Aims In cystic fibrosis, Pseudomonas aeruginosa (PA) infection is associated with significant morbidity and mortality. Unfortunately, inpatients who cannot expectorate sputum current diagnostic methods are unreliable, unpleasant or invasive. This has lead to increased interest in using the cyanogenic properties of PA to develop a non-microbiological method for its detection. Prior to this development it needs to be determined if cyanide production varies according to PA strain.

Methods The hydrogen cyanide (HCN) released into the gas phase by 96 genotyped PA samples was measured using Selected Ion Flow Tube Mass Spectrometry after 24, 48, 72 and 96 h of incubation. The HCN produced by a range of non-PA cultures and incubated blank agar plates was also measured.

Results The 96 samples included 26 different strains; four were previously described epidemic strains (Liverpool, Midlands1, Midlands2 and Stoke). Extremely low levels of HCN (<10 parts per billion) were produced by the control samples. All the PA strains produced more HCN than the controls. Across all four time points the non-mucoid samples produced more HCN than the mucoid samples (p=0.003), they also produced more HCN after 24 h when the time points were analysed separately (p=0.008). When samples were separated according to strain and phenotype, some strains (Liverpool and Stoke) continued to show higher HCN production by non-mucoid samples but others (Midlands1) showed higher production by mucoid samples. Clear differences were apparent in the HCN production between the different strains. Multivariant analysis of the three commonest strains (Liverpool, Midlands1 and Stoke) revealed a significant effect of strain (p<0.001) and a borderline interaction of strain and phenotype (p=0.051).

Discussion This supports previous studies showing PA is one of a limited number of organisms to produce cyanide and SIFT-MS is a sensitive and reproducible way of analysing this. It is the first study to demonstrate that cyanide production is dependent on the PA strain and that the effect of phenotype may vary between strains. As all the PA strains produced more HCN than the controls, it creates further interest in using the cyanogenic properties of PA to develop a diagnostic test.