Table 2.

Summary on Neu5Ac de novo biosynthesis.

Host	Pathways	Strategies	Carbon sources	GlcNAc titer	ManNAc titer	Neu5Ac titer	Reference
E. coli MG1655	NeuC pathway	1) removing the native Neu5Ac catabolic pathway 2) expressing neuC and neuB from N. meningitidis	glucose	N.M	N.M	1.7 g/L (shake flask)	[9]
E. coli DH5α	AGE pathway	1) relieving the feedback inhibition of glmS 2) increasing the GlcNAc accumulation 3) eliminating lactate formation	glucose	13.51 g/L	3.49 g/L	7.85 g/L (5 L bioreactor)	[93]
E. coli DH5α	AGE pathway	1) evolution of an in vivo Neu5Ac biosensor 2) fine-tuning of AGE pathway via RBS engineering 3) evolution of NeuB using the Neu5Ac biosensor	glucose	∼10 g/L	∼2 g/L	8.31 g/L (two-stage fermentation)	[94]
E. coli DH5α	AGE pathway	1) pathway optimization using an evolved Neu5Ac riboswitch with high-threshold 2) evolution of age using the Neu5Ac riboswitch	glucose	∼8 g/L	∼1 g/L	14.32 g/L (shake flask)	[95]
E. coli DH5α	AGE pathway	1) optimizing the amino sugar metabolism 2) optimizing the expression of AGE and NeuB 2) improving PEP precursor supply	glucose	∼13 g/L	∼3 g/L	16.7 g/L (shake flask)	[96]
E. coli MG1655	NeuC pathway	1) deleting the nanATEK operon 2) applying biosensors based on NanR	glucose	N.M	N.M	1.4 g/L (shake flask)	[97]
E.coli BL21 (DE3)	NeuC pathway	1) constructing a plasmid-free engineered strain 2) optimizing GlmM and GlmU-GlmSA metabolic modules 3) optimizing NeuB expression	glycerol	N.M	N.M	23.46 g/L (3 L bioreactor)	[98]
B. subtilis	AGE pathway	1) introducing a glucose and malate coutilization pathway 2) balancing cell growth and Neu5Ac biosynthesis	glucose	N.M	N.M	2.18 g/L (shake flask)	[89]
B. subtilis	AGE pathway	1) fine-tuning expression of GNA1, pfkA, and pyk via N-terminal coding sequences	glucose	N.M	N.M	2.75 g/L (shake flask)	[99]
B. subtilis	AGE pathway	1) dynamic regulation through genetic code expansion-based cell growth and biosynthesis balance engineering strategy	glucose	N.M	N.M	4.72 g/L (3 L bioreactor)	[100]
B. subtilis	AGE pathway	1) enriching the producing cell subpopulation through an inducible population quality control system	glucose	N.M	N.M	4.23 g/L (3 L fermenter)	[101]
B. subtilis	AGE, NanE, and NeuC pathways	1) combining three Neu5Ac synthesis pathways and modularly optimizing using a synthetic multiplexed pathway engineering 2) optimizing the fermentation conditions	glucose	39.7 g/L	14.4 g/L	30.10 g/L (5 L bioreactor)	[90]
B. subtilis	NeuC pathway	1) reducing GlcNAc accumulation by constructing the NeuC pathway 2) improving NeuB and PEP supply to reduce ManNAc accumulation 3) static regulation to balance Neu5Ac biosynthesis and cell growth	glucose and glycerol as dual-carbon sources	0 g/L (shake flask)	2.2 g/L (shake flask)	7.9 g/L (shake flask) 21.8 g/L (3 L bioreactor)	[91]

N.M: Not mentioned.