This form is of great importance in view of the fulness of our knowledge regarding it, of its practical consequence as the causative agent in a widely spread disease, and of the recently devised methods of treatment by blood serum.

Fig. 148. - Blood from case of relapsing fever, showing corpuscles and spirillum Obermeieri. x 750. (Carter).

The bacillus varies somewhat in size. It is usually about the same length as the tubercle bacillus but broader. The most common form is that of a straight or slightly curved rod a little thicker at one end than the other. In some forms there* is clubbing of the ends of the rods; in others the ends are tapered off. (Fig. 149.) The bacillus is stained by watery solutions of the ordinary aniline dyes, but especially by Loeffier's methyl-blue. The irregular disposition of the protoplasm in stained preparations is a striking and constant feature, conferring a beaded or streaked appearance which is very characteristic. The bacillus is not motile. It grows with great rapidity in ordinary culture media, but especially in a special preparation of blood serum, or in glycerine agar. It grows in milk and in sterilized urine. A temperature between 20° and 42° C. is required.

The bacillus is found in the exudation in all cases of true diphtheria, and it can be so readily cultivated and recognized that its presence is used for diagnostic purposes. It may linger even for months about the parts after the attack is over, thus constituting a prolonged danger of infection. It is associated in some cases with the Streptococcus and Staphylococcus pyogenes so as to produce a Mixed infection. Such cases are peculiarly frequent in young children and are very severe, the disease tending to spread downwards to the lungs (Roux and Yersin). In scarlet fever the throat condition, which resembles that in diphtheria, is due to the streptococcus alone. The bacillus is pathogenic in many animals, especially guinea-pigs, sheep, goats, dogs, and many others. Horses are somewhat tolerant of the cultures. Inoculation in the trachea, vagina, etc., of guinea-pigs produces local phenomena similar to those observed in man. There are also the general toxic symptoms, and in prolonged cases the paralyses such as are met with in patients.

The bacillus frequently extends to the lungs, especially in cases where the larynx and trachea are involved. In the lungs it product's an acute inflammation or broncho-pneumonia, and may do so of itself. On the other hand, it may be associated in the lungs with pyogenic cocci in a mixed infection, or the latter may alone be the cause of the pneumonia. The bacillus is englobed by the epithelial cells of the alveoli and not by the leucocytes, although the latter form the chief constituents of the exudate. The bacilli have been found in the spleen, bone marrow, and blood as well as in the lungs, the last mentioned being their place of entrance.

Fig. 149. - Bacilli of diphtheria. Various forms are shown, x about 1000.

Immunity is producible in animals by the graduated administration of increasing doses of the cultures or of the toxine free from the microbes. The cultures themselves may be used in the horse, which is less susceptible than some other animals. The blood serum of immunized animals is effective in protecting susceptible animals by means of the antitoxine which it contains. The serum is tested on susceptible animals, as the guinea-pig and rabbit, so as to determine the degree of its efficiency.

The mouth and pharynx of healthy persons not infrequently contain a bacillus which resembles that of diphtheria but is not virulent.