Page 113 - Biology_F5

Biology for Advanced Level Secondary Schools

Classification of bacteria based on exist, namely spherical (cocci), rod
their morphology (bacilli), spiral (spirilla), and comma
Bacteria can be classified according to (vibrio).
their morphology or shapes. They can
exist singly or in a series of interconnected Cocci (singular coccus)
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individual cells called colonies. This These are spherical-shaped bacteria. They
should not be confused with multicellular may exist as a single cell, or in pairs, forming
organisms, because each cell is capable of a double-celled (diplococcus) organism.

carrying out all its essential life processes, The diplococus may be surrounded by a
and each can survive independently if capsule, for example the pneumococcus
separated from the colony. The cells bacteria (Diplococcus pneumoniae) a
causative agent of pneumonia. They may
normally associate to form a colony after also occur in chains of interconnected
binary fission, as the two resulting cells individuals or in bunches (cluster cells),
adhere to each other. Some bacteria cells such as Streptococcus and Staphylococcus
associate in tubular sheaths, forming respectively (Figure 3.6 (a), (b), (c), and

filaments which undergo binary fission (d)).
simultaneously. Four shapes of bacteria

(a) (b) (c) (d)

Figure 3.6: Structures of cocci bacteria (a) single-celled (b) double-celled (c) colon and (d) chain

Bacilli (singular bacillus) tendency of forming endospores of various

These are rod-shaped bacteria, which shapes, position and size. Examples include
may occur singly as in Escherichia coli; oval spore without swelling at the center as
a common gut-living symbiont and in Bacillus anthracis, and spherical spore
Salmonella typhi which causes typhoid fever. with a swollen terminal as in Clostridium
The bacilli may also occur in chains as seen tetani; the causative agent of tetanus or with
in nitrogen fixing bacteria (Azotobacter) sub-terminal swollen example Clostridium
and the anthrax causing bacteria (Bacillus botulinum the causative agent of botulism

anthracis). The bacilli usually have a (Figure 3.7 (a), (b) and (c)).

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