Structure of bacteria

E. Coli bacteria structure, Bacterial morphology, Microbiology

1. BACTERIAL MORPHOLOGY R. NITHYA M. Sc., M. Phil., PGDCA., Ph. D ASSISTANT PROFESSOR IN BIOTECHNOLOGY SRI ADI CHUNCHANAGIRI WOMEN’S COLLEGE, CUMBUM.

2. BACTERIA (E. Coli) – STRUCTURE E. coli is Escherichia coli. It is a colon bacterium. It is rod shaped, Gram negative, facultative anaerobic, motile bacteria. It is 1.5 um broad and 6.5 um long. It is harmless. But occasionally it attains virulence and produce enterotoxin. It produces diseases like diarrhea, septicemia meningitis and urinary tract infections.

3. GLYCOCALYX - CAPSULE AND SLIME LAYER It is found in most bacteria; slimy or gummy substance that becomes the outermost layer of the cell envelope; a thick glycocalyx is often called a capsule a thin glycocalyx is often called a slime layer FUNCTION protection from drying out   helps a cell adhere to a surface where conditions are favorable for growth    provide protection against phagocytosis (engulfment and destruction by cells) - a slippery glycocalyx makes it difficult for the phagocyte to grab hold of the bacterium.

4. CAPSULE Capsule is the outer covering of the cell. It is tightly attached to cell wall. It is made up of disaccharides and polypeptides. The bacteria contain capsule are called capsulated bacteria with out capsule are called non-capsulated bacteria capsulated bacteria - smooth colonies non-capsulated bacteria - rough colonies Functions Protect the cell from drying Prevent attachment of phages Helps in attachment to surface Give protection against antibacterial agents Acts as anti-phagocytic Provide immunological specificity to bacterial cells.

5. CELLWALL It is the dense layer lying above plasma membrane. It gives shape and rigidity to the cell The cell wall is made up of strong fibres composed of peptidoglycans ( glycopeptides ). The peptidoglycans consists of alternating monosaccharide units N-acetyl glucosamine (G) N-acetyl muramic acid (M) The units are linked by β 1,4 linkages. A tetrapeptide is attached to N-acetyl muramic acid. Peptidoglycans are cross linked. A pentapeptide bridge links the tetrapeptides of adjacent peptidoglycans. The interstices of peptidoglycan carries many chemical compounds such as lipids, mucopeptides , teichoic acid, protein, etc.

6. PLASMA MEMBRANE Outer plasma membrane Outer plasma membrane is present beneath the capsule. It is found only on Gram negative bacteria. It is a unit membrane. It is three layered membrane. It is composed of two outer layers of phospholipids and a middle layer of protein. Phospholipids ha a polar head on the surface and the tails are in the interior. The protein components are embedded within the phospholipids. Functions Serves as a barrier to lysosome It acts as an endotoxin. It bears O antigens. Permeable to small molecules such as nucleosides, oligosaccharides, monosaccharide's, peptides and amino acids. The porins , a protein present in membrane serve as receptors for the attachment of bacteriophages and bacteriocins .

7. INNER PLASMA MEMBRANE It is a unit membrane. It is made up of proteins and phospholipids. The outer two layers are phospholipids and the middle layer is protein . Functions It is selectively permeable. It contains permeases , functioning as transporting system. It is the Centre of energy production. It is site of electron flow in respiration and photosynthesis. Phosphorylation occurs here. It functions as the energy plant. It provide a specific site for the attachment of chromosome. Replication starts from here.

8. CYTOPLASM The cell membrane encloses the cytoplasm. It is colloidal in nature. The cytoplasm does not show streaming movement. It contains ribosomes, mesosomes , chlorosomes , magnetosomes , etc. Golgi complex, mitochondria, lysosomes, endoplasmic reticulum, etc. are absent.

9. RIBOSOME Ribosomes are ribo-nucleo protein particles found in all cells. Ribosomes are assembly shops for protein synthesis. They are also described as protein factories . They are found in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes were first observed by Claude in 1941 and named as microsomes . Palade in 1955 named them as ribosomes . Ribosomes are spherical in shape. Bacterial ribosomes are 70S type and eukaryotic ribosomes are 80S type. Each ribosome consists of two sub units, namely large subunit and a small subunit. POLYSOME Generally 5 or more ribosomes line up and join an mRNA chain. Such a string of ribosomes is called polyribosome or polysome

10. RIBOSOME

11. MESOSOMES Mesosomes are intracytoplasmic membranous structures. They are vesicular, convoluted or multilaminated structures, formed of invaginations of the plasma membrane into the cytoplasm. They contain vesicles, tubules or lamellar whorls. Functions of mesosomes It is involved in septum formation during binary fission. It is involved in the export of exocellular enzymes. It forms a link between the plasma membrane and the nuclear material.

