Comparative pathogenicity of different Actinobacillus suis O/K serotypes

The pathogenicity of Actinobacillus suis serotypes O1/K1 (strain SO4), O1/K 2 (strain C84), and O2/K2 (strain H91-0380) was evaluated in specific-patho gen-free (SPF) piglets challenged by intraperitoneal inoculation with appro ximately 1 x 10(7) colony-forming units per mt. All 3 strains produced peri tonitis, but differences were observed in the composite histopathologic sco res (P = 0.001) and in their ability to spread (P = 0.008) at 7 h post chal lenge. The O2/K2 strain caused the most severe peritonitis and disseminated most widely to other tissues. Moderate lesions were seen with the O1/K2 st rain while the O1/K1 strain caused mild lesions and remained largely locali zed to the peritoneum. In an attempt to explain the basis of observed diffe rences, the serum sensitivity of 9 A. suis strains with different O and K t ypes was assessed. Regardless of the O/K type, all of the isolates tested w ere serum resistant. Moreover, most A. suis isolates grew as well or better in complement-replete sera as they did in complement-depleted sera. These observations indicate that although O2 and K2 strains had a greater propens ity to cause a disseminating septic inflammatory response in pigs, they wer e no more resistant to complement-mediated killing than O1 strains.