Microorganisms use us for food, shelter and their reproduction

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

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