Selective isolation and study on the global distribution of the genus Planobispora in soils

Planobispora strains, including the type strain, produced abundant sporangia on humic acid-vitamin gellan gum medium when more than 50 colonies grew on a plate. However, these strains did not produce sporangia on the aerial mycelium, a diagnostic characteristic of this genus, when fewer than 20 colonies grew on a plate. Trace salts, such as FeSO4 x 7H2O, MnCl2 x 4H2O, ZnSO4 x 7H2O, and NiSO4 x 6H2O, stimulated sporangium formation of Planobispora strains. Better sporangium formation of Planobispora strains was observed on the humic acid-trace salts gellan gum medium at pH 9.0 than at neutral pH. Moreover, an alkaline medium repressed the growth of three out of six Streptomyces strains so that this condition was effective for selective isolation of Planobispora strains. An alkaline flooding solution of skim milk, five antimicrobial agents, and dry heat treatment at 90 degrees C for 60 min were effective for selective isolation. Using these techniques, we examined the distribution of Planobispora strains by using 1467 soil samples collected from Japan and other parts of the world. One hundred and nineteen Planobispora strains were isolated from 51 soil samples (3.5% of the samples tested) that were collected in Ecuador, Egypt, French Guiana, India, and Madagascar. Planobispora strains were recovered only in tropical and subtropical soils. To our knowledge, this is the first record that Planobispora strains have been isolated from locations other than Venezuela, Namibia, or South Africa.