Bacteria: Structure, Nutrition and Respiration

Bacterium was first observed by Anton Von Leeuwenhoek in 1683. Bacteria are smallest living organisms. They measure 2 to 10 micron in length. Their average diameter is 1.25 micron. The smallest bacterium belongs to genus Mycoplasma and largest bacterium is Thiomargarita namibiensis.

On the basis of shape, bacteria are classified into four types namely: Cocci, bacilli, vibrio and spirilla.

Coccus are spherical or ovoid shape in shape. Bacillus are rod shaped bacteria. They occur individually or in chains. Vibrio are short and curved. Spirillum are spiral shaped or twisted.

On the basis of their reaction with gram stain, bacteria are classified into two types namely: Gram positive bacteria and gram negative bacteria.

Gram positive bacteria have a thick layer of peptidoglycan which traps the crystal violet dye and appears blue/purple in colour. Gram negative bacteria have a thin layer of peptidoglycan.

Structure of bacteria comprises cell wall, capsule, plasma membrane, cell envelope, cytoplasm, nuclear material, plasmid, flagella and pili.

Some bacteria respire aerobically whereas, some respire anaerobically.

Based on the mode of nutrition, bacteria are classified into following two types: autotrophic bacteria and heterotrophic bacteria. Autotrophic bacteria are further categorized into chemosynthetic autotrophs and photosynthetic autotrophs. Similarly, heterotrophic bacteria are further categorized into saprophytes, parasites and symbionts.

Bacteria which are chemosynthetic autotrophs include: nitrifying bacteria, denitrifying bacteria, sulphur bacteria, iron bacteria, hydrogen bacteria, carbon bacteria and methane bacteria.

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