ml), chloramphenicol (median, > 32 microgram/ml), kanamycin (median, 16 microgram/ml), and trimethoprim-sulfonamide combiantion (median, 4 microgram/ml; 76 microgram/ml). Differences between the disk diffusion and microdilution methods in distinguishing resistant isolates of P aeruginosa from sensitive isolates were minor. Complete agreement between the two methods was obtained in 87.0% of the observations.