Imipenem induced high levels of beta-lactamase production in Pseudomonas aeruginosa biofilms. Piperacillin also induced beta-lactamase production in these biofilms but to a lesser degree. The combination of beta-lactamase production with other protective properties of the biofilm mode of growth could be a major reason for the persistence of this sessile bacterium in chronic infections.

The development of significant mechanisms of resistance to beta-lactam antibiotics in Pseudomonas aeruginosa in cystic fibrosis (CF) patients have been studied in ten CF patients during a two week course of anti-pseudomonal beta-lactam antibiotic therapy. Sputum samples were collected on days 1, 7 and 15. Entire homogenized sputum samples were examined directly for the number of bacteria resistant to different levels of antibiotics. This allowed the detection of pre-existing resistant subpopulations of bacteria as well as following the changes in beta-lactam antibiotic susceptibility during treatment. P. aeruginosa isolates were characterized by means of sero-grouping, phage- and pyocin-typing. Outer membrane proteins of paired sensitive and resistant strains were characterized. Sonicated extracts of cells were assayed for basal and induced beta-lactamase activity. Beta-lactamase activity was further characterized by isoelectric focusing and inhibition profiles. Our observations were in accordance with the hypothesis that the sensitive inducible population was overrun by the pre-existing resistant subpopulation, during treatment. The resistant in-vivo selected P. aeruginosa population exhibited stable partially derepression but the beta-lactamase inhibitor tazobactam restored beta-lactam antibiotic activity.

At the Danish CF Center patients with chronic Pseudomonas aeruginosa lung infection were treated 3–4 times a year (from 1976) with a 2‐week intravenous antipseudomonal course which included preferentially an aminoglycoside and a β‐lactam antibiotic. We investigated the development of antibiotic resistance in P. aeruginosa strains isolated from Danish CF patients over a period of 18 years by testing the in vitro efficacy of carbenicillin, piperacillin, ceftazidime, tobramycin and ciprofloxacin against P. aeruginosa strains collected in 1973 (51 strains), 1980 (80 strains), 1985 (58 strains), and 1991 (100 strains). All the strains were screened and assayed semiquantitively for β‐lactamase activity by use of nitrocefin. We found a significant (p<0.005) increase in the MIC values of the P. aeruginosa strains against piperacillin and ceftazidime. However, no significant correlation was found between the MIC and the number of antipseudomonal courses of antibiotics. The proportion of resistant in vivo selected P. aeruginosa strains, presumed to be stably derepressed producers of chromosomal β lactamase, also increased significantly during the period studied. Our results confirm that the β lactamase production is an important mechanism of antibiotic resistance in P. aeruginosa.

The in vivo activity and source of I-lactamase in sputum samples from 43 patients with cystic fibrosis (CF) during a 2-week antipseudomonal treatment were studied. A colorimetric method, based on the conversion of nitrocefin, was used for quantitation of the sputum ,B-lactamase activity. P-Lactamases in sputum were characterized by isoelectric focusing and inhibition profile and were compared with the ,-lactamases extracted from Pseudomonas aeruginosa isolated from the paired sputum samples. We found that the ,B-lactamase activity increased to high levels in sputum from patients with CF during the course of piperacillin, ceftazidime, cefsulodin, or imipenem therapy. Aztreonam therapy lead to opposite results because the j-lactamase activity decreased and aztreonam was able to mask j-lactamase activity by acting as an inhibitor. All sputum j-lactamases displayed characteristics indicative of a class I enzyme, identical to the (-lactamases extracted from P. aeruginosa. The presence of P-lactamase at such levels could lead to in vivo inactivation of j-lactam antibiotics. This study supports the hypothesis that P-lactamase production is an important in vivo resistance mechanism in P. aeruginosa-infected patients with CF.