In this article we will discuss about the Structure of a Bacterial Cell.
It is a primitive type of cell in which genetic material is not organised in the form of nucleus but instead lies freely in a naked super-coiled state in the cytoplasm whence it is known as pro-chromosome or nucleoid.
Prokaryotic (= prokaryotic) cells are known for their rapid multiplication. The average size is 2.0-2.6 µm long and 1.1-1.5µm wide. In shape, bacterial cells are of many types (Fig. 8.7). Mycelial form is found in actinomycetes.
1. Coccus (Gk. kokkos- berry):
Coccus bacteria are spherical or ovoid in outline.
Depending upon their grouping they are called:
(i) Monococcus (occurring singly),
(ii) Diplo coccus (in twos),
(iii) Tetracoccus (in tetrads), Streptococcus (in chains), Staphylococcus (irregular grape-like clusters) and
(vi) Sarcina (3-dimensional geo-metrical forms).
2. Bacillus (L. bacillus-small rod):
The bacterium is straight and cylindrical like a rod with ends being flat rounded or cigar shaped.
It has three special types:
(i) Diplobacillus (in twos),
(ii) Palisade Bacillus (like astack) and
(iii) Streptobacillus (in chains).
3. Spirillum (L. spira-coil):
The bacterium is coiledillum, Spirochaete. Aggregation does not occur.
4. Vibrio:
The body of the bacterium is like a comma, curved rod or single turn of the spiral e.g. Vibrio cholerae. Like spirillum bacteria, the vibrio forms live singly.
5. Stalked:
The bacterium possesses a stalk e.g. Caulobacter.
6. Budding:
The bacterium is swollen at places, e.g. Rhodomicrobium.
Flagellation:
Depending upon the presence or absence of flagella, bacteria are grouped into flagellate and non flagellate types.
The various forms of flagellation (Fig. 8.8) are as follows:
(a) Atrichous:
Flagella absent.
(b) Monotrichous:
A single flagellum occurs sat or near one end of bacterium.
(c) Amphitrichous:
A flagellum at each of the two ends.
(d) Lophotrichous:
A group or tuft of flagella is found only at one end.
(e) Cephalotrichous:
A tuft or group of flagella occurs at each of the two ends or poles. The term amphitrichous is also used for this condition.
(f) Peritrichous:
A number of flagella are distributed all over the surface.
Gram Positive and Gram Negative Bacteria:
The grouping is based on the reaction of bacteria to Gram’s stain (Christian Gram, 1884). Bacteria are stained first with weakly alkaline solution of crystal violet or gentian violet, when all of them pick up blue colour. They are then treated with 0.5% iodine solution followed by washing with water and then absolute alcohol or acetone.
Bacteria which retain blue or purple colour are known as Gram (+) bacteria (e.g., Bacillus subtilis). Bacteria which do not retain any stain and become colourless are termed as Gram (-) bacteria (e.g., Escherichia coli). (Gram-ve bacteria are commonly stained with safranin). Washing of the stain in Gram —ve bacteria is due to high lipid content of cell wall which gets dissolved in organic solvents like acetone.