Inducing Secondary Metabolite Production by the Endophytic Fungus <i>Fusarium tricinctum</i> through Coculture with <i>Bacillus subtilis</i>

Abstract

Coculturing the fungal endophyte <i>Fusarium tricinctum</i> with the bacterium <i>Bacillus subtilis</i> 168 trpC2 on solid rice medium resulted in an up to 78-fold increase in the accumulation in constitutively present secondary metabolites that included lateropyrone (<b>5</b>), cyclic depsipeptides of the enniatin type (<b>6</b>–<b>8</b>), and the lipopeptide fusaristatin A (<b>9</b>). In addition, four compounds (<b>1</b>–<b>4</b>) including (−)-citreoisocoumarin (<b>2</b>) as well as three new natural products (<b>1</b>, <b>3</b>, and <b>4</b>) were not present in discrete fungal and bacterial controls and only detected in the cocultures. The new compounds were identified as macrocarpon C (<b>1</b>), 2-(carboxymethylamino)­benzoic acid (<b>3</b>), and (−)-citreoisocoumarinol (<b>4</b>) by analysis of the 1D and 2D NMR and HRMS data. Enniatins B1 (<b>7</b>) and A1 (<b>8</b>), whose production was particularly enhanced, inhibited the growth of the cocultivated <i>B. subtilis</i> strain with minimal inhibitory concentrations (MICs) of 16 and 8 μg/mL, respectively, and were also active against <i>Staphylococcus aureus</i>, <i>Streptococcus pneumoniae</i>, and <i>Enterococcus faecalis</i> with MIC values in the range 2–8 μg/mL. In addition, lateropyrone (<b>5</b>), which was constitutively present in <i>F. tricinctum</i>, displayed good antibacterial activity against <i>B. subtilis</i>,<i> S. aureus</i>, <i>S. pneumoniae</i>, and <i>E. faecalis,</i> with MIC values ranging from 2 to 8 μg/mL. All active compounds were equally effective against a multiresistant clinical isolate of <i>S. aureus</i> and a susceptible reference strain of the same species