Ac.ti.no.ba.cil' lus. Gr. n. actis a ray; L. dim. masc. n. bacillus a small staff or rod; M.L. masc. n. Actinobacillus ray bacillus or rod.

Proteobacteria / Gammaproteobacteria / Pasteurellales / Pasteurellaceae / Actinobacillus

Cells, measuring 0.4 ± 0.1 × 1.0 ± 0.4 µm, are spherical, oval, or rod-shaped (Phillips, 1984). Most often bacillary but sometimes interspersed with coccal elements that may lie at the pole of a larger form, producing the characteristic “Morse-code” form. Cell forms up to 6 µm in length may appear when grown on media containing glucose or maltose. Cells are single or arranged in pairs or, more rarely, in chains. Endospores are not formed. Gram negative, but staining is irregular. Not acid fast. India ink may demonstrate small amounts of extracellular slime in wet preparations. Nonmotile. Organisms are aerobic, microaerobic, facultatively anaerobic, or chemoorganotrophic, having both respiratory and fermentative types of metabolism. After growth for 24 h on blood agar, translucent colonies, usually 1–2 mm in diameter appear. Surface colonies have low viability and may die in 2–7 days. Growth may be very sticky upon primary cultivation, making it difficult to remove colonies completely from the agar surface. Optimum growth temperature is 37°C. Temperature range for growth is 25–42°C.

The mol% G + C of the DNA is: 35.5–46.9.

Type species: Actinobacillus lignieresii Brumpt 1910, 849.