Abstract:

Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the effects of different
levels of cadmium (Cd2 +) on Lactococcus lactis subsp. lactis cells were examined. The SEM results showed that when the
mass concentration of Cd2+ was 0 and 10 mg/L, the bacterium displayed an oblong shape with smooth surface and grew
vigorously. With increasing concentration of Cd2+, white granules appeared on the surface and the number of surviving cells
began to fall (i.e., OD was reduced from 1.336 to 0.515). Under the stress of Cd2+ at 200 mg/L, almost no visible bacteria
were seen and a small amount of prismatic crystals appeared. The TEM results revealed that when the mass concentration
of Cd2+ was 0–50 mg/L, the cell structure of this strain was intact and the cellular contents were evenly distributed, and the
strain grew normally. When exposed to 100 and 200 mg/L Cd2+, it began to change abnormally such as ruptured cells and
release of cellular contents from holes in the cellular membrane. In addition, plasmolysis was observed. Together, these
results indicated that Lactococcus lactis subsp. lactis was hardly affected by Cd2+ at low concentrations (≤ 50 mg/L) but
inhibited at high concentrations (100 and 200 mg/L).

Key words: Lactococcus lactis subsp. lactis, cadmium stress, electron microscope, subcellular structure