Actinobacillus equuli
Cultural characteristics
Biochemical characters
Phylum Proteobacteria, Class Gammaproteobacteria, Order Pasteurellales, Family Pasteurellaceae, Genus Actinobacillus,
Actinobacillus equuli subsp. equuli  (van Straaten 1918) Haupt 1934,
Actinobacillus equuli subsp. haemolyticus  Christensen et al. 2002 (3 biovars).

Historical synonyms:
Bacillus equuli van Straaten 1918, Bacillus nephritidis equi Meyer 1910, Bacterium viscosum equi Magnusson
Bacillus pyosepticus equi de Blieck and van Heelsbergen 1919, Bacillus equirulis de Blieck and van Heelsbergen 1919,
Bacterium pyosepticum viscosum equi Lütje 1921, Bacterium pyosepticum Miessner and Berge 1922, Bacterium equi Weldin and
Levine 1923,
Bacillus pyosepticus Clarenburg 1925, Eberthella viscosa Snyder 1925, Shigella equi (Weldin and Levine) Weldin 1927,
Shigella viscosa (Snyder) Bergey 1930, Shigella equirulis (de Blieck and van Heelsbergen) Edwards 1931, Shigella equuli (van
Straaten) Dimock, Edwards and Bruner 1947, Bisgaard taxon 11, Bisgaaerd et al., 1984.
Rod-shaped cells, but vary markedly depending upon the growth medium. In media
with fermentable carbohydrates, long bacillary or filamentous forms, similar to those
A. lignieresii, can be seen. Capsule is not produced, nonmotile, Gram-negative.
Viscous colonies, and the stickness is not lost upon repeated subculturing. When
first isolated from clinical material, colonies are usually rough, but they may become
smooth on repeated subculture. The viscous character is also present in liquid
culture, and it is not lost on repeated subculture.
Nonhemolytic and CAMP-negative (
A. equuli subsp. equuli) or weak beta-hemolytic
and CAMP-positive (
A. equuli subsp.haemolyticus)
Aerobic, facultatively anaerobic. Can grow on MacConkey agar. V-factor independent.
Growth occurs between 20 and 39 ºC. Some strains will grow at 44 ºC.
Occurs as a commensal in the equine intestinal tract and mouth.
Pathogenic for horses and pigs.
Causes “sleepy foal disease” or “joint ill” (septicemia in association with nephritis,
arthritis, pneumonia, pleuritis or enteritis in young foal). Adult horses may also be
infected (with arthritis , pneumonia, endocarditis, pericarditis, nephritis, meningitis,
and metritis) and abortions may occur in pregnant animals.  
A. equuli may also infect
pigs . In piglets that are a few days old,
A. equuli may cause septicemia, while older
pigs may suffer from arthritis , pneumonia, endocarditis,  nephritis, and osteomyelitis.
May also cause systemic infections in monkeys, calves, dogs, and rabbits.
A. equuli-like bacterium has been isolated from an infected horse bite wound in a
human being.
Not pathogenic for rabbits, guinea pigs and rats.
A. equuli subsp. equuli was isolated from horses and pigs. A. equuli subsp.
strains have so far been isolated from horses only.
  1. Olsen I., Moller K., 2004. Genus II. Actinobacillus. In: Bergey’s Manual of Systematic Bacteriology, Second edition,Vol two, part B,
    George M. Garrity (Editor-in-Chief), pp. 866-883.
  2. W.M. Janda, R. Mutters, 2006. Pasteurella, Mannheimia, Actinobacillus, Eikenella, Kingella, Capnocytophaga, and other
    miscellaneous Gram-negative rods. In: Topley & Wilson’s Microbiology and Microbial Infections, 10 edition, Vol. 2, Bacteriology,
    Edward Arnold Ltd.
  3. J. G. Holt et al., 1994. Facultatively Anaerobic Gram-Negative Rods. Subgroup 3. Family Pasteurellaceae. In: Begey’s Manual of
    Determinative Bacteriology, 9th-edition, Williams & Wilkins, pp 194-196.
  4. Carmen Pinzaru, 2009. Identificarea genurilor Pasteurella, Mannheimia, Actinobacillus.In:D.Buiuc, M.Negut (Ed.),Tratat de
    Microbiologie Clinica, Ed. Medicala, Bucuresti, 859-868.
  5. Buchanan R.E., Gibbons N.E. (co-editors), 1975. Bergey’s  Manual of Determinative Bacteriology, eight edition, The Williams &
    Wilkins Company, Baltimore.
  6. H. Christensen, M. Bisgaard, J. E. Olsen, 2002. Reclassification of equine isolates previously reported as Actinobacillus equuli,
    variants of A. equuli, Actinobacillus suis or Bisgaard taxon 11and proposal of A. equuli subsp. equuli subsp. nov. and  A. equuli
    subsp.haemolyticus subsp. nov.IJSEM 52, 1569-1576.
Positive results for nitrates  reduction, oxidase, alkaline phosphatase, urease, acid production from: D(+) xylose, mannitol, maltose, D
(-) fructose, D(+) galactose, D(+) mannose, melibiose, trehalose, raffinose, sucrose (saccharose), lactose (delayed 5-7 days), dextrin
& D(+) glucose (without gas).

Negative results for gelatinase, arginine dehydrolase, lysine decarboxylase, ornithine decarboxylase, Tween 20, Tween 80, alpha-
fucosidase, alpha-galactosidase, alpha-glucuronidase, alpha-mannosidase indole production, acid production from: xylitol, dulcitol,
meso-inositol, L(+) rhamnose, inulin, esculin & salicin.

Variable results for catalase, ONPG, Voges-Proskauer, H
2S production, acid production from: glycerol, L(+) arabinose, D(-) sorbitol &

Cellobiose negative (
A. equuli subsp. equuli, A. equuli subsp. haemolyticus biovar 3) or  positive (A. equuli subsp.haemolyticus
biovar 1 & 2
Alpha-glucosidase (PNPG) positive (
A. equuli subsp. equuli, A. equuli subsp.haemolyticus biovar 1 & 3) or negative (A. equuli subsp.
haemolyticus biovar 2
(c) Costin Stoica
Culture media
Biochemical tests
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