12. CHROMATOPHORES(CHLOROSOMES) Chlorosomes are pigment bearing membranous structures. They are in the form of thylakoids. They are involved in photosynthesis. They are found in photosynthetic bacteria .

13. MAGNETOSOMES Magnetosomes are cellular inclusions present in some bacteria. They are sensitive to magnetic field. They help to orient(adjust, face) the cell in a magnetic cell.

14. NUCLEOID The bacterial chromosome remains in a part of cytoplasm. This is not surrounded by a nuclear membrane. The nuclear material without a nuclear membrane is called nucleoid. The bacterial chromosome is made up of a single double stranded circular DNA .

15. PLASMID Plasmids are small, extra chromosomal, double-stranded, circular, self replicating DNA molecules. Almost all the bacteria have plasmids containing low copy number (1-4 per cell) or a high copy number (10 – 100 per cell). The size of plasmids varies from 1 – 500kb. Usually plasmids contribute to about 0.5 – 5.0 % of the total DNA of bacteria. On the basis of conjugative transfer plasmids are classified into two categories. They are Conjugative plasmids Non conjugative plasmids On the basis of function plasmids are classified into five types. They are F-plasmid, R-plasmid, col –plasmid, Degradative plasmids and virulence plasmids

16. TYPES OF PLASMIDS Plasmids are classified in to main types, namely F plasmids or sex plasmids have the ability to transfer chromosomal genes to other cells. R Plasmids has the gene for resistance to one or more antibiotics. Col Plasmids have the ability to synthesize a toxin called colicins . Some plasmids are capable of integrating with bacterial chromosome. Such plasmids are called episomes . DIFFERENT FORMS OF PLASMIDS The plasmids exist in supercoiled form or open circle or linear duplex

17. FUNCTIONS OF PLASMIDS Plasmids make toxins They resist various antibiotics They resist various environmental factors They use unusual chemical compounds as nutrients.

18. FLAGELLUM Bacteria possess one or more long unbranched appendages called flagella. They are used for locomotion. The flagella may be polar when it is arranged at one or both ends TYPES OF FLAGELLUM Monotrichous - Flagellum present at one end. Eg . Pseudomonas aeruginosa . Amphitrichous - Flagella present at both ends . Lophotrichous - A tuft of flagella is present . Peritrichous - Flagella are uniformly distributed throughout the cell Atrichous - Flagellum is absent

19. FLAGELLUM - structure Flagellum consists of three components, namely a basal body, a hook and a shaft. The basal body consists of two sets of rings connected by a rod. The rings are M = ring (Membrane ring) S = ring (super membrane ring) P = ring (peptidoglycan ring) L = ring (lipopolysaccharide ring) The hook connects the basal body with the shaft. The flagella are used for locomotion.

20. STRUCTURE OF A FLAGELLUM

21. PILI (FIMBRIAE) Pili are short hair –like appendages arising from the surface of bacteria. They are shorter than flagella and are also much more numerous. They are straight and not hooked. They are found in Gram negative bacteria. Pili arise from the cytoplasm. They are made up of protein subunits called pilin of fimbrin . The pilin subunits are arranged in a helical manner. Pili divided into two types, namely normal pili and sex pili .

22. FUNCTIONS OF PILI They are involved in the clumping of agglutination of cells. They are involved in the transfer of genetic materials. They serve as phage receptor site. They help in infection by attaching to human epithelial cells.

23. Sex Pili Sex pili are hair –like structures present on the surface of some bacteria. They are longer than normal pili . They have an axial hole. They have a knob at the terminal end. They are determined by plasmids. The bacteria containing sex pili are called F+ cells . One or more sex pili are produced by each F+ cells . The cells donot contain sex pili are called F- cells . Sex pili of F+ cell binds to F-cell during conjugation. DNA is passed from F+ to F- cell through sex pili . There are two types of sex pili . They are F- type and P -type F – type are long and flexible 7 to 8 nm in diameter. P- type pili are shorter than F- pili . They are rigid, with a diameter of 8 - 12 nm.

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