c. Step 3. The third step involves formation of a cross wall which grows
inwardly from the cell wall and splits the cross plate so that each of the newly formed
cells has a continuous cytoplasmic membrane. The cross wall then divides, providing
each daughter cell with a complete cell wall.
d. Step 4. The last step in cell division is the separation of the sister cells. The
cells of many bacterial species separate shortly after the cross wall is formed. Such
bacteria generally appear as single cells and form smooth colonies on solid culture
media. Other species have a cell wall which does not tend to split between cells and
which withstands the stresses induced by continued growth. Such bacteria do not
separate easily and form chains or other groupings such as sheets, packets, or irregular
clumps. Colonies from rod-shaped bacteria forming long chains generally appear rough
or wrinkled because of the buckling of the chains as they meet resistance to their
1-15. CELL GROWTH
If conditions are favorable, cell division is normally followed by a period of cell
growth or enlargement. The cell grows to its original size through absorption of water
and food and through manufacture of protoplasm. This growth is at right angles to the
plane in which the division takes place. Quite naturally, during such a period of cell
division and growth in a culture, the new cells are only half as long as the mature cells.
In some spherical or coccus forms, the new daughter cells are hemispheres when first
formed and are then restored to full spheres through growth. In some short rod cells,
the cell elongates first and then divides.
1-16. GROWTH CURVE
Growth of microorganisms refers to an increase in the number of unicellular
organisms. Under favorable conditions of nutrition, oxygen, pH, moisture, and
temperature, some kinds of bacteria may double in number about every 20 minutes.
This time interval is called generation time. Simple arithmetic shows the magnitude of
the result if this rate of increase were to continue for only a few days. Fortunately, there
are several factors that help control the situation. In some cases, the food supply may
become depleted or the accumulation of waste products may slow the process.
However, when these conditions are controlled by the continuous addition of nutrients
and removal of waste products, the bacterial population always reaches a maximum
before the medium in which it is growing becomes a solid mass of cells.
a. Four Phases. Different species of bacteria show various shapes of growth
curves depending on the generation time and the maximum population attainable under
the prevailing growth conditions. The growth curve is determined by plotting the
numbers of bacteria per milliliter of culture against incubation time. The counts are
plotted with logarithms of numbers. The growth curve of microorganisms is composed
of four phases (figure1-5).