- The gene encoding xylulose-5-phosphate/fructose-6-phosphate phosphoketolase (xfp) is conserved among Bifidobacterium species within a more variable region of the genome and both are useful for strain identification. Yin, X., Chambers, J.R., Barlow, K., Park, A.S., Wheatcroft, R. FEMS Microbiol. Lett. (2005)