Biofilm formation by lactobacilli and resistance to stress treatments

We investigated biofilm formation by six lactobacilli isolates and tested the resistance of biofilm and planktonic Lactobacillus plantarum B1 and Lactobacillus paracasei W1 to ethanol (ET) and acetic acid (AA). The viability of both isolates was considerably affected by ET, with a higher reduction observed in the planktonic cells, whereas AA was harmful only to L. paracasei W1. Cells that had intact (Syto (R) 9(+)PI(-)) and compromised (Syto (R) 9(-)PI(+)) membranes, and adopted intermediate states with different concentrations of the probes, i.e., Syto (R) 9(+)PI(+) and Syto (R) 9(-)PI(-), were identified following flow cytometry. We linked Syto (R) 9(+)PI(+) to the planktonic cells and clarified on Syto (R) 9(-)PI(-) cells, which after being sorted could regain growth better in biofilm cells. Based on confocal laser scanning microscopy, numerous cells had damaged membranes in the ET-treated biofilm, while half the cells in the AA-treated biofilm maintained their membrane integrity.