Staphylococci develop opaque, paint-like colonies of varying size, with or without hemolysis, and so do other less frequently found bacteria give more or less distinctive colonies. The heated blood agar does not show these differences.
The colonies most liable to be confused with those of B. influenzæ are, therefore, those of B. xerosis, immature colonies of the Gram negative cocci and certain colonies of the streptococcus viridans group. Transfers should always be made to heated blood agar of all colonies suggestive of B. influenzæ, or when the growth of the B. influenzæ has only occurred in the more crowded portions of the plate, and it is difficult to pick pure cultures, attempted pickings should be made to this medium for further platings. It is frequently necessary to make further blood agar plates from the original blood agar, blood broth or heated blood agar cultures after longer incubation periods, depending on the findings in smears from these media. The heated blood agar is the best of these to encourage the growth of B. influenzæ. It must, however, be used at once, or within a very few days of its preparation, and cannot be kept on hand as a stock medium. I have not found it as useful for plating because of the difficulty of differentiating colonies. The phenomenon of the star-like and more luxuriant growth of the colonies of B. influenzæ about colonies of other bacteria has often been noted, and will be referred to in a later portion of this report. Here it may be said that this is at times a marked feature of certain mixtures and must be recognized in studying the plates. The finding of B. influenzæ in picks from apparently isolated colonies of other forms is not uncommon, and is the same type of difficulty which I have discussed in papers on streptococci. It is important to recall, in connection with cultures taken from the lungs at autopsy, the experimental work of Norris and Pappenheimer, who showed that B. prodigiosus put in the mouth immediately after death could be recovered from the lungs in over 50 per cent. of the cases studied.
In Table I are shown my results from the 32 cases which came to autopsy. The B. influenzæ was isolated from one or more sources in 25, making a total of 78 per cent. Most of the negative cases probably also had this organism, but I did not grow it from the material which I used for culturing. The work of others would indicate that it may have been present in other regions, such as the sinuses of the head or other portions of the lung and respiratory tract. The positive results show B. influenzæ present in 20 out of 30 cases from the bronchi; in 13 of 28 from the lungs; in 2 of 14 from the pleural cavity; in 9 of 26 from both bronchi and lung where both were cultured; in 8 of 26 from the bronchi with the lung negative; in 3 of 26 from the lung with the bronchi negative; once of 10 from the pleural cavity with both the bronchi and the lung negative, and once from all three sources.