Colony Morphology and Biofilm Formation by Food Spoilage Bacteria Lactobacillus plantarum

Introduction: The lactic acid bacterium Lactobacillus plantarum is widely used in food manufacturing as beneficial bacterium, while it can also cause food spoilage. In its natural environment, L. plantarum forms biofilms, which structurally are composed of multicellular communities of bacteria, and such biofilm formation confers resistance to environmental stresses and chemical treatments. Moreover, we found two types of colony with different morphologies, wild type (WT) and mucoid variant (MV), emerge in populations of L. plantarum derived from environmental isolates.

Purpose: In this study, we evaluated the effects of differences in colony morphology on biofilm structure and resistance to chemical treatment in L. plantarum.

Methods: WT and MV cells isolated from a population of L. plantarum were incubated in MRS broth at 30°C for 24 h, and their biofilm structures were examined using confocal reflection microscopy and scanning electron microscopy. To evaluate the resistance to chemical treatment, the survival rates of the biofilm cells were measured by counting the numbers of colony forming units after 1 h exposure to 8% acetic acid or 15% ethanol at room temperature. The WT and MV cells were also stained with Indian ink to observe cell-associated polysaccharides (CPS).

Results: The WT formed a thin, dense biofilm, while the MV formed a thick biofilm with a lower cell density than the WT biofilm. The WT biofilm exhibited greater resistance to ethanol than the MV biofilm, whereas the MV biofilm was more resistant to acetic acid than the WT biofilm. Moreover, compared with the WT cells, the MV cells produced increased amounts of CPSs.

Significance: These results suggest that the coexistence of WT and MV cells in L. plantarum populations affects the biofilm structure and confers resistance to various environmental stresses and chemical treatments. These findings have important implications for the control of L. plantarum and food spoilage by environmental lactic acid bacteria.