Airborne bacteria detection: Species-specific concentration measurement using colorimetric detection of free antibodies

This study presents a novel colorimetric detection method for species-specific quantification of airborne bacteria. Using a filtration-based approach combined with antibody-conjugated horseradish peroxidase (HRP), this method enhanced the detection of target bacteria (Escherichia coli and Staphylococcus aureus) collected via an aerosol-to-hydrosol sampler. Bacteria–antibody–HRP complexes were captured on a membrane by filtration, and the free antibody-conjugated HRP in the filtrate was subsequently quantified. Experimental results demonstrated that the absorbance increased as the airborne bacterial concentration decreased. Absorbance values were obtained for bacterial samples at varying concentrations, ranging from 10⁰ to 10⁷ colony-forming unit (CFU) per 1 mL of liquid. A correlation between the absorbance and CFU·mL⁻¹ was established for each bacterial species, resulting in a limit of detection (LOD) of 21.5 CFU·mL⁻¹ for E. coli and 246 CFU·mL⁻¹ for S. aureus. The absorbance values were measured for the bacteria collected using a commercial air-to-liquid sampler with an experimental setup that generated airborne bacteria in a controlled laboratory environment. The liquid bacterial concentrations (CFU·mL⁻¹) were converted to airborne bacterial concentrations in CFU·m⁻³ air by dividing by enrichment ratio of the air sampler, yielding an LOD of 545 CFU·m⁻³ air for E. coli and an LOD of 953 CFU·m⁻³ air for S. aureus.