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THE bacteriology of this disease has only been worked out within a comparatively recent period. For although the infectious nature of cholera had been recognised for a long period, nothing was known of the true nature of the
Fig. 10.–Vibrio Choleræ. From a film prepared from cholera
“rice-water stool”. [From Curtis's Essentials of Practical Bacteriology.]
disease. It was not till 1883 that Koch, by his brilliant researches in Egypt and India, discovered a peculiar organism, the "comma bacillus,” which is now generally admitted as the causa causans of cholera. This organism was constantly met with in all cases of true cholera, and
in no other disease. Where cholera causes most marked changes, i.e., in the lower half of the small intestine, the bacilli were most numerousabove it they diminished more and more. This constant occurrence of the comma bacilli and their limitation to the choleraic process cannot be regarded as an accidental coincidence. On the contrary, the organism and the cholera process must be related to each other as cause and effect.
The comma bacillus, or rather cholera vibrio or spirillum (Frontispiece, Fig. III.), occurs as a curved rod, singly or in pairs, the latter giving rise to half-circles or S-shaped curves. It is actively motile, and usually possesses a single flagellum. The organism can be readily stained by various aniline dyes, but is decolorised by Gram's method. It does not form spores, although Hueppe claims to have seen small brilliant bodies, which he called “arthospores”.
Stab cultures in gelatine are characteristic: there is a whitish growth along the needle track with gradual liquefaction, which at first is more marked near the surface, so that a funnel-shaped depression is formed from the resulting evaporation. Liquefaction is comparatively slow,
cholerze. site this
but after some days it has progressed so far as to destroy the appearance just described.
On agar-agar there is a superficial slimy growth offering no special Non
liquefied features. Gelatine plates are very marginal characteristic, and by some are con- portion sidered to have a distinct diagnostic Fig. 12.—Islet of
Vibrio Choleræ on a value. The young colonies show un
gelatine plate, in proeven margins, and the surface looks cess of liquefaction.. as if it were covered with little frag
Bacteriology.] ments of broken glass.
While the nutrient media on which the cholera spirilla are expected to prosper must be of a marked alkaline reaction, these organisms possess a notable capacity of accommodating themselves to acid media, provided the acid is of a vegetable origin. The surface of boiled potato has often a feeble acid reaction, and yet the vibrio develops on it at the body temperature (37° C.).
Broth and milk are excellent media for the growth of this organism. It also grows readily in peptone water, forming indol and nitrites, which, on the addition of concentrated sulphuric acid, gives rise to a characteristic red colour (cholera-red or indol reaction).
Although a resting stage is absent, yet these organisms are more resistant than is generally supposed. In fact, they are so little fastidious in their requirements that, between 17° C. and 40° C., they will grow on almost anything. They, however, thrive best from 35° C. to 37° C., and especially if the medium is faintly alkaline.
In sterilised distilled water the vibrios quickly die out, but the addition of common salt greatly prolongs their existence. When added to sewage they are soon overpowered by the vulgar saprophytic bacteria, but may live from two to four weeks. In ordinary moist soils they
remain alive from one to two months; but in dry soil and peat they die out in a few days.
The vibrios thrive exceedingly well on carbohydrates (e.g., rice), and on such fruits as melons and cucumbers. On the whole, however, the acidity of fruits favours the death of the organism.
They are rapidly killed by desiccation, high temperature (55° C. for ten minutes), sunshine, and the ordinary antiseptics.
Is the Cholera Vibrio the Specific Germ of Cholera ?(1) It has already been remarked (see p. 60) that an important evidence in favour of the specificity of an organism consists in the experimental production of disease by the use of pure cultures. Now, the lower animals appear to be immune to cholera, and it is, therefore, difficult to reproduce this disease by artificial means. In Bengal, where cholera is endemic, and where domestic animals live in close association with the people, the animals remain remarkably free from cholera.
Koch thought that the acidity of the gastric juice and the intestinal peristalsis were the two conditions which prevented the organism from gaining a foothold in the intestine. To counteract these factors he neutralised the former with sodium carbonate, and paralysed the latter by an injection of opium. On subsequent inoculation with a cholera culture, he was able to produce in guineapigs a condition closely akin to cholera. The post-mortem appearances more or less resembled those of cholera, but it was noticed that the animals died without having vomited or passed watery evacuations. Guinea-pigs, however, do not vomit; and the absence of diarrhoea is probably due to the extraordinary size of their cæcum, which is capable of retaining considerable quantities of the intestinal contents.
Metschnikoff, struck by the fact that animals very sensitive to subcutaneous or intraperitoneal injections enjoyed an immunity against vibrios introduced by the mouth, thought that this protection was due to the influence of the intestinal flora. Having ascertained the fact that the intestinal canal of newly born rabbits is almost sterile, he fed the young sucklings with cholera cultures, and was thus able to reproduce the classical symptoms of cholera. When the suckling stage is passed, the microbic contents increase with the change of food, and the results are far less satisfactory.
But although the question of experimental inoculation is beset with many difficulties, there are on record many instances of accidental infection in man. In some of these cases a severe attack of cholera followed, and in others death resulted from the infection. On the other hand, several experimenters have swallowed cultures without any injurious effects. But this is in accordance with the fact that not every one exposed to cholera infection is attacked by the disease.
(2) A criticism of a different kind comes from Cunningham of Calcutta, who denies the vibrionic unity of cholera, and contends that the organisms isolated from fresh evacuations are not of one, but of several species, distinct morphologically and biologically.
But these observations do not prove anything more than that cholera organisms are susceptible of great variations. Indeed, the variability of the cholera germ is observed not only in different cases of the disease, but also in the same case. Thus, when we examine the intestinal contents of a deceased cholera patient, we find vibrios “in which the external forms, instead of the characteristic comma or. spirillum, will vary between a coccus and a single thread; the number and disposition of flagella, the secretion of