The robust establishment of biofilm on surfaces in water systems poses risks to industry and human health, yet their removal is challenging owing to their high tolerance towards conventional chemical strategies. Germicidal UV-C irradiation from LEDs presents new opportunities for inhibiting biofilm on surfaces. By connecting to side-emitting optical fibers, UV-C light could be distributed to any shapes of designed geometries (i.e., curved plumbing fixtures). However, the application of UV-C exposure for biofilm inhibition necessitate knowledge of 1) biofilm growth rates relative to UV-C intensity; and 2) mechanism of microbial response to varying intensities of UV-C stress. Herein we created a continuous UV-C gradience from excessive UV-C intensity to zero intensity in single 50-cm reactor corresponds to the attenuation from optical fibers. Surface biovolume obtained from optical conference tomography was directly linked to light intensity. Biofilm formation gets inhibited inside the reactor along the length of optical fiber, until the UV-C intensity drops to 5-10 μW/cm<sup>2</sup>, where biofilm start expending. Biofilm samples collected from area continuously irradiated by high, moderate, low and zero UV-C intensities was analyzed by RNA sequencing. We found that over-excessive UV-C intensities inhibit functional genes expression related to protection compared to dark control. Level of gene depression was lower but follows a similar trend as the UV-C intensity decreases as long as its above 5-10 μW/cm<sup>2</sup>. However, biofilms behave unique survival strategies at places where inhibition failed with relatively lower UV-C stress. During the situation with non-lethal UV-C exposure, biofilm generates more EPS for protection and UV-C shielding; the persister cell-related and SOS response-related genes are overexpressed to defend the stress. Limiting amount of reactive oxygen species from UV-C exposure leads to significantly higher oxidative stress resistance. Meanwhile, bacteria perform higher mobility to pay extra effort for surface attachment. These results imply the importance of maintaining enough UV-C exposure onto surface for microbial control in many scenarios.

Quantifying biofilm response to UV-C light by side-emitting optical fibers and RNA sequencing analysis

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