Page 111 - Biology_F5

Biology for Advanced Level Secondary Schools

Cyanobacteria are grouped in the same Classification of bacteria based on
division with other true bacteria, because their staining properties
they share many features. For example, In this case, the bacterial cell is smeared on
both possess prokaryotic cell structures, a microscope slide and stained with crystal
reproduce by binary fission, their cell violet solution followed by iodine solution,
walls contain peptidoglycan (murein), and and later washed with organic solvent such as
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acetone or ethanol and counterstained with
they have mucilaginous sheet. a red dye called safranin or carbolfuchsin.
This staining based classification of bacteria
In other classification systems, cyanobacteria was invented by Hans Christian Gram, a
form a division of their own called Division Danish microbiologist in 1884, and was
Cyanobacteria. This is because, unlike therefore named “Gram stain test.” This
the bacteria, they have chlorophyll a for simple test classifies bacteria into two broad
photosynthesis and they produce oxygen groups, namely; Gram negative and Gram-
positive (3.4 a and b). The Gram-positive
during photosynthesis which resembles bacteria appear blue or purple, because they
algae more than bacteria. In addition, their retain the crystal violet colour of the dye
chlorophyll lamellae and DNA lie free in used to stain them. In contrast, the Gram-
the cytoplasm. Cyanophytes can occur as negative bacteria do not retain the colour
free living cells or as colonies. They have of the stain (crystal violet).
specialised cells called heterocyst which are
capable of fixing nitrogen. These bacteria The staining differences are based on
variation in the cell wall chemistry,
form blooms in ponds. Sometimes, they particularly in the amount of peptidoglycan.
are found in symbiotic association called The Gram-positive bacteria have a
lichen with fungi. simple cell wall with a thick layer of
peptidoglycan (polysaccharides and
Classification of bacteria protein). Moreover, due to differences
Bacteria are single celled microbes, with in cell wall constituents, the Gram-
a simple cell structure that has no nucleus positive bacteria cell wall is susceptible
or membrane bound organelles. Although to lysozymes and antibiotics. The Gram-
genetic divergence specifies the evolutionary negative bacteria are resistant to antibiotics
relationships among the bacteria, reaction and lysosomes. This is because, although
they have thinner cell walls, they are more
to Gram stain, mode of nutrition, complex, by having additional outer layer
morphological, motility, and biochemical on the outside of murein layer which is
features of bacteria remain important in thin, smooth membrane-like containing
the identification and classification of these lipid and polysaccharides that protect the

organisms. Three major ways can be used bacteria from the host’s lysozyme, as well
to identify bacteria; based on their cell wall as antibacterial enzymes found in tears,
chemistry or composition, which makes saliva and other body fluids. Examples of
them respond differently to Gram stains, Gram-positive bacteria include Bacillus
difference in morphology of their cells, and sp., Clostridium sp., Streptococcus sp.,
their modes of nutrition. and Staphylococcus sp., while Gram-

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