Optofluidic Lab-on-a-Chip Monitoring of Subsurface Bacterial Transport

Abstract

An optofluidic lab-on-a-chip system and subsequent sampling procedure were developed for detecting bacteria from soil samples utilizing light scattering detection of immunoagglutination assay. This system and protocol detected the presence of Escherichia coli K12 from soil particles in near real-time (10 min) with a detection limit down to 1 CFU mL-1, which is superior to the conventional methods, such as plate counting or polymerase chain reaction (PCR) assays. E. coli solutions were applied to the surface of a mock soil system and incubated overnight. The light scattering immunoagglutination assays using the optofluidic lab-on-a-chip showed two E. coli peaks over the soil depth, one at 1 cm and the other at 4 cm. Comparison with bacterial viability assay and Bradford protein assay revealed that smaller E. coli colonies were found at 1 cm depth and larger colonies at 4 cm, while free antigens adsorbed and desorbed more reversibly at both locations. The two peaks were explained by the two-step process of protein-surface interaction and gravitational force. The target molecules with small sizes (free antigens and single cells) arrived at the soil particle surface faster according to the diffusion model, and the larger E. coli colonies arrived later where the soil surface was already occupied. Because the free antigens adsorbed and desorbed in a more reversible manner, they could be found throughout the depth of the mock-up soil system, whereas the larger E. coli colonies traveled through the void space within soil particles via gravitational force and accumulated at the bottom of where the liquid reached. This work also demonstrates a device and procedure that could be potentially implemented in field studies. With proper soil sample handling protocol and light scattering detection of immunoagglutination assay in an optofluidic lab-on-a-chip, developing more complete bacteria subsurface transport models with actual field results can be achieved.