The morphological, cultural and biochemical characteristics
of 138 strains of Actinpbacillus equuli from various sources have been
investigated and evaluated in order to provide useful criteria for the
identification of this taxonomically disputed species of microorganism.The antigenic structure of all these strains was investigated by
agglutination and agglutinin-absorption tests and 98.6 per cent, of them
can be arranged into 28 groups on the basis of their heat-stable
antigens (O antigens). The heat-labile antigens associated with
extracellular slime have been demonstrated and found to be relatively
common to organisms of different O groups. Although agglutination
tests offer a means of classifying this serologically heterogeneous
group of organisms, the autoagglutinability of rough strains and
inagglutinability due to its viscous nature have often been found to
interfere with such tests. Thus the feasibility of applying other
serological procedures to overcome the difficulties has been sought and
a passive haemagglutination test and an immunodiffusion precipitin
test for serogroupiny of A. equuli were developed. The conditions
for the passive haemagglutination test have been investigated and are
discussed in some detail with special reference to the soluble
antigens of A. equuli. Heat extracts of the organisms were used in
the immunodiffusion precipitin test to group A. equuli strains
into 28 O-groups. The immunodiffusion precipitin test appeared to
be the test of choice for the serogrouping of isolates of A. equuli
because of its specificity and its simplicity and rapidity in
performance. Moreover, the immunodiffusion precipitin test has a
great advantage over the other serological methods in showing
a clear proof of an antigenic relationship between 0 groups.The work has shown that serological classification of the organism
is fairly well correlated with certain biochemical characters of the
organism and the majority of the strains can be arranged into two
main divisions. The term biotype is applied for the differentiation
of these divisions of A« equuli. Of the 138 strains studied, 84 were
of Type I, 42 were of Type II and the remaining 12 were intermediate
in type. Type I strains gave a positive reaction with mannitol,
whilst their reaction with salicin and cellobiose were negative.
All the strains comprising Type I were aesculin-negative and, with the
exception of one, non-haemolytic. These strains fell into O groups
1-22 inclusive. On the other hand, Type II strains were mannitol
negative, whilst the majority of the strains fermented salicin and
cellobiose and hydrolysed aesculin, and all haemolysed sheep
erythrocytes. The Type II strains were made up of 0 groups
24 - 28 inclusive.An antigenic relationship between A. equuli, A. suis and
A. lignieresi has been demonstrated. Comparative biochemical and
serological studies of these 3 species of Actinobacillus have shown
that A. suis is closely related to Type II of A. equuli and that
A. suis differs sufficiently from both A. lignieresi and Type I
of A. equuli to warrant its separation as a different species within
the genus.Further work is required to characterise haemolytic actinobacilli
from horses and pigs, but the results obtained in the work appear to
suggest the not infrequent occurrence in the upper respiratory tract
of normal horses of organisms resembling A. suis and the association
of such organisms with disease in this host species on occasions
