NAJRAN UNIVERSITY College of Medicine

1. NAJRAN UNIVERSITY College of Medicine Microbiology &Immunology Course Lecture No. 2 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

2. What is Microbiology? Too small object to be seen by the ordinary eyes • Refers to life Means science Microbiology is the science studying very small living cells that could not be seen by ordinary eyes. These cells are called: microbes, organisms, microorganisms, pathogens, or bugs.

3. Bacteriology Mycology Microbes Virology Parasitology

4. Bacterial structure Bacteria are microscopic, rigid‑walled, unicellular organisms

5. Nomenclature of microorganisms A kingdom → Orders → Families → Genera → Species → Strains. Depends on 2 names; the name of the genus and the name of the species (e.g. Staphylococcousaureus, Escherichia coli, etc..)

6. Size of bacteria • Lenghtscale commonly used to describe the microbial world: • Micrometer (µm), known as micron = 10-6 meter • Nanometer (nm) = 10-9 meter • Bacterial cells range in size from 10 -100 µm

7. Shape of bacteria • Sphere (coccus, cocci) • Rod (bacillus, bacilli) • Coma shaped rod (Vibrios) • Branching rod • Spiral (spirillum, spirilli) • Spore-forming rod • Pleomorphic

8. Cell arrangement • Single cell • Pairs • Tetrads • Chains • Clusters

10. Structure of bacteria • Bacterial cell is composed of 4 essential components: • Cell wall • Cell membrane (cytoplasmic membrane) • Cytoplasm • The nuclear body • Some bacteria contain: • capsule, flagella, fimbriae or spores

12. Bacterial Cell Wall • ‑ It is the outermost basic structure of the bacterial cell. • ‑ It is rigid due to its high content of a mucopeptide polymer (peptidoglycan). • ‑ Osmotically insensitive, protecting the cell from bursting (because the cytoplasm has a high osmotic pressure). • The main functions of the cell wall are: • 1- Protection of bacterial cell (due to its rigidity & osmotic insensitivity). • 2- Maintaining the shape of the cell (due to its rigidity). • 3- Responsible for the staining affinity of the organism. • 4- Plays a role in cell division (forms a transverse cell wall or a septum)

13. The composition of the cell wall is different among bacteria. ‑ In Gram‑positive organisms the cell wall is formed of a thick layer of poptidoglycan and a thin layer of techoic acid. ‑ In Gram negative organisms the cell wall is composed of 3 layers: ‑ A thin layer of peptidoglycan surrounding the cytoplasmic membrane. ‑ An outer layer composed of lipopolysaccharide (LPS) called endotoxin. This LPS layer is subdivided into 3 layers (an inner highly toxic lipid A, which is attached to a middle polysaccharide core, and the outer polysaccharide side‑chains known as the somatic (0) antigen). ‑ A periplasmic space, which separates the inner peptidoglycan layer from the outer lipopolysaccharide layer. This space is filled with a gel‑like substance.

15. Cytoplasmic Membrane ‑ A semipermeable delicate membrane that surrounds the cytoplasm, located interior to the cell wall. The main functions of the cytoplasmic membrane are : 1. Selective permeability (it is osmotic semipermeable barrier). 2. Active transport of substances that cannot enter the cell passively. 3. Energy production (by oxidative phosphorylation and electron transport). 4. Excretion of toxins and hydrolytic enzymes outside the cell. 5. Biosynthesis of cell wall and other proteins. 6. Plays an important role during cell division (by being the site of DNA attachment during replication and by forming the transverse septum).

16. The Mesosomes • Convoluted invaginations of the cytoplasmic membrane. • Functions: • 1- They are the sites of attachment of the chromosome during cell division. • 2- They are the sites of respiratory activities of the cell.

17. The Cytoplasm It is rich in ribonucleic acid (RNA). it also contains the nuclear body,and it is rich in ribosomes and cytoplasmic granules Ribosomes: They are the sites of protein synthesis. Cytoplasmic granules: ‑ Accumulation of food reserves in cytoplasm ‑ e.g. volutin or metachromatic granules in Corynebacteriumdiphtheriae.