1. 1) If the decimal reduction time (D) of E. coli at 121 Celsius degrees is 1.5
ID: 207826 • Letter: 1
Question
1.1) If the decimal reduction time (D) of E. coli at 121 Celsius degrees is 1.5 mins, how many cells would have died after 1.5 mins of exposure to heat of a population of 900,000 E.coli bacterial cells?
2) If I am preparing culture media to use in the laboratory, which method would be more suitable to sterilize this liquid media? Why?
3) In the laboratory we also prepare culture media containing urea (a heat labile component). Which method would you recommend being used to remove all bacteria from this medium? What specific instructions referring to the materials used in the process would you provide? 1.
1) If the decimal reduction time (D) of E. coli at 121 Celsius degrees is 1.5 mins, how many cells would have died after 1.5 mins of exposure to heat of a population of 900,000 E.coli bacterial cells?
2) If I am preparing culture media to use in the laboratory, which method would be more suitable to sterilize this liquid media? Why?
3) In the laboratory we also prepare culture media containing urea (a heat labile component). Which method would you recommend being used to remove all bacteria from this medium? What specific instructions referring to the materials used in the process would you provide? 1.
2) If I am preparing culture media to use in the laboratory, which method would be more suitable to sterilize this liquid media? Why?
3) In the laboratory we also prepare culture media containing urea (a heat labile component). Which method would you recommend being used to remove all bacteria from this medium? What specific instructions referring to the materials used in the process would you provide?
Explanation / Answer
1) Decimal reduction time (D) is the time required to reduce a population of microbes by 90% (10 fold) at a specified sterilization condition. At the end of one cycle reduction, 90% of the organisms will be killed and only 10% remains. In the problem, the decimal reduction time is given as 1.5min. Therefore at the end of 1.5 min of sterilization, 90% of the organism will be killed and only 10% will be alive.
If 900,000 E.coli cells were present initially, then the number of organisms killed will be calculated as,
900,000 x 90/100 = 810,000.
2) Culture media prepared in the laboratory which does not contain heat labile constituents can be sterilized by Autoclaving. Autoclaving uses pressurised steam to heat the material to be sterilised. This is a very effective method that kills all microbes, spores and viruses.
Autoclaving kills microbes by hydrolysis and coagulation of cellular proteins, which is efficiently achieved by intense heat in the presence of water.
3) For media containing heat labile components, filtration is a great way of sterilization. Filteration works by passing the solution through a filter with a pore diameter that is too small for microbes to pass through.
Filtration through a membrane with 0.2 micron or smaller pore size removes biological contaminants, including bacteria, mold and yeast.
The various types of filters are as follows:
Membrane filters are thin filters that are made of cellulose. They can be used for sterilization during injection by placing the membrane between the syringe and the needle.
Seitz filters are usually made of asbestos. They are pad-like and thicker than membrane filters.
Sintered glass filters are an alternative type of filter that are made of glass and hence do not absorb liquids during filtration.
Candle filters are made of clay-like mud. This special mud has tiny pores made by algae. The microbes get stuck during their travel through the pores.
Filtration has to be done carefully because glass filters are very brittle and can break easily and membrane filters rupture easily. In Sietz Filters, the solution might get absorbed by the filter pad itself and clogging may occur.
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