The unique staphylococci, the E46 and E97 strains described in the preceding papers, were examined at cellular level, for the mechanism of high virulence for mice. An
in vitro test with cultured mammalian cells was also studied to determine whether it was available as a criterion for the grading of virulence of these strains in comparison with the typical strains of
Staphylococcus aureus, E111 and B40, and of
S. epidermidis, E241 and B217. The cystopathic effects of all these living organisms, their culture filtrates, and nuclease were investigated when these materials had been inoculated onto the monolayers of HeLa and FL cells cultured in the YLE medium supplemented with 5% calf serum. The results obtained were as follows.
1) In the case of living organisms, cultured cells were invaded most vigorously by the
S. aureus strains, moderately by the unique strains, and very slightly by the
S. epidermidis strains. The consequent appearance of cystopathic effect was in parallel with the above grading of invasivess among those strains.
2) The cytopathic effect of the culture filtrate was also the most conspicuous in the
S. aureus strains. The unique staphylococci, however, came next with a small difference. It was noted for them to cause an obvious karyolysis in host cells. The
S. epidermidis strains gave rise to the slightest effect.
3) The susceptibility of the cultured cells to cytopathic effect due to the culture filtrate was proved considerably stronger in HeLa cells than in FL cells. Such a difference, however, was not recognized in the invasiveness of organisms into their host cells.
4) Addition of the nuclease isolated from the B40 strain to HeLa cells resulted in a conspicuous karyolysis in these host cells.
The results of the cell culture method mentioned above were not identical with those of the virulence test by intraperitoneal inoculation of mice reported in the preceding paper. Thus it may be concluded that such test
in vitro by cell culture is not satisfactory for the determination of the grading of virulence of the unique staphylococci, but that it would be available, to some extent, for checking the cytopathic effect of the extracellular metabolites, mainly nuclease.
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