WJEC Biology for A2: Student Bk

The genetic causes of some cancers are described on 214. Link The structure of the bacterial cell wall is illustrated on p57. Link YOU SHOULD KNOW › › › ››› Antibiotics can be bactericidal or bacteriostatic ››› The structure of the bacterial cell wall ››› The modes of action of penicillin and tetracycline ››› How antibiotics have given rise to antibiotic-resistant bacteria Key Term Antibiotic : A substance produced by a fungus, which diminishes the growth of bacteria. ▪ Cell transformation: viral DNA can integrate into the host chromosome. If the DNA inserts into a pro-oncogene or tumour suppressor gene, it can result in the cell undergoing rapid, uncontrolled division, i.e. becoming cancerous. An example is HPV, the human papilloma virus, which can cause cervical cancer by inserting into the tumour suppressor gene, TP53. ▪ Immune suppression: – Suppression of the reactions that cause B and T lymphocytes to mature. – Reduction in antibody formation, e.g. HIV destroys a group of T helper cells, so B lymphocytes can no longer make antibodies. People with HIV infection are therefore immuno-compromised and highly susceptible to infection. – Reduction of phagocytic cells engulfing microbes. Antibiotics Types of antibiotic Compounds that inhibit the growth of bacteria are called antimicrobials. They are: ▪ Antiseptics used on living tissue, e.g. Dettol. ▪ Disinfectants used on non-living surfaces, e.g. bleach. ▪ Antibiotics. Antibiotics are produced by fungi and act on bacteria, but not on viruses and not on eukaryotic cells. They can therefore treat bacterial infection without harm to the patient. Broad-spectrum antibiotics , such as ampicillin and tetracycline, affect many different Gram-positive and Gram-negative species but narrow-spectrum antibiotics are much more selective, e.g. penicillin G kills Gram-positive bacteria only. Different antibiotics affect different aspects of bacterial metabolism. Their use in medicine has allowed them to be classified: ▪ Bactericidal antibiotics kill bacteria, e.g. penicillin, which destroys bacterial cell walls. ▪ Bacteriostatic antibiotics prevent bacterial multiplication, but do not cause death, e.g. sulphonamides, which are competitive enzyme inhibitors and tetracycline, which inhibits protein synthesis. The bacteria resume their normal metabolism when the antibiotic is no longer present. Bacterial cell walls ▪ Peptidoglycan, sometimes called murein, forms part of the bacterial cell wall. As the name suggests, it contains polysaccharide and short chains of amino acids. Transpeptidase enzymes cross-link the polysaccharide molecules by attaching them to the side chains of the amino acids. The cross-linking makes the cell wall strong, gives the cell its shape and allows it to resist bursting due to the osmotic uptake of water. ▪ The cell walls of Gram-positive bacteria are made of a thick layer of murein, which makes almost 90% of the cell wall. Pores in the murein close during the decolorisation stage of the Gram stain protocol, and so crystal violet is retained with in the cell, which stains violet. Safranin is used as a counter-stain and turns the violet Gram-positive cells purple. The murein is accessible to molecules outside the cell, making it susceptible to attack by lysozyme and penicillin. ▪ The cell walls of Gram-negative bacteria have a thin layer of murein, no more than 10% of the cell wall, surrounded by a layer of lipoprotein and lipopolysaccharide. These lipid- containing molecules are disrupted by the decolorisation stage of Gram staining and the crystal violet stain leaks out of the cell, leaving them unstained. Safranin, the counter- stain, turns the Gram-negative cells red. The lipid-containing layer protects the murein from antimicrobial agents including lysozyme and penicillin. WJEC A2 Biology: Chapter 14 268