Pasteurella multocida represents a heterogeneous group of Gram-negative bacteria that are commensals in the upper respiratory tract of many mammals and birds. The organism is also a primary or secondary pathogen and is responsible for a range of economically important diseases in domesticated animals throughout the world. Important infections caused by P. multocida include pneumonia of cattle, sheep and pigs, progressive atrophic rhinitis of pigs, fowl cholera of poultry and haemorrhagic septicaemia of cattle and water buffaloes in certain enzootic areas of Asia and Africa. In addition, P. multocida is responsible for infections in deer, causes respiratory tract disease in rabbits and is associated with human infections resulting from cat and dog bites.
P. multocida strains express a polysaccharide capsule on the cell surface and the antigenic specificity of the capsule determines the organism's serogroup (A, B, D, E or F). It has long been recognised that strains of certain capsular serogroups are associated with specific diseases and animal species, which suggests that the capsular polysaccharide type plays a role in host and disease specificity. However, host-adaptation is a multi-factorial process and capsular serogroup alone is unlikely to account for host or disease predilection. To better understand the molecular basis of host-pathogen interactions in P. multocida we need first to have a more comprehensive understanding of the evolutionary relationships of isolates associated with different host species.
We have previously examined capsular serotypic and outer membrane protein (OMP) variation among 446 isolates of P. multocida originating from cattle (153 isolates), sheep (35), pigs (158) and birds (100) (Davies et al. 2003a; 2003b; 2003c; 2004). We demonstrated that the majority of cases of disease in cattle, sheep and pigs are caused by a restricted number of clones; avian isolates exhibit greater diversity. However, such studies do not provide any information about the underlying evolutionary relationships among strains. For example, are serogroup A strains associated with bovine and porcine pneumonia closely related or not? What is the relationship of these to avian serogroup A strains responsible for fowl cholera? What is the relationship of strains responsible for atrophic rhinitis and pneumonia of pigs?
To address such questions, we have developed a MLST scheme to investigate evolutionary relationships among bovine, ovine, porcine and avian isolates of P. multocida. The scheme could also be used to study isolates from other host species. In the present study, 35 bovine, 14 ovine, 35 porcine and 34 avian isolates were carefully selected to represent the various disease manifestations and all of the clonal groups previously identified by capsular typing and OMP analysis. These isolates are, effectively, representative of 446 isolates recovered from these four host species. In addition, we have included in the MLST scheme the type strains of the three P. multocida subspecies (i.e. multocida, gallicida and septica) as well as four isolates recovered from cases of haemorrhagic septicaemia in Africa and Asia. The MLST scheme is based on comparative nucleotide sequence analysis of fragments from the seven housekeeping enzyme genes adk, aroA, deoD, gdhA, g6pd, mdh and pgi. Concatenated sequences are 3990 bp in length. Preliminary details of the scheme were previously published in a study of evolutionary relationships among 35 bovine isolates of P. multocida (Davies et al., 2004).
- Davies, R.L., MacCorquodale, R., Baillie, S. and Caffrey, B. (2003a) Characterization and comparison of Pasteurella multocida strains associated with porcine pneumonia and atrophic rhinitis. J Med Microbiol 52:59-67.
- Davies, R.L., MacCorquodale, R. and Caffrey, B. (2003b) Diversity of avian Pasteurella multocida strains based on capsular PCR typing and variation of the OmpA and OmpH outer membrane proteins. Vet Microbiol 91: 169-182.
- Davies, R.L., Watson, P.J. and Caffrey, B. (2003c) Comparative analyses of Pasteurella multocida strains associated with the ovine respiratory and vaginal tracts. Vet Rec 152: 7-10.
- Davies, R.L., MacCorquodale, R. and Reilly, S. (2004) Characterization of bovine strains of Pasteurella multocida and comparison with isolates of avian, ovine and porcine origin. Vet Microbiol 99:145-158.