BACTERIAL STRUCTURES

• The Endosymbiotic Theory proposes that the forerunner of eukaryotic cells were associations of small symbiotic prokaryotic cells existing within larger prokaryotic cells.
• The Endosymbiotic Theory focuses primarily on the origin of mitochondria and chloroplasts, which multiply in a way similar to that of bacteria.
• Good evidence indicates that the ancestors of mitochondria were probably oxygen-requiring heterotrophic bacteria.
• Chloroplasts are believed to be descendants of photosynthetic prokaryotes.
• Prokaryotic cells have various external structures such as pili, flagella, and glycocalyx.
• Pili are protein structures that aid in attachment to surfaces and genetic transfer, and are found primarily in gram-negative bacteria.
• Flagella are protein structures that contribute to motility, and are present in many rods and spirilla, with a few cocci, and can vary in number and placement.
• Glycocalyx is a polysaccharide and small protein structure that serves as a buffer to the environment, and contributes to attachment to surfaces.
• The cell envelope consists of the cell wall and the plasma membrane, and is found in bacterial gram-positives, which have a thick peptidoglycan and teichoic acid, and in gram-negatives, which have a little.
• The site of activity of penicillin and lysozyme is the cell envelope of gram-negative bacteria.
• It has been suggested that oxygen-requiring heterotrophs and photosynthetic prokaryotes both gained entry into the large prokaryotic cell as undigested prey or internal parasites.
• The nuclear envelope and other internal membranes may have evolved from infoldings of the plasma membrane.
• The bacterial glycocalyx is a viscous, gelatinous polymer that is external to the cell wall and is composed of polysaccharide, peptide or both.
• The chemical composition of the bacterial glycocalyx varies widely within species and is mostly made within the cell and excreted to the cell surface.
• If the substance is organized and is firmly attached to the cell wall, the glycocalyx is described as a capsule.
• Capsules are important in contributing to bacterial virulence and often protect pathogenic bacteria from phagocytosis by the cells of the host.
• K antigen helps capsules to escape from host self-defense mechanisms.
• Flagella are hair-like, helical appendages that protrude to the cell wall and are responsible for the swimming motility; much thinner than the flagella or cilia of eukaryotes.
• Flagella are made up of a basal body (associated with the cell wall and cytoplasmic membrane), a short hook and a filament which is helical in shape and usually longer than the cell.
• The 4.7 million base pairs in the genome of E. coli, if opened and linearized, would be about 1mm in length, yet the E. coli is only about 2-3um long.
• Chromosomes exist as a single, circular molecule of double stranded DNA, complexed with small amounts of proteins and RNA, but are not associated with histones and do not contain introns.
• Plasmids are relatively small, circular pieces of double stranded DNA which exist separately from the bacterial chromosome and are capable of autonomous replication.
• Spores and endospores are thick-walled, highly refractile bodies produced by Bacillus, Clostridium and other few genera.
• Endospores are extremely resistant to dessication, staining, chemical disinfection, radiation and heat.
• All endospores contain large amounts of calcium dipicolinic acid (Ca DPA), which plays a role in the heat resistance of the endospores.
• During germination, endospores lose their resistance to heat and staining.
• Stages of spore formation include Stage 0, where at initiation of sporulation, the vegetative cell contains 2 chromosomes; Stage 1, where the 2 chromosomes subsequently condenses to form a single broad filament; Stage 2, where the chromosomes separate and one moves terminally in the cell, followed by the formation of a septum between the two, but without cell wall formation; Stage 3, where the larger of the protoplasts engulfs the smaller; the smaller protoplast, or forespore, then lies within the cytoplasm of the larger and is surrounded by two membranes; Stage 4, characterized by the develo
• Exospores do not contain DPA but are also heat and dessication resistant and are formed by cells of the methane-oxidizing genus such as Methylosinus.
• Conidiospores and sporangiospores are a group of bacteria such as the actinomycetes that form branching hyphae; spores develop singly or in pairs.
• Spores are enclosed in a sac and are called sporangiospores, if not they are called conidiospores (or conidia).
• Conidiospores and sporangiospores are not heat resistant as endospores but can survive long periods of drying.
• Gram negative bacteria have cell walls that are chemically more complex than those of the gram positive bacteria, contain less peptidoglycan (10% to 20% dry weight of the cell wall), and possess a second “membrane” structure, composed of proteins, lipids and lipopolysaccharides (LPS) located exterior to the peptidoglycan layer.
• The periplasmic zone, or periplasm, is an enzyme-containing compartment located between the outer and cytoplasmic membranes; it acts as a molecular sieve to retain enzymes, principally hydrolases.
• The outer membrane of gram negative bacteria is similar in morphology and constitution to other biological membranes, but contains less phospholipid and fewer proteins, and possesses a unique constituent, lipopolysaccharide.
• Lipopolysaccharide is composed of a disaccharide of glucosamine to which are bound fatty acids called Lipid A, a core polysaccharide, and a specific polysaccharide region, the O specific side chains.
• Outer membrane proteins in gram negative bacteria can be categorized into three major types: pore-forming proteins, nonporin proteins, and lipoproteins.
• The cytoplasmic membrane of gram negative bacteria is located beneath the cell wall and is primarily composed of phospholipids (about 20 - 30%) and proteins (about 60 - 70%).
• The cytoplasmic membrane of gram negative bacteria is a hydrophobic barrier to penetration by most water-soluble molecules, and contains various enzymes involved in the respiratory metabolism and the synthesis of capsular and cell wall components.
• The protoplast is a gram positive bacteria devoid of cell wall.
• The spheroplast is a gram negative bacteria devoid of cell wall only.