Influence of exogenous cholesterol in Pythiaceae resistance to inhibition by Trichoderma antibiosis

Abstract

The purpose of this study was to determine if exogenous cholesterol availability influenced Pythiaceae resistance to antibiosis. Characterisation of an isolate of Phytophthora erythroseptica and Pythium ultimum for tolerance to antibacterial compounds found that 0.05 g.l −1 chloramphenicol inhibited mycelial growth by 96.6 % and 23.5 % respectively. However, the addition of cholesterol (0.01 g l −1 ) to potato dextrose agar (PDA) containing 0.05 g l −1 chloramphenicol was found to increase mycelial growth of P. erythroseptica, indicating a role for cholesterol in tolerance to inhibitory antibacterial compounds. To determine if this property extended to suppressive effects of a potential biocontrol agent, P. erythroseptica and P. ultimum were then tested against a cell-free filtrate of diffusible metabolites produced by a suppressive Trichoderma harzianum isolate in the presence and absence of cholesterol in PDA. In the absence of cholesterol, diffusible metabolites of the T. harzianum isolate were found to inhibit mycelial growth of P. erythroseptica and P. ultimum on PDA by 98 % and 63.6 % respectively (P < 0.0001). However, the inhibitory effect of the metabolites was mitigated when 0.005 g l −1 of cholesterol was present in PDA, w i t h m y c e l i a l g r o w t h o f P. u l t i m u m a n d P. erythroseptica reduced by only 60.4 % and 41.8 %, respectively (P < 0.0001), much less inhibition than was observed in the absence of cholesterol. These results demonstrated that access to exogenous cholesterol can influence the sensitivity of Pythiaceae species to antibiosis by positively influencing mycelial growth.