If we start taking about microscopic organisms there are a large number of organisms with slide to far variety in them. We can say microorganisms are heterogeneous group of several different class of living organisms. In the beginning when these groups are started to study, they were classified with plant and animal kingdom. When we start knowing more about microorganisms, it proved that classifying them with plant and animals is unjustified, so they were grouped in third kingdoms protista. Further based on cellular organization and biochemistry of living organisms in protista, it is further decided into two groups prokaryotes and eukaryotes. Microorganisms like bacteria are called prokaryotes while other microorganisms like fungi, other algae, slime mounds and protozoa are considered as eukaryotes.
Size of bacteria
The most common unit of measurement used in bacteriology is micron (micrometer )
1 micron or micrometers = one thousandth of a millimeter.
1 millimicron or nanometer = one thousandth of a micron or one millionth of a millimeter.
1 Angstrom unit = one tenth of a nanometer.
Human unaided eye can see up to 200 micron, bacterial cells are much smaller than that. So these can be visibalised only under magnification. Bacterias of medical importance either harmful or beneficial are typically measured between 0.2 – 1.5 micrometer in diameter and about 3 – 5 micrometer in length.
Shape of bacteria
There are so many bacterias obviously many of them different type of shape. If we start grouping them according to there shape, there can be several groups formed. Some common groups of bacteria as per their shape are as follow:
- Cocci (from kokkos meaning berry) are spherical and oval shape cells.
- Bacilli (from bacillus meaning rod) are rod shaped cells.
- Vibrios are comma shaped, curved rods and derive the name from their characteristic vibratory motility.
- Spirilla are rigid spiral forms.
- Spirochetes (from spears meaning coil and chaite meaning hair) are flexuous spiral forms.
- Actinomycetes are branching filament out bacteria.
- Mycoplasma are bacteria that are cell wall deficient and hence do not possess a stable morphology.
Bacteria some times show characteristic cellular arrangement and group their cells. Thus cocci may be arranged in pairs (diplodococci), chains (streptococci), groups of four (tetrads) or as grape-like clusters (staphylococci).
Other then cocci, some bacilli too may arrange in chane and forming (streptobacilli). Some other may arrange in angle to each other, presenting a cuneiform or Chinese letter pattern.
In most of the cases the cellular arrangement is determined by the plane through which binary fission take place and by the tendency of the daughter cells to remain attached even after division.
Growth and multiplication of bacteria
In general bacterial cell divide by binary fission. When a bacterial cell engulf enough nutrient and reaches to a certain size it divides to form two daughter cells. Nuclear division precedes cell division, and therefore, in a growing population many cells carry two nuclear bodies.
In the process of binary fission cells divides by a constrictive or pinching process, or by the ingrowth of a transverse septum across the cell. In some species, daughter cells may remain attached partially to a parent cell, after cell division.
This is the time required bacteria to give birth to a new daughter cell under optimum conditions. This is also known as population doubling time.
In coliform bacilli and many other medically important bacteria, the generation time is about 20 minutes. Some bacteria are slow growing, e.g. the generation time of tubercle bacilli is about 20 hours and lepra bacilli as long as about 20 days.
Bacterial growth can be considered in two ways, increase in size of a single cell and increase in total number of cells. When a bacterial cell reaches to its critical size it tend to divide and produce daughter cells. Growth in number of bacterial cell can be studied by bacterial count. Two type of bacterial count can be made – total count and viable count.
As the name indicate, total count gives the total number of cells in a sample, irrespective whether they are living or not.
The viable count measures the number of living cells, that is, cells capable of multiplication. Viable count of bacterial sample can be done by dilution or plating methods.
The bacterial cells are living cells and have almost same biochemistry as higher organism’s cells have. They need food for energy. The basic constituent of bacterial cell is water. Water makes almost 80% of bacterial cell protein, polysaccharides, lipids, nuclei acids, mucopeptides and low molecular weight compounds make up the rest. Bacterial metabolism is also closely similar to the metabolism o the higher organisms, exemplifying the ‘unity of biochemistry’.
To be alive and multiply bacteria need some minimal nutrients those include water, a source of carbon, a source of nitrogen and some inorganic salts. Here water act as the medium to get in all other nutrients, other nutrients dissolved in water and inhaled by bacterial cells.
On the bases of nutritional requirements or their energy requirement, bacteria can be grouped (classified) .
Bacteria which derive energy from Sun (sunlight) are called phototropism and those obtained energy from chemical reactions are called chemotrophs. Bacteria those can synthesize all their organic compounds are called autotrophs. Bacteria those depend on others for some or all organic compounds are called heterotrophs. Autotrophs are able to utilize atmospheric carbon dioxide and nitrogen. Autotrophic bacteria are capable to exist independently in water or soil.
Bacteria also need a supply of inorganic salts, particularly the anions phosphate and sulphate, and the cation sodium, potassium, magnesium, iron, manganese and calcium. These are generally present in the natural environment where bacteria live. Some ions such as cobalt is needed in traces.