The NCYC in the UK recommend growing in YM broth shaking for 72 hours then mix 50:50 with YM broth with 10% glycerol. These store at -80°C for >5 years. ed. Kirsop, B.E.; Snell, J.J.S. (1984) Maintenance of Microorganisms - a manual of laboratory methods. Chapter 12, Pages 109-130. Academic Press Inc. (London) Ltd., London, UK. Pd