Summary: 492a and 492c were two strains of isolated from the sputum of a patient with cystic fibrosis. The strains were closely related but expressed different antibiograms. 492c was hypersensitive (10-100 times more sensitive than 492a) to the β-lactam antibiotics carbenicillin, methicillin, flucloxacillin, mecillinam and cefuroxime and the non-β-lactam, nalidixic acid. 492c also showed enhanced sensitivity (4–8 times more sensitive than 492a) to chloramphenicol, trimethoprim and novobiocin. 492a and PAO8 expressed similar levels of antibiotic resistance, except for trimethoprim, to which 492a was five times more sensitive than PAO8. Two genes associated with antibiotic hypersensitivity were mapped in the 30 min region of the chromosome, by means of R68.45-mediated plate matings between a Leu mutant of 492c and PAO8, followed by transductional analysis using phage F116L. The first of these genes, was closely linked to and in a PAO background, was associated with hypersensitivity to the β-lactams and a moderate increase in sensitivity to chloramphenicol, trimethoprim, nalidixic acid and novobiocin. A further increase in sensitivity to the latter three antibiotics was associated with the second gene, , which mapped between and . This gene could also be transferred to PAO from 492a, thus 492c could have arisen from 492a following a single chromosomal mutation at the locus. Isolation of a mutant of PAO969 provided further evidence for this theory.


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