Transmission electron microscopy was used to investigate the effect of nisin on the ultrastructure of S. suis MGGUS2 (ST1). The electron micrographs of S. suis cells untreated and treated (1 h) with nisin are presented in Fig. 2. At MIC, nisin was found to induce rupture of the cytoplasmic membrane, although most of the bacterial cells kept their shape. Moreover, a proportion of bacteria remains unaffected. When the concentration of nisin was increased to 2-fold MBC, ultrastructural damages appear more important resulting in breakdown of the cytoplasmic membrane and cell wall with leakage of the cytoplasmic content of S. suis. Interestingly, the ultrastructure of the capsule layer was not altered even in the cell wall disrupted cells, confirming the absence of direct consequences on this structure in the presence of nisin.