Biofilm Enlèvement des Aérosols de dioxyde de carbone sans purge d'azote

Biofilms are complex bacterial community structures in which bacterial cells are embedded within self-produced matrices of extracellular polymeric substances, held together, and protected from the external environment. Biofilms can present a public health risk and cause economic losses because they can form on various surfaces, including medical implant materials and devices, food processing equipment, and heat exchangers. In fact, biofilms have been found to be associated with 65% of all bacterial infectious diseases in humans, according to the Centers for Disease Control and Prevention¹.

Autoclaves and disinfectants such as chlorine have generally been used for the inactivation of biofilms. However, the use of an autoclave is limited for surfaces that can neither withstand high temperature steam nor be placed into the autoclave. Disinfectants are not suitable for surfaces sensitive to chemical treatment or prone to collecting toxic oxidation products². In addition, the biofilm should not only be inactivated, but also removed in order to prevent the attachment of new cells onto the surface, thereby forming a new biofilm³. However, it is difficult to remove biofilms using methods based on viscous fluid shear because the flow velocity near the surface is almost zero and the shear force usually cannot overcome the adhesive forces of micron- and submicron-sized substances. Moreover, the biofilm matrix is known to act as a physical and chemical barrier¹.

Many physical and mechanical techniques have been developed to remove biofilms from surfaces, including ultrasonic vibration¹, electric currents⁴, laser irradiation⁵, and high-pressure water sprays⁶. Each technique has its own pros and cons. Ultrasonic vibration and electric current can be used to control biofilm formation; however, they require a particular configuration and conductive surfaces, respectively, requiring additional shear stress^{1, 4}. Laser irradiation can be applied to a limited area and to hard surfaces; however, some live and dead cells remain on the surface⁵. High-pressure water sprays effectively remove biofilms; however, their high momentum can cause damage to soft substrates⁶.

Biofilm removal using CO₂ aerosols has been previously proposed. It has shown promising results, with high removal efficiencies within a short time^7-11. CO₂ aerosols are generated by adiabatic expansion of a high-pressure CO₂ gas through a nozzle, and they are applied to the surfaces contaminated with a biofilm. This cleaning technique utilizes the momentum transfer of solid CO₂ particles and the solvent action of the melted CO₂ liquids, followed by the aerodynamic shear force of the CO₂ gas¹². Compared with high-pressure N₂ gas jets, CO₂ aerosol jets at the same stagnation pressure are much more effective in removing E. coli biofilms⁷. Moreover, although the momentum of the solid CO₂ that is delivered to the bacteria is considerably high, the momentum of the total aerosol jet applied to the solid surface is substantially lower than that of water jets. Therefore, damage-free cleaning is possible using this CO₂ aerosol technique.

In this protocol, periodic jets of CO₂ aerosols without nitrogen purges were used to remove Pseudomonas putida biofilms from a polished stainless steel surface. In fact, nitrogen purges have been used in many CO₂ aerosol studies to increase the removal efficiency by reducing the moisture condensation generated during the cleaning, even though heating with a hot plate or infrared lamps and employing dry boxes have also been adopted¹². The surface biofilm formation and the optimized cleaning procedures are described below. The removal efficiency was evaluated by measuring the fluorescence intensity of the biofilm on the surface.