Fig. LPS in Gram negative bacteria

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1 Structure of bacterial cell Dentistry college - first class Medical biology- Lec.3 Lecturer D. Hanan S A- Cell wall ***Chemical composition of the cell wall Bacteria are divided into two separated groups Gram positive and Gram negative based on staining properties. Gram stain developed in 1884 by Christian Gram, the most widely employed in bacteriology Lab. A- Gram positive bacteria cell wall composed of : *Peptidoglycan This layer is very thick in G +ve bacteria constituting 50-80nm of cell wall and responsible for rigidity of cell wall and retention of crystal violet dyes during the Gram stain procedure. **Teichoic acid and thin layer of lipid B- Gram negative bacteria cell wall composed of : *Inner layer of peptidoglcan This layer is thin constituting of (5-10) nm of cell wall which can not retain the crystal violet stain. ** Outer layer of lipopolysaccharides (LPS) cotaining of lipid A (endotoxin) and polysaccharide (fig.). Fig. LPS in Gram negative bacteria *** Periplasmic space between the inner and outer layers It is filled with gel and is crossed by lipoprotein molecules to link the peptidoglycan layer and LPS layer 1

2 Structure of cell wall in G ve and G+ ve bacteria The stain may be classified into simple and differential, the simple stain colored all parts of the cell e.g. methylene blue while the differential stain are so selected that react with specific groups of different parts of the cell for example Gram stain helps in the identification and differentiation of bacteria in G+ ve and G-ve especially the pathogenic agents that causing disease. Many theories have been proposed to explain the observed difference in Gram staining, one of the most common theory is that based on variation in the chemical composition of bacterial cell wall. G+ve bacteria contain magnesium- RNA-protein carbohydrate complex which form an insoluble substance with the crystal violet and iodine, this complex is not washed with alcohol, while the lipid content of the cell wall being 10 times in G-ve as much as in G+ve ones, this lead to the solubility of lipids in alcohol and increase of cell wall porosity in G- ve and crystal violet iodine complex can be extracted, so, G-ve cells become colorless after alcohol washing and take the another color safranin (cell turn pink) as shown in the below figure. G + ve bacteria G ve bateria 2

3 B- Cytoplasm membrane It is a thin elastic semi permeable membrane that surrounds the cytoplasm, it is formed of two layers of phospholipids in which proteins and enzymes are embedded. Mesosomes are invagination of plasma membrane in the shape of vesicles it is thought has a role in cell division because they are the origin of the cross walls during cell division.the most widely accepted model for membrane structure is fluid mosaic model of Singer and Nicholson in protein Phospholipids bilayer Fig. fluid mosaic model of plasma membrane Functions of plasma membrane 1-Selective permeability to different molecules across the membrane. 2-It supply the cell with energy. 3-Secretion,excretion of toxins and hydrolytic enzyme outside the cell. 4-It provides the enzymes needed for cell wall synthesis. 5- Bacterial cell deficient of mitochondria so, the plasma membrane is the site of respiration in the cell. C- Intra cytoplasmic structure 1-nucleoid It is a primitive nucleus, not surrounded by nuclear membrane and a single circular double stranded DNA molecule located in a region of the cell known as the nucleoid and a small amount of RNA. It is carries the genetic coded information necessary for growth metabolism and survival of the cell, additional genetic information may be carried in plasmid an extra -chromosomal DNA lies within the bacterial cytoplasm and replicate independently of the chromosome, plasmid are not required for host growth and reproduction but encodes for traits that are given advantage to the cell such as drug resistant antibiotic or give them new metabolic pathway ( Fig.). 3

4 2 - Ribosome They are spherical, non membranous particles in cytoplasm of bacterial cell, the collections of ribosome are called polysomes. Chemically they are complex structures composed of several RNA molecules 60% and proteins 40%, it is an active center for protein synthesis. 3- Storage granules The cytoplasm contains granules which represent accumulation of food or energy reserve e.g. the metachromatic granules. 1-Flagella D-Appendages of bacterial cell An extra cellular long thin filamentous structure responsible for motility of pathogeic bacteria, can play role in production of disease because has an antigenic property. Bacterial cells, may carry a single flagellum described monotrichous, if the bacterium carries a single tuft of flagella it is said to be lophotrichous. When the tuft appears at both ends of the cell, the bacteria is amphitrichous. Bacteria that are covered all over body in flagella are said to be peritrichous as shown in the figure below. Monotrichous monotrichous lophotrichous amphitrichous peritrichous 4

5 2-Fimbriae The structure is hair like projections but shorter and thinner than flagella, it is found mainly in G-ve bacteria.they are two types of pilli divided according to their functions : 1- Ordinary pili which play a role in attachment of mucous membrane. 2- Sex pili their function was transfer DNA between conjugated bacteria. Gram negative pathogen bacteria may be covered with fine hair called fimbriae helps to stick to body surfaces (Fig.). 3-Capsule Fig. fimbriae and flagella of bacterial cell Most bacteria contain some sort of polysaccharide layer outside of the cell wall, this layer is called capsule, protects the bacterium even within phagocytes, helping to prevent the bacterial cell from being killed. Encapsulated bacteria grow as " smooth colonies", where as colonies of bacteria that have lost their capsules appear rough. Fig. bacterial capsule Some bacteria produce slime to help them to stick to surfaces, usually made up from polysaccharides,produced by Streptococcus mutants enables stick to the surface of teeth, were helps to form plaque, leading to dental carries. Dental plaque is a general term for the diverse microbial community (predominantly bacteria) 5

6 found on the tooth surface, embedded in a matrix of polymers of bacterial and salivary origin. Fig. dental carries Fig. slime of bacteria 4- Spores Some bacteria can develop a highly resistant structure called endospore as a response to unfavorable growth environmental condition such as radiation, heat, and desiccation for ex., Clostridium, Bacillus. The spore is formed inside the parent vegetative cell incorporating the nuclear material, acquiring a thick covering layer is called cortex and an outer spore coat that contains calcium and is impermeable to water as shown in figure. Spores may vary in : - Shape : oval or round. - Site : terminal, sub terminal or central as seen in figure below. - Size : the same size or bulging of the vegetative cell. Fig. types of spore sites in bacterial cell 6