Staining Reaction.-Stains well by Gram's method and by the ordinary aniline dyes. The capsule may be stained, in fresh specimens from the blood of inoculated animals, or from sputum by means of MacConkey's capsule stain, or by the glacial acetic acid. method. Glacial acetic acid is poured on the film prepared in the ordinary way and immediately poured off, and the slip plunged without washing into aniline-gentian-violet.

Biological Characters.-Aerobic facultative anäerobic, nonliquefying, non-motile pathogenic coccus.

Gelatine Plates, 22° C.-Round, grey, 1-2 mm. in four days, deep colonies round, entire, slightly granular microscopically.

Gelatine Stab, 22° C.-Thread-like (filiform), later beaded, no liquefaction, little surface growth.

Agar Plates, 37.5° C.-Macroscopically like gelatin. Microscopically, deep, round, lenticular, greyish-black, coarsely punctate; surface round, entire, translucent, finely punctate.

Blood Agar Streak, 37.5° C.-Greyish, punctate, edge entire or slightly dentate. The colonies generally remain separate.

Blood Serum, 37.5° C.-Slimy, transparent, no liquefaction. Potato, 37.5° and 22o C.-No growth, except occasionally in old laboratory cultures.

Litmus Milk, 37.5° C.-Sometimes an acid reaction, with or without coagulation, but variable.

Broth, 37.5° C.—Faintly turbid with a slight flocculent precipitate.

Note. The growth on gelatin plates and stab given above is taken from Lehmann and Neumann's " Bacteriology." On the other hand, Dr. Eyre assures me that only the non-virulent, or slightly pathogenic forms with which he has worked grow at all on gelatin, and even then only upon gelatin streaks, never upon plates or stab cultures. Fully virulent forms do not grow at all upon gelatin at 22° C.

Pathogenesis.-A small quantity of an agar culture inoculated into a mouse, rabbit, or guinea-pig produces death in one or two days. Injection of saliva containing the pneumococci, rusty sputum from lobar pneumonia, or a piece of lung tissue from croupous pneumonia produces the same result. At the autopsy signs of general infection are present. At the site of inoculation there is generally a well-marked fibrinous or gelatinous exudate, often half an inch in thickness. The pleural, pericardial, and peritoneal cavities are generally full of fluid. The spleen, liver, lungs and other organs contain the cocci in large numbers. They are also

present in the heart blood and general circulation-in other words, a general septicemia occurs. The lungs show no distinct pneumonic changes, although Marli has claimed to produce pneumonia in animals by the injection of pathogenic cultures of the pneumococcus into the trachea.

The cocci exhibit the typical capsulated form when stained from the heart blood of an animal dead of pneumococcic infection.

The amount of immunity produced by infection with the pneumococcus is very brief, and although many observers have attempted the isolation of a definite toxine from the cultures of the organism, no success has so far rewarded their efforts; it is probable, however, that such a toxine does exist in rather minute quantities.

G. and F. Klemperer showed that the serum of immunised animals protected animals against infection with the pneumococcus.

Washbourn has also prepared an anti-pneumococcic serum which will protect against one hundred times the fatal dose of pneumococci.

The pneumococcus is capable of attaining an enormously high degree of virulence, and in the experiments conducted by Washbourn and Eyre this "standard virulence" was 0.000001 of a loop holding 0.5 milligram of culture, this amount invariably producing death in rabbits, and an anti-pneumococcic serum was obtained of such strength that it protected against 1,000 times this fatal dose. Considerable benefit has resulted from the use of this serum in certain cases of pneumonia. Washbourn and Eyre are continuing their experiments with the pneumococcus in several directions, and have recently published further notes upon the Pathology of Pneumococcic Infection.

(5) MICROCOCCUS TETRAGENOUS.

This coccus was originally observed and studied by Koch and Gaffky, who found it in the lung cavities of tubercular persons.

It is often present in normal saliva, and is not infrequently present in dento-alveolar abscesses. Biondi found it three times in fifty cases examined, Miller also has met with it frequently, as have Park, Vangel and Steinhaus.

Morphology.-Cocci spherical and arranged in groups of four, due to division in two planes at right angles to each other. The individual cocci are about 1 μ in diameter; the whole group is generally surrounded with a gelatinous a gelatinous capsule. The typical

arrangement and the capsule are only constantly present in the tissues of inoculated animals.

Staining Reactions.-Stains by the ordinary aniline dyes and by Gram's method.

Biological Characters. Aerobic facultative, anäerobic, nonmotile coccus, forming tetrads. Does not form spores; no liquefaction of gelatin occurs.

Gelatin Stab, 22° C.-A well marked convex yellowish viscous mass appears on the surface in two to four days; growth occurs along the needle track as white globular colonies which may become confluent, but generally remain distinct. No liquefaction takes. place.

Gelatin Streak, 22° C.-Thick grey-white layer, no liquefaction. Gelatin Plates, 22° C.-Lenticular, irregular and finely granular and irregular under microscope.

Gelatin Shake, 22° C.-Minute colonies, discrete and globular, lenticular.

Agar Streak, 37.5° C.- Well-marked viscous white layer; colonies may remain distinct.

