topic 6 Flashcards
[6.1]
Why must we use aseptic techniques when handling cultures of microorganisms?
To prevent:
● potentially harmful microorganisms escaping from your cultures into air + infecting humans
● microorganisms in the air entering and contaminating your cultures
aseptic techniques:
● all equipment (including agar + petri dishes) should be sterile
● flaming equipment in a bunsen flame ensures sterility
● inoculation should be done with a flamed instrument
● lids should be replaced as quickly as possible
● work near a bunsen burner flame
● disinfect surfaces
Culture?
when microorganisms are provided with the nutrients, level of O2, pH + temp they require to grow large numbers, so they can be observed + measured.
steps + techniques involved when culturing microorganisms:
- obtain culture from microorganism
- provide it with right nutrients + environment (eg carbon,nitrogen,protein,minerals) needed to grow
- transfer to the nutrient medium (nutrient broth [liquid] / nutrient agar [jelly] ) using sterile inoculating loop
- close dish airtight
inoculating when using a nutrient broth..
-make a suspension of the bacteria to be grown + mix a known volume of it with the sterile nutrient broth in the flask
- close quickly (cotton wool) to prevent contamination w air
- flask incubated at suitable temp + is often shaken regularly to allow O2 to the growing bacteria
inoculating when using nutrient agar….
if inoculating agar, either:
- Make a streak plate
- Make a spread plate
streak plate?
Sterilise inoculating loop by flaming, dip in culture, sterile plate, at least 3 streaks straight / zig-zag, turn, streak which must overlap with first streak, turn, streak to try to obtain single colonies.
: aim to obtain single colonies by rotating the plate to build layers of the culture on at least 3 separate streaks.
Spread plate?
distributing microorganisms evenly with a sterile spreader.
spread the sample evenly over the entire plate –> allow sample to absorb thoroughly (at least 5 minutes) before inverting the plate for incubation.
selective mediums?
medium containing a very specific balance of nutrients = only very specific microorganisms with those particular requirement will grow in it = mutant strains won’t
= SM important in identifying particular mutant strains of microorganisms + antibiotic resistance
+ / - of using broth medium over agar?
+ Broth can provide anoxic conditions as well as oxygen closer to the surface = can provide information about what kind of oxygen requirements the microbes have.
+ They can also grow a much larger volume of bacteria.
- However, u can’t get a single, discrete, pure colony from a broth to inoculate with/study.
Different methods of measuring growth of bacteria?
- cell counts
- dilution plating
- turbidity (mass + optical methods)
In Cell counts, bacteria + single-celled fungi can be counted directly using a haemocytometer.
What’s a haemocytometer?
bacteria + single-celled fungi can be counted directly using a haemocytometer
= a thick microscope slide engraved with a grid + a rectangular chamber which holds 0.1mm3 volume of liquid.
- cell count
How to conduct a cell count?
- Cells counted using a haemocytometer
- The sample of broth is diluted 1:1 with trypan blue , which stains dead cells blue = so u can identify + count only the living cells.
- cells viewed + counted using microscope.
count cells in each of the 4 sets of 16 squares + take a mean.
Haemocytometer has been pre-calibrated so number of bacteria cells in 1 set of 16 squares = number of cells x 10^4 per cm3 pr broth. - This is repeated at regular intervals throughout growth = shows changing cell numbers.
+ / - of using cell count
+ useful because it counts only viable (living) cells + is accurate.
- slow
- equipment involved is expensive
- large margin for human error
- Diluting plating
- Dilution plating works on the principle that every colony is grown from a single, viable microorganism .
- Immediately after culturing, colonies cannot be counted because a single mass is often present.
= So that single colonies can be seen, the original culture is serially diluted, a lawn plate made and the colonies counted.
-This is then multiplied by the dilution factor to obtain a cell count.
dilution plating is used to find the total viable cell count.
What’s the total viable cell count?
A measure of the number of cells that are alive in a given volume of a culture.
+ / - of using diluting plating
+ only counts viable / living cells
+ no complex / expensive equipment needed
- slow as incubation period needed + serial dilutions needed
What’s turbidimetry?
Turbid?
Turbidimetry:
- Is a specialised form of colorimetry
= measures the concentration of a substance by measuring the amount of light passing through it.
Turbid:
- Is opaque, or thick with suspended matter (bacteria cells)
- Optical methods
[Turbidity]
- As turbidity⬆= transmission⬇+absorbance⬆ (measured in Au).
- This value can be linked to cell count by measuring absorbance of samples with a known cell count (by counting cells with haemocytometer / dilution plating), + using a calibration graph to obtain the cell count in an unknown sample.
+ /- of using turbidity measurement:
+ Quick + can be conducted in the field
- expensive equipment
- counts both living + dead cells
- Requires calibration curve from known samples
- Assumes density of cells is equal across culture
How to assess growth when fungi is used instead of bacteria?
measure diameter of the patches of mycelium
= used to compare growth rates in different conditions + find the optimum
Generation time?
The time between bacterial divisions
Analysing the data:
What are the different phases of a bacteria growth curve?
- Lag phase
- Log Phase / Exponential phase
- Stationary phase
- Death phase / Decline phase
- Lag phase
- when bacteria are adapting to new environment = not yet reproducing at their maximum rate