infection

• What are pathogens?

  Disease causing micro- organisms
• Most common pathogens that affect the human body

  • Bacteria
  • Viruses
  • Fungi
  • Animal parasites
• Bacteria


  • Prokaryote, unicellular organisms with a simple internal structure
  • Lack a nucleus
  • DNA floats freely in the cytoplasm or is in the form of circular plasmids
  • Single-celled organisms
• The great majority of bacteria are harmless to humans - NON PATHOGENIC
• Many bacteria are essential to life on earth, through their role in the decomposition of organic material and the cycling of the elements
• Huge numbers of bacteria live on our skin, in our alimentary canal and in other parts of the body
• Bacteria size


  Diameter ranging from 0.5-2.0 micrometres (1micrometre=1x10^-6 m) and length ranging from 1 to 10 micrometre
• Bacteria can only be seen with a microscope
• Viruses


  • Not considered alive, cannot replicate their own genetic material and must instead rely on a host
  • An infectious agent, too small to be seen with a light microscope, consisting of a protein sheath surrounding a core nucleic acid
• Virus size


  • 20-750nm (nanometres 1nm= 1 x 10^-9 m)
  • Too small to be seen with an ordinary light microscope
• All viruses found to contain genetic material in the form of a molecule of either DNA or RNA but NEVER both
• Viruses are not living things, cannot reproduce themselves
• How viruses reproduce


  • They infect a living cell and its DNA or RNA induces the cell to manufacture more virus particles
  • New virus particles are then able to leave the host cell to infect others, during this process the cells become damaged or changed or die
• Viruses differ in the type of cell they invade, therefore the symptoms shown relate to the tissue affected
• Some viruses multiply in bacterial cells causing death of bacterium- known as bacteriophages
• Communicable diseases


  Caused by pathogens such as bacteria and viruses and are spread from one person either directly or indirectly
• Ways pathogens can be transferred


  • Contact (direct or indirect)
  • Ingestion of contaminated food/drink
  • Body fluids
  • Droplets
  • Airborne
  • Vectors (insects, ticks, mites)
• Bactericidal antibiotics


  Kill bacteria by changing the structure of the cell wall/membrane or by disrupting the action of essential enzymes
• Bacteriostatic antibiotics


  Stop bacteria from reproducing usually by disrupting protein synthesis
• Broad spectrum antibiotics


  Affect a wide range of different types of bacteria
• Narrow spectrum antibiotics


  Effective only against specific types of bacteria
• Antibiotic resistance (Multiple Drug Resistance)


  When some strains of bacteria have gradually evolved and become resistant to most or all of the antibiotics that have been used to fight them. Such bacteria are now referred to as 'super bugs'
• Causes of antibiotic resistance


  • Overuse of antibiotics in medicine and agriculture
  • Failure to consider patients weight and previous use of antibiotics
  • Failure to take the complete course of treatment
  • Failure to take the course at the prescribed daily intervals
  • Failure to rest for sufficient recovery time
• First line of defence (external)


  Physical barriers such as the skin and mucous membranes
• Skin


  • An effective barrier covering the outside of the body, very good at stopping the entry of micro-organisms, provided its not broken by cuts and abrasions
  • Bacteria that lives on skin at all times, so potential pathogens find it difficult to be established
  • Secretes sebum and sweat which contain substances that kill some pathogenic bacteria
• Mucus


  Mucous membranes line body cavities that open to the exterior and secrete mucus, which traps particles and therefore inhibits the entry of micro-organisms to the organs of the body
• Hairs


  Found in the nasal cavity and ears, trap up to 90% of particles inhaled when breathing
• Cilia


  Tiny hair-like projections from cells that are capable of a beating motion, move mucus containing trapped particles and micro-organisms towards the throat
• Acids


  Stomach juices, vaginal secretions, urine and sweat are slightly acidic and kill many bacteria
• Lysozyme


  An enzyme that kills bacteria, found in tears, saliva, sweat, secretions of the nose and tissue fluid
• Cerumen (ear wax)


  Protects the outer ear against infection by some bacteria, it is slightly acidic and contains lysozyme
• Movement of fluid


  The flushing action of body fluids helps keep some areas relatively free of pathogens, such as urine flowing through the urethra
• Protective reflexes


  Automatic, involuntary responses to a stimulus that help protect the body from injury or infection, including sneezing, coughing, vomiting and diarrhoea
• Second line of defence (internal)


  Non-specific defences that work to eliminate pathogens that get past the external defences
• Phagocytosis


  Organisms that penetrate the external defences are attacked by phagocytes (specialised leucocytes that engulf and digest micro-organisms and cell debris)
• Types of phagocytes


  • Monocytes and Macrophages
  • Neutrophils
  • Dendritic cells
• Inflammatory response


  Occurs when tissue is damaged or infected, leading to increased blood flow, vessel permeability and phagocytosis, resulting in heat, redness, swelling and pain
• Steps of the inflammatory process


  1. Mast cells activated

  2. Histamine released (increases blood flow and permeability)

  3. Heparin released (prevents clotting)

  4. Complement proteins attract phagocytes

  5. Pain receptors stimulated

  6. Pus forms

  7. Mitosis and tissue repair
• Fever


  An elevation of body temperature due to a resetting of the body's thermoregulatory centre, often accompanying an infection
• Fever


  An elevation of body temperature due to a resetting of the body's thermostat, controlled by the hypothalamus, to a level higher than normal