Katie Entry #2 ODs and Growth Rates

Time is flying by at the Cheung Lab!  I have finished my third week in the lab and I feel like I just got here.  I am starting to see the results of my work which is very exciting! This week I continued to grow the 61 potentially cold-sensitive MRSA strains that had difficulty growing in cold temperatures on agar plates the previous week in liquid media at three different temperatures to better observe their growth. I tested around 5 different strains per day. Every hour for 6-7 hours, I checked the optical density (OD) of each culture (I also made 10 fold dilutions when necessary since the spectrophotometer does not give accurate readings once the optical density reaches 0.7). Next, I made a growth curve using the resulting ODs.  I also began using the ODs to find the growth rates of each strain at the three temperatures to determine if and how much temperature has affected the growth rate of the mutants and to back up the information provided by the growth curves.  At first, I had a few challenges trying to calculate the growth rate because depending on which data points I used, the growth rate did not accurately reflect the growth curves that I’ve been making.  However, after some trial and error I determined which ODs were best to use in order to find a more accurate growth rate.  

These are the mutant S. aureus strains grown at three different temperatures.  As you can see, the culture on the left (which was grown at the optimal temperature for S. aureus, 37 C) is more turbid, showing faster cell growth, and the culture on the right (which was grown in cold conditions) is the least turbid, showing the slowest cell growth.

I have also started researching some of the known genes that were knocked out of the potentially cold-sensitive MRSA strains to learn more about the proteins that they encode and their relation to bacterial cold shock response.  However, some of the genes encode "conserved hypothetical proteins", which  basically means that the proteins' functions are unknown. This was also exciting to find because these unknown proteins could possibly be cold shock proteins.

I am almost finished observing the growth of the potentially cold-sensitive MRSA strains (I will probably finish next week) and I am excited to move on to the next step of my experiment, transduction!