Experimental pneumonia with Pseudomonas aeruginosa in immunosuppressed guinea pigs as a model for biofilm-associated infection

Abstract

To establish a model where the role of bacterial biofilms in chronic pneumonia with Pseudomonas aeruginosa could be investigated, hydrocortisone-treated guinea pigs were given P. aeruginosa, strain 2126 by inhalation which were used throughout this study, in planktonic form. In these animals, the bacteria were recovered only from the lungs more than 4 weeks after infection. The persistence in bacterial colonization in the lungs coincided with the formation of glanulomatous lesions that surrounded spherical grains consisting of outer shell and inner bacterial colonies. The outer shell of grain was stained with ruthenium red and was presumed to be polyanionic and therefore to be a biofilm-like material. In normal animals without hydrocortisone-treatment, the number of neutrophils recovered from bronchoalveolar lavage fluid increased significantly from 3 hours after infection and subsequently the inhaled bacteria were eliminated from the lungs by day 3 of infection. This early influx of neutrophils into the lungs tended to be suppressed by treatment with hydrocortisone. The formation of grains did not take place in the lungs of normal animals, indicating the significant role of grain-formation in the initiation and the prolongation of bacterial colonization in the lungs. P. aeruginosa, strain 2126, incubated in saline formed thick biofilms on the surface of teflon piece. Levofloxacin (LVFX), a quinolone antibacterial, exhibited killing activity against the bacteria in in vitro-forming biofilms at MIC. In contrast, gentamicin (GM), an aminoglycosid antibiotic, and ceftazidime (CAZ), a beta-lactams antibiotic, showed no such killing activity at MIC. Treatment of this model with oral LVFX achieved complete eradication of the bacteria, whereas subcutaneous injection of GM or CAZ was hardly effective. The pharmacokinetic study on these antibacterials revealed that the doses used in this study were sufficient to obtain the pulmonary levels of these drugs far above MIC even in GM and CAZ. These data indicate that the outer shell of grains, a characteristic finding in the pulmonary lesions of this model, may be one of the forms of pseudomonal biofilms and that this model represents the significant role of biofilm mode of growth of P. aeruginosa in persistence in pulmonary colonization.