Effects of manganese (II) on Bacillus licheniformis ATCC 9945A physiology and gamma-poly(glutamic acid) formation

Abstract

Bacillus licheniformis ATCC 9945A was cultivated in shake flasks using citrate (12 gl-1), glutamate (20 gl-1) and glycerol (80 gl-1) as carbon sources for cell growth and gamma-poly(glutamic acid) (gamma-PGA) production. The effect of the MnSO4 concentration in the medium over a range from 0.0 to 615 microns was studied. The number of viable cells increased for all concentrations of MnSO4 from approximately 10(5) to 10(9) colony-forming units (cfu) ml-1 by the early stationary phase (24 h). However, after 50 h, the cell viability decreased rapidly for relatively lower MnSO4 concentrations (0.615 and 0 microns). The utilization of carbon sources by B. licheniformis was greater for cultures containing 33.8 and 615 microns MnSO4 relative to cultures with no added MnSO4. For example, cultures with 615 microns MnSO4 utilized 37, 54 and 93% and cultures with no added MnSO4 utilized 19, 10 and 17% of glutamate, glycerol and citrate, respectively. The gamma-PGA volumetric yield increased from approximately 5 to 17 gl-1 for corresponding increases in MnSO4 concentration from 0 to 33.8 microns and then decreased at higher MnSO4 concentrations. The stereochemical content of gamma-PGA was found to vary inversely with MnSO4 concentration, and ranged from 59 to 10% L-glutamate units for MnSO4 concentrations of 0 and 615 microns, respectively. For all of the MnSO4 concentrations investigated, the gamma-PGA molecular weights decreased rapidly as the gamma-PGA volumetric yield simultaneously increased for cultivation times from 24 to approximately 50 h. Mw and Mn values after approximately 50 h cultivation times, determined by gel permeation chromatography (GPC), were 1.3 to 1.6 and 0.5 to 0.8 million g mol-1, respectively. A complex gamma-PGA molecular weight distribution that appeared bimodal by GPC analysis due to the presence of a low-molecular-weight product fraction was observed in cultures containing 33.8 and 61.5 microns MnSO4 at extended cultivation times. A high-molecular-weight fraction and the unfractionated gamma-PGA sample from the 33.8 microns MnSO4 culture contained 13 +/- 4 and 30 +/- 1% L-repeat units, respectively. A relationship between the product molecular weight and its stereochemical composition was thus established.