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colour, and on this account the white and black varieties of the disease are recognised.
When examined under the microscope the little bodies are seen to be fungoid in nature. It appears that the two varieties are not caused by the same organism, but by distinct species of streptothrix, both of which, however, are closely related to the actinomyces.
As in actinomycosis, the disease is communicated through the skin when the latter is injured by thorns, prickly plants, etc., infected with the fungus.
IF a serum tube be inoculated with a swabbing from the throat of a patient suffering from diphtheria, and then incubated at 37° C., a growth takes place in the course of a few hours. On staining with methylene blue the specific organisms, or Klebs-Löffler bacilli, are readily
FIG. 18.—Bacillus Diphtheriæ. A, long variety; B, short form.
[From Curtis's Essentials of Practical Bacteriology.] seen under the microscope.
is diagnostic of the disease, but their absence does not prove the contrary.
The bacilli occur as straight or slightly curved nonmotile rods, which are frequently “clubbed at one or
both extremities, and show
“ beaded appearance on staining. In film preparations the organisms are usually arranged in characteristic groups (Chinese letter formation). The individual bacilli may be long or short; and various involution forms may not unfrequently be seen in the same culture. These irregular forms are very commonly observed, and may
be said to be characteristic of this organism.
The diphtheria bacillus is ærobic and does not form spores. It grows best on blood serum, although it can also grow on gelatine, agar, broth, and other media. On blood serum
small circular white colonies develop within a few hours. Similar colonies also develop on agar-agar, but the growth takes place rather less rapidly. In broth it produces a turbidity which, however, soon settles to the bottom. Milk is a favourable medium and may serve as a carrier of infection.
The organism is rapidly killed under the influence of light, heat, and various germi
Fig. 19. — Bacillus Diph
theriæ. Pure culture on
serum, about 36 hours old. [From Curtis's Essentials of
cides, but in a dried diphtheritic membrane it may retain its vitality for weeks or months.
Diphtheria is essentially a local disease, the specific bacilli being present only at the seat of inoculation, and very rarely in other tissues. There they manufacture the toxins which, being absorbed into the blood, produce the clinical symptoms of the disease. The paralytic symptoms which occur in the course of the disease have been experimentally produced in animals by the use of separated toxins. But it appears that the bacilli, even in pure cultures, cannot induce the formation of a false membrane unless the mucous membrane is directly injured. It is, probably, in this manner that the pyogenic cocci, so frequently met with in the false membrane, injure the tissues and pave the way for diphtheritic infection. The cocci grow side by side with the specific organisms, and may thus increase the virulence of the latter, and bring about some of the complications of this disease.
The antitoxin or serum treatment has proved eminently successful in the cure of diphtheria. But the treatment, to be successful, must be commenced at the earliest opportunity in order to prevent the combination of the toxin with the tissue elements, for which it has a selective affinity. As the interval between infection and the introduction of antitoxin increases, so the value of the latter rapidly diminishes, till finally it can no longer prevent death. The antitoxin, however, only neutralises the toxin of diphtheria, and has no action against the septic symptoms produced by the associated streptococci. For this reason it has been recommended to administer antistreptococcus serum along with the diphtheria antitoxin.
Organisms are occasionally met with in healthy throats which are indistinguishable from the true diphtheria
bacilli in their morphological and cultural characters, but differ from them in the fact that they are virulent. It has been suggested that these pseudodiphtheria bacilli form a distinct species, but the balance of opinion inclines to the view that they are merely the attenuated forms of the Klebs-Löffler bacillus.