ABSTRACT
Invasion of chicken embryo fibroblast (CEF) cells by the virulent encapsulated Pasteurella multocida strains P-1059 (serovar A:3) and X-73 (serovar A:1) and an avirulent noncapsulated derivative P-1059B (serovar -:3) was investigated. The number of intracellular bacteria increased for all the strains after 2, 4 and 6 h post-inoculation to CEF cells. By 6 h post-inoculation, the number of invaded bacteria of encapsulated strains was significantly higher than noncapsulated strain and reached 150- and 112-fold for strains P-1059 and X-73, respectively, while it was 9-fold for strain P-1059B as compared to the number of invaded bacteria recovered after 2 h post-inoculation. Electron microscopy of invasion by encapsulated strains showed that the bacteria were adhering to CEF cells membrane after 1 h of inoculation. By 4-h, one or two bacteria were detected within membrane-bound vacuoles of the intracellular space. The number of intracellular bacteria markedly increased at 14 h post-inoculation. Invasion of all strains was inhibited significantly when the monolayers were treated with periodic acid (P<0.001) or trypsin (P<0.05). The treatment of bacteria with hyaluronidase did not affect invasion. The present results indicate that avian P. multocida capsular type A strains are invasive and that the receptor on CEF cell surface might be glycoprotein.
