I don't have a plate around, but S. marcescens ( don't capitalize the marcescens) is gram negative and doesn't ferment lactose. Gram positive cultures don't grow on MacConkey. So you should see growth but not the mucoid pink colonies that would indicate a lactose fermentor. ( Really fast fermentors like E. coli even get a pink halo in the agar .) Non-fermentors ususallly have un-colored or white colonies. What makes it tricky is Serratia produces a brick-colored pigment normally. I'm not sure if the brick-colored pigment is suppressed or not on MacConkey. My notes just say "growth".