Characteristics of the organism and its antigens

F. tularensis is a small coccoid or ellipsoid nonmotile gram-negative bacterium. It has a capsule which can be dissociated from the cell wall by incubation of the bacteria in hypertonic NaCl.

It grows poorly on most ordinary culture media. Growth is, however, well supported on blood agar enriched with L-cysteine and glucose. Chocolate agar, such as used for the isolation of gonococci, also maintains the growth of F. tularensis. F. tularensis is strictly aerobic, weakly catalase-positive, and oxi-dase-negative. Carbohydrates are weakly catabolized with the production of acid but no gas. The organism has a characteristic pattern of cellular fatty acids, including long-chain (C20-C26) acids and the hydroxy acids 2-hydroxydecanoate, 3-hydroxyhexa-decanoate and 3-hydroxyoctadecanoate.

F. tularensis seems to be unique among clinically important gram-negative bacteria in so far as more than 50% (wt/vol) of its capsule and cell wall are lipids. Accordingly, ether-water extraction is an efficient way to release antigen. The reactivity of immune serum with such an extract is due mainly to carbohydate determinants, whereas determinants responsible for immunospecific lymphocyte stimu lation reside in proteins of the extract. Antibodies to membrane proteins of F. tularensis can be demonstrated in immune serum by the immunoblotting technique.

F. tularensis is strongly agglutinated by commerci-ably available antisera. By immunodiffusion of sonicated F. tularensis against hyperimmune serum, several antigenic determinants are disclosed. Minor cross-reactions are found with Brucella, Yersinia and Proteus. The only strongly cross-reacting species seems to be Francisella philomiragia.

Proteins of F. tularensis able to induce cellmediated immunity reside mainly in the bacterial membranes. According to in vitro studies, several membrane polypeptides of the vaccine strain F. tularensis LVS induce a T cell response in LVS-vaccinated individuals as well as in individuals previously-undergoing tularemia. Experiments with T cell clones suggest that there is a heterogeneity of T cell determinants among these polypeptides.

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