Lecture 4 Flashcards
Values on graphs and the final of Physical control methods
What is the rule for temp and time for killing cells
As temp rises time decreases. Death is exponential so increasing the temp makes death occur more rapidly
Define the thermal death point
The lowest temp required to kill all cells in a standard suspension of bacteria in a liquid culture in 10mins
Define the thermal death time
The length of time required to kill all cells in a standard suspension of bacteria in a liquid culture at a given temperature
Define the decimal reduction time/D value
The length in time it takes to have a 10-fold reduction in the number of bacteria in a standard suspension liquid culture (90% killed/ 1 log difference)
Define the death rate constant/K and how do you calculate it
K is the death rate constant which is also the slope of the curve. It can be calculated by K= 2.3/t x log10 (Nt/No)
where t is time, Nt is number of surviving cells at time t and No is the number of cells at 0 time.
K should be negative as the slope is going down
Define the z value
The temp change required for one log10 reduction in the d value
What states of bacterial culture can be put through filtration
Liquids and Gases
What is the standard pore size for filtration
0.2 micrometres
Define a depth filter
A random array of overlapping fibres of paper, glass or asbestos. It is thick compared to others (5-7mm high)
disadvantages- things are trapped randomly, retains the liquid coz too thick, wastes expensive product
advantages- high dirt handling capacity, traps lots of things
typically used in accordance with another filter and is used as HEPA filters for biosafety air decontamination
Define a membrane filter
Typically used with the depth filter, smaller with a definite pore size and is the most common filter used. Its made up of cellulose acetate or cellulose nitrate and acts like a sieve trapping everything on the surface.
Disadvantages- easily blocked due to pore size
advantages- doesnt retain fluid and has a definite pore size
example is tissue culture media
Define a nucleopore filter
Smallest of the filters and is made up of polycarbonate film to keeps the thinness
disadvantages- slow flow rate due to the size of the pores
advantages- the extremely define pore size
example is preparation of samples for electron microscopy
Define non-ionising radiation
Typically UV radiation with a wavelength of 260nm, it damages the DNA in a cell by forming pyrimidine dimers or by direct protein damage.
this only achieves disinfection meaning it doesnt kill endospores.
It also can only be used on surface level liquid of other surfaces
What type of bond forms between the thymines and what effects does this have
Covalent bonds from between thymines to make a thymine dimer also known as a pyrimidine dimer. This MAY result in death but it is not a constant result
What are the bacterial repair systems that respond to thymine dimers being formed
Nucleotide excision repair- UvrABC endonuclease enzymes remove damaged nucleotides. Resulting single stranded gaps are then filled by DNA polymerase I and DNA ligase joins the fragments
direct repair- uses photoreactivation (visible light and photolyase)
recombinational repair- corrects damaged DNA using Rec A
sos repair- last resort effort, a transcriptional repressor protein is destroyed, this involves 40+ genes and is extremely error prone/ may damage the cell further
Define ionising radiation
A much smaller and more intense wavelength than non ionising. Kills indirectly by forming reactive free radicals by breaking individual molecules into ions (IONising radiation). Eg. H2O is broken down by gamma rays, into hydroxyl (oxidising) and Hydrogen(reducing) free radical that are extremely strong. Its these that react with the bacteria.
This is used as sterilisation
What are the advantages and disadvantages of using ionising radiation for sterilisation
Adv- its penetrating meaning it can be used in the food industry to sterilise even after packaging has already occurred
dis-expensive because of the safety requirements
