Bacterial Cell Structure and Function PDF

Chapter 2: Bacterial Cell structure and Function Medical Microbiology (BIO 219) Dr. Mohammad Fayyad-Kazan Spring 2023  Bacterial Cell structure and Function  Learning Outcome...

Chapter 2: Bacterial Cell structure and Function Medical Microbiology (BIO 219) Dr. Mohammad Fayyad-Kazan Spring 2023  Bacterial Cell structure and Function  Learning Outcome By the end of this lecture, you should be able to: 1. Understand the different components of Microbes 2. Understand the function of each Microbial component 3. Discriminate between different Microorganisms  Bacterial Cell structure and Function Microorganisms Parasites Viruses Bacteria Yeast Algae (Protozoa) Bacterial Cell structure and Function  Bacterial Cell structure and Function Bacteria: size, shape and arrangement Bacteria are: 1. Prokaryotic cells as they have a primitive nucleus, 2. Unicellular structures, 3. Exist in different sizes, shapes and arrangements  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 1. Cocci Cocci: It is a plural form, which represents a large number of bacteria possessing a spherical shape. Coccus is a singular form, which represents a single spherical- shaped bacterial cell. Cocci are small, round and slightly flattened in shape. They can appear singly or in pairs, chains, groups, clusters etc. Examples: Staphylococcus aureus, Streptococcus hominis, Neisseria gonorrhoea  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 1. Cocci 1. Staphylococcus aureus bacteria, which is a common cause of skin infections. Staph can also cause other infections throughout the body.  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 1. Cocci 2. Neisseria gonorrhoea is a sexually transmitted disease (STD) caused by infection with the Neisseria gonorrhoeae bacterium. N. gonorrhoeae infects the mucous membranes of the reproductive tract, including the cervix, uterus, and fallopian tubes in women, and the urethra in women and men.  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 2. Bacilli 1. Coccobacilli: They are oval and rod-shaped bacteria, where the division occurs in one plane. 2. Bacillus is a rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus 3. Diplobacilli: They appear and exist in pairs after the cell division. In diplobacilli, cell division occurs in only one plane. 4. Streptobacilli: They have a rod shape and exist in the form of a long chain. In streptobacilli, cell division occurs in one plane. 5. Palisades: These are rod-shaped bacteria arranged in a side by side sequence, which closely resembles the matchsticks.  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 2. Bacilli  Escherichia coli (E. coli) bacteria: Normally live in the intestines of healthy people and animals. Most types of E. coli are harmless or cause relatively brief diarrhea. But a few strains, such as E. coli O157:H7, can cause severe stomach cramps, bloody diarrhea and vomiting. You may be exposed to E. coli from contaminated water or food — especially raw vegetables and undercooked ground beef.  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 3. Spirilla The spirillum cells have a spiral structure and they usually exist singly. In the spiral bacterial cells, twisting occurs within the cell, which eventually gives it a helical shape. They are of two types, based on arrangement and number: 1. Vibrio: They appear comma-shaped and form after one complete twist. Examples: Vibrio cholera. 2. Spirochetes: It is another type, which forms after two or three complete twists. Spirochetes are highly flexible in structure. Examples: Spirochaeta sp , Treponema sp , Borrelia sp etc.  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 3. Spirilla  Vibrio cholera: The bacterium that causes cholera (an acute, diarrheal illness caused by infection of the intestine) Is usually found in food or water contaminated by feces from a person with the infection Cholera is mostly found in the tropics — in particular Asia, Africa, Latin America, India, and the Middle East. They called it the blue death.  Bacterial Cell structure and Function Bacteria: size, shape and arrangement 4. Other shapes Bacterial Cell structure and Function Bacteria: Cell structure  Bacterial Cell structure and Function Bacteria: Cell structure Bacteria:  Prokaryote 1. Lacki well-defined nuclei 2. Lack membrane-bound organelles, 3. Genome composed of a single closed DNA circle. Bacterial Cell structure and Function Bacteria: Cell structure  Bacterial Cell structure and Function Bacteria: Cell structure 1. The cytoplasmic membrane, also called a cell membrane or plasma membrane 2. The bacterial plasma membrane is a fluid phospholipid bilayer, composed of phospholipids (40%) and peripheral and integral proteins (60%) molecules. 3. About 7 nanometers (nm; 1/1,000,000,000 m) thick. 4. It lies internal to the cell wall and encloses the cytoplasm of the bacterium.  Bacterial Cell structure and Function Bacteria: Cell structure The phospholipids are molecules with a polar hydrophilic glycerol "head" attached via an ester bond to two non-polar hydrophobic fatty acid tails. The phospholipid bilayer is arranged such that: 1. The polar ends of the molecules form the outermost and innermost surface of the membrane 2. The non-polar ends form the center of the membrane  Bacterial Cell structure and Function Bacteria: Cell structure Dispersed within the bilayer are various structural and enzymatic proteins, which carry out most membrane functions. Some membrane proteins are located and function on one side or another of the membrane (peripheral proteins). Other proteins are partly inserted into the membrane, or possibly even traverse the membrane as channels from the outside to the inside (integral proteins).  