We examined the effects of subinhibitory concentrations of ciprofloxacin, tobramycin, and ceftazidime on Pseudomonas aeruginosa exoenzyme expression in vitro and in vivo. Exotoxin A, exoenzyme S, phospholipase C, elastase, and total protease activities were suppressed by antibiotics at concentrations as low as 1/20 of the MIC over a 24-h period in broth. Continuous 10-day exposure of P. aeruginosa DG1 broth cultures to antibiotic levels equal to 1/10 of the MIC reduced exoenzyme S activity in all treatment groups. Elastase activity was reduced only by ciprofloxacin and tobramycin treatment. This suppressive effect of the antibiotics persisted throughout the 10 days and was not influenced by the increase in MIC of ciprofloxacin detected during the course of the experiment. Rats chronically infected with P. aeruginosa were treated with subinhibitory doses of antibiotics and compared with untreated controls. Bacterial numbers in lung homogenates from each of the four study groups were identical. However, the lungs from antibiotic-treated rats had significantly less histological damage than those from control rats (P < 0.001). The protective effect was greatest for ciprofloxacin and tobramycin. Further, P. aeruginosa isolates from ciprofloxacin-and tobramycin-treated rats demonstrated significantly less exoentyme S and elastase activity than isolates from untreated rats (P < 0.001).Isolates from ceftazidime-treated lungs expressed less exoenzyme S activity (P < 0.001) but an equivalent amount of elastase activity as isolates from controls. The suppression of P. aeruginosa exoenzymes may arrest progressive lung injury during chronic P. aeruginosa lung infections.Cystic fibrosis is the most common lethal genetic disorder of Caucasians (14). Individuals with cystic fibrosis suffer from chronic infection and progressive destruction of their airways (15), and once they are colonized by Pseudomonas aeruginosa there is a relentless decline in pulmonary function leading to respiratory failure and death (21). During exacerbations of their respiratory disease, treatment with antipseudomonal antibiotics frequently leads to clinical improvement, which occurs in the absence of a significant or sustained bacteriological response (5, 25). The replication of P. aeruginosa in bronchial secretions may not be as important as the production of extracellular enzymes, which are believed to play an important role in the pathogenesis of disease due to this organism (36). Suppression of these exoenzymes may limit the progressive lung damage experienced by patients with cystic fibrosis.Exoenzymes phospholipase C, exotoxin A, the proteases, and in particular, elastase and exoenzyme S are important virulence factors for P. aeruginosa lung infections (34). Aminoglycosides, in subinhibitory concentrations, have been shown to inhibit the secretion of proteases in vitro (7,9,11,30), and ciprofloxacin has been shown to reduce levels of proteases and exotoxin A both in vitro and in vivo in the rat granuloma-pouch model for chronic local P. aeruginos...

Two studies were conducted to determine whether Pseudomonas aeruginosa exoenzyme expression was increased during pulmonary exacerbations of cystic fibrosis (CF) and if it was reduced by antibiotic therapy. The first study was retrospective comparing in vitro exoenzyme levels expressed by P. aeruginosa sputum isolates from seriously ill, hospitalized patients with CF to those from P. aeruginosa strains isolated from CF clinic patients who were in relatively better health. Exoenzyme values were significantly greater in P. aeruginosa strains isolated from ill, hospitalized patients than in clinic patients (P = 0.0001). In the prospective study, in vitro exoenzyme levels were measured from sputum P. aeruginosa isolates of 9 hospitalized patients with CF. Exoenzyme values were greatest in nonmucoid strains on admission (P < 0.0025), and P. aeruginosa exoenzyme expression decreased significantly during hospitalization (P < 0.0025). Deterioration in CF lung disease was accompanied by increased P. aeruginosa exoenzyme production, especially by nonmucoid strains. Antibiotic treatment during hospitalization resulted in mean improvement of % predicted forced expiratory volume in 1 sec (FEV1) from 39 to 53% (P = 0.002). Thus, antibiotics may improve pulmonary function in patients with CF by decreasing P. aeruginosa exoenzyme expression.

Using the agar-bead rat lung model, we evaluated the effects of subinhibitory antibiotic treatment upon Pseudomonas aeruginosa exoenzyme expression and lung injury in vivo. One hundred and twenty-eight animals were separated into two groups of 64 animals. One group was inoculated with P. aeruginosa DG1, and the other with P. aeruginosa 3740. Each of these two groups was divided into four subgroups of 16 animals on the basis of ten-day antibiotic treatment with ciprofloxacin, tobramycin and ceftazidime or untreated controls. P. aeruginosa DG1 is non-mucoid and expresses significant yields of exoenzyme S and elastase. P. aeruginosa 3740 is a mucoid organism isolated from the sputum of a cystic fibrosis patient, and demonstrates modest elastase activity only (10% of DG1 levels). Lung bacterial counts were similar in treatment and control groups. Lungs from antibiotic-treated rats demonstrated fewer histological changes than those from untreated animals (P less than 0.001). DG1 lung isolates from antibiotic-treated animals yielded less elastase and exoenzyme S compared with isolates from untreated animals (P less than 0.001). No detectable decrease in elastase or mucoid phenotype was observed in 3740 lung isolates from antibiotic treated animals. Thus, antibiotic protection against lung injury by P. aeruginosa may involve modulation of virulence factors.