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Diseases > Bacteria
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Bacteria (single celled, prokaryotic)

  • Only small number are pathogens
  • Asexually reproduction by binary fission / 2 identical daughter cells
  • Grow best at optimum conditions (human body)
    • Constant temperature
    • Neutral pH
    • Constant supply of food, H2O, O2
    • Mechanism removing waste
  • Most bacteria are aerobic / obligated aerobes
  • Aerobic bacteria growing with absent oxygen / facultative anaerobes
  • Bacteria which find oxygen toxic / obligate anaerobes

Sigmoid growth curve shows the number of bacteria plotted against time

  • Bacteria MUST grow in closed system and nutrient medium → BACTERIAL CULTURE
    • 1) Population grows slowly - LAG PHASE
    • 2) Rapid increase of population growth - LOG PHASE
    • 3) Reaches equilibrium when number remains constant - STATIONARY PHASE
  • Lag Phase → initial phase
    • Low number of reproducing organisms
    • Bacteria increase in size before division
    • This requires nutrients which need to be digested
    • Digestion requires enzymes, proteins → activation of genes → time consuming process
  • Log Phase → exponential phase; max growth rate; steep curve
    • Optimum conditions: no limiting factors, waste does not accumulate to a toxic level
    • Bacteria most susceptible due to production of new cells
      • Antibiotics inhibit cell wall formation
      • Antibiotics inhibit DNA replication
      • Antibiotics inhibit protein synthesis
  • Stationary Phase → reduced growth rate
    • New cell production balanced by death of cells
    • Limiting factors, declining nutrients, accumulating waste influence population size
  • Decline Phase → bacteria stop dividing
    • Death rate increases; numbers may fall to zero
    • Lack of nutrients, build up of toxic waste products
  • Aseptic conditions
    • Sterilise equipment, instruments, thus, to prevent contamination with the culture.
    • Use high temp and disinfectants

Total cell count

  • Number of cells whether living or dead
  • Count cells with haemocytometer
  • Stop bacteria entering the flask with a stopper. Important as bacteria would have been caused reduced growth rate of yeast/killed yeast and competed for space/nutrients
  • Culture is shaken to achieve a uniform distribution of yeast cells/spread out yeast
  • Larger number is taken to avoid anomalies/produce an average

Measurement of growth

Generation time: time taken for a bacterial population to double

  • Rate of population
    • = increase in number of yeast cells/time
    •  = number larger - number less/(time larger - time less)
  • Suppose the number of cells in one square are 6 8 9 5 7
  • The sum of the cells in 5 squares is 35
  • The mean for one type B square is 7
  • Therefore 25 squares have 25 x 7 = 175 in 0.1mm3
  • In 1mm3 there will be 1750 cells or 1,8 x 103

Viable cell count

  • Only living cells since these are the only ones capable of dividing
  • 1cm3 original sample is diluted in 9cm3 distilled H2O
  • Mix 1cm3 from last dilution with 9cm3 distilled H2O - serial dilution
  • 1cm3 of each dilution is put on an agar plate and counted. Number is multiplied by the dilution factor

Measurement of growth

  • Number of colonies on the 10-3 dilution plate = 35
  • Number of viable colonies in 1cm3 of 10-4 dilution of milk
    • 35 x 1/0.1 = 350
  • Sample was diluted by 10-3
  • Number of bacteria in 1cm3 of the original sample = 350 x 103

Biological factors

  • Bacteria are effected in growing by nutrients: C, H, N, P, S
  • Temp: low → low speed of enzyme reactions; high → denaturisation of enzymes
  • pH → tolerate a wider range of pH than plant and animal cells
  • O2 → some grow better in presence, but some grow in absence

Koch's Postulates1

  • A list of postulates (criteria) must be fulfilled to proof an infective cause for a disease
    • "Organism must be sufficiently abundant in every case to account for the disease
    • Organism associated with the disease can be cultivated artificially in pure culture
    • Cultivated organism produces the disease upon inoculation into another member of the same species
    • Antibodies to the organism appear during the course of the disease"1
  • Exceptions are possible
    • Number of organism causing disease might be very low (eg tuberculosis)
    • Cultivation might be difficult
    • Animals must be used as it is unethical to infect a human with a causative organism
    • Antibodies may not appear if the immune system is inhibited

Entry of Microorganisms (Pathogens) into the Body

  • Damaged skin
    • Skin acts as a barrier to infections
    • Tetanus occurs when the bacterium Clostridium tetani enters a wound
  • Mucus membrane of respiratory tract
    • Air containing droplet of infectious material are breathed in
    • Mycobacterium tuberculosis causes tuberculosis
  • Digestive track
    • Vibrio cholerae causes cholera when drinking water infected with faeces
    • Salmonella enteritis causes food poisoning when eating undercooked food
    • These organisms are resistant to acidic conditions in the stomach
    • Acid protects against microorganisms by providing a hostile environment
  • Others
    • Transmission by vectors (e.g. malaria via Plasmodium parasite when mosquito vector takes blood)
    • Direct entry through the intact skin (e.g. Schistosomiasis where the larval stage schistosome burrows through the skin of the feet)

Pathogenesis: How Microorganisms Cause Disease

  • Damage or destroy host cells - e.g. HIV, Salmonella
    • Organism is taken up by epithelial cells in the intestine
      • HOST SPECIFIC: ligand on pathogen must fit onto receptor proteins on host
      • Some hosts are more susceptible than others because proteins depend on gene coding
    • Destroy brush border of microvilli
    • 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 → diarrhoea
  • Produce toxic waste - e.g. Vibrio cholerae
    • Are harmless but produce harmful "exotoxins" - toxins released from the cell
    • Causes loss of chloride and hydrogencarbonate ions from the intestinal cells
    • Osmotic loss of up to 10 litres of water per day
    • Impaired absorption of water and salt from the gut
    • This explains severe watery diarrhoea and death from dehydration
  • Body's own immune response to the presence of microorganisms which produce the symptom
    • e.g. Mycobacterium tuberculosis
    • Body tries do destroy the invading bacteria
    • This causes inflammation and damage to the surrounding cells occur
    • Lesions may become hard or spongy, leaving "holes" in the lungs, sometimes damaging blood vessels
  • Some bacteria will cause all of the 3 ways above; Some require a large number of bacteria for a disease; Some will only a few number of bacteria
  • Microorganisms may enter the lymphatic system via tissue fluid and are carried around the body in this way
  • Ability of bacteria to cause disease relies 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 cause disease

Tuberculosis (Myobacterium tuberculosis)

  • Lung most common infected organ
  • Inhaling droplets exhaled from a carrier during coughing causes the infection
  • Latent period: bacteria may lie inactive for up to 30 years and become active as primary tuberculosis (TB)
  • Symptoms are fever, loss of weight and persistent coughing
  • Bacteria destroy lung tissue and cause accumulation of fluid in the pleural cavity
  • Coughing up blood is common because bacteria destroy lung tissue
  • Treatment
    • Antibiotics for a six-month period to ensure eradication
    • Combinations of drugs prevent development of resistant strains
    • Vaccine is a live attenuated strain of TB

Salmonella

  • Symptoms: typhoid fever, intestinal infections
  • Food poisoning caused from uncooked poultry, beef, and eggs
  • "Salmonella enters the body in contaminated food/drink
  • The bacterium passes from the esophagus, through the stomach, into the intestine
  • It enters cells lining the small intestine to multiply population increases
  • Some bacteria die and release an endotoxin
  • This causes (symptoms) diarrhoea, vomiting, nausea, abdominal pain (food poisoning)"1