Frankia is difficult to isolate directly from soil, so most strains originate from root nodules. Two factors limit success, one is that Frankia strains grow slowly, and the other is that fast-growing contaminants are common. To minimize the second problem, nodules are disinfected with dilute sodium hypochlorite and then peeled. Vesicle clusters can be separated from plant tissue by differential screening (Benson, 1982) or density centrifugation (Baker & O'Keefe, 1984). Clusters are best pour-plated on a variety of media and followed microscopically until they begin to grow over a period of ten days to three weeks. Monitoring the outgrowth of hyphae microscopically improves the chances of obtaining a monoculture. Contaminants are spatially removed from the slower-growing Frankia colony.
The medium used in isolating new Frankia strains is important but universal, or selective, media have not been reported. Effective mdia range from defined propionate media (DPM; Baker, 1984), to the complex QMod medium of Lalonde and Calvert (Lalonde, 1979). Antifungal agents, like cycloheximide or nystatin, can minimize fungal contamination. Other additions include root-lipid extracts to stimulate growth during isolation (Quispel, 1983).
Frankia strains are heterotrophic aerobes having generation times of 15 or more hours. Some strains, like HFPCcI3, prefer lower O2 levels than others and grow poorly in vigorously aerated media, particularly at low cell density. As a consequence of their filamentous morphology, the growth kinetics of Frankia strains generally consist of a stationary phase after transfer, followed by a short 'exponential' phase, and then by a slower increase in biomass over time. Problems typical of growing other filamentous organisms apply to Frankia strains. Care must be taken to avoid nutrient and waste gradients across mycelia and a flocs or pellet formation should be avoided by frequent homgenization (Benson and Schultz,1990).
Growth and maintenance media range from simple basal salts media containing pyruvate or propionate, to complex media containing, in addition to basal salts, yeast extract, bovine serum albumin, malt extract, beef extract, NZ Amine A, casamino acids, vitamin supplements, or Tweens (Lechevalier, 1990).
Virtually all Frankia strains isolated require no growth factors, and thus grow well in defined minimal medium (FDM) . Some are inhibited by undefined media additives such as yeast extract (Lechevalier, 1990). Another general medium for Frankia is defined propionate minimal medium (DPM) medium of Baker and O'Keefe (Baker, 1984), or BAP medium (Murry et al., 1984). We use a modified FDM medium for growing CcI3 on plates called CB-CcI3.
Most strains are grown and maintained in liquid culture, and generally grow slowly on solid media. Colonies from spores or mycelial fragments after about seven to ten days, or longer, after plating, under the best of conditions. Some types of agar may be inhibitory, while gellan gum (Phytogel) has proven to be a suitable substitute, especially for spore germination studies.