MICR221 Lecture 4 - Physical Control Methods continued Flashcards
what is moist heat sterilisation?
sterilisation that involves the use of hot water vapour as a sterilising agent
what is dry heat sterilisation?
sterilisation by conduction as heat is absorbed by the exterior surface of an item and then passes inward to the next layer
what is 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 10 minutes
what is thermal death time?
the length of time required to kill all of the cells in a standard suspension of bacteria in a liquid culture at a given temperature
what does it mean if there is no growth on the agar plate or of the control sector?
if there is no growth present on the agar and no control sector then the bacterial culture may not of been viable or the bacteria have a very short thermal death time
what is decimal reduction time (D value)?
the length of time taken to obtain a 10-fold reduction in the number of bacteria in a standard suspension of bacteria in a liquid culture
what is a 10-fold reduction in the number of bacteria?
when 90% of bacteria are killed that is equivalent to a 1 log difference
what is the countable number range on an agar plate to assess bacterial death?
25-250 colonies
what declines as bacteria die out?
time
how is a sample to assess bacterial death counted?
the measured sample is spread out and the colonies are counted
what is K?
the death rate constant that creates the slope of the curve within a graph
what is the equation for K (death rate constant)?
K= 2.3/t x log(Nt/No)
t is time
Nt is the number of surviving cells at time t
No is the number of cells at time zero
what is Nt?
the number of surviving cells at time t
what is No?
the number of cells at time zero
what is the Z value?
the temperature required for a 1 log reduction in the D value
what is the filtration filter (mechanical removal)?
the filtration filter contains pores too small for microorganisms but large enough to allow liquid or air to pass through
what is the pore size to filter out bacteria in filtration filters (mechanical removal)?
0.20um
what is the pore size to filter out mycoplasma in filtration filters (mechanical removal)?
0.1um
what are mycoplasma?
genus of bacteria that lack a cell wall around their bacterial membrane
what are the 3 types of filtration filters?
depth filter (eg HEPA)
membrane filter
nucleopore filter
what are depth filters?
a random array of overlapping fibres that made of paper, glass and oespestos
what is the advantage of depth filters?
that particles get trapped in the fibres because they are overlapping
what are the disadvantages of depth filters?
they are not selective therefore they will randomly trap microbes and they also retain some of the solution you are filtering through
what are depth filters used as?
used as a prefilter as they can capture a-lot of particles
what is an example of a depth filter?
a high efficiency particulate absorbing filter (HEPA)
where are HEPA depth filters conducted?
conducted in a class II biological safety cabinet that protects the specimen, user and the environment from contamination
what does HEPA depth filters protect from contamination?
protects the user, specimen and the environment
what is used for basic specimen protection in HEPA depth filters?
a laminar flow hood
what are the 2 steps involved that HEPA depth filters use to sterilise the air?
1) blows across the workspace area and to the user and provides no initial protection for the user, only protection for the specimen
2) using the HEPA depth filter, the air is double filtered and this protects the user, environment and the specimen
what are 5 examples of solutions that can be filtered by HEPA depth filters?
tissue culture media serum (for lab use) antibiotic solutions gases filtered beer
what are membrane filters?
thin paper discs made up of fibuloaccetate
where are microbes trapped in membrane filters?
microbes are trapped on the surface of the filter because it is so thin
what is the advantage of membrane filters?
the filter is so thin that it does not retain fluid
what are nucleopore filters?
filters that are made up of thin polycarbonate film that is arranged vertically
what is the relationship between nucleopore filters and the number of pores?
nucleopore filters contains a low number of pores
what is the disadvantage of nucleopore filters?
has a very slow rate due to the low number of pores
what are the 2 methods used to control microbial growth?
filtration (mechanical removal) and radiation
what is an example of non-ionising radiation?
UV radiation
what is non-ionising radiation?
radiation with a wavelength of 260nm that damages cell DNA by forming pyrimidine dimers or causes direct protein damage
what is pyrimidine dimers?
covalent chemical bonds that form between adjacent thymine molecules (thymine+thymine)
what is non-iodising radiation used for?
used only for the sterilisation of benches and air
what is the limitation of non-iodising radition?
it cannot penetrate a significant depth in large volumes of liquids
what is the function of a pyrimidine dimer in non-iodising radiation?
to degrade/affect DNA to potentially cause the death of the bacterial cell
what are the 3 repair systems bacteria use to DNA damage?
nucleotide excision repair, direct repair and SOS repair
what is nucleotide excision repair?
the UVrABC endonuclease enzyme removes damaged nucleotides resulting in a single-stranded gap. The single-stranded gap is filled by DNA polymerase I and DNA ligase joining fragments together
what is direct repair?
photo-reactivation (visible light and photolyase-enzyme) is used to break the phosphodiester backbone
what is SOS repair?
a transcriptional repressor protein called Lex A is destroyed and this involves many genes and if often error prone
what is an example of ionising radiation?
x rays and gamma rays
what is ionising radiation?
radiation that kills indirectly by inducing reactive chemical radicals (free radicals) from the breakdown of individual molecules into ions
what causes atoms to ionise in ionising radiation?
the very short wavelengths that are high energy
what is gamma radiation?
3.14 Gy/min (dose-rate) using caesium-137 for non-thermal sterilisation of reagent preparations or small equipment
what occurs when gamma rays interact with water?
free radicals are formed
what is a strong oxidising agent in terms of free radicals?
hydroxyl free radicals
what is a strong reducing agent in terms of free radicals?
hydrogen free radicals
what is the advantage of ionising radiation?
ionising radiation penetrates which allows for sterilisation even after products have been packaged
what are the disadvantages of ionising radiation?
it is expensive to operate, requires elaborate safety precautions such as lead shielding of operators
what are 4 uses of ionising radiation?
used to sterilise lab products (petri dishes), treatment of sewage, treatment of industrial sludges and in food preservation
