Why do antibiotics have a special importance when taking lactulose?
From the Wikipedia article (I'm sure there's a scholarly reference, but I think this is well-known):


  Lactulose works by relieving constipation through a number of different methods. Lactulose is made up of sugar molecules and is partly broken down by the bacteria that live in the lower part of the gut. 


So, certain antibiotics affect (and reduce) the intestinal flora (and neomycin especially so, which is I believe the reason why it is mainly used in ointments), leaving the lactulose undigested and not as effective in forcing the body to retain water in the colon, which is the mechanism through which the drug acts.