Lecture 2: Structure of Bacterial Cell PDF

General Microbiology and Immunology Lecture 2: Structure of Bacterial cell Dr./ Shimaa Mohamed Ali Sphinx University  Structure of Bacterial Cell Smaller and simpler in structure than eukaryotic cells, with no recognizable organelles. All the activ...

General Microbiology and Immunology Lecture 2: Structure of Bacterial cell Dr./ Shimaa Mohamed Ali Sphinx University  Structure of Bacterial Cell Smaller and simpler in structure than eukaryotic cells, with no recognizable organelles. All the activities performed by organelles also take place in bacteria, but they are not carried out by specialized structures. The small size, simple design, and broad metabolic capabilities of bacteria allow them to grow and divide very rapidly and to inhabit and flourish in almost any environment. They were first seen under a microscope by Anton van Leeuwenhoek in 1676. As microscopes have improved, scientists have come to understand bacterial cell structure better. Bacterial cell structure ❑Four main essential components of the bacterial cell : 1. Cell wall. 2. Cell or cytoplasmic membrane( plasma membrane). 3. Cytoplasm. 4. Nucleoid.  1. Cell wall It is a rigid structure which retains bacterial cell shape in the face of all physical and chemical treatments. Most of the metabolic activities of the cell depend on its structure. Structure of bacterial cell wallː A. Peptidoglycan layer. B. Teichoic acid layer (only in Gram-positive bacteria). C. Outer membrane (only in Gram-negative bacteria). D. Lipoprotein molecules.  A. Peptidoglycan layer Peptidoglycan = (polysaccharides + protein) The backbone of the peptidoglycan molecule is composed of two derivatives of glucose arranged in long chain : N-acetylglucosamine (NAG) N-acetyl muramic acid (NAM). The NAG and NAM strands are connected by inter peptide bridges. Its synthesis inhibited by penicillins and cephalosporins and it is degraded by lysozyme. Amount and location of peptidoglycan in the cell wall determines whether a bacterium is Gm +ve or Gm -ve. In Gram-Positive Bacteria Cell Wall: Peptidoglycan layer forms about 50% of the cell wall material. There are as many as 40 sheets of peptidoglycan in Gm +ve cell wall.  In Gram-Negative Bacteria Cell Wall: Peptidoglycan makes up only 5 – 20% of the cell wall (one or two sheets) and is not the outermost layer but lies between the plasma membrane and an outer membrane.  It gives rigid support to the cell. Function of Peptidoglycan??? Protect against osmotic pressure. (Prevent cell lysis and cell ?????? dehydration) It confers Gram reaction. ( Gram +ve or Gram –ve)  B. Teichoic acid layer Teichoic acids is perpendicular to the peptidoglycan sheets which are unique to the Gram-positive cell wall. It is a major surface component composed of a polymer of glycerol or ribitol phosphate. ❑Function: Major surface antigen in Gm +ve bacteria only. Induces TNF –α, IL-1 production. TNF –α , IL-1 Chemical messenger induce inflammation. Secreted by inflamatory cell like mature macrophage. Attachment of Gram-positive bacteria to mucosal cells ( host). C. Outer membrane ❑Found in the structure of cell wall of Gram-Negative bacteria only. ❑An outer layer of lipopolysaccharides (LPS) contain very tiny pores called Porins. lipopolysaccharides (LPS) which composed of 3 units: A phospholipid called lipid A (responsible for the toxic effects). A core polysaccharide. An outer polysaccharide (somatic or O antigen). ▪ These differences in structure can produce differences in antibiotic susceptibility ▪ Ex: Vancomycin can kill only Gram +ve bacteria and is ineffective against Gram -ve pathogens.  Function of outer membrane (LPS) 1.Endotoxin: It is responsible for shock; it is released only when bacterial cells are lysed. 2.Can bind specifically to receptors to activate macrophages. 3.Can nonspecifically activate B cells without the help of T cells. 4.Can be serotyped to classify bacteria⟾ Somatic antigen (O - antigen). Porins: Tiny holes or openings found in outer membrane (Gram –ve). Porins function: 1.Transport channels across outer membrane. 2.block the entrance of harmful chemicals and antibiotics, making Gm-ve bacteria much more resistant than Gm+ve bacteria.  D. Lipoprotein molecules (LPP) Set of membrane protein linked by lipid. In gram –ve bacteria it cross-link the peptidoglycan layer and the LPS layer. Function: 1.Adhesion to host tissue during infection. 2.Virulence. 3.Antibiotic resistance 4.Cell division Periplasmic space. Present only in Gm-ve bacteria a concentrated gel-like It's the site of B- matrix in the space lactamases that degrade between the inner penicillins and other B- cytoplasmic membrane lactam drugs and the bacterial outer (cephalosporine). membrane.  It's a rigid structure that maintains Functions shape of bacteria. Protects the cytoplasmic membrane of the cell from bursting in hypotonic solutions. wall Detects the response to Gram stain. Play a role in cell division. Structure of bacterial cell wall  Cell wall deficient bacteria A. Protoplast, spheroplast, L. forms: There are bacteria which are treated to remove the cell wall to reproduce new generation lacking the cell wall. The process can be induced by: 1. Destruction of the cell wall by treating the organism with an enzyme i.e. lysozyme. 2.Cultivation of the organism in presence of penicillin which inhibits the synthesis of cell- wall substance without interfering with the growth of the organism. If such treated cells are liberated from Gram positive cells, they are called protoplasts. If such treated cells are liberated from Gram negative cells, they are called spheroplasts. If these cell wall deficient cells are allowed to grow and divide, they are called L-forms. L-forms can revert to the parental form upon removal of cell wall inhibitor. B. Mycoplasma: The only bacterial species that is naturally deficient in cell wall. Hence, they are pleomorphic and resistant to penicillin. External structures 1. The Capsule 2. Flagella 3. Pili (fimbriae) 1. The Capsule Functions of capsules: 1. Virulence factor of many bacteria as it protects bacteria against phagocytosis. 