Isocitrate lyase supplies precursors for hydrogen cyanide production in a cystic fibrosis isolate of Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa colonizes and can persist in the lungs of cystic fibrosis (CF) patients for decades. Adaptation of P. aeruginosa to the CF lung environment causes various genotypic and phenotypic alterations in the bacterium that facilitate persistence. We showed previously that isocitrate lyase (ICL) activity is constitutively upregulated in the P. aeruginosa CF isolate FRD1. We show here that high ICL activity in FRD1 contributes to increased hydrogen cyanide (HCN) production by this isolate. Disruption of aceA, which encodes ICL, results in reduced cyanide production by FRD1 but does not affect cyanide production in the wound isolate PAO1. Cyanide production is restored to the FRD1aceA mutant by addition of glyoxylate, a product of ICL activity, or glycine to the growth medium. Conversion of glyoxylate to glycine may provide a mechanism for increased cyanide production by P. aeruginosa growing on compounds that activate the glyoxylate pathway. Consistent with this hypothesis, disruption of PA5304, encoding a putative d-amino acid dehydrogenase (DadA), led to decreased cyanide production by FRD1. Cyanide production was restored to the FRD1dadA mutant by the addition of glycine, but not glyoxylate, to the growth medium, suggesting that loss of the ability to convert glyoxylate to glycine was associated with the dadA mutation. This was supported by increased glycine production from toluene-treated FRD1 cells with the addition of glyoxylate compared to FRD1dadA cells. This study indicates a larger role for ICL in the physiology and virulence of chronic isolates of P. aeruginosa than previously recognized.

FIG. 1.

ICL is required for cyanide production by FRD1. Cyanide concentrations were normalized to the CFU of bacteria recovered from PIA plates. Values represent the averages of three independent experiments conducted in duplicate with standard deviation bars. FRD1aceA⁺ is FRD1aceA complemented with a wild-type copy of aceA in cis.

FIG. 2.

Glyoxylate and glycine restore HCN production. Cyanide concentrations were normalized to the CFU of bacteria recovered from PIA plates. Values represent the averages of three independent experiments conducted in duplicate with standard deviation bars.

FIG. 3.

d-Amino acid dehydrogenase is required for HCN production. Cyanide concentrations were normalized to the CFU of bacteria recovered from PIA plates. Values represent the averages of three independent experiments conducted in duplicate with standard deviation bars.

FIG. 4.

ICL supplies precursors for HCN production. Pathway showing role of ICL in HCN production.