Ionic Liquid and NanoSuit® Methods Prevent Surface Alterations in the Probiotic Lacticaseibacillus paracasei Strain Shirota Caused by the Critical Point Drying Process on Field Emission Scanning Electron Microscopy

Field emission scanning electron microscopy (FE-SEM) may be used to visualize the surface morphology of samples that are permeable to electron beams, including biological samples. Probiotics attenuate host physiological functions and are characterized by their three-dimensional surface structures. In this study, we determined the effect of critical point drying (CPD) on FE-SEM observations of the surface of Lacticaseibacillus paracasei strain Shirota (LcS). We also assessed ionic liquid (IL), a non-volatile liquid salt that retains moisture, and NanoSuit®, which forms a protective polymer membrane around the sample, through FE-SEM observation of these probiotics. The results indicate that dehydration during CPD leads to reticular structures on the probiotic surface, potentially affecting the characteristics observed by FE-SEM. In addition, we examined IL and NanoSuit®, which do not involve dehydration. The initial examination involving optimal dilution using silica particles revealed that 5–10% IL and 5–20% NanoSuit® solutions maintained particle size consistency. We examined LcS specimens under these conditions and observed smooth surfaces, not reticulate structures. These results indicate that CPD affects LcS surface morphology, whereas the IL and NanoSuit® methods preserved it. This suggests their applicability for probiotic preparation before FE-SEM observations.