Strain performance without a neutralizing agent
Considering the promising result of the previous experiment, the LA2
strain was selected for further observation. Figure 3 shows
that with and without the addition of calcium carbonate, the LA2 strain
produced lactic acid in concentrations of 43.2 and 33.2
g·L-1, respectively. Even though the accumulation of
lactic acid declined, the removal of calcium carbonate did not cause a
significant decrease in the rate of glucose uptake because the glucose
was completely consumed within only 9 h under both conditions.
Nonetheless, as shown in Table 2 , these results were relatively
competitive by comparison with those of the microbial hosts described in
other studies (Ilmén et al., 2013; Ishida et al., 2005; Mitsui et al.,
2020; Park et al., 2018). Despite its lower yield, the LA2 strain
maintained the ability to utilize glucose rapidly and exhibited an even
more stable level of productivity when the neutralizing agent was
removed. With the addition of a neutralizing agent, the productivity was
4.80 g·L-1·h-1. Meanwhile, under a
non-neutralized condition, the productivity remained high (3.68
g·L-1·h-1), although the pH of the
medium dropped to 3.00 after 9 h of cultivation (∆pH = + 1.5,
compared with a neutralized condition). Other studies included inTable 2 have exhibited a drop in productivity by as much as
50%, or more, when no neutralizing agent was supplemented into the
medium.
Based on the result shown in Figure 3 , lactic acid generation
was terminated because the glucose was completely consumed within 9 h.
However, as depicted in Figure 4 , the LA2 strain could still
accumulate lactic acid when glucose was fed to the medium even without
supplementation with a neutralizing agent. After being fed with
concentrated glucose three times at 6 h, 10 h and 14 h, the productivity
of engineered strain stood at 3.45
g·L-1·h-1(C LA = 62.0 g·L-1) and 2.35
g·L-1·h-1(C LA = 42.2 g·L-1) under the
neutralized and non-neutralized condition, respectively. Akin to the
results from batch cultivation, this strain exhibited high productivity
under both conditions, with a drop in value less than 35% when the
neutralizing agent was removed from the medium.