It retains its vitality in cultures and sometimes in the throat for months. Three or four weeks is the average length of time for its existence in the membrane, but, owing to the difficulty of killing it _in situ_, it may live on for as long as a year. All the conditions in the throat--mucous membrane, blood-heat, moisture, air--are extremely favourable to the bacillus; but it is very materially modified in virulence. It is secured for diagnostic purposes by one of two methods: (_a_) Either a piece of the membrane is detached, and after washing carefully examined by culture as well as the microscope; or (_b_) a "swab" is made from the infected throat and cultured on serum, and incubated at 37° C. for eighteen hours and then microscopically examined. Both methods--and there is no further choice--present some difficulties owing to the large number of bacteria found in the throat. Hence a negative result must be accepted with reserve.
We have already referred at some length to the question of _toxins_ in diphtheria, and need not dwell further upon that matter. Still a word or two may be said here summarising the general action of the bacillus. Locally it produces inflammatory change with fibrinous exudation and some cellular necrosis. In the membrane a ferment is probably produced which, unlike the localised bacilli, passes throughout the body and by digestion of the proteids produces albumoses and an organic acid which have the toxic influence. The toxins act on the blood-vessels, and nerves, and muscle fibres of the heart, and many of the more highly specialised cells of the body. Thus we get degenerative changes in the kidney, in cells of the central nervous system, in the peripheral nerves (post-diphtheritic paralysis), and elsewhere, these pathological conditions setting up, in addition to the membrane, the signs of the disease. The bacillus is pathogenic for the horse, ox, rabbit, guinea-pig, cat, and some birds. Cases are on record of supposed infection of children by cats suffering from the disease. The horse, it will be remembered, yields the antitoxin which has saved so many lives (_Metropolitan Asylums Board Report_, 1896).
The influence of drainage, milk, and schools must not be forgotten by sanitary authorities any more than the essential importance of adequate isolation hospital accommodation. Mr. Shattock's experiments on the effect of sewer air upon attenuated Klebs-Löffler bacilli have been mentioned (see p. 105). Nevertheless there can be no doubt that emanations from defective drains have a materially predisposing effect, not, it is true, upon the bacilli, but upon the tissues. Sore throats thus acquired are _par excellence_ the site for the development of diphtheria.