Bacterial Cell Components: Bacterial Cell Anatomy, Morphology and Reproduction Eukaryotic Cells VS Prokaryotic Cells: comprises of: I. Outer Membrane Eukaryotic Cells: CELL ENVELOPE notable characteristics of Eukaryotes are the presence of the membrane enclosed cell organelles that specific cellular function. Such as: Nucleus-provide membrane closure for chromosomes Lysosomes- provide environment for controlled enzymatic degradation of intracellular substances. Mitochondria- generate energy (ATP) Golgi Bodies-processes substances for transport outside the cell Endoplasmic Reticulum – process and transport proteins Prokaryotic Cells CELL WALL They do not have organelles. All functions take place in the cytoplasmic membrane of the cell. Cell walls of most prokaryotic cells are made up of peptidoglycan layer. Bacterial Morphology mostly found in Gram-negative bacteria function: cell’s initial barrier to the environment; serve as permeability barriers to hydrophilic and hydrophobic compounds. it is a membrane bi-layered structure composed of lipopolysaccharide –gives a surface of a Gram negative bacteria a net negative charge. plays a significant role in a certain ability of the bacteria to cause a disease. Porins are water-filled protein structures that are scattered throughout the lipopolysaccharide that control the passage of nutrients and other solutes including antibiotics through the outer membrane. most clinically relevant species range in a size of 0.25 to 1 um in width and 1 to 3 um in height. differences in the cell wall provide the basis for the Gram Stain, which is the most fundamental test used in bacterial identification schemes. Gram Stain - The staining procedure separates almost all bacteria that are medically important bacteria into two different types: Gram-positive: deep blue to purple color Gram-negative: pink to red color Also referred to the Peptidoglycan layer or Murein layer. Gives the bacteria cell shape and strength to withstand changes in the environmental osmotic pressure that would otherwise result in cell lysis and also protects the cell against mechanical disruption. This feature has been the primary target for the development and design of antibiotics. The structure of the cell is composed of disaccharide pentapeptide subunits. The notable difference between the cell walls of gram-positive versus gram negative cell wall is that peptidoglycan layer of the gram-positive bacteria is thicker. Gram positive cell wall also contain Techoic acids. some Gram positive bacteria like the Mycobacterium is rich in mycolic acid that make their cells refractory to toxic acids. PERIPLASMIC SPACE Only found in Gram negative bacteria It is bounded by the internal surface of the outer membrane and the external surface of the cellular membrane. It consists of gel-like substances that help secure nutrients from the environment and also contain enzymes that degrade macromolecules and detoxify environmental solutes including antibiotics that enter through the outer membrane. CYTOPLASMIC INNER MEMBRANE Present in both Gram-positive and Gram-negative bacteria and is the deepest layer of the cell envelope. And the structure of the cell membrane for both are similar. It is functionally similar to that of a Eukaryotic cell’s organelles. Functions include: Common bacterial morphology: cocci (round), coccobacilli (round), bacillus (rod-shaped), fusiform (pointed end) Transport solutes into and out of the cell. Housing enzymes involved in the outer membrane synthesis, cell wall synthesis, and the assembly and secretion of extracytoplasmic and extracellular substances Generation of chemical energy (like the ATP). Cell motility Mediation of chromosomal segregation during the replication. Housing molecular sensors that monitor chemical and physical changes in the environment. CELLULAR APPENDAGES -play a role in causing infections and in laboratory identification, varies among bacterial species and even among strains of the same species. Capsule- Immediately exterior to the peptidoglycan/murein layer of gram-positive bacteria and outer membrane of the gramnegative bacteria. Often referred to as the slime layer Composed of high molecular weight polysaccharides whose production may depend on the environment and growth conditions surrounding the bacterial cell. It does not function as an effective permeability membrane barrier or add strength to the cell envelope but only protects the bacterial from attack by cells of the human defense system. (Immune system) Fimbriae or Pili- It is a hair-like, proteinaceous structures that extend from the cell membrane into the external environment. some may be up to 2 um in length. Inclusions- It includes storage reserve granules. Two common types of granules: a. Glycogen- storage form of glucose. b. Polyphosphate granules- a storage form of inorganic phosphates that are microscopically visible in certain bacteria stained with specific dyes. Unlike eukaryotic chromosomes, bacterial chromosomes exist as a nucleoid- highly coiled DNA intermixed with RNA, polyamines, and various protein that lend structural support Depending on the stage of cell division, there may be more than one chromosome per bacterial cell. Plasmids- are the other genetic elements that exist independently in the cytosol and their numbers vary from none to several per bacterial cell. Endospore- Under adverse physical and chemical conditions, or when nutrients are scarce some bacterial genera are able to form spores (sporulate). Sporulation involves substantial metabolic and structural changes in the bacterial cell. Fimbriae- Bristle-like. Present in multiple numbers, Adhere to hot tissues Pili- Bristle-like, Longer, Present singly on pairs There are two general types: The spore state is maintained until favorable conditions for growth are