Abstract
Cell buoyant densities were determined by centrifugation in Percoll gradients containing exponential-phase cells of Streptococcus faecium ATCC 9790 grown at a mass doubling time of about 33 min. This bacterium showed the highest average density values (1.13 g/ml) measured to date for any eucaryotic or procaryotic organism. Fractions having the highest densities were enriched with cells that were in the process of dividing or had just divided. These high-density fractions were also enriched with cells that had newly initiated sites of cell wall growth. It appears that S. faecium shows minimum cell densities in the midportion of its cycle.
Highlights
Cell buoyant densities were determined by centrifugation in Percoll gradients containing exponential-phase cells of Streptococcus faecium ATCC 9790 grown at a mnass doubling time of about 33 min
Values for the variance in the buoyant density of Escherichia coli during its cell cycle have decreased from an early estimate of -5%, which was obtained from an analysis of the distribution of cells in Ludox gradients, to the currently accepted figure of
In contrast with E. coli, it appears that the buoyant density of Saccharomyces cerevisiae varies significantly during its cell cycle [1, 6]
Summary
To analyze accurately cells banded in Percoll equilibrium density gradients, it was necessary to adjust the osmolarity of the Percoll solution This done by adding concentrated growth medium to stock solutions of Percoll before gradients were made. 2 ,ug of dry mass per ml [21] were used to inoculate tubes containing 20%o growth medium and were allowed to undergo six exponential-phase mass doublings At this time the culture was used to inoculate 10 ml of prewarmed 20% growth medium with sufficient cells to bring the culture to the equivalent of ca. After being chilled or fixed, cells were concentrated by centrifugation (8,000 x g, 10 min, 4°C), and the equivalent of 2.15 mg of dry cellular mass was suspended in 1 ml of the lowest-density Percoll-growth medium stock used to form a gradient. At least 100 cells were reconstructed from each gradient fraction analyzed
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