Prokaryotic structure cell The Structure within Cytoplasm Cytoplasm The Nucleoid c) Plasmid d) Ribosomes e) Endospore a) b) A typical bacterium usually consists of: a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane; a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and often various external structures such as a glycocalyx, flagella, and pili. Cytoplasm In bacteria, the cytoplasm refers to everything enclosed by the cytoplasmic membrane. About 80% of the cytoplasm of bacteria is composed of water. Within the cytoplasm can be found nucleic acids (DNA and RNA), enzymes and amino acids, carbohydrates, lipids, inorganic ions, and many low molecular weight compounds. The liquid component of the cytoplasm is called the cytosol. The Nucleiod The bacterial genome is composed of chromosomal deoxyribonucleic acid or DNA and represents the bacterium's nucleoid. the bacterial nucleoid has no nuclear membrane or nucleoli the bacterial nucleoid does not divide by mitosis In general it is thought that during DNA replication, each strand of the replicating bacterial DNA attaches to proteins at what will become the cell division plane. The nucleoid is one long, single molecule of double stranded, helical, supercoiled DNA The chromosome is generally around 1000 µm long and frequently contains as many as 3500 genes E. coli, a bacterium that is 2-3 µm in length, has a chromosome approximately 1400 µm long. Electron Micrograph of Nucleiod DNA Function of nucleiod? The nucleoid is the genetic material of the bacterium. Genes located along the DNA are transcribed into RNA that, in the case of mRNA, is then translated into protein at the ribosomes. In other words, DNA determines what proteins and enzymes an organism can synthesize and, therefore, what chemical reactions it is able to carry out. Plasmid Plasmid- Small molecules of autonomously replicating, circular, extrachromosomal DNA found in many bacteria. F: They are transferable genetic elements that can be transferred from one organism to another - through a process called conjugation, the conjugation pilus enables the bacterium to transfer a copy of the Rplasmids(g-ve) to other bacteria, making them also multiple antibiotic resistant and able to produce a conjugation pilus. Ribosome Ribosomes are composed of ribosomal RNA (rRNA) and protein. Composed of two subunits with densities of 50S and 30S. ("S" refers to a unit of density called the Svedberg unit.) The two subunits combine during protein synthesis to form a complete 70S ribosome about 25nm in diameter. A typical bacterium may have as many as 15,000 ribosomes. Ribosomes function as a workbench for protein synthesis, that is, they receive and translate genetic instructions for the formation of specific proteins. During protein synthesis, mRNA attaches to the 30s subunit and amino acid-carrying transfer RNAs (tRNA) attach to the 50s subunit (see Fig. 1). Protein synthesis is discussed in detail in Microbial Genes Chapter. Endospore Endospores are dormant alternate life forms produced by the genus Bacillus, the genus Clostridium, and several other genera of bacteria including Desulfotomaculum, Sporosarcina, Sporolactobacillus, Oscillospira, and Thermoactinomyces. Bacillus species are obligate aerobes that live in soil while Clostridium species are obligate anaerobes often found as normal flora of the gastrointestinal tract in animals. Under conditions of starvation, especially the lack of carbon and nitrogen sources, a single endospores form within some of the bacteria. The process is called sporulation. The completed endospore consists of multiple layers of resistant coats (including a cortex, a spore coat, and sometimes an exosporium) surrounding a nucleoid, some ribosomes, RNA molecules, and enzymes. Endospores are quite resistant to high temperatures (including boiling), most disinfectants, low energy radiation, drying, etc. The endospore can survive possibly thousands of years until a variety of environmental stimuli trigger germination, allowing outgrowth of a single vegetative bacterium Organelles Used in Bacterial Photosynthesis There are three major groups of photosynthetic bacteria: cyanobacteria, purple bacteria, and green bacteria. The cyanobacteria carry out oxygenic photosynthesis, that is, they use water as an electron donor and generate oxygen during photosynthesis.The photosynthetic system is located in an extensive thylakoid membrane system that is lined with particles