SUMMARY: Phase-contrast microscopy of living bacterial cells under appropriate conditions is of value in determining cellular structure, bacterial nuclear processes, methods of division and the effects of cultural conditions. Different species of bacteria differ in their methods of division (for example by constriction or by eross-septation) and in their apparent internal structure. Considerable differences may be seen also in the same organism grown under different cultural conditions. The same species of bacteria may exist as short oval bacilli in an agar medium and long-chained or filamentous forms in broth, with equally characteristic differenees in the distribution of refractile material within the cells. The incorporation of special chemical compounds in the culture medium raises the possibility of vital ‘staining’ by the deposition of refractile material in defined sites within the bacteria during growth, thus rendering visible by phase-contrast microscopy certain cellular structures in living bacteria. The inclusion in culture media of tellurite and selenite has this effect of showing up internal structure in the bacteria grown in their presence. Apart from the possibility of developing new procedures for detecting internal structure, phase-contrast microscopy is of use in evaluating fixation and staining methods by comparing the fixed and stained specimens with living cells, and thus recognizing distortions and artefacts introduced by preparative treatments.


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