http://www.mycology.adelaide.edu.au/Fungal_Descriptions/Yeasts/Candida/

Looks like candida keyfyr is the only strain in the report that can somewhat ferment lactose.

Seems like lactulose would mainly feed bacteria:

Wikipedia:

"Lactulose is metabolized in the colon by bacterial flora to short chain fatty acids including the production of the lactic acid and acetic acid. This partially dissociates, acidifying the colonic contents (increasing the H+ concentration in the gut).[7] This favors the formation of the nonabsorbable NH4+ from NH3, trapping NH3 in the colon and effectively reducing plasma NH3 concentrations."