Rapid development in vitro and in vivo of resistance to ceftazidime in biofilm-growing Pseudomonas aeruginosa due to chromosomal beta-lactamase

The aim of this study was to examine the development of resistance of biofi lm-growing P. aeruginosa during treatment with ceftazidime. Biofilms were e stablished in vitro using a modified Robbins device (MRD) and in vivo in th e rat model of chronic lung infection. Three P. aeruginosa strains isolated from the lungs of cystic fibrosis (CF) patients (MICceftazidime-basal/indu ced beta -lactamase activity: PAO 579 = 0.8 mg/1-19/550 milliunits, 19676A= 50 mg/l-38/957 milliunits, 17107B=100 mg/l-504/947 milliunits) were studied . After 1 or 2 weeks of continuous or intermittent (4 h/day) administration of ceftazidime to biofilms established in MDR, a statistically significant development of resistance to ceftazidime in PAO 579 or 19676A bacterial po pulations occurred. When ceftazidime was administered 4 h/day (200 mg/l) fo r 2 weeks, the frequency of resistant 19676A having MIC>25 mg/l was 4.4.10( -1) compared to 6.0.10(-5) in the control biofilm. The same trend was obser ved after continuous administration of ceftazidime. MICceftazidime of the m ore resistant variants was increased 500-fold for PAO 579 and 8-fold for 19 676A, and the specific basal beta -lactamase activities from 19 to 1,400 un its for PAO 579 and from 38 to 10,000 units for 19676A. Chronic P. aerugino sa lung infection was established with alginate-embedded PAO 579, 19676A or 17107B in 146 Lewis rats which were treated with ceftazidime 4 g/kg intrap eritoneally twice a day for 1 week. A statistically significant development of resistance was observed for all three strains. The MICceftazidime of th e more resistant variants was increased 15-fold for PAO 579, 8-fold for 196 76A, and 4-fold for 17107B, and the specific basal beta -lactamase activity from 19 to 100 units for PAO 579, from 38 to 1,300 units for 19676A, and f rom 500 to 1,300 units for 17107B. It was shown that, during treatment with ceftazidime, biofilm-growing P. aeruginosa had the capacity to develop res istance due to the production of chromosomal beta -lactamase.