Mechanisms of Inhibition of Fungi in Agar by Streptomycetes

Abstract

SUMMARYOf 20 unidentified Streptomyces isolates tested, all inhibited Mucor ramannianus and 18 inhibited Glomerella cingulata in agar. Agar discs from inhibition zones, or paper discs placed beneath inhibition zones, of nine of the Streptomyces isolates caused new inhibition zones when transferred to fresh, seeded agar plates. Transferable inhibition zones were not produced by the other I I isolates. Inhibitory substances were produced in liquid cultures by eight of the nine isolates which produced them in agar media, whereas no antibiotics were detected in liquid cultures of the other I I isolates. Glomerella cingulata conidia, which do not require exogenous nutrients for germination, germinated in liquid cultures of non-antibiotic-producing streptomycetes, but failed to germinate in cultures of antibiotic-producing streptomycetes. Inhibition zones produced by non-antibiotic streptomycetes decreased in size with increasing concentration of nutrients, whereas those of antibiotic streptomycetes were unchanged. Glucose and glutamic acid levels in agar rapidly decreased adjacent to streptomycete colonies. Agar, leached of nutrients by sterilized distilled water running slowly through a groove cut in the agar, developed clear inhibition zones. Therefore, inhibition of fungi by streptomycetes in agar, in some cases, appears to be due to nutrient deprivation.

Bacteria and Actinomycetes Antogonistic to Fungal Root Pathogens in Australian Soils

“…Sometimes, too, the inhibitory zones produced by Streptomyces spp. result from depriving the fungus of nutrients and not from an antibiotic effect (Hsu and Lockwood 1969).…”

“…Using the methods of Hsu and Lockwood (1969), approximately 80% of Streptomyces spp. tested inhibited fungal growth by antibiotic production while the remainder deprived the fungus of nutrients.…”

Detection of Antibiotic-Producing Streptomyces Inhabiting Forest Soils

“…Two other cultures exhibited initial activity detectable by day 2 (Fig. 2a) or day 6 (Fig. 2e), followed by a maximum activity which remained constant with one culture and slowly decreased with the other.…”

“…Problems with these methods include obtaining a medium suitable for the growth of both antagonist and test organism and using a medium which enhances antibiotic production while differentiating between antibiotic activity and other antagonistic effects. Hsu and Lockwood (6) demonstrated that clear zones of inhibition may result from nutrient deprivation as well as antibiotic production, and they indicated that the only certain method for differentiating between the two effects was by preparing an extract from the inhibitory zone in the agar and examining it for retention of antimicrobial activity. In addition, inhibition can be caused by an alteration in the pH of the overlay layer and has been shown to be dependent upon both the composition and quantity of the overlay agar (9).…”

“…The inoculated media were incubated at 270C; beginning on day 2 and at each 2-day interval for 20 days thereafter, the overlay procedures (as previously described) were performed on the plates, and the paper disk assay was performed on the shake flasks. To standardize the inoculum, all cultures were initially grown and shaken for 7 days at 27°C in yeast extract-malt extract broth (10), after which the cells were removed by 8,000 x g centrifugation for 20 min, washed twice in sterile, distilled water, and then resuspended in 6 ml of the same broth. The suspension was diluted to yield 10 mg (dry weight) of mycelial suspension per ml, and all plates and flasks were inoculated with 0.1 ml of this suspension.…”