The Genus Actinobacillus

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The Genus Actinobacillus JANET I. MACINNES AND EDWARD T. LALLY

Introduction Members of the genus Actinobacillus are small, Gram-negative, pleomorphic, coccobacillary rods that are facultatively anaerobic, indole negative, -galactosidase and urease positive and reduce nitrates (Olsen and Møller, 2004a). Apart from A. pleuropneumoniae and some A. suis strains, most Actinobacillus isolates grow on MacConkey agar. The optimum temperature for growth is 37°C and all have complex nutritional requirements. These bacteria are always associated with mucous membranes and the host range of Actinobacillus spp. tends to be limited. Although members of the genus Actinobacillus can be benign commensals of the respiratory, alimentary, and genital tracts, Actinobacillus equuli, A. lignieresii, A. pleuropneumoniae and A. suis can cause important diseases in domestic animals. (Actinobacillus) actinomycetemcomitans is the only member of the genus that routinely colonizes humans; A. hominus and A. ureae are associated with rare opportunistic infections in compromised individuals.

Phylogeny When the family Pasteurellaceae was first described in 1981 by Pohl et al., it was comprised of three genera: Pasteurella, Haemophilus and Actinobacillus. Since that time it has undergone considerable revision and two new genera, Lonepinella and Mannheimia (formerly Pasteurella complex) have been added (Osawa et al., 1996; Angen et al., 1999). Recent 16S rRNA studies of more than 120 strains of the family Pasteurellaceae revealed 20 genus level groups (I. Olsen et al., manuscript in press). On the basis of this analysis, the Actinobacillus group is comprised of A. suis, A. equuli, A. hominis, A. ureae, A. ligneiresii and A. pleuropneumoniae. In addition, several other species/strains including H. parahaemolyticus, Actinobacillus spp. CCUG 19799 and CCUG 28015R, Bisgaard’s taxon 8 CCUG 16494, Bisgaard’s taxon 9 CCUG 24862, Actinobacillus (capsuIatus) MCCM 00149 and H. parainfluenzae belong to the group.Actinobacil-

lus minor, A. porcinus, A. rossii and A. indolicus are closely related to the Actinobacillus group while (A.) capsulatus, A. muris, A. seminis and (A.) actinomycetemcomitans are only distantly related When 16S RNA, sequence data, DNA— DNA hybridizations, and phenotypic analysis are considered together, Christensen and Bisgaard (2004) suggest that Actinobacillus sensu stricto should be limited to A. ligneiresii, A. pleuropneumoniae, A. equuli subsp. equuli, A. equuli subsp. haemolyticus, (taxon 11 of Bisgaard), A. hominis, A. suis, A. ureae, A. arthriditis (taxon 9 of Bisgaard), Actinobacillus genomospecies 1 and 2 and taxa 8 and 26 of Bisgaard. Further studies using additional strains and other molecular techniques are needed to further clarify the phylogenic relationships within the genus Actinobacillus, but division of the family Pasteurellaceae, and even genera within the family, into phylogenically and phenotypically coherent groups may remain problematic.

Taxonomy In the ninth edition of