Aims: To devise and evaluate a method for isolating the rare, zoosporic actinomycetes, Actinokineospora spp. in soil and plant litter. Methods and Results: The newly developed method consists of two enrichment stages followed by plating on a selective medium. The source material is initially incubated with calcium carbonate to multiply the population of Actinokineospora spp., and is then air‐dried. The second stage consists of rehydration‐centrifugation, in which the amended substrate is immersed in phosphate buffer‐soil extract to liberate actinomycete zoospores, and nonmotile microbial associates are then eliminated by centrifugation. Portions of the supernatant enriched with zoospores are plated on humic‐acid vitamin agar supplemented with fradiomycin, kanamycin, nalidixic acid and trimethoprim. We examined 39 soil and plant‐litter samples taken from fields, forests and stream banks. The proposed method consistently enriched and selectively isolated Actinokineospora spp. in 17 samples. Evidence for antimicrobial activity was found in most of the isolates. Conclusions: A combination of enrichment and a medium containing selective antibiotics can be used successfully for efficient isolation of certain rare actinomycete taxa. Significance and Impact of the Study: The development of new methodologies with which to isolate rare actinomycetes is of great importance to extend our understanding of their ecology, taxonomy and bioactivity.

Tetrad distributions for 108 different gene pairs in 1346 asci of 113 diploids heterozygous for various combinations of 24 genes in Saccharomycodes ludwigii were investigated. Tetratype tetrads occurred only rarely and the 24 genes tested were classified into seven linkage groups. Electrophoretic karyotypes of three independent strains of S'codes ludwigii showed seven bands of chromosome‐sized DNA having molecular sizes of 0·8 to 2·3 Mb with strain‐specific polymorphic chromosomal DNAs as determined based on their migration distances.

The present paper describes a simple enrichment technique which enables rapid and selective isolation of diverse zoosporic actinomycete genera directly from soil and plant litter. This technique, designated the rehydration and centrifugation (RC) method, consists of immersing the air-dried source material in 10 mM phosphate buffer containing 10% soil extract, letting the preparation stand at 30 degrees C for 90 min, followed by centrifugation of the fluid at 1,500 x g for 20 min. Portions of the supernatant containing actinomycete zoospores are plated on the humic acid-vitamin agar which is supplemented with nalidixic acid and trimethoprim as the selective inhibitors for Gram-negative bacteria and bacilli. The phosphate buffer-soil extract solution significantly promoted liberation of motile zoospores from the source material. The centrifugation stage greatly eliminated streptomycetes and other non-motile actinomycetes from the liquid phase, thereby facilitating selective growth of rare, motile actinomycetes on the isolation plates subsequent to inoculation. Ten different soil and leaf-litter samples, taken from fields, forests, and stream banks, were examined. The RC method consistently achieved preferential isolation of motile actinomycetes in all samples, which accounted for 37-86% of the total microbial population recovered. The most frequently isolated motile actinomycetes were Actinoplanes and Dactylosporangium. Strains of Actinokineospora, Catenuloplanes and Kineosporia were also recovered, depending on the nature of the samples examined. Other motile actinomycetes that were occasionally isolated in small numbers included Actinosynnema, Geodermatophilus and Sporichthya.