Bacteria And Disease Flashcards
What is culturing a micro organism
Growing large numbers of the micro organisms so they can be measured in some way
What are the risks of culturing micro organisms
-Even if the micro organism is harmless there is a risk of a mutant strain arising that may be pathogenic
-There is a risk of contamination of the culture by pathogenic micro organisms from the environment
What is a selective medium
A medium in or on which only a select group of micro organisms with those particular requirements will grow
How do you inoculate an agar plate
Sterilise the inoculating loop by holding it in the bunsen burner until it glows red hot and then leave it to cool
Dip the sterilised loop in the suspension of the bacteria
Streak the loop across the surface of the agar, avoiding digging into the agar
Replace the Petri dish lid, tape closed and label
Turn the dish upside down
How to grow a pure culture
Grow a culture under conditions only the bacteria you want to culture will survive in e.g. anaerobic conditions to grow anaerobic bacteria
How can you count bacteria and single celled fungi
With a microscope and a haemocytometer
What is a haemocytometer
A specialised thick microscope slide with a rectangular chamber that holds a standard volume of liquid. The chamber is engraved with a grid of lines.
What stain is used when using a haemocytometer, and what are the benefits of this?
Trypan blue - it stains the dead cells dark blue so you can count the living cells
What are optical methods of counting cells?
Turbidity
What is turbidimetry
A specialised form of colorimetry.
As the numbers of bacterial cells in a culture increase it becomes more turbid.
As a solution gets more turbid it absorbs more light so less light passes through it.
A colorimeter measured the turbidity showing how much light is absorbed and therefore how many micro organisms are present.
A calibration curve is then produced by growing a control culture and taking samples at regular time intervals.
What is dilution plating
A method used to find the total viable cell count.
It involves diluting a culture to a point where the colonies can be counted.
The accuracy of this can be checked using a haemocytometer
How to measure growth of fungi when culturing
Measure the diameter of the patches of mycelium.
This can be used to compare growth rate in different conditions.
What is the generation time
The time between bacterial divisions
Why do we use a logarithmic scale when considering the growth of bacteria
Because bacteria grows exponentially so the numbers are enormous, therefore the only way to capture this on a scale is when its logarithmic.
What are the 4 stages in a growth curve and what are they
Lag phase - when bacteria are adapting to their new environment and are not reproducing at their maximum rate
Log phase - when the rate of bacterial reproduction is at its theoretical maximum (doubling)
Stationary phase - when the total growth rate is 0 as the number of new cells is equal to the number of deaths
Death phase - when reproduction has almost ceased and the death rate of cells is increasing
What are the reasons for a slow in growth rate of bacteria
A reduction in the amount of nutrients available
A build up of waste products
What are the two main types of toxins bacteria release
Endotoxins and exotoxins
Endotoxins
Lipopolysaccharides that are part of the outer layer of the cell wall of gram negative bacteria.
The lipid part causes disease.
Bacterial endotoxins can cause salmonella spp.
Antibiotic treatment can lead to further endotoxin release due to lysis of the cell wall, meaning the lipopolysaccharide component is released.
Salmonella spp.
Can’t be treated with antibiotics, they usually just reduce symptoms.
Best way to deal with salmonella spp. is to avoid it by washing hands after handling raw meat, cooking meat thoroughly etc.
Exotoxins
Usually soluble proteins produce and released by bacteria.
Produced by both gram positive and gram negative bacteria.
The effects can be more widespread than endotoxins: some damage cell membranes causing cell breakdown or internal bleeding, some act as competitive inhibitors or neurotransmitters, some directly poison cells.
Staphylococcus spp.
Only cause disease if they get inside tissues of the body or the person has a compromised immune system.
Gram positive bacteria that produce exotoxins which can cause anything from mild skin diseases to rapid death.
Can be treated with antibiotics if diagnosed quickly.
Becoming increasingly resistant to antibiotics.
What is host tissue invasion
A third way in which bacteria act as pathogens, in which they invade host tissues and damage cells.
It is the response of the host organism to the cell damage that causes the symptoms of disease.
Mycobacterium tuberculosis
Spread by droplet infection.
People who have a compromised immune system are more prone to TB.
It affects the respiratory system and also suppresses the immune system so the body can’t respond.
Primary infection - invade the cells of the lungs and multiply slowly.
Immune response forms a mass of tissue called a tubercle containing dead bacteria and macrophages.
Immune system usually controls the bacteria after 8 weeks.
Mycobacterium tuberculosis is adapted to avoid the immune system and survive the primary infection stage, staying dormant until the person has a suppressed immune system.
What is selective toxicity
A substance is toxic against some types of cells or organisms but not others, how antimicrobial drugs work
Action of sulfonamides
Antimetabolites interrupt metabolic pathways such as blocking nucleic acid synthesis causing death
Action of tetracyclines, chloramphenicol
Protein synthesis inhibitors interrupt or prevent transcription or translation of microbial genes so protein production is affected
Action of beta lactams e.g. penicillin
Cell wall agents prevent formation of cross linking in cell walls so bacteria are killed by lysis
Action of some penicillins, cephalosporins
Cell membrane agents damage the cell membrane so metabolites leak out or water moves in, killing the bacteria
Action of quinolone
DNA gyrase inhibitors stop bacterial DNA coiling up so it no longer fits with the bacterium
What is bacteriostatic
The antibiotic completely inhibits the growth of the microorganisms
Tetracycline is bacteriostatic
