Aeration selects for mucoid phenotype of Pseudomonas aeruginosa

A mucoid strain of Pseudomonas aeruginosa isolated from a patient with cystic fibrosis and its nonmucoid revertant were grown in a chemically defined alginate-promoting medium under batch and continuous culture conditions. Selection for the mucoid and nonmucoid phenotype was accomplished by varying the levels of air available to the culture. The addition of air at a rate of 0.5 liters/min to the nonmucoid revertant growing under batch or continuous culture conditions resulted in a greater than 50% decrease in viability over a 10-h incubation period. In contrast, aeration of the mucoid culture maintained a totally mucoid population and there was no decrease in viability over a 55-h incubation. Aeration of a mixed population of the mucoid and nonmucoid phenotype (1:1) resulted in selection for the mucoid phenotype within the first 20 h of cocultivation. The correlation between the mucoid phenotype and alginic acid was demonstrated by the production of 580 micrograms of uronic acid per mg (dry weight) of cells by the mucoid phenotype and less than 1 microgram of uronic acid per mg (dry weight) of cells by the nonmucoid revertant. These results suggest that nonmucoid revertants may have an unusual sensitivity to aeration, which may indicate a mechanism for natural selection of the mucoid phenotype in vivo.