OBJECTIVETo determine the ability of adhering and invading to host cells and the related pathologic changes of Leptospira spp. with different virulence.

METHODSA special Fontana silver staining method was developed to observe the ability of L.interrogans serogroup Icterohaemorrhagiae serovar lai strain 56601 and serogroup Pomona serovar pomona strain 56608, and L.biflexa serogroup Samaranga serovar patoc strain Patoc I to adhere Vero and J774A.1 cells. Ultrastructural lesions of the infected cells were examined by electron microscopy. By using flow cytometry with fluorescein labeling of FITC-Annexin V/PI,apoptosis and necrosis of the Vero and J774A.1 cells induced by the leptospiral strains before and after inactivation with ultraviolet treatment were detected, respectively.

RESULTSThe adhering rates of L.interrogans strains 56601 and 56608 to the Vero cells were 24.2% and 22.9% (P>0.05), while to the J774A.1 cells were 49.0% and 46.9% (P>0.05), respectively. L.biflexa strain Patoc I did not adhere to these two host cells. After the two strains of L.interrogans invaded two different cell lines, the special phagosomes containing the Leptospira were formed and similar cell ultrastructural lesions, such as chromatilysis and chromatin condensation, vacuolar degeneration, mitochondrion swelling and mitochondrial crista disappearance, and endoplasmic reticulum swelling and paramembranous ribosome disappearance, were observed. The apoptosis rates in the Vero cells caused by the L.interrogans strains 56601 and 56608 before and after ultraviolet inactivation were 84.4%, 82.8% and 77.9%, 86.1%, respectively. The L.interrogans strain 56601 mainly induced the terminal apoptosis in Vero cells with the rates of 68.0% and 52.9% before or after inactivation, while the L.interrogans strain 56608 mainly induced the early apoptosis in Vero cells with apoptosis rates of 64.1% and 50.1% before or after inactivation. In the J774A.1 cells, the L.interrogans strain 56608 caused cell necrosis (before and after inactivation) and apoptosis that was dominated by terminal apoptosis.

CONCLUSIONThe established Fontana silver staining method can be used to observe adhesion of L.interrogans.L.interrogans can invade host cells through endocytosis which causes untrastructural lesions. The necrosis or apoptosis induced by L.interrogans are affected by different host cell lines but not the virulence of L.interrogans strains.