How do I prevent satellite colonies from appearing on my ampicillin plates?
Satellite or “feeder” colonies appear on ampicillin plates near ampicillin resistant colonies because the enzyme responsible for antibiotic resistance, beta-lactamase, is secreted from the cells. The use of other penicillin derivatives containing beta-lactam rings sometimes alleviates this. It is a matter of debate whether the enzyme is inhibited from leaving the cell, or the derivatives are more resistant to the secreted form of the enzyme. In any case, the number of feeder colonies can be reduced by using 200 ug/ml carbenicillin, or a mixture of ampicillin 20 ug/ml and methicillin 80 ug/ml. Here is a list of suppliers that sell carbenicillin from the the 1996 Cold Spring Harbor Lab Manual Source Book. The Source Book is a buyer’s guide for lab reagents, supplies and equipment. It is distributed free from CSHL Press. You can request a free copy at the Source Book’s WWW site http://www.biosupplynet.com.
