Protocol for Quantitative Estimation of Hydrogen Cyanide Production from Bacteria

Hydrogen cyanide (HCN) is a volatile, nitrogen-containing secondary metabolite produced by various bacterial species, primarily during the idiophase of growth under nutrient-limiting or competitive conditions. It plays a significant ecological role as a biocontrol agent by inhibiting the respiratory enzymes of plant pathogens and modulating microbial competition in the rhizosphere. Although protocols for detecting HCN production have existed for over a century, they have largely remained qualitative and are rarely optimized for quantitative assessment. This is mainly due to the volatile nature of HCN, unidentified stable reference standards, and the absence of a robust, universally accepted protocol that ensures consistency across diverse microbial types. In this study, we present a simplified and efficient colorimetric method to quantify HCN production in both Gram-positive and Gram-negative bacteria. Qualitatively, HCN production was observed by a color change due to the isopurpurate complex. This compound was then eluted and quantified by measuring absorbance at 625 nm. The method uses potassium ferrocyanide as a standard, whose slow dissociation constant enables a stable and controlled release of cyanide ions for calibration, unlike highly dissociative salts like KCN that introduce early volatilization errors. This protocol demonstrated high sensitivity, capable of detecting HCN at concentrations as low as ppm levels, with strong correlation to the standard curve (R² > 0.99). Achieving such sensitivity with other conventional methods, such as gas detection tubes or electrochemical sensors, often requires more sophisticated instrumentation and strict handling conditions. In contrast, this approach offers a cost-effective, reproducible, and user-friendly alternative. While a universally adopted method is still lacking due to standardization challenges and HCN volatility, the proposed protocol marks a significant advancement toward accurate and accessible quantitative assessment in microbiological and agricultural applications. Key features • Enables both qualitative detection and quantitative estimation of hydrogen cyanide (HCN) production in bacteria using a colorimetric assay. • Utilizes a low-dissociation reference compound, potassium ferrocyanide, to create a stable and accurate standard curve for reproducible measurement of HCN concentration. • Offers a simple, cost-effective, and broadly applicable method suitable for screening HCN-producing bacteria in both Gram-positive and Gram-negative groups. • Highly sensitive and accurate HCN detection at sub-ppm levels, ensuring rapid colorimetrical results and reproducibility.