I have recently been having some contamination problems in my tissue culture. I have not been using anitbiotics in the media as I know it's not good practice in the long-term (damage to cells and increasing chances of antibiotic resistant bacterial strains etc.).
However, it has been suggested that I could hit the contamination with a treatment of pen/strep for a short while until the contamination is sorted and my cells are happy again.
Having searched the net though, I can't find the answer to a question I ask out of curiosity really: Why is a pen/strep combo used as standard in tissue culture media? why not gentamicin or kanamycin? I've come across a couple of (very old!) papers which say that gentamicin is better and a guidance sheet on invitrogen states that gentamicin and kanamycin have slightly broader antibiotic spectrums.