Aluminium foil and parafilm - why? (Apr/01/2009 )
Thinking back at one of my labs at school last year I remembered that the teacher instructed us to wrap parafilm around the petri dishes to prevent contamination and keep the dish at high humidity.
I understand the humidity part, but how can you get contamination with a closed petri dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
-josse-
josse on Apr 16 2009, 03:25 AM said:
Thinking back at one of my labs at school last year I remembered that the teacher instructed us to wrap parafilm around the petri dishes to prevent contamination and keep the dish at high humidity.
I understand the humidity part, but how can you get contamination with a closed petri dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
I understand the humidity part, but how can you get contamination with a closed petri dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
OK, consider this scenario:
You have successfully transformed you cells, and they now lie on the upturned petri dish. Gradually, over a week, say, moisture condenses on the lid and some of it condenses where the upper and lower parts meet. This is not a perfect seal, and spores can land in the gap between the lip of the lid (now facing atmosphere) and the lip of the plate ( equally, I suppose, you might be able to argue that the spores have been there since you plated out your cells, the week before). Now, what do you think the fungal spores are going to do with the ampicillin, tet ,cam or kan in the plates? Not a lot, they'll just keep on growing...
As for whether the humidity depends on the organism, only to a marginal degree, unless the cells are especially sensitive to dessication. When you think of bacteria, that's not going to be likely.
-swanny-
Nabi on Apr 14 2009, 08:59 AM said:
it is dark inside the incubator but others might also be sharing it so there might be frequent opening and closing of the door/lights of the incubator. So, if anything is light sensitive, then it is better to wrap with a foil.
agree with that. All our freezers, incubators, growth chambers are opened and closed frequently, the light is switched on and off etc. There are light sensitve media (I even have a medium which is poured in infrared light) and organisms which wont develope "right" when incubated in light.
josse on Apr 15 2009, 07:25 PM said:
I understand the humidity part, but how can you get contamination with a closed petri dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
Just one word for contamination: mites
If you are working with environmental samples, you can easiely get an invasion of these little beasts.....they are eating your fungi or bacteria, are breeding and are wandering round your incubator. But they do not clean their shoes before entering a new plate and voila.....all of a sudden you are the owner of beautiful mixed cultures. And they are very difficult to get rid off......sometimes the strategies used resemble some obscure cultic behaviour 
Humidiy might be quite important, especially when incubating something for a long time at high temps. Without sealing or humid chambers your agar plate will get dry within 1 - 2 weeks at 37 °C, and your culture will die. But for O/N icubations it is not necessary.
-gebirgsziege-
swanny on Apr 17 2009, 08:01 AM said:
josse on Apr 16 2009, 03:25 AM said:
Thinking back at one of my labs at school last year I remembered that the teacher instructed us to wrap parafilm around the petri dishes to prevent contamination and keep the dish at high humidity.
I understand the humidity part, but how can you get contamination with a closed petri dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
I understand the humidity part, but how can you get contamination with a closed petri dish?
And about the humidity, does this depend on the type of micro-organims you have in your dish?
OK, consider this scenario:
You have successfully transformed you cells, and they now lie on the upturned petri dish. Gradually, over a week, say, moisture condenses on the lid and some of it condenses where the upper and lower parts meet. This is not a perfect seal, and spores can land in the gap between the lip of the lid (now facing atmosphere) and the lip of the plate ( equally, I suppose, you might be able to argue that the spores have been there since you plated out your cells, the week before). Now, what do you think the fungal spores are going to do with the ampicillin, tet ,cam or kan in the plates? Not a lot, they'll just keep on growing...
As for whether the humidity depends on the organism, only to a marginal degree, unless the cells are especially sensitive to dessication. When you think of bacteria, that's not going to be likely.
You store the dishes still upside down after inoculation? why?
(we do not put them upside down after we placed our fungus on it)
And why are the dishes anyway kep upside down after pouring the agar?
-josse-