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Growth at the margin of my agar plates - (Jul/04/2014 )



on some of my plates after streaking and incubation, I could observe growth at the periphery of my agar plates.

I wonder wether this was contamination.



One problem I had was that the agar shrank during the incubation. For this reason, I soaked paper tissue with tap water and put it and the agar plates into a plastic bag. I sealed the bag and incubated. the agar did not shrink anymore.


Another problem is that the agar rotates against the petri dish. Before incubation I draw a line on the bag of the dish to divide it into to halfs. Then I inoculate each half with one bacterial species. After incubation it might happen that the streaks of inoculation crosses the lines of the petri dish. The agar seems to have moved.



Could anyone help me out with these two issues?



Many thanks!


Isn't my english clear enough? not understandable? smile.png


No one responds because there is little to say. Yes, you likely have contamination on your plates. Who knows why your plates are rotating and pulling away from the edges. One (random) guess is that you may be pouring the plates when the agar is too hot. If you want people to help, you have to give them all of the information that is available. We are not mind readers. Tell us what the agar is, how you are autoclaving it, how you are cooling it, are you adding antibiotics, what temperature you are pouring the plates at, how you are storing your plates.


Also make sure you are pouring enough agar on the plates. Have heard a funny story about a fairly big pharmaceutical spending alot of money investigating a problem like this (tracking the transport of the plates the humidity etc) and 10ml more of agar per plate solved the issue of the plates drying up