Antagonistic bacteria can act as biocontrol agents against various phytopathogens. Recently, Arthrobacter spp. demonstrated antifungal activity, but were not further characterized. In this study, the antimicrobial activity of Arthrobacter humicola strains M9‐1A, M9‐2, and M9‐8, and Arthrobacter psychrophenolicus strain M9‐17 were evaluated against nine plant pathogens in vitro, and their cell‐free filtrates were additionally assessed for inhibition of Alternaria alternata and suppression of black mold disease on tomato fruit. Results indicated that A. humicola M9‐1A and A. psychrophenolicus M9‐17 were the most inhibitory, reducing growth of seven of the pathogens studied. Cell‐free filtrates of A. psychrophenolicus M9‐17 reduced the growth of most pathogens. All cell‐free bacterial filtrates, except those from A. humicola M9‐2, suppressed black mold on tomato fruit. Disk diffusion assays with ethyl acetate soluble culture filtrate extracts of all bacteria reduced the mycelial growth of A. alternata. Clear inhibition zones were observed for A. psychrophenolicus M9‐17 extracts using drop bioassays. The antifungal compound N‐acetyltryptamine was purified and characterized from the A. psychrophenolicus M9‐17 cell‐free ethyl acetate soluble extract. This study suggests that antibiosis may play a key role in the antimicrobial activity of Arthrobacter spp.

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In an effort to describe bioactive antifungal compounds from antagonistic bacteria with potential for biocontrol of plant pathogens, a strain of Arthrobacter psychrophenolicus was collected from plant disease suppressive compost prepared from composted material of marine origin. Few natural products have been characterized from the non-filamentous Actinobacteria genus Arthrobacter. A new cyclic tetrapeptide, cyclo-(L-Pro-L-Leu-L-γHyp-L-Tyr); arthropeptide A (1), was isolated from the EtOAc soluble culture filtrate extract of A. psychrophenolicus M9-17 grown in MOLP broth. Its structure was confirmed by HRMS, interpretation of NMR data, and a modified Marfey's method. Arthropeptide A (1) displayed antifungal activity towards Alternaria alternata, the causal agent of disease in numerous host plant species, which had shown previous susceptibility to A. psychrophenolicus. The newly identified compound may be responsible, in part, for the inhibitory activity of the bacterium against fungal plant pathogens.

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