The Effect of Environment on Virulence Factors and Resistance of Mucoid Strains of Pseudomonas Aeruginosa

Abstract

M7, a mucoid variant of Pseudomonas aeruginosa NCTC 6750 isolated in vitro, was compared to its non-mucoid parent strain (WT). Their nutritional requirements showed slight differences. The non-mucoid had a growth rate advantage over the mucoid variant. M7 retained its mucoid character both in batch and in continuous culture. Using continuous culture, the mucoid organism was shown to produce several phenotypes which showed differences in their susceptibility to carbenicillin, polymyxin and deoxycholate with and without added magnesium. WT produced two major phenotypes which differed in colony size and appearance on nutrient agar. Both showed differences in the production of the virulence factors, protease, elastase, lipase, phospholipase C and pyocyanine and fluorescent pigments under different nutrient limitations. Growth conditions also affected exopolysaccharide production by M7. In general, phosphate and iron limitation resulted in enhanced production of the extracellular products, while magnesium limited cells produced the least amounts. M7 showed lower amounts of extracellular enzymes in culture supernatants but higher levels of cell bound enzymes. The sensitivity to polymyxin was markedly affected by nutrient limitation. The resistance of magnesium > sulphate > carbon > phosphate > iron = nitrogen limited cells. The order was the same for both organisms. Sulphate and magnesium limited cells of M7 were more resistant than similarly limited WT. In contrast, iron and nitrogen limited M7 were more sensitive than WT cells. The results suggest differences in cell envelope structures between M7 and WT. Batch grown M7 was more resistant than WT to the killing effect of blood, but this difference was reduced when cells were grown in nutrient depleted medium. Using chemostat cultures, magnesium limited cells showed highest sensitivity while iron and nitrogen limited cells were least sensitive. Iron limited cells showed an optimum dilution rate at which both M7 and WT cells were least sensitive to the killing effect of blood. The results demonstrate differences between mucoid and nonmucoid P. aeruginosa, and the need for defined growth conditions on work on virulence determinants and pathogenic mechanisms of P. aeruginosa and probably other bacteria.