Blood Serum, 37.5° C.-Similar to agar, no liquefaction.

Potato, 22° C.-Well-marked white layer in forty-eight hours, viscous.

Litmus Milk, 37.5° C.-No coagulation of casein. Slight acid reaction.

Broth, 37.5° C.- Thick stringy viscous deposit and general turbidity.

Pathogenesis. White mice particularly susceptible, a minute quantity of pure culture causing fatal septicæmia; the organism may be recovered from the heart blood, spleen, liver and other organs. The tetra-cocci are well marked in the tissues. Guinea-pigs are more resistant and generally only develop a local abscess. House mice, field mice and dogs are immune.

The organism probably hastens the tissue necrosis in pulmonary tuberculosis.

(6) BACILLUS DIPHTHERIA.

The Klebs-Læffler bacillus occurring typically in the membranous exudation of faucial diphtheria may also be found in the anterior part of the buccal cavity, and frequently in individuals who exhibit no clinical or pathological signs of the disease the organisms have

been observed in a fully virulent condition. Thus Aaser1 found the diphtheria bacillus present in 17 out of 895 soldiers in a cavalry regiment. Park and Beebe2 found that of 330 persons examined at random, 8 had fully virulent bacilli and 24 characteristic but nonvirulent bacilli in their throats. Meade Bolton, among 214 persons more or less exposed to the disease, found virulent bacilli present in 41.5 per cent., and the literature teems with similar cases.

3

It by no means follows that all the persons in whose throats the diphtheria bacilli are found are suffering at that moment from clinical diphtheria. In a large school of 800 children during an epidemic of sore throat and clinical diphtheria I found 33 per cent. of the whole school had characteristic bacilli present in their throats, while only 14 out of the total number of children examined showed clinical symptoms of the disease. In three of the cases in which no clinical symptoms had at any time manifested themselves, the organisms were in a fully virulent condition, causing the death of injected guinea-pigs in forty-eight hours, with all the characters of infection with the diphtheria bacilli. The importance of those œco-parasites lies in the ease with which they may be transferred from one mouth to another until a susceptible individual becomes the recipient, when grave, often fatal, disease may result; it is, moreover, these "bacillusträgende " persons who may come under the care of the dental surgeon and form an unrecognised centre of infection.

Occurrence.-The Klebs-Loeffler bacilli are found most frequently upon the throats of persons suffering from faucial diphtheria, but are also found occasionally in open wounds, causing wound diphtheria, and upon the conjunctiva.

The bacilli have also been found in milk, which is an excellent medium for their development, several epidemics having been traced to contamination of the milk supply from infected persons.

The organisms rapidly die when introduced into water, and they have never been found in samples of water submitted to examination, nor have they been found in sewage, or in drain and sewer air, or in the emanations of decomposing animal or vegetable matter.

The bacilli will withstand drying for several weeks, and may undoubtedly remain in the dust of rooms in a virulent condition.

'Deutsch. Med. Woch., 1895, p. 357. 2 New York Med. Record, xlvi., 1894. Med. and Surg. Reporter, lxxiv., p. 799. + Trans. Epidem., 1900, p. 99.

They are easily destroyed by the action of germicides, and by a temperature of 58° C. for ten minutes.

When grown in a current of air, Fernbach found that the growth was more luxuriant, but the life cycle shortened. The organism will also grow when air is entirely excluded, and is therefore äerobic and facultative anäerobic. The bacilli may be cultivated upon the ordinary laboratory media, but are morphologically most typical upon coagulated blood serum, the medium largely used for diagnosis.

Loeffler's blood serum gives even better results; this medium consists of blood serum (liquid) 3 parts, glucose (1 per cent.) broth 1 part.

Coagulation and sterilization are carried out as for ordinary blood serum.

Diagnosis. In the routine examination of suspected throats for diphtheria bacilli, now largely carried out at the public expense in most British towns, a "sterilized swab" (consisting of a wire, the end of which is wrapped round with cotton wool, and kept till use in a sterilized and cotton wool plugged tube) is introduced into the throat and the surface touched with the sterile wool of the swab. Blood serum tubes are then inoculated, incubated and the culture examined in eighteen to twenty-four hours, when the typical bacilli are sought for. If diphtheria bacilli be present, the colonies, at the end of twenty-four hours will be larger and more defined than those of other bacteria present.

Ohlmacher recommends the examination of the culture in five hours, even though no growth be visible, but a negative result by this means would hardly be of sufficient value to obviate a further examination at twenty-four hours; still a positive result obtained at this time (five hours) is certainly of value. Equally good results may be obtained by examining coverslip preparations of the throat membrane direct and by this method the cultural diagnosis may be forestalled in about 30 per cent. of cases, the stain used being Neisser's two solutions.

Varieties. Two distinct varieties of the diphtheria bacillus are known, one the short variety, usually considered the least pathogenic, and the "long variety" or most virulent; the ends of the long variety are more frequently swollen and clubbed than the short variety. Both varieties form very short rods upon agar, whilst upon blood serum the "long" variety grows out into rods of 5 to 8 μ in length, or even longer.