- Both, bacteria and viruses, are called microbes
- Infectious or communicable diseases are caused by pathogens
- Pathogens gain entry; colonise tissues; resist defences; damage host tissues;
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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
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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, … to prevent contamination with the culture.
Use high temp and disinfectants
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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
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Measurement of growth |
| Generation time: time taken for a bacterial population to double |
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= increase in number of yeast cells/time
= number larger - number less/(time larger - time less) |
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- 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
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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
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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
- Sample was diluted by 10-3
- Number of bacteria in 1cm3 of the original sample = 350 x 103
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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
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Viruses (200nm) |
- Consists of a core containing genetic material DNA or RNA
- This is surrounded by a protective coat of protein called capsid (subunits: capsomeres)
- The capsid is (sometimes) surrounded by an envelope of lipoprotein
- Antigens, glycoproteins on its surface recognize receptors on T-lymphocytes
- They cause damage by taking over the host cell for multiplication
- Do not have a cellular structure / don't respire or need food
- Transmitted via sexual contact; infected woman passing it to her baby through the placenta
- Also by receiving blood from an infected person
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Human Immunodeficiency Virus HIV |
- AIDS (Acquired Immune Deficiency Syndrome)
- All T-helper cells infected (and destroyed)
- Number of T lymphocytes decrease dramatically / sign for the disease
- People highly susceptible to infections, diseases and cancer
- Retrovirus: core
contains reverse transcriptase and its genetic material as RNA
- HIV can change its surface proteins and evade the immune system / vaccination is difficult
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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 the capsid and for the envelope are synthesised by the infected host cell
- New viruses assembled with RNA and proteins leave the cell by exocytosis - viral envelope is constructed from the cell membrane of the host cell
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Association of microorganisms with disease |
| 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
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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)
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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
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