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Comparative studies of natural groups of organisms

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(a) (b) (c)

Figure 3.7: The structure of bacilli bacteria (a) single rods (b) chained rod and (c) Bacilli with endospores
Spirilla (singular spirillum) Vibrio
Bacteria in this group have a long This group of bacteria includes various
cylindrical cell, coiled into a spiral or curved bacteria, which appear superficially
helical body. The individuals under this like a comma. That is why they are
group are also called spirochaetes; the sometimes named as comma-shaped
name that came from the fact that they bacteria. These bacteria possess a single
contain distinctive double membrane, and flagellum, which is used for locomotion, as
most of which have long helically coiled seen in Vibrio cholerae (Figure 3.9).
structure (corkscrew-shaped or spiraled).
They are also characterised by the presence
of unique axial filaments composed of
flagella, running from one end of the cell,
where they are directly attached to the cell
wall. The flagella are used for locomotion.
Bacteria under this group include free
living bacteria found in water and muds,
as well as parasitic spirochaetes, such as
Treponema pallidum that causes syphilis Figure 3.9: The structure of a comma-shaped
in humans and Borrelia burgdorferi that bacterium
causes Lyme disease (Figure 3.8(a) and (b)). Classification of bacteria based on
their mode of nutrition
Bacteria obtain their food from their
environment in different ways. Some of them
can synthesise their own food using energy
from the sunlight, while others use chemicals
as their energy source. Some of the bacteria
cannot synthesise their own food and they
thus rely on organic substrates from other
(a) (b) organisms as their source of food. Therefore
Figure 3.8: The structure of (a) Treponema pallidum in this regard, bacteria are classified into
and (b) Borrelia burgdorferi autotrophs and heterotrophs.

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