Effect of ultrafine bubbles on pseudomonas aeruginosa and staphylococcus aureus during sterilization of machining fluid

Bacterial growth is one of the common causes of putrefaction and deterioration of water-soluble machining fluid. The 16S ribosomal DNA metagenome analysis of the bacterial species composing the microbial flora present in the machining fluid derived after processing demonstrated a high amount of species belonging to the Pseudomonas genus. Therefore, we prepared two types of ultrafine bubbles water (gas species: air and CO₂) containing different types of gas and confirmed the bactericidal effect on Pseudomonas aeruginosa (ATCC 10145), a typical Pseudomonas species. The grinding fluid was prepared using sterile purified water containing ultrafine bubbles (hereafter referred to as UFB) as diluted water, and the Pseudomonas aeruginosa was inoculated to obtain 10⁶ CFU/mL. The sterilization rate of the number of bacteria was determined immediately after immersion in each fluid and subsequently after two hours. The sterilization rate was determined to be 100% in the test group using the ultrafine bubbles water of CO₂ (CO₂-UFB water). As a comparative control, a similar test was performed on Staphylococcus aureus IFO12732, and the sterilization rate was determined as 0%. Fluorescence microscopic observation of bacteria after immersion in the CO₂-UFB water demonstrated damage to the cell wall as the cause of death of the Pseudomonas aeruginosa. Therefore, CO₂-UFB demonstrated sterilization of machining fluid by killing Pseudomonas aeruginosa in the machining fluid. The bactericidal mechanism of UFB involved the induction of damage in bacterial cell walls. This can be attributed to crushing due to the increase in the particle size of UFB.