if self.knowledge_base.cell.parameters.get_one(id='mean_volume') is not None:
            mean_volume = self.knowledge_base.cell.parameters.get_one(id='mean_volume').value
        else:
            mean_volume = 0.000000000000000067
            print('"mean_volume" parameter is missing, using Mycoplasma pneumoniae value (6.7E-17L).')