1st Year Practicals: Their Role in Developing Future Bioscientists
B3. Microbiology
Introduction to Microbiology Tutorial (originates from Julie Rattray, Glasgow Caledonian University; j.rattray@gcal.ac.uk) Please attempt these before you come to the tutorial, when I will go over the answers with you. (This type of material will be included in a Lab test.)
Question 1: A bacterial culture was counted using two methods. (a) The cells were enumerated using a standard cell counting chamber where the conversion factor was 4 x 106. The original sample proved to be too dense a suspension so 0.5 ml of it was added to 4.5ml of a suitable diluent and counted instead. Result: 316 cells were counted; they were present in 12 squares. (b) A dilution series was set up and the cells were counted using spread plates. The colonies were counted after incubation (average of two plates per dilution). Results: 672 colonies on 10-5; 86 colonies on 10-6; 11 colonies on 10-7. Calculate the total and viable counts of the original culture and hence determine, as a percentage, the proportion of cells in the culture which were viable.
Question 2 : You are asked to estimate the dry weight of a single bacterial cell (not a colony). How would you tackle this experimentally? Assume that you are provided with a large number of plates covered with bacterial growth. You can also assume that the lab has the normal range of equipment (but not a balance able to weigh picogramme amounts!) Hint: obviously you could pile the bacteria into a mass which was large enough to be weighable. But what would you then need to know about what was in the mass? Review the types of growth estimation that you have been told about, or have performed in the lab, in terms of what you need to know, eg what would be more appropriate, an estimation of viable or total cell numbers. Then take your selected method(s) and adapt them to the particular requirements of this problem.
Question 3: You are supplied with a culture of bacteria which need to be counted by the pour plate method. It is known to contain about 107 viable bacteria per ml. You will need to make a dilution series. Which three dilutions should you plate out to give a suitable set of colony counts?
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