Bacteria

Bacteria are a large domain of prokaryotic microorganisms . Bacteria are present in most habitats on Earth , growing in soil, acidic hot springs , radioactive waste water, organic matter and live bodies of plants and animals. Bacteria have many shapes and sizes. Bacterial cells are about one tenth the size of eukaryotic cells and 0.5–5.0  micrometres in length. Most bacterial species are either spherical, called cocci or rod-shaped, called bacilli . Some rod-shaped bacteria are slightly curved called vibrio or comma-shaped. Many bacterial species exist as single cells and associate in characteristic patterns such as form pairs calleddiploids, form chains, and group together in clusters. Bacteria can also be elongated to form filaments.

The bacterial cell is surrounded by cell membrane, which encloses the contents of the cell and acts as a barrier to hold nutrients, proteins and other essential components of the cytoplasm within the cell. They lack a true nucleus, mitochondria, chloroplasts, Golgi apparatus and endoplasmic reticulum. Most bacteria do not have a membrane-bound nucleus, and their genetic material is typically a single circular chromosome located in the cytoplasm in an irregularly shaped body called the nucleoid .The nucleoid contains the chromosome with associated proteins and RNA. The bacteria contain ribosomes for the production of proteinsbut different from those of eukaryotes and Archaea. Some bacteria produce intracellular nutrient storage granules, such as glycogen, polyphosphate, sulfur or polyhydroxyalkanoates.These granules enable bacteria to store compounds for later use. Certain bacterial species, such as the photosynthetic Cyanobacteria produce internal gas vesicles which they use to regulate their buoyancy – allowing them to move up or down into water layers with different light intensities and nutrient levels. The cell wall is present on the outside of the cytoplasmic membrane. A common bacterial cell wall material is peptidoglycan which is a polymer contains two sugar derivatives N-aetylglucosamine(NAG) and N-acetylmuramic acid (NAM) joined by glycosidic bond. A peptide chain of four alternating D- and L-amino acids called tetrapeptide is connected to the carboxyl group of the NAM. The amino acids present in the tetrapeptide include L-alanine, D-alanine, D-glutamic acid, and either lysine or diaminopimilic acid (DAP).The carboxyl group of terminal D-alanine is connected directly to amino group of DAD. The peptide interbridge connects the tetrapeptide chains.

There are two different types of cell wall in bacteria, called Gram-positive and Gram-negative. Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. They have many surface structures such as flagella, pili and fimbriae. Flagella are rigid protein structures about 20 nanometers in diameter and up to 20 micrometres in length that are used for motility. Flagella are driven by the energy released by the transfer of ions down an electrochemical gradient across the cell membrane. Fimbriae are fine filaments of protein, just 2–10 nanometers in diameter and up to several micrometers in length. They are distributed over the surface of the cell, and resemble fine hairs. Fimbriae are involved in attachment to solid surfaces or to other cells and are essential for the virulence of some bacterial pathogens. Pili are cellular appendages, slightly larger than fimbriae that can transfer genetic material between bacterial cells in a process called conjugation. Capsules or slime layers are produced by bacteria to surround their cells, and vary in structural complexity such as disorganizedslime layer and highly structured capsule or glycocalyx. These structures protect cells from engulfment by eukaryotic cells, such as macrophages. They can also act as antigens and be involved in cell recognition. Gram-positive bacteria, such as Bacillus, Clostridium, Sporohalobacter, Anaerobacter and Heliobacterium, can form highly resistant, dormant structures called endospores. Endospores have cytoplasm containing DNA and ribosomes surrounded by a cortex layer and protected by an impermeable and rigid coat. Endospores can survive extreme physical and chemical stresses, such as high levels of UV light, gamma radiation, detergents, disinfectants, heat, freezing, pressure and desiccation thereby help in surviving in harsh conditions. The bacteria mainly reproduced by binary fission which involves chromosome replication followed by cell division.