2. It is an immunogenic.( stimulate immune response of host ) 3.used for specific identification of organisms. 4. Capsular polysaccharide used as antigens in certain vaccines ⟾ Capsular antigen (K- antigen) 5. Capsule and slime layers may play a role in the adherence of bacteria; e.g. adherence of Viridans streptococci to the surface of teeth which lead to the formation of plaque and dental caries. 2. Flagella These are long, delicate, appendages attached to the bacterial cell. They are composed of protein (flagellin) have antigenic charecters ⟾ Flagellar antigen (H- antigen) They are organs of locomotion (but some organisms like the spirochaetes may move without flagella). They are several times the length of the bacterium. Functions of flagella: 1.Motility of bacteria towards nutrients and other attractants. 2.Identification of some bacteria by antibodies against flagellar proteins (Flagellar antigen (H- antigen)  Arrangement and Types of Bacterial Flagella There are six types of flagellar arrangement: Monotrichous: Single polar flagellum e.g. Vibrio cholerae, Campylobacter spp. Amphitrichous: Single flagellum at both ends e.g. Alcaligenes faecalis Lophotrichous: Tuft of flagella at one ends e.g. Spirilla spp. Cephalotrichous: Tuft of flagella at both ends Peritrichous : Flagella surrounding the bacterial cell. All the members of family Enterobacteriaceae, e.g. Salmonella Typhi, Escherichia coli, Proteus spp. Atrichous: has no flagella.  These are very thin and short thread-like structure found on the surface of Gram-negative bacterial cells. It is composed of subunits of a protein called pilin. These are two types of pili: 3. Pili (fimbriae) a) Sex pili (Fertility-pili): They are highly specialized hair-like structure found in many bacteria e.g. E. coli. They play a role in the transfer of part of the genetic material from one cell to another thus giving rise to a simple form of sexual reproduction. a) Ordinary pili ( common pili): (colonization antigens) They mediate adherence of bacteria to specific receptors on human cell surface which is a necessary step in initiation of infection of some organisms. Cytoplasmic membrane (Cell membrane) It is a semipermeable phospholipid double layered structure around the cytoplasm. It lies inside the peptidoglycan layer of the cell wall. Functions: 1. Active transport of molecules into the cell. 2. Energy generation by oxidative phosphorylation (ATP). 3. Synthesis of precursors of cell wall. 4. Secretion of enzymes and toxins. 5. It plays a role in DNA replication. Cytoplasm Constituents of Cytoplasm ▪ Proteins including enzymes ▪ Vitamins ▪ Ions ▪ Nucleic acids and their precursors ▪ Amino acids and their precursors ▪ Sugars, carbohydrates and their derivatives ▪ Fatty acids and their derivatives  Nucleoid Unlike the eukaryotic (true) cells, bacteria do not have a membrane enclosed nucleus. The nucleoid is a region of cytoplasm where the chromosomal DNA is located.  Plasmids Small extra-chromosomal DNA. Contain genes for antibiotic resistance or virulence. Structure: Similar to most bacterial chromosomes, but considerably smaller. Plasmids are covalently closed circular DNA. In a few species linear plasmids found. Size: Chromosomal DNA is typically about 4000 kbp. Plasmid DNA ranges from 1-200 kbp. Number of plasmids: 1-700 copies of plasmid in a cell.  Ribosomes Consists of RNA and protein Abundant in cytoplasm Often grouped in long chains called polyribosomes. Smaller than the ribosomes in eukaryotic cells-but have a similar function Bacterial ribosomes have sedimentation rate of 70S; their subunits have rates of 30S and 50S. Function: responsible for protein synthesis. Mesosome Mesosome are covonluted or multilaminated membranous bodies visible by the electron microscope. They develop by complex invagination of cytoplasmic membrane into the cytoplasm. Function: Functioning in the compartment of DNA at cell division and at sporulation. Having a function analogous to the eukeryotic cell----providing a cell membranous support for respiratory enzyme. Bacterial Spores They are highly resistant structures formed under unfavorable conditions by two medically important genera of Gram-positive bacilli, the genus bacillus and the genus clostridium. Sporulation and germination : Spores are formed in adverse environmental condition. ( temp., humidity, oxyen level, depletion of carbon and nitrogen , pH ) The spores are formed inside the cell and contain bacterial DNA, small amount of cytoplasm, cell membrane, peptidoglycan and a thick keratin like coat responsible for resistance to heat, dehydration, radiation and chemicals. Protein and calcium dipicolinate are found in the spore core. Once formed, the spore has no metabolite activity and can remain dormant for many years. When water and nutrients are available, specific enzymes degrade the coat, so nutrients enter and germination to vegetative cell occurs. Medical importance of spores: The medical importance of spores is due to their resistance to heat as they can be sterilized only by autoclave for at least 20 minutes. 1. Spores are used as indicator of sterilization process to ensure complete sterilization. 2. Spores are used as biological weapon. NB: Spores are not seen in clinical specimens from infected patients because the supply of nutrients is adequate. Try to solve 1.Enumerate bacterial structures which act as adherence factors. 2.Mention cell structures play a role in bacterial antigenic classification. 3.Compare between Gm –ve and Gm +ve bacteria from the point of: 1. cell wall 2. antibiotic resistance 4.Illustrate the function of LPP. 5.Describe the plasmid , porins. 6.Write on medical importance of spores. Thank You