Haemophilus, Actinobacillus, Pasteurella: mechanisms of resistance and antibiotic therapy.

Clinical isolates of members of the family Pasteurellaceae show resistance to drugs used for therapy of common infectious diseases of animals. Veterinarians want to use an antimicrobial that is effective against the pathogen, continues to provide therapy for several days, and is not too expensive. Resistant bacteria have complicated the problem of selecting the best antibiotics for treatment of livestock. Resistance to some antibiotics, such as those which inhibit ribosome function, may be encoded on the chromosome; however most antibiotic resistance that involves enzymatic pathways is mediated by genetic elements encoded on plasmids and/or transposons. Members of the genera Haemophilus, Actinobacillus and Pasteurella contain transferable plasmids and transposons that confer antibiotic resistance. This means that clones of pathogenic Pasteurellaceae have antibiotic resistance that fluctuates due to transferable plasmids as well as more permanent resistance mediated by chromosomal changes. Effective therapy requires treatment with a combination of long-acting agents.