Objective    To investigate the effect of clpP gene deletion on the virulence of Streptococcus pneumoniae.     Methods    clpP-deficient strain was constructed by LFH-PCR and identified by PCR and Western bloting. Relative quantitative RT-PCR was used to measure the mRNA levels of autolysin A

pneumococcal surface adhesion A

pneumolysin and choline binding protein A. The clpP-deficient strain was applied to infect BALB/c mice to determine its virulence.     Results    clpP gene was replaced completely by erm cassette. The expression of virulent factors in clpP-deficient strain was lower than that of the wide-type (P0.05). Mice virulence experiments showed median lethal time of wide-type was 22 h

while that of clpP-deficient mutant was 8 d (P0.01). The elimination of clpP-deficient mutant from mice blood was faster than that of wide-type.     Conclusion    The deficiency of clpP gene decreases the expression of virulent genes of Streptococcus pneumoniae

thus resulting in reduction of bacterial virulence.