Tobramycin resistance of Pseudomonas aeruginosa cells growing as a biofilm on urinary catheter material.

When disks of urinary catheter material were exposed to the flow of artificial urine containing cells of Pseudomonas aeruginosa, a thick adherent biofilm, composed of these bacteria and of their exopolysaccharide products, developed on the latex surface within 8 h. After this colonization, sterile artificial urine containing 1,000 micrograms of tobramycin per ml was flowed past this established biofilm, and a significant proportion of the bacterial cells within the biofilm were found to be still viable after 12 h of exposure to this very high concentration of aminoglycoside antibiotic. Planktonic (floating) cells taken from the test system just before the exposure of the biofilm to the antibiotic were completely killed by 50 micrograms of tobramycin per ml. The MIC of tobramycin for cells taken from the seeding cultures before colonization of the catheter material, and for surviving cells recovered directly from the tobramycin-treated biofilm, was found to be 0.4 micrograms/ml when dispersed cells were assayed by standard methods. These data indicate that growth within thick adherent biofilms confers a measure of tobramycin resistance on cells of P. aeruginosa.