First, I would like to thank all the many people that answered my
question. It seems that this problem is not rare, and many people have
encountered it at some point.
Just to add some details that I left out: I know the basic procedure,
hence I use stock solution that is no older than one week, I add the
antibiotic only when the medium is cool enough to hold with bare
hands, I use plates that are no older than three weeks, and I do not
incubate the plates for more than overnight.
Now, it seems to me that the probable cause is the age of the powder.
It is about two years old, and has been kept in a dessicator in a 4
degrees room all the time, and has been opened only last month.
However, I suspect the the shelf life of ampicillin (we use the sodium
salt) powder is shorter that I would like it to be.
The suggestions of using carbenicillin or a mixture of
ampicillin/methcillin seem very atractive, not only for eliminating
satellites, but also for prolonging the shelf life of plates, and I
will try it as soon as we get the antibiotics. I will post about the
Thank you all once again,
Scripps Institution of Oceanography Phone (619) 534-0638
La Jolla CA 92093-0202 Fax (619) 534-7313