Environmental Engineering Reference
In-Depth Information
Cocci (spherical shapes)
Singles (random arrangement)
Pairs (
Diplococcus
)
Tetrad
Cubical packets
of eight (
Sarcina
)
Chains (
Streptococcus
)
Grape-like clusters
(
Staphylococcus
)
Bacilli (cylindrical or rod shaped)
Side-by-side (palisading)
One-half spiral
turn vibro
Loosely wound
spiral
Tightly wound
spiral
Figure 6.1
Bacterial shapes and arrangements.
Bacterial cells are usually measured in microns (µ), or micrometers (µm);
1 µm = 0.001 or 1/1000 of a millimeter (mm). A typical rod-shaped coliform
bacterial cell is about 2 µm long and about 0.7 µm wide. The size of a cell
changes with time due to growth and death.
Viewed under a microscope, bacterial cells may be seen as separate (indi-
vidual) cells or as cells in groupings. Depending on the species, cells may
appear in pairs (diploids), chains, groups of four (tetrads), cubes (sarcinae),
or clumps. Long chains of cocci result when cells adhere after repeated
divisions in one plane; this pattern is seen in the genera
Enterococcus
and
Lactococcus
. In the genus
Sarcina
, however, cocci divide in three planes, pro-
ducing cubical packets of eight cells (tetrads). The exact shape of rod-shaped
cells varies, especially at the end of the rod. The end of the rod may be flat,
cigar shaped, rounded, or bifurcated. Although many rods do occur singly,
they may remain together after division to form pairs or chains (see
Figure
6.1
). Frequently, these characteristic arrangements are useful in bacterial
identification.
