THE DIPHTHERIA BACILLUS The exciting cause of diphtheria is a bacterium belonging to the group of the actinomyces, whose form, growth and virulence show marked changes according to the culture-medium and the age of the culture.
describes it as a non-motile, straight or slightly curved rod—with an average length of that of the tubercle ba cillus but about twice as broad showingrounded or often swollen ends, and under certain conditions appearing stratified or granular through an irregular absorption of the stain (thickening of the chromatin, ap pearance of degeneration, involution forms). In a smear preparation, the rods lie parallel or at angles to each other in groups of various sizes, frequently forming letter-like figures, as V, W, X or Y. In sec tions of the pseudomembrane they are seen grouped in large or small nests or arranged somewhat like a fish's tail. In addition to the typical Leffler form of the long bacillus, there are frequently seen young, shorter forms which arc not granular and are wedge-shaped or cylindrical, and also giant-forms, two or three times as large, with bulbous ends which are always decidedly granular and stratified, so that they are easily mistaken for cocci. Under certain ex ceptional conditions of growth there, are seen thread-like forms which may or may not branch (Abbott and Gildersleeve).
diphtheria bacilli take all the aniline stains readily, and resist Gram's with only a short application of the iodiniodide of potash solution. A good picture is furnished with Loffler's alkaline methylene blue solution (30 c.c. concen trated alcoholic solution of methylene blue, 60 c.c. potassium hydroxide solution, 1 : 10,000) which ought to be made fresh every four to six weeks. When stained with Ziehl's solution or aniline-gentian violet, the bacilli appear more plump through a swelling of the plasma. Double staining is recom mended to bring out the Babes-Ernst granulations and especially to differentiate the chromatin from the enveloping substance; Roux uses dahlia violet and methyl green, Neisser's method being acetic acid methylene blue, crystal violet and chrysoidin. The value of double staining is discussed in the section on diagnosis.
diphtheria bacillus needs for its growth a culture medium with slightly alkaline reaction. The limits of its growth lie
between 19°-42° C. (66°-107° F.), the most favorable temperature being from 33°-37° C. (91°-9S.6° F. ). It grows most luxuriantly on an albuminous medium, especially blood-serum, which is used as a selec tive medium; for the bacillus develops on this more rapidly than other bacteria which may accompany it. From eight to twelve hours after inoculation a growth of minute colonies is seen on the surface of the serum looking like ropes of droplets which soon become confluent and in about two days cover the culture medium with a thick white overgrowth, the edges of which are scalloped. In bouillon the effect is first to give an acid reaction and after not less than eight days a return to alkaline. The bacilli grow luxuriantly in milk without curdling it. With the development of an acid reaction in the culture medium, the growth and the formation of toxin (Madsen) lessen. A similar result follows a change of temperature to that above or below the limits mentioned.
The sensibility of the bacilli to thermic influences is very varied. Cold, even the action of winter-temperatures for months, is well borne (Abel), but in the opposite direction, death soon follows the action of a temperature of 50° C. (122° F.). The bacilli arc just as sensitive also to the action of certain chemicals, especially those used ordinarily in disin fection in the usual strengths :—alcohol, lvsol, phenol, tincture of the chloride of iron, chlorine water, corrosive sublimate, cyanide of mercury. Much less powerful are boric acid and permanganate of potash, but peroxide of hydrogen is very useful.
Although the bacilli resist heat and chemicals so feebly, their re sistance to drying is in the inverse ratio, especially if they are enclosed in bits of membrane and are not exposed to diffused daylight. Positive cultures have been obtained after weeks and months from toys and books, from furniture, dishes, floors and walls of previously infected dwellings, especially if dark and clamp. It is noteworthy that the bacilli are able to endure a temperature of OS° C. (20S° F.) for an hour in the dried pseudomembranes (Roux and Yersin).