Comparison of Pasteurella multocida serotype 3,4 isolates from turkeys with fowl cholera.

Nine isolates of Pasteurella multocida serotype 3,4 were isolated from turkeys that had been vaccinated against fowl cholera but that subsequently experienced outbreaks of fowl cholera. These isolates were compared with the CU vaccine strain. Characteristics examined were antimicrobial susceptibility profiles; carbohydrate fermentation; production of bacteriocin, siderophore, and hemolysin; resistance to serum complement; membrane proteins; enzyme activities; presence of plasmids; transferability of R-plasmids; and pathogenicity. Differences were observed between the CU strain and virulent isolates of P. multocida serotype 3,4 for resistance to serum complement, presence of plasmids, hemolysis, membrane proteins, and pathogenicity. Other parameters compared did not differ significantly between the two groups. The presence of plasmids and resistance to the lytic action of complement of the isolates of P. multocida serotype 3,4 correlated with the mortality rates caused by these bacteria.