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Gelatin Plates, 22° C.-In three days minute colonies appear, not much larger than a pin's point. The colonies spread a little, and feathery processes extend into the medium. The deep colonies are often surrounded with a series of fine rays. No liquefaction of the gelatin occurs.

Gelatin Streak, 22° C.-In three days a slight beaded growth occurs, which later sends processes into the medium. The gelatin is not liquefied, but may become a little softened around the colonies.

Gelatin Stab, 22° C.-Slight granular growth along the line of puncture, radiating processes may be formed. No liquefaction occurs.

Gelatin Shake, 22° C.--No gas bubbles are formed.

Potato, 37.5° C.-In forty-eight hours slight shining appearance upon the inoculated surface. The organisms show considerable involution. .

Agar Plates, 37.5° C.-Minute grey colonies in forty-eight hours, round and regular or erose edge, and central nucleus brownish and raised. Microscopically, brownish-yellow with central nucleus faintly granular and regular edge. Under anäerobic conditions the colonies are larger.

Broth, 37.5° C.-In twenty-four hours slight general turbidity with a flocculent precipitate, which increases whilst the turbidity does not. No pellicle is formed. No indol produced in ten days; no H,S formed.

Litmus Milk, 37.5° C.-In twenty-four hours no change; in forty-eight hours solid clot, lower portion decolourised, the top showing a marked acid reaction. The clot is not re-dissolved.

Glucose broth, lactose broth, starch broth, maltose broth: strong acid reaction in forty-eight hours.

Anäerobiosis.-Well marked growth on glucose formate agar in Buchner tubes. The colonies are much larger than on äerobic media, and are brownish in colour and have a well marked nipplelike central projection (umbonate). No gas is produced on glucose formate broth, but the turbidity is well marked.

Pathogenesis.-Not determined.

Spore Formation.No spores stainable. Cultures three weeks old, heated to 70° C. for half an hour, gave no subsequent growth.

Optimum temperature, 37.5° C. Thermal death point 60° C.

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FIG. 54.–BACILLUS PLEXIFORMIS. Twenty-four hours' cultivation of decalcified dentine. X 1,000.

(27) BACILLUS PLEXIFORMIS (GOADBY). Found occasionally in carious dentine.

Morphology.-Curved and twisted bacilli on most media ; may be associated in pairs or grow out into long threads 30 u or more long. In gelatine cultures the bacilli are short and tend to stain irregularly (see fig. 53), while on slices of decalcified dentine long threads are formed (see fig. 54). Motility well marked but flagella not stained.

Staining Reactions.Does not stain by Gram's method, stains by ordinary aniline dyes. No spores observed.

Biological Characters. Gelatin Plates, 22° C.-Minute white to grey colonies (punctate or effused). Gradual liquefaction occurs.

Gelatin Stab, 22° C.-Filiform growth to bottom of stab, liquefaction only in upper part. Stratiform, well marked flocculent deposit and general turbidity of fluid.

Gelatin Streak, 22° C.-Well marked liquefied groove in two to three days.

Agar Plates, 37.5° C., 2 to 3 mm.- Translucent colonies raised and round (pulvinate), edge entire, of rather viscous consistency.

Agar Streak, 37-5° C.-- Raised, moist, gummy, confined to needle track.

Blood' Serum, 37.5° C.—Dirty brown streak, eventually slight liquefaction.

Potato, 22° C.—Brownish slimy growth, slow in appearance, confined to streak. Not spreading.

Litmus Milk, 37-5° C.- Slightly alkaline reaction, no clot.

Broth, 37.5 C°.- General turbidity with somewhat flocculent deposit, no pellicle. Indol reaction well marked.

Glucose formate media.—No anäerobic growth and no gas bubbles formed.

CHAPTER IX.

Bacteria in Tooth Pulps.

The channel of infection of the tooth pulp is along the dentinal canals, and may occur with but slight and almost imperceptible signs of caries in the tooth surface (see fig. 47, inroads of organisms well shown).

When once the dentine has been reached the organisms are able to make their way along the dentinal tubules, at the same time that their products penetrate to the pulp surface by capillarity. I have often found that cultivations and microscopical examination of pulps which had succumbed to caries showed no bacteria, while the dentine at a short distance from the pulp chamber gave positive results. Miller has several times pointed out that cultivations made from tooth pulps gave no evidence of living organisms even after most careful examination. The soluble products of bacterial activity may therefore produce death of the pulp, accompanied with fatty or other degeneration, without the living organisms themselves actually coming into play, and the familiar clinical observation of the ease with which arsenious acid gains access to the pulp demonstrates the permeability of sound dentine.

In the largest number of cases, however, bacteria are present in dead pulps; some of them have been found by Miller to be pathogenic for animals, generally producing local necrosis or suppuration when injected subcutaneously. Miller examined fifty cases of pulp gangrene, and notes several of the organisms met with. Many of these are gas-forming bacteria, and most of them capable of fermenting carbohydrates. Among the known pathogenic bacteria that have been found in tooth pulps, the streptococcus has been observed by most of the workers : Miller, Siebeth, Dobrzyniecki ? and myself have constantly met with it, but I am inclined to think that it is the

i Dental Cosmos, July, 1894. 2 Central. für Bak., 1900, xxviii., p. 302. 3 Central. für Bak., 1898, xxiii., p. 670.

ordinary streptococcus of the mouth (S. brevis) rather than the pathogenic streptococcus.

Miller also met with Micrococcus tetragenous, whilst Zierleri occasionally found sarcinæ.

All these organisms are met with from time to time in carious dentine, so that their presence in dead pulps is not surprising, and I can confirm the occasional presence of both sarcinæ and micrococcus tetragenous from my observations. Staphylococci are not infrequent, the most common variety being the Staphylococcus albus, although the staphylococcus aureus does also occur. I have occasionally (four times), met with S. aureus in pure cultivations in the abscesses of roots in which the pulp had been dead for a considerable time. B. necrodentalis is also often obtainable, as is B. gingivæ pyogenes. The cultural characters of these bacteria have been already given (pages 127 and 161) in the chapter on Dental Caries.

Miller, in the paper already referred to, gives the following list of four organisms which he has frequently met with :

(1) Cocci and diplococci (pathogenic).
(2) Bacilli, curved and often forming threads.
(3) Short rods with bipolar staining.
(4) Micrococcus tenuis.

He found these organisms frequently present, but does not state how-often in pure culture, or in combination with the others.

A number of other organisms were also observed, some of which grew upon agar, others upon gelatin, but it is not clear which. The cultural reactions were unfortunately omitted.

Miller concluded that the cocci present were probably more concerned in pulp destruction than the bacilli, but that some symbiotic relation existed between the bacilli and cocci; injection of animals with mixed cultures producing more marked effect than the pure cultures alone. Inoculation with masses of putrid pulps generally resulted in the death of the animals (mice and rats) in three or four days, with local tissue necrosis and occasionally septicæmia.

Miller did not meet with the pneumococcus during any of his investigations; although more than one hundred and fifty mice were inoculated with putrid pulps, in no case did the pneumococci appear in the animals' blood after death. Schreier” claims to have observed the pneumococcus in dead

Med. Rundschaw Ber., 1900, p. 534.
. ? Oest. Vng. Vietelj. für Zahnarzte, 1893, Heft ii.

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