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Bacteria and Viruses

  • Both, bacteria and viruses, are called microbes
  • Infectious or communicable diseases are caused by pathogens
    • Gain entry, colonise tissues, resist defences, damage host tissues
  • Microscopy
    • Magnification → increases the size of an object
    • Resolution/resolving power → ability to distinguish between adjacent points
  • Calculating magnification
    • X = size of picture (measure the size of the diagram in the question)
    • Y = size of object in real life (often given in exam question)
    • Make sure Y has the same unit as X!
      • If X = mm and Y = μm
      • Convert mm to μm = X * 1000
    • Magnification = Xμm / Yμm

Feature

Optical microscope

Electron microscope

Radiation

Light

Electrons

Magnification

400x (max1500)

≈500 000x

Resolution

2µm

1nm / 0,001µm
Electrons have a small wavelength
Thus, higher resolution

Vacuum in microscope

Absent

Present

Specimen is

- Alive or dead
- Stained

- Dead (vacuum!)

Transmission microscope:
Electrons pass through internal
structure of specimen

Scanning microscope:
Beams of electrons are reflected
off specimens surface. Allows a
three dimensional view

Bacteria

  • Grow best at optimum conditions (human body)
    • Constant temperature
    • Neutral pH
    • Constant supply of food, H2O, O2
    • Mechanism removing waste
  • Classification
    • Most bacteria require oxygen to survive: aerobic bacteria
    • Bacteria that are growing in the absence of oxygen: anaerobic bacteria
  • Bacteria are prokaryotes
    • Nucleus (5µm)
      • Contains chromosomes (genes made of DNA which control cell activities)
      • Separated from the cytoplasm by a nuclear envelope
      • The envelope is made of a double membrane containing small holes
      • These small holes are called nuclear pores (100nm)
      • Nuclear pores allow the transport of proteins into the nucleus
    • Undergo asexually reproduction by binary fission / 2 identical daughter cells
  • Only a small number are pathogens. Pathogens cause disease by:
    • Damaging our cells; or
    • Producing toxins; or
    • Directing our immune system against our own cells

Salmonella

  • Damage host cells
  • Found in eggs and poultry
  • Causes disease if food is expired and/or undercooked
  • Organism is taken up by epithelial cells in the intestine
    • Severity depends on ability to invade host cell
    • Some people are more susceptible than others
    • Ligand on pathogen must fit onto receptor proteins on host
    • Structure of receptor protein depends on an individual’s genetic coding
  • Host creates a ruffled surface
    • Invaded cells detach from intestinal wall, creating inflamed lesions
    • Secretion of large amounts of watery fluid into the lumen of the gut
    • This causes watery diarrhoea
  • Other symptoms: vomiting, abdominal pain
  • Management
    • High fluid intake
    • Often self-limiting, don’t require treatment
    • Severe cases, take stool culture and use antibiotics

Vibrio Cholera

  • Produces enterotoxins released from bacteria
    • Enters enterocytes (cells lining the surface of the intestine) by endocytosis
    • Activates the CFTR protein (cystic fibrosis transmembrane regulator)
    • Causes secretion of sodium, chloride and bicarbonate ions from enterocytes
    • Water follows sodium into the intestinal lumen
  • Osmotic loss of up to 10L of water per day!
    • Results in severe watery diarrhoea of sudden onset
    • Dehydration leads to death within hours if untreated
  • Giving oral sodium would cause more water to be secreted into the intestine, worse!
  • Giving oral glucose and sodium (oral rehydration therapy)
    • Glucose is still absorbed through the intestinal wall
    • This is done by a glucose-sodium co-transporter
    • Carries one glucose molecule and one sodium ion across the intestine into the blood
    • Water always follows sodium
    • Diarrhoea is less severe and body becomes rehydrated
  • Oral rehydration therapy (ORT) also contains potassium and bicarbonate ions
    • Prevents electrolyte imbalance
    • Prevents metabolic acidosis

Tuberculosis (TB)

  • Cause an attack by the body's own immune system
  • Symptoms
    • Weight loss
    • Night sweats
    • Cough
    • Rash
  • Transmitted by coughing and sneezing
  • Body tries to destroy the invading bacteria in the lungs
  • But the inflammation causes damage to the surrounding cells
  • Lesions may become hard or spongy, leaving "holes" in the lungs
  • Treated with a cocktail of antibiotics for 6 month

Ability to Cause Disease

  • Depends on
    • Location - what tissue is colonised
    • Infectivity - how easily a bacterium can enter the host cell
    • Invasiveness - how easily a bacterium or its toxin spreads within the body
    • Pathogenicity - how a bacterium causes disease

Antibiotics

  • Produced as natural secretions by bacterial or fungal cells
    • Bacteria and fungi are secondary metabolites (produce antibiotics during a late stage of their life cycle)
    • Antibiotics inhibit growth of natural competitors
    • Gives antibiotic-secreting population an advantage in colonising it
  • Antibiotics harm pathogenic bacteria by
    • Bacteriostatic antibiotics that are slowing down their growth rate
    • Bactericidal antibiotics that kill pathogenic bacteria (in correct concentration)
  • Narrow-spectrum antibiotics (e.g. penicillin) are only effective on a few pathogens
  • Wide-spectrum antibiotics (e.g. chloramphenicol) are effective on many pathogens
  • Prevent formation of bacterial cell wall
    • Bacteria occupy a solution with a more negative water potential than their own cytoplasm
    • Without the cell wall, bacteria are exposed to this hostile environment
    • As a result, bacteria will swell, burst and die
    • Revision: Water Potential from Unit 1 Section 3-1-3(b)
  • Prevent formation of bacterial proteins
    • By inhibiting DNA transcription or mRNA translation
    • Bacteria are unable to synthesise proteins → affects the metabolism of bacteria
  • NOTE: Antibiotics do not affect viruses
    • Viruses have no cellular structure (such as a cell wall)
    • Viruses use the host to reproduce and synthesize proteins

Viruses

  • Transmitted via
    • Sexual contact
    • Placenta (infected woman passing it to her baby)
    • Receiving blood from an infected person (IV drug abuse)

Human Immunodeficiency Virus (HIV)

  • Structure
    • Retrovirus: core contains reverse transcriptase and RNA (2 single strands)
    • Core is surrounded by a protective coat of protein called capsid
    • Capsid is covered by a lipid membrane (acquired when HIV leaves cell after replication)
    • This lipid membrane has antigens and glycoproteins on its surface
    • Those projections recognize receptors on T-lymphocytes
  • AIDS (Acquired Immune Deficiency Syndrome)
    • All T-helper cells infected and destroyed
    • Without T-helper cells, no immune response
    • People highly susceptible to infections and cancer
  • HIV can change its surface proteins and evade the immune system / vaccination is difficult

Cycle of Infection

  • HIV enters body from HIV +ve persons via body fluids such as blood or semen
  • Viral glycoprotein attaches to receptors on cell membrane of T-helper cells
  • HIV enters cell by endocytosis, releasing its RNA and reserve transcriptase into the cytoplasm
  • Reverse transcriptase copies viral RNA strand
  • This forms a double stranded viral DNA in the nucleus of T-helper cell / now called "provirus"
  • Viral DNA is integrated into the host DNA / host cell replicates with provirus
  • Latency period (variable period of time) → infection of more cells, but no symptoms
  • Outbreak
    • Host DNA is transcribed to make new viral RNA
    • Proteins necessary for capsid and envelope are synthesised by infected host cell
  • New viruses assembled with RNA and proteins leave the cell by exocytosis
  • Viral envelope is constructed from cell membrane of host cell