Cells were grown at 21°C in HL5 medium (14.3 g/L peptone (Oxoid, Hampshire, England), 7.15 g/L yeast extract, 18 g/L maltose monohydrate, 0.641 g/L Na2HPO4.2H2O, 0.490 g/L KH2PO4) and subcultured twice a week to maintain a maximal density of 106 cells/ml.