Growth and development of Frankia spp. strain CcI3 at the single-hypha level in liquid culture

Filamentous actinobacteria from the genus Frankia grow by hyphal tip extension and branching. The growth kinetics and branching pattern of Frankia are not well studied, especially at the early stages of mycelial development. Here, we compare the growth of Frankia sp. strain CcI3 in liquid cultures with and without proteose peptone #3 (PP3) using time-lapse photomicrography and image analysis. Individual hyphae showed a pseudolinear increase in length at early stages of development, whereas at the mycelial level, the aggregate length of hyphae described an exponential rate before slowing. Growth based on optical density or microscopic observations was similar in medium with or without PP3. However, PP3 altered the pattern of mycelial development by increasing branching. Distances between the hyphal apex and first branches were on average shorter in PP3-containing media. The final interbranch distances were also shorter in PP3 medium indicating that hyphae tended to branch earlier and more often when supplemented with PP3 to give a more compact mycelium. Vesicle development in nitrogen-fixing cultures limited cell expansion as a result of vesicles truncating growth on new branches. The results provide some explanation for the growth kinetics of Frankia and some indication of how growth rates may be improved.