Induction of viable but putatively non-culturable Lactobacillus acetotolerans by thermosonication and its characteristics

Abstract

This study aimed at investigating for the first time whether thermosonication (TS), a non-thermal processing technique, could induce the beer-spoilage bacterium, Lactobacillus acetotolerans CGMCC 7.150, into the viable but putatively non-culturable (VPNC) state. After determining culturable, viable and total cell counts, we found that L. acetotolerans in beer could enter the VPNC state by TS at a constant frequency of 24 kHz at 50 °C at three relatively low volumetric powers (0.6, 1.2 and 1.8 W/mL, but not 2.4 W/mL). Furthermore, with the increase of volumetric power, the time required for L. acetotolerans cells to enter the VPNC state shortened. The addition of Tween 80 at a concentration of 0.1% v/v into de Man Rogosa Sharpe agar enabled the VPNC cells to regain culturability with a recovery time of approximately 3 days. However, their recovery capability decreased with the increasing volumetric power. Scanning electron microscopy analysis showed that cells decreased in size and gradually changed morphology from short rods to coccoids upon entering the VPNC state. An insignificant difference in beer-spoilage ability was found between the exponential-phase and VPNC cells. The resistance of these VPNC cells to 0.025 M NaOH and 0.025 M KOH was enhanced compared with the exponential-phase cells.