Removability of bacterial spores made adherent to solid surfaces from suspension with and without drying

Spores of Geobacillus stearothermophilus and Bacillus subtilis adhering to the surface of stainless steel (SS) and polypropylene (PP) were subjected to rinsing in water or NaOH solution to study their removability. The spores were made adherent through deposition from suspension under humid and dry conditions. The humid condition kept the surface wet throughout the deposition, whereas the dry condition made them dry up finally. Experimental results demonstrated that drying significantly enhanced both quantity and strength of the spore adhesion for both types of surface. No removal of the spores adhering to surfaces exposed to dry environment was observed through water rinsing with shear force application. Even with 0.2% NaOH, they were hardly removed irrespective of temperature. Although 1% NaOH removed or inactivated most of spores on SS surface, it left considerable viable spores on PP surface. Thus enhanced alkali tolerance was indicated for spores adhering to surfaces, especially made of PP, exposed to dry environment.