# Biological Production of 3-Hydroxpropionic Acid: An Update on the Current Status
## 2 Production of 3 HP from Glycerol
### Other Microorganisms  
It is clear that most of the research is focused mainly on either _E. coli_ or _K. pneumoniae_ as host microorganism for the construction of a cell factory capable of producing 3 HP from glycerol. However, there are other microorganisms that could be suitable and efficient cell factories for efficient production of 3 HP. For example, _Lactobacillus reuteri_ is a good candidate, due to its ability to naturally produce the coenzyme B\({}_{12}\) and to its high acid tolerance--both of which are required when producing 3 HP. Moreover, the _pdu_ operon is endogenous in that specific species [40], minimizing the need for gene transfer. The use of a recombinant _L. reuteri_ strain with a mutation in the catabolite repression element (CRE) was investigated by [PERSON] et al. [23], to improve the metabolic flux of glycerol to 3 HP and 1,3-propanediol. During the flux analysis for the different steps of the _pdu_ operon, it was found that the glycerol dehydration to 3-hydroxypropionaldehyde was ten times faster than the subsequent oxidation and reduction of 3-hydroxypropionaldehyde to 1,3-propanediol and 3 HP. Thus, establishment of an optimal feeding rate of glycerol was crucial to avoid 3-hydroxypropionaldehyde accumulation and direct the flux of glycerol towards 3 HP and 1,3-propanediol formation. The final titers were 10.6 g/L 3 HP and 9.0 g/L 1,3-propanediol under anaerobic conditions in the fed-batch bioreactor when resting cells of _L. reuteri_ were used (Table 3).  
The same recombinant strain of _L. reuteri_ was also used for the preparation of crosslinked, cryostructured monoliths that could be used as a biocatalyst for the conversion of glycerol to 3-hydroxypropionaldehyde, 3 HP, and 1,3-propanediol [14]. Different crosslinkers were tested for the preparation of monolith columns of resting _L. reuteri_ cells, with only the mixture of synthetic macromolecular structures of activated polyethyleneimine and modified polyvinyl alcohol (Cryo-PEI/PVA) demonstrating enhanced biocatalytic activity, mechanical stability, and sustained viability. Under optimal conditions, 3.3 g/L 3 HP was produced during fed-batch feeding of the immobilized cells. Finally, a two-step process involving the cultivation of _L. reuteri_ and _G. oxydans_ has also been proposed [41]. More specifically, in the first step, the anaerobic cultivation of _L. reuteri_ in fed-batch mode resulted in the production of equimolar quantities of 3 HP and 1,3-propanediol, whereas in the second step, the 1,3-propanediol in the cell-free supernatant was selectively oxidized to 3 HP by _G. oxydans_ under aerobic batch cultivation. At the end of the first step, 14 g/L 3 HP and 12 g/L 1,3-propanediol were produced, with the corresponding conversion yields being 0.48 g/g and 0.42 g/g, respectively. Finally, the oxidation in the second step resulted in the quantitative conversion of 1,3-propanediol to 3 HP, with a final 3 HP concentration of 23.6 g/L and an overall conversion yield of glycerol to 3 HP of approximately 1 mol/mol.  
Another host strain that has been used is _Bacillus subtilis_, which shows high growth rates, and is a non-pathogenic microorganism classified as GRAS (generally recognized as safe) [42]. The first attempt to use this host microorganism for 3 HP was described by [PERSON] et al. [42]. The constructed strain performed well in shaker flasks, producing up to 10 g/L with average yield 0.79 g/g, and showing good tolerance towards the 3 HP produced. One drawback of the use of this strain as host is the lack of any native ability to produce the coenzyme B\({}_{12}\) (as with _E. coli_); the authors suggested that this can be solved by transferring the necessary genes from _Bacillus megaterium_. This work demonstrated that _Bacillus_ strains can also be used as cell factories for 3 HP production, and more work using these cells will probably be conducted in the future.