Mechanism for the effect of agitation on the molecular weight of hyaluronic acid produced by Streptococcus zooepidemicus

Various views persist about the effect of agitation on hyaluronic acid (HA) molecular weight, and the mechanism remains unclear. In this paper, the effect of agitation on HA molecular weight was investigated. The HA molecular weight increased with impeller speed, due to a mass transfer enhancement; however, higher impeller speed caused a decrease in the HA molecular weight. According to the experimental results, it is proposed for the first time that reactive oxygen species (ROS) under higher impeller speed in the bioreactor, which were generated by NADH oxidase, could cause the reduction in the HA molecular weight. ROS generation increased remarkably during the exponential growth phase when the impeller speed increased from 450 rpm to 1000 rpm, and the NADH oxidase activity increased correspondingly. Finally, it was proved that the addition of salicylic acid could increase the HA molecular weight, but inhibit the efficiency of the HA synthesis.