BIOLOGYINDEEP

• 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);

Salmonella;