Page 866 - Veterinary Immunology, 10th Edition

VetBooks.ir Serology of Bacterial Infections

Bacterial infections may be diagnosed by detecting specific
antibodies in serum. Thus the agglutination test is widely employed

to detect bacterial infections, particularly those involving Gram-
negative bacteria such as Brucella and Salmonella. The usual
procedure in bacterial agglutination tests is to titrate serum
(antibody) against a standard suspension of antigen. Bacteria are
not, of course, antigenically homogeneous but rather are covered by

a mosaic of many different antigens. Thus motile bacteria will have
flagellar (H) antigens, and agglutination by antiflagellar antibodies
will produce fluffy cotton-like floccules as the flagella stick

together, leaving the bacterial bodies only loosely agglutinated.
Agglutination of the somatic (O) antigens results in tight clumping
of the bacterial bodies so that the agglutination is finely granular in
character. Many bacteria possess several O and H antigens, as well
as capsular (K) and pilus (F) antigens. By using a set of specific

antisera, it is possible to characterize the antigenic structure of an
organism and consequently to classify it. It is on this basis, for
instance, that the 2400 or so different serovars of S. enterica are

classified.
Flagella (H) antigens are destroyed by heating, whereas O
antigens are heat resistant and therefore remain intact on heat-
killed bacteria. K antigens vary in their heat stability: the L antigen
of E. coli, which is a capsular antigen, is heat labile, whereas another

K antigen, antigen A, is heat stable. S. enterica Typhi possesses an
antigen called Vi that, although heat stable, is removed from the
bacterial cells by heating. The presence of K or Vi antigens on an

organism may render them O-inagglutinable and thus complicate
agglutination tests. It should also be pointed out that rough forms
of bacteria do not form stable suspensions and therefore cannot be
typed by means of agglutination tests.
Bacterial agglutination tests may be performed by mixing drops

of reagents on glass slides or by titrating the reagents in tubes or
wells in plastic plates. Tube agglutination tests are commonly used
for such diseases as salmonellosis, brucellosis, tularemia, and

campylobacteriosis. Slide agglutination tests are commonly used as

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