We next addressed whether the recombinant H. jecorina strain had the ability to produce NeuNAc in vivo. To test this, the strain was grown on GlcNAc in shake flasks and cultivated on colloidal chitin in a bioreactor. Data on the corresponding cultivation monitoring are provided in additional file 2. As a positive control, an enzyme assay using the GST fusion proteins was again performed and resulted in the detection of ManNAc using HPLC-MS analysis as shown in Figure 4a1 . Notably, the intermediate ManNAc was detected in the recombinant strain, regardless of the carbon source (Figure 4a2 und Figure  4a4 ), whereas the parental strain did not form ManNAc (Figure 4a3 ). In the parental strain, only the first metabolite, GlcNAc, was detected, and it was present because it was either directly used as a carbon source or formed by degradation of the biopolymer chitin due to the native chitinolytic activity of the fungus. The synthesis of the product NeuNAc was analyzed using HPLC-MS/MS analysis (Figure 4b) . As a positive control, the reaction products (ManNAc, NeuNAc) generated by the use of the GST fusion proteins in an enzymatic assay are shown (Figure 4b1) . Importantly, the recombinant H. jecorina strain formed NeuNAc using either carbon source, Figure 2 The analysis of possible metabolization of NeuNAc in H. jecorina. The parental strain was pre-cultured on glycerol, and half of the mycelia were autoclaved. Living (light grey) and dead (dark grey) mycelia were transferred to MA media containing glycerol or MA media that lacked a carbon source, and NeuNAc (a) or GlcNAc (b) (as positive control) was added to both media. Samples were collected at 0 and 8 h. For NeuNAc analysis supernatants were derivatized using DMB befor analyses using HPLC. The presented values are the means of two biological duplicates that were derivatized in duplicate. Error bars indicate the standard deviations. GlcNAc or chitin (Figure 4b2 and 4b4) , whereas in the parental strain no formation of NeuNAc was detected (Figure 4b3 ). This analysis allowed us to estimate that 13 μg NeuNAc per g mycelium (dry weight) was formed in the recombinant strain. Thus, on its own, this would not be a competitive production process, but it does demonstrate the possibility for engineering a saprophyte and using it as a whole-cell catalyst that expresses a bacterial enzyme cascade. This method has enormous potential considering its use of a cheap starting material and the relatively simple, inexpensive cultivation of a fungus.