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L-fucose isomerase, the first enzyme needed to degrade L-fucose to L-fuculose, was identified in two samples. The adjacent gene, L-fuculo[http://www.selleckchem.com/products/ldk378.html Ceritinib] kinase, responsible for conversion of to 1-fucose 1-phosphate, was not identified. L-fucose aldolase, also encoded in this region, converts 1-fucose 1-phosphate to L-lactaldehyde; this protein was also not identified, but the adjacent fructose operon regulator was identified in one sample. Although both UC1CITs have pathways for the anaerobic degradation of rhamnose as well as fucose, the genes for rhamnose degradation (and transport) were not detected. The protein encoded by the next gene in the anaerobic fucose degradation pathway converts L-lactaldehyde to 1,2-propanediol (1,2-PD). This protein was identified in all samples. Citrobacter also can convert 1,2-PD to propionyl-CoA, and  probably does so within a well-characterized organelle (a microcompartment), which prevents the accumulation of toxic aldehyde intermediate (Kerfeld and Erbilgin, 2014). Shell proteins for this microcompartment, specifically shell protein PduA, were consistently identified in samples from most days. Propionyl-CoA is likely degraded to propionyl phosphate then to propionate (a SCFA) as propionate kinase, the final enzyme that converts propyionyl phosphate to propionate, was consistently identified in both UC1CITs. Propionate may then be excreted, likely to be absorbed by the infant (Tan et al., 2014). Interestingly, Serratia does not  appear to have the capacity to ferment either fucose or rhamnose, which may be the metabolic basis of their niche separation. Another prominent fermentative pathway found in the Citrobacter proteomes involves enzymes that degrade glycerol, several of which are vitamin B12-dependent. Production of vitamin B12 (cobalamin) is unique to bacteria and archaea, and is an essential cofactor for many forms of life. We consistently detected proteins required for the biosynthesis of vitamin B12, specifically CbiG and CbiK, from UC1CIT. The UC1CITs are the only relatively abundant organisms in the infant's gut that encode cobalamin biosynthesis genes, and consistent expression of this pathway suggests it to be a key role in the community (Supplemental Figure  1). Notably, these cobalamin biosynthesis enzymes operate under anaerobic conditions, a further indication of anaerobic niches in the gut during this phase of colonization. Additionally, enzymes were identified for a fermentation pathway that converts glycerol to 1,3-propanediol (1,3-PD) and other SCFA by-products (using the glycerol dehydratase complex: EC:4.2.1.30; three subunits, all of which were identified by proteomics in all samples). Citrobacter is one of a small number of bacterial genera with the glycerol fermentative pathway (others include Klebsiella, Clostridium, and Lactobacillus). The SCFA by-products of this pathway are acetate and sometimes butyrate (Abbad-Andaloussi et al., 1996).