How to tell if competent cells or SOC are contaminated? - (Jan/17/2006 )
I dont know why just all troubles happen at once....not only that i dont have my transformed colonies but my negative control is contaminated with a lot of bacteria... 
i have prepared electrocompitant cells BL21 for the first time in my life and have been told that working near fire is not necessary... 
....so i didnt ....and i also have prepared new SOC and didnt autoclave it..(was not recomended in the procedure) ....so how do I tell now if my cells have some contaminants or my SOC? do I simly plate my electrocompitant BL21 on the plate and see??thanx a lot
You could plate out the SOC media to see how contaminated it is. Or you could even take a sample of the SOC media and have a look under a microscope to see if any cells are present. You should always autoclave media (or filter sterilize depending on the medium)....SOC is one that is easily autoclaved. Are the colonies on the negative control different looking to BL21 cells. If they are, you could tell by plating the SOC and BL21 cells and looking for the contaminating colonies.
i have prepared electrocompitant cells BL21 for the first time in my life and have been told that working near fire is not necessary... ....so i didnt ....and i also have prepared new SOC and didnt autoclave it..(was not recomended in the procedure) ....so how do I tell now if my cells have some contaminants or my SOC? do I simly plate my electrocompitant BL21 on the plate and see??thanx a lotthanx for the reply....i will plate only SOC to see...but yes colonies do look different than those of positive control...much bigger...
I would put my money on the SOC; the glucose makes it prime food for wandering bacterias 
thanx aimkins, I ll plate them today on a plate but im pretty sure too its SOC....so i guess I ll have to prepare a new one...or maybe just filtering will be ok?
by the way if i want to test my cells should i just take them out of -70 thaw and plate on the plate? or i should transfer them to some medium first?
I would say you can plate your cells directly
about the SOC...never ever use contaminated media. you don't know how much the chemistry has changed with the contaminants. it's not too hard to make, and if I were you I would autoclave each future batch
Kathy, have you read 'Molecular Cloning'? I know that it's old and it's tedious stuff to read, but based on some of what you have been posting lately I think it could be helpful for you. You seem to be picking up many new techniques at once without too much guidance and I think those books could give you leg up.
about the SOC...never ever use contaminated media. you don't know how much the chemistry has changed with the contaminants. it's not too hard to make, and if I were you I would autoclave each future batch
Kathy, have you read 'Molecular Cloning'? I know that it's old and it's tedious stuff to read, but based on some of what you have been posting lately I think it could be helpful for you. You seem to be picking up many new techniques at once without too much guidance and I think those books could give you leg up.
thanx aimkins, yes i am reading molecular cloning we have 3rd edition here in the lab....my problem is that i am in japan and poeple here dont speak much english so i have to find out many things by myself....anyway i didnt read your post yesterday and i have dissolved my cells in media and then plated it.....ok today news are: SOC is not contaminated but my cells are...
....now that ive used the media im afraid that it is my media that is contaminated so i guess ill make another plate today of just a media but how can it be possibly contaminated with ampicillin resistant bacteria?!?!?! ampicillin is kind of wimpy. very temperature-sensitive. too much time at 37C and you'll get all sorts of colonies...none positive 
if you can get it, carbenicillin is a good alternative. still shows the same thing but is much more stable
if you can get it, carbenicillin is a good alternative. still shows the same thing but is much more stable
but SOC's plate had no colonies?
I can only tell you what I know from experience.
if you do a transformation and plate out some reasonable dilutions, at a 'short overnight' say 15 or 16 hrs, you'll get some small colonies. put the plate back in for the rest of the day, or until the next morning, and you will get e coli...but probably without your plasmid
I think some of the transformants typically hang out on top of the media, and begin to grow once the amp breaks down if they have managed to survive. the classic example of this is satellite colonies; this occurs when your comp cells are not contaminated!
if you saw no colonies on the SOC (this is your negative control?) then I would guess that the SOC is not your contaminating factor (you already surmised this) and therefore had no bacteria which could take over and grow