Antimicrobial Activity of Actinobacteria Isolated From the Guts of Subterranean Termites

Abstract

Subterranean termites need to minimize potentially pathogenic and competitive fungi in their environment in order to maintain colony health. We examined the ability of Actinobacteria isolated from termite guts in suppressing microorganisms commonly encountered in a subterranean environment. Guts from two subterranean termite species, Reticulitermes flavipes (Kollar) and Reticulitermes tibialis Banks, were extracted and plated on selective chitin media. A total of 38 Actinobacteria isolates were selected for in vitro growth inhibition assays. Target microbes included three strains of Serratia marcescens Bizio, two mold fungi (Trichoderma sp. and Metarhizium sp.), a yeast fungus (Candida albicans (C.P. Robin) Berkhout), and four basidiomycete fungi (Gloeophyllum trabeum (Persoon) Murrill, Tyromyces palustris (Berkeley & M.A. Curtis) Murrill, Irpex lacteus (Fries) Fries, and Trametes versicolor (L.) Lloyd). Results showed both broad and narrow ranges of antimicrobial activity against the mold fungi, yeast fungus, and S. marcescens isolates by the Actinobacteria selected. This suggests that termite gut-associated Actinobacteria produce secondary antimicrobial compounds that may be important for pathogen inhibition in termites. Basidiomycete fungi were strongly inhibited by the selected Actinobacteria isolates, with G. trabeum and T. versicolor being most inhibited, followed by I. lacteus and T. palustris The degree of inhibition was correlated with shifts in pH caused by the Actinobacteria. Nearly all Actinobacteria isolates raised pH of the growth medium to basic levels (i.e. pH ∼8.0-9.5). We summarize antimicrobial activity of these termite gut-associated Actinobacteria and examine the implications of these pH shifts.

Keywords: Reticulitermes spp.; Streptomyces spp.; basidiomycetes; mold fungi; pH.

Fig. 1

(a) Experimental design for inhibition of potential entomopathogens; (b) experimental (top) wells showing no- to complete inhibition of mold fungi (ratings: 0–3) and controls (bottom). Metarhizium sp. used as an example for ratings 0 and 2, Trichoderma sp. used as an example for ratings 1 and 3.

Fig. 2

Phylogenetic tree based on partial 16S rRNA sequences of Actinobacteria isolated from subterranean termite guts compared to other publically available Streptomyces and bacterial strains. Isolates used in this study are shown in green (Hazel Green, WI isolates; R.t.), blue (Janesville, WI isolates; R.f.), and orange (Muscoda, WI isolates; R.f.) and assigned accession numbers from the NCBI database are given in brackets. Other termite-associated strains are labeled with the termite species in parentheses in brown. Bootstrap values are based on 100 replicates. Percent support is given for all nodes ≥ 70%.

Fig. 3

Median (cm) zone of inhibition (d) values for all Actinobacteria isolates by basidiomycete fungus. Boxes represent the 25% and 75% quartiles.

Fig. 4

Growth of S. marcescens and mold fungi on YPM plates with pH ranging from 5–10.

Fig. 5

Growth of basidiomycete fungi on YPM plates with pH ranging from 5–10.