The work reported in this paper relates to the bacteria which can be cultivated from the blood and spleen of guinea pigs at different stages of infection with the virus of typhus fever.

The studies show that during the period of incubation and before the onset of fever no ordinary bacteria appear in the cultures, while on the 1st day of the febrile reaction different bacteria were found in 6 of 26 guinea pigs cultured; on the 2nd day, in 10 of 16; on the 3rd day, in 3 of 4; and on the 4th day in cultures of all of the 4 guinea pigs observed. The findings indicate that the virus of typhus fever is distinct from ordinary cultivable bacteria, and, as the disease set up by the virus progresses, the infected guinea pigs become subject to invasion by secondary or concurrent bacteria which thus induce a mixed infection.

The bacteria which under the influence of the virus of typhus fever thus invade the body of the guinea pig are of several kinds, and vary not only among themselves, but also with the day of the fever on which the examination is made. Thus, on the 1st day of the fever Plotz' bacilli were recovered twice and anaerobic streptococci, proteus bacilli, aerobic diphtheroids, Gärtner type bacilli, and Staphylococcus aureus each once. On the 2nd day Plotz' bacilli were found four times, anaerobic streptococci three times, Gärtner type bacilli, aerobic diphtheroids, Bacillus welchii, aerobic Gram-positive diplobacilli, and Staphylococcus aureus each once. On the 3rd day Plotz' bacilli were recovered once, as were anaerobic streptococci and Grtner type bacilli. On the 4th day Staphylococcus aureus was found twice and Plotz' bacilli and Bacillus proteus each once.

This variation in the kind of bacteria as well as the lack of predominance of one kind over another during the different stages of the febrile reaction in guinea pigs leads us to infer that they occur concurrently with the typhus virus. And since the more unusual of these organisms, the Plotz bacillus, the anaerobic streptococcus, the aerobic diphtheroid, and the diplobacillus are non-pathogenic for guinea pigs, while the more common bacteria such as the Gärtner type bacillus, Welch's bacillus, the proteus bacillus, and the staphylococci induce distinctive effects, and since all the bacteria could be suppressed without their reappearance in guinea pig passages of the virus containing them, we believe that they are independent and unrelated to the true virus of typhus fever.

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