Actinomycetes University of Udine, Mycology Department ISSN: 0732-0574 Vol. 10, Num. 1, 2000, pp. pp. 10-12

R. SRIVIBOOL

Institute of Marine Science, Burapha University, Chonburi, 20131, Thailand

ABSTRACT. One hundred and seventy Actinomadura strains isolated from various sites of the Samae Sarn, Juang, Raet and Kram islands were tested for antimicrobial metabolites. Seventeen strains showed activity against Gram positive bacteria and/or fungi.

For years actinomycetes have been a rich source of antibiotic metabolites. These are mostly obtained from Streptomyces spp., however representatives of other genera, such as Actinomadura (Terekhova et al., 1982), have also yielded numerous and interesting substances. In the present note the activity of some members of the genus, isolated from the soils of four tropical islands, is reported.

Isolation. Soil samples were collected in the Samae Sarn, Juang, Raet and Kram islands. Ten grams of each soil were pre-treated at 55°C for 15 min and at 100°C for 60 min, suspended in 100 ml peptone water solution (Ruan, 1994a; Athalye et al., 1981), diluted and spread on Actinomycetes Isolation Agar and Starch Casein Agar plates incubated at 32°C.

Morphological characteristics were determined examining by light microscopy the culture plates and coverslips inserted into the agar.

Chemical characteristics. Cultures were grown for 4-5 dd in a Glucose-Yeast Extract medium. Diaminopimelic acid and sugars of whole cell hydrolysates were determined according to Ruan (1994a,b).

Test organisms. Antibiotic activity was tested against Bacillus subtilis, ATCC 6633, Staphylococcus aureus, ATCC 13150, Micrococcus luteus, ATCC 934 and Candida albicans, DSM 70014, and other fungi, obtained from the TISTR Culture Collection, Thailand.

The total number of actinomycetes and of Actinomadura representatives is summarised in Table 1. Samae Sarn and Raet island soils show the highest numbers of both organisms. The soil of these islands is richer in humic matter and supports a wider range of vegetation than the sandy soil of Juang island, characterised by annual herbs and desert plants.

Table 1. Number of organisms isolated and characterised in the different locations (cfu: colony forming units).

Table 2. Antibiotic activity of some Actinomadura isolates.

From 55 soil samples, 117 strains of Actinomadura were isolated and 17 of these showed activity against the test strains. The cultures are characterised by a yellowish brown substrate mycelium and short, hooked to spiral spore chains. The results are summarised in Table 2. The five strains isolated from Samae Sarn island soils (S14-11, 12, 16, 17 and 18) possess antifungal properties but no activity against the organisms listed in the table.

Notwithstanding the general belief that tropical soils are richer in actinomycetes (Alexander, 1961), the number of organisms isolated in the present investigation is low. This is probably due to the poor quality of the soil in terms of structure, pH and humic substances. The same seems to apply to the low population of 'rare' actinomycetes in contrast to that of richer soils (Hayakawa et al., 1988). The proportion of antibiotic producers is also low (14%). This again is possibly due to the soil characteristics. In fact in a recent survey carried out on mangrove soils (Srivibool, 1998) a 78% proportion of antibiotically active Actinomadura strains was observed.

However the low proportion of active strains may not necessarily be a sign of poor 'quality' of the isolates and work is now in progress on the characterisation of their metabolites.

ACKNOWLEDGEMENTS. The financial assistance of the Royal Conservation Project, an initiative of HRH Princess Mahachakri Sirindhorn and the assistance of the Special Warfare Group of the Royal Thai Naval Fleet are acknowledged.

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