Antonie Van Leeuwenhoek was the first to describe the morphology of bacteria. They may be rod-shaped called bacilli or spherical called cocci and their size depends on the shape. The structure of a bacteria cell can be described as under-
Capsule– surrounds the cell wall and is a thick coat of gelatinous material. It may be diffused to be called Slime Layer. It is compared largely to polysaccharides and serves as an additional protective layer.
Cell Wall- consists of a single layer and is made up of a nitrogenous substance called Chitin. The cell wall of certain bacteria shows a characteristic reaction to the stain devised by C.Gram. Those bacteria which retain the stain are known as Gram-positive and those bacteria which do not retain the stain are called Gram-Negative. Chemically, the cell wall contains N-acetyl glucosamine and diaminopimelic acid.
Cytoplasmic Membrane- is a thin, delicate, permeable cell membrane just beneath the cell wall.
Mesosomes- are vesicular, convoluted structures formed by the invaginations of the plasma membrane in the cytoplasm and function like that of mitochondria and are the sites of respiratory enzymes in bacteria.
Ribosomes- are small dot-like structure scattered throughout the cytoplasm and help in protein synthesis. They may occur in series forming polyribosomes.
Metachromatic Granulesor Chromatophores- are present in photosynthetic bacteria and contain photosynthetic pigments.
Gas Vacuoles- are present in certain bacteria. There are filled with gases and help bacteria to float or sink.
Nucleus- The bacterial nucleus is not surrounded by a nuclear membrane and does not have nucleolus. The genome consists of a single molecule of double-stranded DNA arranged in the form of a circle. It may open under certain conditions to form a long chain of about 1000 microns. Bacteria may sometimes have extranuclear genetic material called plasmid or episome because of which some bacteria show toxicity and drug resistance.
Flagella- are long sinuous contractile filamentous appendages and arise from the basal body just inside the cell wall and contain a cavity. They are organ of locomotion and are also antigenic. The number and arrangement of flagella are characteristic of each bacterium.
Fimbriae or Pili- project from the cell surface as straight filaments. They are smaller and thinner than flagella and do not contain a cavity. They are organs of adhesion and conjugation.