4.1

• Pathogens include bacteria and viruses, which are the main disease causing pathogens in humans.
• Bacteria are prokaryotic cells with no membrane bound organelles and no nucleus, their genetic information is stored in the form of a circular strand of DNA.
• Viruses consist of just nucleic acid enclosed in a protein coat and their genetic material can take the form of DNA or RNA.
• Bacteria do not require a host to survive, whereas viruses are entirely dependent on their hosts and cannot survive without them.
• Bacteria are significantly larger than viruses.
• Antibiotics are only used when needed and their course is completed to ensure that all the bacteria are destroyed and to minimise the selection pressure on bacteria to prevent resistant strains from forming.
• Resistance to antibiotics results in antibiotic resistant bacterial infections in hospitals such as MRSA.
• All staff must follow the code of practice which includes strict hygiene regimes such as washing hands with alcohol based antibacterial gels and wearing suitable clothing which minimises the transmission of resistant bacteria.
• Hospitals have developed various ways of controlling the spread of antibiotic resistant infections, including screening new patients at arrival, isolating and treating them if they are infected to prevent the spread of bacteria between patients.
• Bacteria have a cell membrane, cell wall and cytoplasm as well as other organelles such as ribosomes, plasmids, flagellum and pili, whereas viruses possess no such structures.
• An example of a bacterial disease is tuberculosis, also known as TB, which is caused by a bacterium called Mycobacterium tuberculosis that infects phagocytes in the lungs.
• TB is symptomless in the first infection as the infected phagocytes are sealed in tubercles as a result of an inflammatory response in the lungs.
• The bacteria lie dormant inside the tubercles as they are not destroyed by the immune system as tubercles are covered with a thick waxy coat.
• When the immune system becomes weakened, the bacteria become active again and slowly destroy the lung tissue, leading to breathing problems, coughing, weight loss as well as fever.
• TB can potentially lead to death.
• Meningitis can be caused by bacterial infection of the meninges, a set of protective membranes around the brain.
• Human Immunodeficiency Virus (HIV) is an example of a viral infection which causes AIDS.
• The first symptoms of HIV include fevers, tiredness and headaches.
• After several weeks, HIV antibodies appear in blood thus making a person HIV positive.
• After this period, the symptoms disappear until the immune system becomes weakened again, leading to AIDS.
• Influenza is a viral disease where the virus infects the ciliated epithelial cells of the gas exchange system.
• There are different strains of influenza.
• Physical barriers to infection in animals include the skin which is a tough physical barrier consisting of keratin, stomach acid (hydrochloric acid) which kills bacteria, gut and skin flora which compete with pathogens for food and space.
• Non-specific responses of the body to infection include inflammation which is when histamines released by mast cells in injured tissue cause vasodilation which increases the flow of blood to the infected area and increases permeability of blood vessels, lysozyme action which is an enzyme found in secretions such as tears and mucus which kills bacterial cells by damaging their cell wall, interferon which prevents viruses spreading to uninfected cells by stopping protein synthesis in viruses, phagocytosis which is a process in which white blood cells engulf pathogens thus destroying them by fus
• Chemical defences include menthols produced by mint which are an example of terpenoids, these are essential oils with antibacterial properties.
• Phenols such as tannin interfere with digestion thus inhibiting insects from attacking the plant, alkaloids such as caffeine and morphine have a bitter flavour- preventing herbivores from feeding on the plant.
• After the pathogen is engulfed and destroyed, its chemical markers called antigens are then presented on the surface of the phagocyte, the phagocyte then becomes an antigen presenting cell which activates other types of immune cells, immune response will be stimulated if the antigen is recognised as foreign.
• Necrosis is a process where cells near the site of infection are killed with the help of intracellular enzymes which are activated by injury, this is a variation of necrosis known as canker, the necrosis of woody tissue in the main stem or branch.
• The specific immune response is antigen specific and produces responses specific to one type of pathogen only.
• Defensins are cysteine-rich proteins involved in inhibition of transport channels, hydrolytic enzymes such as chitinases are released with the purpose of breaking down the cell wall of an invading organism.
• Stomata close to prevent entry to leaves, cell walls thicken with cellulose, callose deposits are made between cell wall and cell membrane near the site of infection to strengthen the cell wall.
• Athlete’s foot in humans is caused by a fungus and is spread by direct contact with the spores on the skin surface or other surface.
• Malaria is an example of indirect transmission via vector in the form of a female Anopheles mosquito.
• Antibodies also possess a variable region which differs for each type of antibody, of varying amino acid sequence which is responsible for the specificity of antibody for one particular antigen.
• Bacteriostatic antibiotics inhibit the growth of bacteria by stopping protein synthesis and production of nucleic acids so the bacteria can’t grow and divide.
• There are also two subtypes of immunity; natural or artificial: Natural active immunity arises from being exposed to an antigen/getting the disease whereas natural passive immunity is the result of crossing of mother’s antibodies through the placenta and their presence in breast milk.
• T helper cells stimulate B cells and T killer cells to divide.
• There are two types of antibiotics: Bactericidal antibiotics kill bacteria by destroying their cell wall thus causing them to burst.
• Immunity can either be active or passive; active immunity results from the production of antibodies by the immune system in response to the presence of an antigen whereas passive immunity results from the introduction of antibodies from another person or animal.
• Antibiotics can also be used to fight infection by killing the bacteria and stopping their growth.