Inactivation of spores and vegetative cells of Bacillus subtilis and Geobacillus stearothermophilus by pulsed light

• PL could be an effective method for inactivating both vegetative and spores forms.
• Inactivation effectiveness of PL depended on the physiological state of cells as well as on their exposure to the incident light.
• Bacillus subtilis was found to be more sensitive to pulsed light than Geobacillus stearothermophilus.

The effect of pulsed light (PL) on the inactivation of vegetative cells and spores of Bacillus subtilis and Geobacillus stearothermophilus at different cell densities was evaluated. The antimicrobial effect of PL decreased when population density increased, both for vegetative cells and spores of B. subtilis and G. stearothermophilus. For low cell densities, vegetative cells were more sensitive to PL than spores. However, lower reductions in vegetative cell counts were shown for higher cell densities, which could be attributed to the fact that vegetative cell suspensions transmitted less amount of light than spores. Concerning the resistance of both microorganisms, lower reduction in G. stearothermophilus than B. subtilis counts were found for the same cell density. When cell suspensions with similar light transmittance were compared, vegetative cells of B. subtilis were found to be more sensitive than the ones of G. stearothermophilus, while the spores of G. stearothermophilus were less resistant to PL.