Growth and conversion to the mucoid phenotype by nonmucoid Pseudomonas aeruginosa PAO1 was studied in a chemostat system under conditions designed to reflect those likely to be present during chronic infection in the lung in cystic fibrosis patients. Mucoid variants were consistently isolated during continuous culture in the presence of 0.3 M NaCl or 5 or 10% glycerol. Mucoid subpopulations were also detected under conditions of carbon, nitrogen, or phosphate limitation. During carbon or nitrogen limitation, mucoid conversion was dependent upon the choice of substrate. Phosphate-limited cultures exhibited an inverse relationship between culture growth rate and number of mucoid organisms detected. Mucoid variants were not detected when dilution rates (D) exceeded 0.173 h-1. Conversely, at a D of 0.044 h-1, 40% of the population expressed the mucoid phenotype. Phosphorylcholine, a product of phospholipase C activity on the major lung surfactant phosphatidylcholine, was also used as a growth substrate in nutrient limitation studies. Under all conditions, growth of PAO1 supplied with phosphorylcholine resulted in isolation of mucoid variants, indicating that the lung may provide at least one nutrient source conducive to mucoid conversion. Continuous culture also resulted in detection of a phage associated with strain PAO1. High titers of phage were present under all conditions, including those which yielded no mucoid organisms, suggesting that environmental conditions rather than the phage regulated the appearance of mucoid variants
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