Destruction of Bacillus stearothermophilus bacteria by weakly ionized low pressure cold oxygen plasma

High resolution scanning electron microscopy (SEM) has been applied to study degradation of Bacillus stearothermophilus bacteria upon exposure to oxygen plasma. Bacteria were grown according to standard prescribed protocol, carefully rinsed with distilled water and deposited onto well polished silicon wafers. The wafers were exposed to oxygen plasma and then characterized by SEM. Plasma was created in an electrode-less RF discharge in the E mode. Plasma parameters were determined with a double Langmuir probe and a catalytic probe. At the working pressure of 75 Pa, the electron temperature was about 3 eV, plasma density about 6 × 10¹⁵ m⁻³ and the density of neutral oxygen atoms about 4 × 10²⁰ m⁻³. SEM investigation revealed slow degradation of the bacterial capsule, which was accomplished in several seconds of plasma treatment. Further exposure to plasma resulted in etching of the bacterial cell wall which was totally destroyed in about half a minute. The remains of the organic material were then slowly oxidized until only ash remained after few minutes of plasma treatment. The organic material degradation was predominantly due to oxidation with O atoms. The dose of atoms needed for damaging of the cell wall was estimated to about 2 × 10²⁴ m⁻² while full degradation was observed after receiving the dose of 5 × 10²⁴ O atoms/m².