called phycobilisomes. Photograph of the cyanobacteria Anabaena. Photograph of the cyanobacteria Oscillatoria. Photograph of the cyanobacteria Anabaena. Photograph of the cyanobacteria Oscillatoria. Cyanobacteria, as well as algae and green plants, use hydrogen atoms from water to reduce carbon dioxide to form carbohydrates, and during this process oxygen gas is given off (an oxygenic process). Cyanobacteria were the first organisms on earth to carry out oxygenic photosynthesis. The green bacteria carry out anoxygenic photosynthesis. They use reduced molecules such as H2, H2S, S, and organic molecules as an electron source and generate NADH and NADPH. The photosynthetic system is located in ellipoidal vesicles called chlorosomes that are independent of the cytoplasmic membrane. The purple bacteria carry out anoxygenic photosynthesis. They use reduced molecules such as H2, H2S, S, and organic molecules as an electron source and generate NADH and NADPH. The photosynthetic system is located in spherical or lamellar membrane systems that are continuous with the cytoplasmic membrane. Structure Outside The Cell Wall a) Glycocalyx b) Flagella c) Pili a) TheGlycocalyx (Capsules and Slime Layers All bacteria secrete some sort of glycocalyx (an outer viscous covering of fibers extending from the bacterium). The possession of a glycocalyx on bacteria is associated with the ability of the bacteria to establish an infection. Can assume several forms. If in a condensed form that is relatively tightly associated with the underlying cell wall, the glycocalyx is referred to as a capsule. A more loosely attached glycocalyx that can be removed from the cell more easily is referred to as a slime layer. Capsule stain of Streptococcus lactis 2 important functions of Glycocalyx The glycocalyx enables certain bacteria to resist phagocytic engulfment by white blood cells in the body or protozoans in soil and water. The glycocalyx also enables some bacteria to adhere to environmental surfaces (rocks, root hairs, teeth, etc.), colonize, and resist flushing. 2) Flagella Outside cell wall Made of chains of flagellin Attached to the protein hook Anchored to the wall and membrane by the basal body •The filament of the bacterial flagellum is connected to a hook which, in turn, is attached to a rod. •The basal body of the flagellum consists of a rod and a series of rings that anchor the flagellum to the cell wall and the cytoplasmic membrane. •In gram-negative bacteria, the L ring anchors the flagellum to the lipopolysaccharide layer of the outer membrane •while the P ring anchors the flagellum to the peptidoglycan portion of the cell wall. •The MS ring is located in the cytoplasmic membrane and the C ring in the cytoplasm. The Mot proteins surround the MS and C rings of the motor and function to generate torque for rotation of the flagellum. Arrangement of Bacterial Flagella 1. monotrichous: a single flagellum, usually at one pole 2. amphitrichous: a single flagellum at both ends of the organism 3. lophotrichous: two or more flagella at one or both poles 4. peritrichous: flagella over the entire surface Flagella are the organelles of locomotion for most of the bacteria that are capable of motility. The bacterial flagellum can rotate both counterclockwise and clockwise. A protein switch in the molecular motor of the basal body controls rotation. Clockwise rotation results in a tumbling motion and changes the direction of bacterial movement. On the other hand, counterclockwise rotation leads to long, straight or curved runs without a change in direction 3) Fimbriae and Pili Fimbriae allow attachment They are found in virtually all gram-negative bacteria but not in many gram-positive bacteria. There are two basic types of pili: 1) short attachment pili, also known as fimbriae, that are usually quite numerous and (fig.a). 2) long conjugation pili, also called "F" or sex pili that are very few in number (fig.b). a) b) Function of pili The short attachment pili or fimbriae are organelles of adhesion allowing bacteria to colonize environmental surfaces or cells and resist flushing. . Because both the bacteria and the host cells have a negative charge, pili may enable the bacteria to bind to host cells without initially having to get close enough to be pushed away by electrostatic repulsion. Once attached to the host cell, the pili can depolymerize and enable adhesions in the bacterial cell wall to make more intimate contact.