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means that the animal easily develops the disease. Thus Pasteur found that the fowl, normally immune to anthrax, became susceptible when its body temperature was reduced by cold water. Guinea

pigs and rats kept in an atmosphere of sewer air showed lowered resistance towards diphtheria bacilli. Animals, such as rats, &c., which have been fatigued by exercise in revolving cages, are found to be much more susceptible to staphylococcal injections than control animals.

Immunity is therefore in some way concerned with the normal functions of the body, but at present very little is known of the subject. It is probable that fresh light will be thrown on the subject by the large amount of enquiry that is at present going forward relative to the anti-bodies produced during artificial immunization, and it is also possible that the property of natural immunity to many diseases is the expression of a gradually developed tolerance to the attacks of micro-organisms evolved over long periods of time, and produced in a manner analogous to artificial immunization. Immunity is, moreover, transmitted from mother to offspring.

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CHAPTER VII.

Pathogenic Bacteria of the Mouth.

FROM time to time various pathogenic bacteria are to be found inhabiting the mouth, and may be obtained from the fluids of that cavity, sometimes by means of cultures alone, but at any rate with certain species, best by the inoculation of animals with saliva.

Most of the bacteria thus obtained prove to be members of well known species which are generally associated with disease and pathological conditions in other regions of the body, a few are members of species as yet little studied, whilst a good many described by various authors have occurred in isolated cases only. It is impossible, in many cases, to find an adequate description of many of this latter series, and the task of connoting all the evidence is particularly difficult; some of the organisms may well belong to known species, modified perhaps by their residence in the mouth in such a manner as to render them difficult to identify in the first cultures obtained.

In noting some of these bacteria in the present chapter, the description given by the observers who originally described them have been rigidly adhered to, and the whole of the data obtainable given. This has been done with two reasons: firstly, to make the chapter as complete as possible; and secondly, by collecting the various scattered researches to enable other workers to confirm or disprove the various statements made. Special note is made of any of these organisms I have met with myself during a somewhat extended series of researches in the last seven years. Many of the pathogenic bacteria which occur in the mouth are apparently living in a state of œco-parasitism, ready under favourable circumstances and environment to act as the liberating impulses of disease.

Amongst the most commonly present the pneumococcus is perhaps the most frequent; its pathogenicity varies within wide limits. It is somewhat interesting to note that this organism, discovered independently by Fränkel,' Pasteur and Sternberg, was first

Zeit. für Klin. Med., 1885.

2 Compend. rend Acad. des Soc., Paris, xcii., p. 159-165.

3 National Board of Health Bull., vol. ii., 1882.

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isolated and described by inoculating rabbits with saliva. Pasteur was searching for the organism associated with rabies, Sternberg for the cause of yellow fever.

Another pathogenic organism whose presence in the mouth is more general than is often appreciated by dental surgeons, and which appears in about 33 per cent. of all persons exposed to infection, is the Klebs-Loeffler or diphtheria bacillus, and moreover the spread of this disease is largely due to the transference of the organism from one individual to another, more particularly children. So much is this the case that notwithstanding the increase of sanitary knowledge and the application of general principles of hygiene to everyday life, the disease, once more common in rural than urban districts, has now become a disease more associated with town than country areas, and shows a most striking relationship to the progressive aggregation of children for educational purposes. It is therefore a disease which should be borne in mind by all dental surgeons. In all mouths, and with no exception as far as I can ascertain, a streptococcus is a normal inhabitant, but apparently exists as a distinct species of a non-virulent type, although at times true pathogenic streptococci are to be met with. B. coli communis is also to be found rarely, while the tubercle bacillus may be found in the subjects of tuberculosis of respiratory tract. The bacillus of blue pus, B. pyocyaneus, is found in a limited number of cases, as is the Micrococcus tetragenous and rarely the Streptothrix actinomyces. These more important pathogenic organisms will be described first and following them a second group, comprising pathogenic organisms observed and described by various authors as peculiar to the mouth.

(1) STREPTOCOCCUS PYOGENES.

This streptococcus is the organism associated with erysipelas and general pyæmia. It has been found also in a large number of other pathological conditions, such, for instance, as infective endocarditis, puerperal septicemia, acute infective periostitis, &c. Associated with other pathogenic organisms it is sometimes found in diphtheria, bronchitis, pneumonia (especially the type known as "septic pneumonia"), as a secondary infection in phthisis, &c. In many cases of obscure febrile type the organisms are to be obtained from the circulating blood.

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FIG. 32.--STREPTOCOCCUS PYOGENES IN BLOOD. × 1,000.

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Twenty-four hours' agar cultivation. Stained Gram. × 1,000. (From Washbourn and Goodall's "Infectious Diseases.")

The streptococci are among the most widely distributed of the pathogenic cocci, and are as a general rule present in all the more acute suppurative conditions found in man.

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Varieties. Formerly the streptococcus of phlegmonous erysipelas, and the S. pyogenes were considered different species, but from a series of extended researches it is now generally admitted that the two species are identical.

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Petruschky placed this beyond doubt, by obtaining a virulent streptococcus from a purulent peritonitis in a patient never having suffered from erysipelas. He then inoculated the cultures into two individuals suffering from cancer, who had never had erysipelas, and produced a definite attack of erysipelas.

Linglesheim,' by a long series of carefully-conducted researches,

Zeitsch. für Hygiene, Bd. x., p. 331.

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