Lecture 4 - physical control methods continued Flashcards
Why is moist heat more effective?
Moist heat = Moist heat is much more effective than dry heat since moist heat uses denaturisation of vital molecules in a bacterial cell such as the nucleic acids and proteins therefore some enzymes are going to be affected
Destroys cells and viruses by degrading nucleic acids and denaturing enzymes and other essential proteins, it also disrupts cells membranes
How does dry heat work?
oxidises bacterial component - slower process - higher temp and longer time
Death from heating is an…
exponential function and thus occurs more rapidly as the temperature is raised
To sterilise or reduce a population it will take longer at a lower temperature than at a higher temperature
An example from the last lecture = batch, flash, UHT (increasing temperature and decreasing time)
Different microbes have
different susceptibility to heat
Thermal death point
First test of efficacy is the Thermal Death Point = the lowest temperature required to kill all cells in a standard suspension of bacteria in a liquid culture within a period of ten minutes (same time, different temperature)
Could later do a second experiment to narrow down the exact temperature of the TDP
Thermal death time
the length of time required to kill all of the cells in a standard suspension (same temperature, different time) (lowest time to kill all)
could do a second experiment from the results of the 1st
Requires only one water bath
Decimal reduction time (D value)
the length of time taken to obtain a ten-fold reduction in the number of bacteria in a standard suspension of bacteria in a liquid culture
Ten-fold reduction = 90% of the population killed = 1 log difference
Measured sample, spread out across an agar plate, count the colonies (because we need to be able to graph and get the ten-fold reduction in the total number)
Dilutions may be required early on because early numbers may be too hard to count
In the example the D value is 1 minute as it takes 1 minute for the numbers to drop 1 logarithmic difference (4-3=1)
Can conclude from the 3 line graph that death from heating occurs more rapidly as the temperature is raised
K is the …
is the death rate constant which is also the slope of the curve
K=2.3/t x log10 (Nt/N0)
Where t=time, Nt is the number of surviving cells at time t, No is the number of cells at zero time
Z value
is the temperature required for one log10 reduction in the D value
In the example the Z value is 5 degrees (120-115 degrees = 5 degrees)
When testing heat and what temperatures kill bacteria -…
Always do a control to ensure that the culture is viable (if no growth then no viable or may have very short TDT etc)
TDP vs TDT
same time, different temperature VS same temperature, different time
Filtration
Mechanical removal
Put through filter with pores too small for microorganisms, but large enough to allow the liquid or air to pass through
Pore size usually 0.2 µm to filter out bacteria (0.1 µm for mycoplasma since it does not have a cell wall therefore can squeeze through small pores of 0.2 µm)
Types of filters
Depth filter e.g. HEPA
Membrane filter
Nuclear pore filter
Depth filter
Structure = Random array of overlapping fibres, usually paper, asbestos or glass, has no regular pore size, structure is quite thick about 10mm high Advantage = stores lots of particles (high dirt handling capacity) Disadvantage = retains fluid in its structure - wouldn’t be used for very small amounts of solution as lots would be lost to the structure also no definite limitation of what size particle will be caught up in there Uses = HEPA (high efficiency particulate air) which is used in biological safety cabinets, or as a prefilter for highly contaminated substances as this has a high dirt handling capacity but would then need to be followed by another filter
Membrane filter
Most common type of filter that is used in sterilisation
Structure = Thin discs made up of cellulose acetate or cellulose nitrate (most common), looks like a piece of paper, acts like a sieve (microbes get trapped on the surface due to the thin nature of the filter)
Advantage = does not retain liquid
Disadvantage = pores can get blocked as it acts like a sieve (low dirt handling capacity) (therefore could use depth filter first as a pre filter)
