_Symbiosis._—The fact that other bacteria can bring to growth the influenza bacillus on media otherwise unsuited to its needs brings up the interesting problem of symbiosis, which is one of the most important characters of the influenza bacillus. Not only do other bacteria make possible the growth of B. influenzæ on media on which the influenza bacillus will not grow, but they stimulate a better growth on blood agar and other more or less favorable media. Grassberger first noted this stimulating character of other bacteria and described and illustrated the very large colonies of B. influenzæ which develop in the neighborhood of colonies of staphylococcus and other bacteria. Staphylococci killed by heat were found to have a similar effect. Meunier nicely described this phenomenon by using the term satellites for the circles of B. influenzæ colonies which develop about the colonies of other bacteria. A great number of workers have since noted this characteristic relationship between B. influenzæ and other bacteria, and occasionally have laid stress on its importance in the problems of the infections by the influenza bacillus. Allen particularly emphasized the probable importance of this in discussing the problem of carriers of B. influenzæ as sources of danger. There seems no doubt that this symbiotic relationship depends on so altering the hemoglobin products as to render them more readily available for the influenza bacillus. This is indicated by the fact that on various media containing hemoglobin, altered so that it encourages the growth of B. influenzæ, no such symbiotic stimulation can be demonstrated. This phenomenon is quite peculiar to this bacillus, distinguishes it from most of the other members of the group, and should be always determined before an organism is classed as B. influenzæ.
_Other Hemophilic Bacteria._—The question of a pseudo-influenza bacillus was first raised by Pfeiffer and has been studied by many workers after him. Grassberger, who carefully investigated this problem, worked more particularly with two strains showing the extreme of variation between the small characteristic morphology of the B. influenzæ and the thread forms supposed to be characteristic of the so-called pseudo-influenza bacillus. The great majority of workers have agreed with him in concluding that this morphological variation is not sufficient nor constant enough to justify separating two such groups. Nevertheless many reports indicate peculiar tendencies of certain strains toward thread formation. There seems to be suggestive evidence that the organism described by Cohen in 1909 under the name B. meningitidis cerebrospinales septicemicus is different from true B. influenzæ. Although the cultural characters were apparently identical, this organism was definitely pathogenic for guinea pigs and rabbits. The involvement of joints in the cases reported by Longo and others would suggest a greater pathogenic power for these strains. Prasek and Zatelli reported a similar bacillus from meningitis, and Davis found that his meningitis strains were more pathogenic for rabbits than were others. Wollstein has studied this question very carefully and found a marked difference between the strains from the meninges and those from the respiratory tract in their pathogenicity for rabbits. The strains with a tendency to thread formation were usually also those grown from the meninges, but she concluded from the results of serological tests that all strains of B. influenzæ are of one race, irrespective of their origin or virulence. The question is still an open one, as Batten and others described strains from the meninges which are non-pathogenic, and Ritchie found his strains from meningitis pathogenic for guinea pigs but not for rabbits. The irregularity and wide divergence in the results of blood cultures may have a definite relationship to these differences in the pathogenicity of strains.