Development of a multilocus sequence typing (MLST) scheme for Mannheimia haemolytica and assessment of the population structure of isolates obtained from cattle and sheep.

Mannheimia haemolytica is an important veterinary pathogen affecting cattle and sheep. Previous typing methods, including restriction enzyme analysis, pulsed-field gel electrophoresis and multilocus enzyme electrophoresis (MLEE) have indicated a clonal population structure. Multilocus sequence typing (MLST) has now almost replaced MLEE and is a definitive and portable typing method, allowing global data exchange. The purpose of this study was to develop a MLST scheme for M. haemolytica. A collection of isolates from 10 countries, including the type strain and reference strains for all recognized serotypes were included in the study. Partial sequences of the housekeeping genes adk, aroE, deoD, gapDH, gnd, mdh and zwf were used to define the MLST scheme. The 95 isolates demonstrated 34 different sequence types (ST) of which 19 were connected in three clonal complexes (CC). ST1 constituted more than one-third of the isolates and was most frequently demonstrated among isolates from bovine sources. The analysis indicated a common evolutionary origin of 33 isolates from the French alps, collected from domestic and wild animals and demonstrating several related STs. An analysis of 17 isolates from the USA demonstrated the same ST in 14 of the isolates. In conclusion, an unambiguous typing scheme is presented for M. haemolytica and results obtained confirm previous observations of a clonal population of the organism.