Why specifically only the E.coli OP50 strain used as a feed for C.elegans??? Why not some other strain of E.coli???
A Google search of ‘elegans why op50’ turns up this link: http://www.wormbook.org/chapters/www_strainmaintain/strainmaintain.html.
or you might would just refer to Brenner’s paper. 
The genetics of Caenorhabditis elegans
www.wormbase.org/papers/31_Brenner74.pdf