We have recently encountered a problem with our OP50. The colonies develop a “crust”, which crumbles when it is touched with a pick. The worms will eat it, but they definitely grow more slowly. N2 has trouble moving through the colony and sort of surfs along top. Wild isolates just burrow through it. I have encountered this phenomenon occasionally in the past, but it always seemed to resolve itself over a short time, which this time it hasn’t. The problem apparently isn’t the OP50 itself (we made a new stock plate from a frozen stock of OP50, problem not solved) or the culture media for the bacteria (fresh media from a different lab’s stock, same problem). Problem also apparently not the ancillary reagents in the plates (we made new stock solutions, same problem). Not the incubator (grew plates in a different incubator, same problem). I have asked several experienced worm hands, they all seem to recognize the phenomenon, but no one knows a solution.