Effect of Pulsed Light Treatment on the Germination of Bacillus subtilis Spores

The loss of cell culturability (spore counts), spore germination and subsequent vegetative growth of Bacillus subtilis were evaluated after pulsed light (PL) treatment. The effect of PL on those mechanisms was dependent on the total fluence applied, decreasing the spore counts when increasing total fluence (0.3–12 J/cm2). Neither spore germination nor vegetative growth was affected after exposure to low total fluence (0.3 J/cm2). A fluence between 1 and 2 J/cm2 did not change the rate of germination, but significantly delayed bacterial growth. The lag period before growth increased with the total fluence. For higher total fluence (5.5 J/cm2), the rate of spore germination was considerably reduced. When B. subtilis spores were subjected to 12 J/cm2, no cultivable cells were detected (the maximum detectable level of cell inactivation was reached), being both spore germination and vegetative growth prevented for at least 48 h. For PL treatments in which germination occurred, a temporary increase in the resistance of B. subtilis cells to PL was found in the early phase of germination except for the lowest fluence (0.5 J/cm2). After this period, cells became more sensitive, having similar resistance to PL during exponential and stationary phase of growth. PL sensitized B. subtilis cells (spore or vegetative forms) to posterior thermal treatments, demonstrating a synergistic effect of heat and PL which points out the potential application of such combination of treatments for microbial inactivation.