again encountered. This survival tactic is demonstrated by a number of clinically relevant bacteria and frequently challenges our ability to sterilize materials and food for human use. a. Common Pili- are adhesins that help bacteria attach to animal host cell surfaces., often as the first step in establishing infection. b. Sex Pili- serves as the conduit for the passage of DNA from donor to recipient during conjugation. Bacterial conjugation- this process occurs between two living cells, involves cell-to-cell contact and requires mobilization of the donor bacterium’s chromosome. Flagella- They are complex structures, mostly composed of the protein flagellin, intricately embedded in the cell envelope. These structures are responsible for bacterial motility. Although not all bacteria are motile, motility plays an important role in survival and the ability of certain bacteria to cause disease. Depending on the bacterial species, a flagella may be: Monotrichous flagella –located at one end of the cell BACTERIAL REPRODUCTION Binary Fission- Most bacteria rely on binary fission for propagation. Morphologic changes during growth: In cell division: Lophotrichous flagella- located at both ends of the cell Peritrichous flagella- entire cell is covered with flagella Cytosol - It is where nearly all the other functions not conducted by the cell membrane occur. It contains thousands of enzymes and is the site of protein synthesis. It has granular appearance caused by the presence of many polysomes (messenger RNA complexed with several ribosomes during translation and protein synthesis). Conceptually this is a simple process; a cell just needs to grow to twice its starting size and then split in two. But, to remain viable and competitive, a bacterium must divide at the right time, in the right place, and must provide each offspring with a complete copy of its essential genetic material. Understanding the mechanics of this process is of great interest because it may allow for the design of new chemicals or novel antibiotics that specifically target and interfere with cell division in bacteria. Most bacteria divide by binary fission into two equal progeny cells. In a growing culture of a rod-shaped bacterium such as E coli, cells elongate and then form a partition that eventually separates the cell into two daughter cells. The partition is referred to as a septum and is a result of the inward growth of the cytoplasmic membrane and cell wall from opposing directions until the two daughter cells are pinched off. The chromosomes, which have doubled in number preceding the division, are distributed equally to the two daughter cells. Although bacteria lack a mitotic spindle, the septum is formed in such a way as to separate the two sister chromosomes formed by chromosomal replication. This is accomplished by the attachment of the chromosome to the cell membrane. According to one model, completion of a cycle of DNA replication triggers active membrane synthesis between the sites of attachment of the two sister chromosomes. The chromosomes are then pushed apart by the inward growth of the septum, one copy going to each daughter cell. In cell groupings: If the cells remain temporarily attached after division, certain characteristic groupings result. Depending on the plane of division and the number of divisions through which the cells remain attached, the following may occur in the coccal forms: chains (streptococci), pairs (diplococci), cubical bundles (sarcinae), or flat plates. Rods may form pairs or chains. After fission of some bacteria, characteristic postdivision movements occur. For example, a “whipping” motion can bring the cells into parallel positions; repeated division and whipping result in the “palisade” arrangement characteristic of diphtheria bacilli. Cyanobacterium diptheriae - its characteristic "palisade arrangement” Other forms of Bacterial Reproduction: Baeocyte Production: It starts out as a small, spherical cell approximately 1 to 2 µm in diameter. This cell is referred to as a baeocyte (which literally means "small cell"). The baeocyte begins to grow, eventually forming a vegetative cell up to 30 µm in diameter. As it grows, the cellular DNA is replicated over and over, and the cell produces a thick extracellular matrix. The vegetative cell eventually transitions into a reproductive phase where it undergoes a rapid succession of cytoplasmic fissions to produce dozens or even hundreds of baeocytes. The extracellular matrix eventually tears open, releasing the baeocytes. Observed in cyanobacterium Staneria Budding: Budding has been observed in some members of the Planctomycetes, Cyanobacteria, Firmicutes (a.k.a. the Low G+C Gram-Positive Bacteria) and the prosthecate Proteobacteria. Although budding has been extensively studied in the eukaryotic yeast Saccharomyces cerevisiae, the molecular mechanisms of bud formation in bacteria are not known. A schematic representation of budding in a Planctomyces species is shown: Intracellular offspring production Epulopiscium spp., Metabacterium polyspora and the Segmented Filamentous Bacteria (SFB) form multiple intracellular offspring - or some of these bacteria, this process appears to be the only way to reproduce. Intracellular offspring development in these bacteria shares characteristics with endospore formation in Bacillus subtilis. Instead of placing the FtsZ ring at the center of the cell, as in binary fission, (A) Z rings are placed near both cell poles in Epulopiscium. (B) Division forms a large mother cell and two small offspring cells. (C) The smaller cells contain DNA and become fully engulfed by the larger mother cell. (D) The internal offspring grow within the cytoplasm of the mother cell. (E) Once offspring development is complete the mother cell dies and releases the offspring.
