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Resistant variants of three clinical Pseudomonas aeruginosa isolates were obtained in the presence of aztreonam. The variants exhibited a four- to eightfold increase in the minimal inhibitory concentrations to beta-lactam antibiotics (except imipenem) to quinolones, such as norfloxacin and fleroxacin, chloramphenicol and tetracycline, but not to gentamicin and polymyxin B. beta-Lactamase production was barely detectable in both wild-type strains and the resistant clones. Only ampicillin, cefoxitin and imipenem increased the production of beta-lactamase, whereas various other beta-lactams did not. Penicillin-binding proteins remained unchanged in the aztreonam-resistant clones. The analysis of the outer membrane proteins did not reveal differences in the outer membrane proteins between the wild-type strains and the aztreonam-resistant clones. Two of the three antibiotic-resistant isogenic clones contained less lipopolysaccharides (LPSs) than their corresponding wild-type strains. Moreover, it could be demonstrated that the ratio of 2-keto-3-deoxy octonate to carbohydrate of the LPS changed in any case between the wild-type strains and the aztreonam-resistant clones. These alterations were accompanied by a decrease in surface hydrophobicity of the resistant clones as compared to the wild-type strains. Therefore, quantitative as well as qualitative alterations in the LPS may provide an explanation for the resistant phenotype observed.
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