Bacterial Cell structure and Function Bacteria: Cell structure Specialized structures called mesosomes are formed from the convoluted invaginations of cytoplasmic membrane. There are two types of mesosomes, septal mesosome and lateral mesosome. The bacterial chromosome is attached to the septal mesosome. During cell division, the septal mesosome participates in the formation of cross-walls. They are believed to be analogous to eukaryotic mitochondria since they are rich in respiratory enzymes.  Bacterial Cell structure and Function Bacteria: Cell structure The layers of cell envelope lying between the cytoplasmic membrane and the capsule are referred to collectively as cell wall. Cell envelope = cell membrane + cell wall + capsule  Bacterial Cell structure and Function Bacteria: Cell structure In gram positive bacteria, the cell wall mainly consists of peptidoglycan and teichoic acid while the cell wall In gram negative bacteria includes peptidoglycan, lipoprotein, outer membrane and lipopolysaccharide layers.  Bacterial Cell structure and Function Bacteria: Cell structure  Most bacteria have a complex cell wall consisting of peptidoglycan (also called murein, mucopeptide).  This complex polymer consists of three parts, 1. A backbone consisting of alternating units of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid). 2. Peptide side chain attached to NAM 3. Peptide cross-bridges, which are short chains of amino acids that crosslink the backbone. Peptidoglycan = {NAG—NAM—pentapeptide}n  Bacterial Cell structure and Function Bacteria: Cell structure  Gram positive bacterial cell wall: There may be as many as 40 sheets of peptidoglycan, comprising up to 50% of cell wall material. Electron micrographs show the peptidoglycan of Gram positive cells to be 20-80 nm thick. Most gram positive cell walls contain additional substances such as teichoic acid. There are two types of teichoic acid, wall teichoic acid (linked to peptidoglycan) and lipoteichoic acid (linked to membrane). The teichoic acid constitutes major antigens of cells that possess them.  Bacterial Cell structure and Function Bacteria: Cell structure  Gram negative bacterial cell wall: Gram negative cells consist of: 1. A relatively thin layer of peptidoglycan (approximately 10 nm). There appears to be only one or two sheets of peptidoglycan, comprising 5-10% of cell wall material. 2. An additional outer membrane. The outer membrane is the major permeability barrier in Gram negative bacteria. The space between the inner membrane and cell wall is known as the periplasmic space, which contains digestive enzymes and other transport proteins.  Chapter 2: Microbial Cell Bacterial structure Cell and structure Function and Function Bacteria: Cell structure  Gram negative bacterial cell wall: Gram negative cell walls contain three components that lie outside the peptidoglycan layer: 1. Lipoprotein, 2. Outer membrane 3. Lipopolysaccharide.  Chapter 2: Microbial Cell Bacterial structure Cell and structure Function and Function Bacteria: Cell structure  Gram negative bacterial cell wall: 1. Lipoprotein stabilizes the outer membrane by anchoring it to peptidoglycan. 2. Outer membrane is phospholipid bilayer in which the outer phospholipids are replaced by lipopolysaccharides. It is structurally similar to cytoplasmic membrane and serves to prevent leakage of periplasmic proteins and protects the cell from bile salts and protelolytic enzymes. The outer membranes contain several important porins, which specifically allow transport of solutes.  Bacterial Cell structure and Function Bacteria: Cell structure  Gram negative bacterial cell wall: 3. Lipopolysaccharide: Consists of a: I. Polysaccharide core, I. A complex lipid called Lipid A II. A terminal series of repeat units.  Bacterial Cell structure and Function Bacteria: Cell structure  Gram negative bacterial cell wall: 3. Lipopolysaccharide: The polysaccharide core is similar in all gram-negative bacteria. Each species contains unique terminal repeat units. Lipopolysaccharide (LPS) is toxic in nature and is called endotoxin because it is firmly bound to the cell wall and released only when cell is lysed. Endotoxin can trigger fever and septic shock in gram negative infections.  Bacterial Cell structure and Function Bacteria: Cell structure Gram positive bacterial cell wall Gram negative bacterial cell wall  Bacterial Cell structure and Function Bacteria: Cell structure 1. Cytoplasm: The cytoplasm or protoplasm is the portion of the cell that lies within the cytoplasmic membrane. It is gel-like in consistency and composed of water (80%). Constituents of cytoplasm include chromosome (DNA), ribosomes, proteins (including enzymes), vitamins, ions, nucleic acids and their precursors, amino acids, carbohydrates and fatty acids. The cytoplasm also lacks organelles such as mitochondria, golgi apparatus or endoplasmic reticulum.  Bacterial Cell structure and Function Bacteria: Cell structure 2. Nucleoid: Nucleoid = Genome = sum of an organism's genetic material (DNA) [orchestra of the cell] Bacterial Nucleoid = one long single circular molecule of double stranded, helical, supercoiled DNA NO: Nuclear membrane, nucleoli, meiosis ; Bacteria only reproduce asexually The chromosome is attached to an invagination of the cytoplasmic membrane called mesosome. The length of E.coli chromosome is approximately 1.4 mm but is condensed inside the cell by supercoiling.  