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The micro-organism, which is the essential cause of acute lobar pneumonia, was discovered by Sternberg, and fully studied by Fraenkel. It is variously known as the diplococcus pneumonie, pneumococcus, and streptococcus lanceolatus (Frontispiece, Fig. I.). This organism is highly pleomorphic, and its morphological characters vary according to the source from which it is derived. Thus, in the blood and exudates from infected animals the individual cocci are lanceolate or like a. grain of wheat, possess a well-defined capsule, and are usually arranged in pairs (diplococci). But in cultivations they occur as oval or spherical bodies, without a distinct capsule, and frequently form a short chain.
Growth takes place most favourably at 37° C. and in the presence of oxygen, although the organism can grow in the absence of this gas. But usually the growth is very feeble on most of the ordinary media, with the exception of blood serum. Blood - - agar
is a favourable medium-the hæmoglobin of the blood is converted into a chocolate-coloured pigment, which diffuses into the agar. It is in this way that the rusty colour of the sputum is produced. The micro-organism also grows in broth and curdles milk.
On artificial media the organisin dies quickly, but in dried sputum it may remain alive for long periods. It
is rapidly destroyed by exposure to 52° C., or by a one per cent. solution of carbolic acid.
Mice and rabbits are very susceptible, while pigeons and fowls are immune. No soluble toxin is formed by the micrococcus ; and death takes place as the result of septicæmia.
The diplococcus of pneumonia is frequently met with in the saliva of healthy individuals, and it has also been detected in meningitis, ulcerative endocarditis, and in acute abscesses.
ANTHRAX was one of the first specific diseases which was proved to be associated with a definite micro-organism, the bacillus anthracis. The bacilli were discovered by
Davaine in 1850, but their etiological rôle was only established by the subsequent researches of Pasteur and Koch.
The bacilli (Frontispiece, Fig. II.) occur as long rods, measuring from 5 u to 20 u in length ; in fact, they are the
largest of all known pathogenic bacteria. They usually grow in the form of long threads, the individual bacilli appearing somewhat enlarged at the ends, so that the appearance of a bamboo cane is produced. They are immobile and ærobic, although they may grow anærobically
Spore formation is an important feature of this organism. Under suitable conditions spores are readily developed in the long filaments, and appear like “peas in their pods”. Spore formation never takes place in the body, owing to the lack of oxygen and of a suitable temperature.
On account of their extreme resistance, anthrax spores are frequently used as test objects for determining the value of various germicides.
If kept dry and away from light the spores retain their vitality for many years, but they are rapidly killed by exposure to a temperature of 100° C., or to 0:1 per cent. .solution of formalin.
In agar plate cultures the bacilli grow into long threads, Fig. 9.–Bacillus Anthracis.
Gelatine stab culture. [From Curtis's Essentials of
which give rise to a convoluted tangled mass of growth. The growth in gelatine stab cultures is characteristic: fine filaments grow out laterally from the track of the needle, while liquefaction of the gelatine takes place slowly from the surface. Upon potato a greyish white layer is produced, with abundant spore formation.
As regards pathogenesis, cattle and most sheep are highly susceptible, but frogs, dogs, and the Algerian sheep are immune. Man occupies an intermediate position between these extremes. In susceptible animals the disease takes a rapid course, with usually fatal results. The bacilli quickly multiply, fill up the lumen of the capillaries, and produce atypical septicemia. The sanguineous discharges of the infected animal are consequently rich in these bacilli, which finally sporulate on the surface of the ground. The spores are blown about and infect pastures, so that animals grazing on these soils are liable to infection. The spores, unlike the corresponding bacilli, are uninjured by the gastric juice and grow out into rodlets, which enter into the circulation and proceed to multiply.
Infection may also result from breathing air containing anthrax spores, as in the wool-sorter's disease of man. The malignant pustule is the result of infection through abrasions in the skin, and is consequently most common in hide workers.
The B. anthracis can be attenuated by various means, as, for instance, by cultivation at 42° C.—a temperature which is higher than its optimum. Pasteur's protective vaccine was first prepared in this manner. There can be no doubt that protective inoculation against anthrax has proved immensely successful in conferring immunity on sheep and cattle.