Bacteria

Bacteria are prokaryotes, evolved first on living earth. They does not contain membrane bound nucleus and organelles. Almost all the bacteria are having circular genome and extrachromasomal DNA which helps in survive different environments. Bacteria reproduce by prokaryotic fission, resulting in two genetically identical daughter cells. Most of the bacteria are unicellular in nature but sometimes they form aggregates. The most common shapes of bacteria are spheres (cocci), rods (bacilli), spirals.

Bacterial cell walls are made of peptidoglycan contains sugar moieties (N-acetylgluscosamine, and N-acetylmuramic acid cross linked with pentapeptide (D-aminoacids). Gram stain is a valuable tool to identify the bacteria based on the cell wall constituents. Gram-positive bacteria have simple cell walls with large amounts of peptidoglycan and which retains crystal violet. Gram-negative bacteria have more complex cell walls with less peptidoglycan and which retains saffron, the counter strain. Presence of lipid layer is a unique characteristic of Gram negative bacteria. Most of the gram negative bacteria are causative agent of many human diseases than Gram positive bacteria. The lipopolysaccharides on the walls of gram-negative bacteria are often toxic, and the outermembrane protects the pathogens from the defenses of their hosts.

Capsules are the slimy layer produced by most of the bacteria which helps them to adhere together and form colonies. Two kinds of filamentous structures may be attached to the cell wall: The bacterial flagellum rotates like a propeller to pull the cell along while movement. Pili help bacteria attach to one another in conjugation, and fimbriae help them attach to surfaces. Many prokaryotes are capable of taxis, movement toward nutrients or oxygen (positive chemotaxis) away from a toxic substance (negative chemotaxis). Some bacteria form resistant cells called endospores when an essential nutrient is lacking in the environment and it may remain dormant but viable for centuries or longer.

Classification based on nutrition and metabolism

Organisms can be categorized by their nutrition, based on how they obtain energy and carbon to build the organic molecules that make up their cells. Organisms that obtain energy from light are phototrophs. Organisms that obtain energy from chemicals in their environment are chemotrophs. Organisms that need only an inorganic compound such as CO₂ as a carbon source are autotrophs. Organisms that require at least one organic nutrient—such as glucose—as a carbon source are heterotrophs. Based on requirement of oxygen they are classified as obligate aerobes (requires O₂ for respiration), facultative anaerobes (can grow both aerobically and anerobically), and obligate anaerobes (does not require O₂).

Bacterial taxonomy

Until the late 20th century, biologists based prokaryotic taxonomy on criteria such as shape, motility, nutritional mode, and Gram staining. Although these criteria may be valuable in culturing and identifying pathogenic bacteria, they may not reflect evolutionary relationships. Applying molecular data to the investigation of prokaryotic phylogeny has been very fruitful. Microbiologists began comparing sequences of prokaryotic genes in the 1970s. Carl Woese and his colleagues used ribosomal RNA (rRNA) as a marker for evolutionary relationships.

In 1923, David Bergey and colleagues set out to publish a definitive book on the identification and classification of bacteria. A Survey of Bacterial Phylogeny and Diversity - based on the 2^nd edition of Bergey's