Liquid media used for culturing microorganisms in the lab is typically sterilized using a small autoclave (essentially, a programmable pressure cooker).
I prepare 5 litres of medium, split this into five one-litre bottles and put these into the autoclave. The autoclave is programmed for a 20 minute/121oC cycle.
My lazy colleague also makes 5 litres of the same medium, but instead puts the whole lot into a single large bottle and autoclaves this using the same programme.
A day-or-so after autoclaving, the medium in my colleague’s bottle has gone cloudy indicating that contaminating bacteria have survived the autoclaving and have grown in the medium. In contrast, the medium in all five of my bottles is clear (i.e. no bacterial growth).
The 5-litre bottle is so large that heat transfer to the interior of the solution in retarded. As a result, the entire solution is not heated up to 121oC for sufficient time. By contrast, if splitting them into 1-litre bottles, the total surface area is larger such that the contents of the bottles are sufficiently heated to the required temperature within the 20-minute cycle.
I would either programme the cycle of autoclave to be longer or set it to a higher temperature. This either allows more time for heating up the bottle, or the bottle can be heated up faster due to larger temperature difference.
Laboratory report: experiment 2 Reducing bacterial contamination
Report your findings. What did the bacterial growth look like on the control (water only) plate? Which of the various treatments were effective in reducing the amount of bacteria? Was there a difference between the Gram positive M. luteus and the Gram negative E. coli?
The size of clear zones (diameter in mm) of different treatments were measured as follows:
|11 mm||6 mm||12 mm||6 mm||none||6 mm||none||none|
The data of another group using M. luteus are as follows:
|22 mm||9 mm||20 mm||none||none||10 mm||none||none|
The class results are as follows (% of groups showing inhibition):
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