L4 Physical control methods Flashcards
define thermal death point
lowest temperature to kill all cells in a standard suspension of bacteria in a liquid culture within 10 minutes
how is a thermal death point tested for
heating bacteria in solution to different temperatures for the same time
streak, incubate and check agar for growth compared to a control section
define thermal death time
LENGTH OF TIME to KILL all of the cells in a standard suspension of bacteria in a liquid culture at a given temperature
how is a thermal death time found
heat cultures at the same temperature for different times
streak, incubate and check agar plates for growth compared to a control (untreated)
define decimal reduction time
LENGTH OF TIME taken to obtain a TEN-FOLD reduction in the number of bacter in a standard suspension of bacteria in liquid culture
-it is independent of the initial population
another name for decimal reduction time
D-value
what does a ten-fold reduction mean?
90% is killed
1 log difference
what term is most commonly used to assess the efficacy of heat treatments
decimal reduction time/ D-value
three terms used to assess the efficacy of heat treatments
thermal death point
thermal death time
D-value/ decimal reduction time
what are the two principles involved in killing microorganisms
- 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 lower temperatures than at higher temperatures
what are the important things to know about a D-value
measured sample
its spread out
the colonies are counted and graphed
what is K in terms of a D-value
death rate constant
what can filtration be used for
sterilising liquid and air
what is the usual pore size for a filter
0.2um
what are the three types of filters
depth (HEPA)
membrane
nucleopore
what does HEPA stand for
high efficiency particulate air filter
what is the structure of depth filter
random array of overlapping fibres
asbestos, glass or paper
5-7mm thick
disadvantages to a depth filter
retains a lot of liquid
what are depth filters usually used for these days
pre-filter to remove large particles or a large number of particles to reduce clogging in final filtration
biological safety cabinets
advantages to depth filter
high dirt handling capacity
structure of a membrane filter
regular pore size (uniform/known diameter)
made from cellulose acetate or cellulose nitrate
thin as paper
what is the most commonly used filter for sterilisation
membrane
how does a membrane filter work
traps on the surface
disadvantages to a membrane filter
low dirt handling capacity (easily blocked)
-so very contaminated solutions need to be pre-filtered
structure of a nucleopore filter
polycarbonate film
completely uniform pores
disadvantages to a nucleopore filter
very slow flow rate
what is a nucleopore filter used for
processing samples for scanning electron microscopy
what are the two types of biological safety methods
lamina flow hood
Class II biological safety cabinet
how does a lamina flow hood work
blows sterilised air over the work space
what does a lamina flow hood protect and how
the speciman but not the user as blows the contaminated air out without sterilisation
what does a Class II biological safety cabinet protect and how
user and speciman
It has a glass screen protecting the user as well and a vent that absorbs air leaving the top and front
It blows sterilised air into the cabinet
what does a Class II biological safety cabinet protect and how
user and speciman
It has a glass screen protecting the user as well and a vent that absorbs air leaving the top and front
It blows sterilised air into the cabinet
what are some examples of solutions that can be filtered
tissue culture media serum antibiotic solution gasses filtered beer
microbe of the day
mycoplasma
facts about mycoplasma
lack a cell wall
resisitant to anti-biotic like beta-lactams due to lack of cell wall
smallest bacteria yet discovered
some are pathogens
what is the bacteria that requires a 0.1um pore when filtering
mycoplasma
why does mycoplasma require a smaller pore hole
no cell wall so it can squeeze through
what are the two types of radiation
ionising
non-ionising
what are some examples of ionising radiation
X-Ray
gamma
cathode
what are some example of non-ionising radiation
UV
what does ionising radiation result in?
sterilisation
what does non-ionising radiation result in?
disinfection
how does non-ionising radiation work
UV light (wavelength 260nm) damages DNA by forming pyrimidine dimers or direct protein damage
application of UV radiation
air and bench serilisation
why cant UV be used to sterilise large volumes of liquid
cannot penetrate it to a significant depth
how does UV light affect DNA
covalent chemical bonds form between adjacent thymine molecules forming a thymine diamer
what are the 4 types of repair systems bacteria use to remove the thymine diamers
nucleotide ecision repair
direct repair
recombinational repair
SOS repair
what happens during nucleotide excision repair
The UvrABC endonuclease removes damaged nucleotides
the gap in the single strand is repaired by polymerase and ligase
what happens during direct repair
photoreactivation
uses visible light and photolyase
what does recombinational repair involve
Uses Rec A enzyme to correct damaged DNA
what does SOS repair involve
transcriptional repressor protien
many genes
which repair system is used as a last resort and why
SOS repair as it is very error prone
when does cell death occur
when damage is greater than repair
order from most to least UV sensitive types of bacteria
gram negative
gram positive
endospores
why is ionising radiation more suitable for control
the shorter wave lengths carry more energy
how does ionising radiation kill bacteria
indirectly by inducing reactive chemicals (free radicals)
it breaks individual molecules into ions hence ionising radiation
what does gamma radiation do to water
water + gamma = positive water + electron
electron + water = negative water
positive water = strong reducing agent H + strong oxidising agent OH
negative water = “
why is ionising radiation indirect
the radiaiton causes molecule to split into free radicals which then go on to react and damage the bacteria
applications of ionising radiation
penetrating radiation so it can sterilise products after packages (food preservation)
sterilise lab products: petri dishes
sewage treatment
disadvantages to gamma radiation
expensive
dangerous (requires lead sheilds)
