Perspectives: challenges and opportunities
Though great progresses have been achieved on the
microbial production of HA with S. zooepidemicus and
the recombinant production systems, several challenges
remain.
1) The continuous rise in the cost of raw materials
weakens the commercial competiveness of microbial
HA production, and thus it is necessary to find a
cheaper substrate replacement to reduce production
cost. Furthermore, the needs of a sustainable society
point to the conversion of renewable resources such as
agricultural derivatives into valuable bioproducts.
Thus, exploring the feasibility of producing HA with
cheap crude materials or wastes from the other industrial
processes is worth investigating. Mussel processing
wastewater (MPW) and tuna peptone (TP) from viscera residue are used for HA production by S. zooepidemicus,
and the economic analysis indicated that
the production cost can be reduced by more than 30%
with the by-products as the culture medium [39]. The
agricultural resource derivatives such as cashew apple
juice was a promising medium for the microbial HA
production [47]. For another example, the large
amount of crude glycerol produced in the biodiesel
industry, if not properly treated, pose a significant
environmental concern. Therefore, we can explore the
potential of microbial HA production with the crude
glycerol as a substrate. Of course, process engineering
for the efficient treatment of crude material and metabolic
engineering of microbes for the efficient utilization
of raw substrates should be considered to achieve
this objective.
2) Whether for S. zooepidemicus or the recombinant
systems like E. coli, B. subtilis, and L. lactis, the key factors
limiting HA synthesis need to be further clarified.
The tools of metabolic engineering, such as metabolic
flux analysis (MFA) and metabolic control analysis
(MCA), can be employed to develop a rational strategy
to improve HA yield and molecular weight. MFA is an
analysis technique used to calculate and analyze the flux
distribution of the entire biochemical reaction network
during a process. MCA quantifies the relation between
genetic modifications or environmental changes and cellular
process responses [48]. MCA introduces the control
coefficients to quantify the fractional change of
cellular output, such as metabolite concentrations and
metabolic fluxes, in response to fractional change of system parameters, such as enzyme activities and growth
conditions [49]. The combination of MFA and MCA
can be used to investigate the metabolic responses of
HA producer to the environmental changes or the
expression of key genes related with HA synthesis. With
the information gathered from MFA and MCA, the
optimal strategies (both process control and key genes
expression) can be determined to improve HA titer and
molecular weight.
3) It is necessary to obtain specially designated molecular
weight or uniform size-defined HA to extend the
applications of HA and make better HA containing biomedical
products. To achieve low polydispersity, we
must know the regulatory mechanisms of initiation and
elongation during the HA polymer synthesis process.
Despite HA polymerization model has been put forward
and some key intracellular metabolites influencing molecular
weight have been clarified, much work needs to be
performed to understand the mechanism of molecular
weight control.