A genus of gram-negative, nonmotile, nonsporulating, facultatively anaerobic coccobacillary to rod-shaped bacteria which are parasitic and often pathogens in many species of mammals, birds, and reptiles. It was named to honor Louis Pasteur in 1887. The organisms grow well on enriched complex media, such as blood agar, and preferably at increased atmospheric carbon dioxide and reduced oxygen pressures. They require organic nitrogen sources and various vitamins, and are capable of both aerobic and anaerobic respiration and carbohydrate fermentation. Nitrates are reduced to nitrites. The base composition of the genome deoxyribonucleic acids range from 38 to 45 mol % guanine plus cytosine. The group is closely interrelated with the genera Haemophilus and Actinobacillus.