Bacterial Cell structure and Function Bacteria: Cell structure 3. Plasmid: Some bacteria possess smaller extrachromosomal pieces of DNA called plasmids. Plasmids replicate independently of the chromosome Carry genes that are not essential for cell survival but may give some advantage to an organism (antibiotic resistance, tolerance to toxic metals). Bacteria may lose or gain them without harm Conjugation  Bacterial Cell structure and Function Bacteria: Cell structure 5. Ribosomes: Bacterial cells can contain thousands of ribosomes, which are the sites of protein synthesis. The distinct granular appearance of procaryotic cytoplasm is due to the presence and distribution of ribosomes. Often they aggregate to form structures known as polysomes. A typical bacterium may have as many as 15,000 ribosomes. Made up of rRNA + proteins Bacterial ribosomes are termed 70 S (Svedberg units) and eukaryotic ribosomes are termed 80S.  Bacterial Cell structure and Function Bacteria: Cell structure 5. Ribosomes:  Bacterial Cell structure and Function Bacteria: Cell structure 6. Endospores: An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria. Endospores enable bacteria to lie dormant for extended periods, even centuries. When the environment becomes more favorable, the endospore can reactivate itself to the vegetative state. Resistant to high temperatures (including boiling), most disinfectants, low energy radiation, drying, antibiotics, etc Examples of bacteria that can form endospores include Bacillus and Clostridium  Bacterial Cell structure and Function Bacteria: Cell structure 6. Endospores:  Bacterial Cell structure and Function Bacteria: Cell structure A gelatinous polysaccharide or polypeptide outer covering of certain bacteria is called glycocalyx. These are the structures that surround the outside of the cell envelope. The glycocalyx is referred to as: 1. A capsule if it is firmly attached to the cell wall, or 2. A slime layer if loosely attached. They are not essential to cell viability and some strains within a species will produce a capsule, while others do not.  Bacterial Cell structure and Function Bacteria: Cell structure 2. Glycocalyx  Significance: 1. Virulence factor. Capsules of pathogenic bacteria inhibit ingestion and killing by phagocytes. It can also prevent complement-mediated bacterial cell lysis. Capsules protect the cells from lysozyme. Mutant strains lacking capsule are avirulent. 2. Permit bacteria to adhere to cell surfaces and structures such as medical implants and catheters. This is a first step in colonization and sometimes leads to disease. 3. Capsules can be a source of nutrients and energy to microbes. Streptococcus mutans, which colonizes teeth, ferments the sugar in the capsule and acid byproducts contribute to tooth decay. 4. Prevent cell from drying out (desiccation) 5. Toxicity to the host cell; capsule of Bacteroides fragilis induces abscess formation. 6. Capsules may protect cells from bacteriophages. 7. Capsules may trap ions.  Bacterial Cell structure and Function Bacteria: Cell structure 2. Glycocalyx  Bacterial Cell structure and Function Bacteria: Cell structure 3. Flagella Some bacteria are motile and some are not. Almost all motile bacteria possess flagella as the organ of locomotion. Such bacteria tend to move towards or away from the source of stimulus. These stimuli can be chemicals (chemotaxis), light (phototaxis), air (aerotaxis) A bacterial flagellum has 3 basic parts: a filament, a hook, and a basal body.  Bacterial Cell structure and Function Bacteria: Cell structure 3. Flagella A bacterial flagellum has 3 basic parts: a filament, a hook, and a basal body. 1) The filament: rigid, helical (flagellin) with a hollow core. 2) The hook is a flexible coupling between the filament and the basal body. 3) The basal body consists of a rod and a series of rings that anchor the flagellum to the cell wall and the cytoplasmic membrane. The basal body acts as a molecular motor (The motor of E. coli rotates 270 revolutions per second!) Bacteria move at average speed of 50µm/sec, the fastest being Vibrio cholerae that moves 200µm/sec.  Bacterial Cell structure and Function Bacteria: Cell structure 3. Flagella Bacterial flagella are 10-20 µm long and between 0.01 and 0.02 µm in diameter and come in a number of distinct arrangements: 1. Monotrichous: a single flagellum, usually at one pole Vibrio cholerae Spirillum undula 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 Spirilla E. coli  Bacterial Cell structure and Function Bacteria: Cell structure 4. Pili Pili or fimbriae are short, hair-like structures made up of protein pilin and are present in virtually all gram-negative bacteria and only few gram-positive bacteria. Fimbriae are shorter and straighter than flagella and are more numerous. Since they are made up of protein, they are antigenic. Adhesive tip enabling attachment to receptors on a host cell. There are two basic types of pili: 1. Short attachment pili = fimbriae (numerous) 2. Long conjugation pili = "F" or sex pili (very few in number)  Bacterial Cell structure and Function Bacteria: Cell structure 4. Pili Conjugation is the transfer of DNA from a donor or male bacterium with a sex pilus to a recipient or female bacterium to enable genetic recombination.