Selective Isolation of Bioactive-Pigmented Bacteria from Saline Agricultural Soil and Assessment of Their Antimicrobial Potential against Plant Pathogens

Abstract

The high incidence of disease and pests and their resistance to chemical control agents pose serious threats to both the agriculture sector and the environment. The present study assessed the antagonistic potential of bioactive pigment-producing bacteria isolated from the saline agricultural fields of Gujrat, Pakistan, against plant pathogenic fungi and bacteria. The seeded agar overlay method was used to selectively isolate bioactive pigment-producing colonies. Isolates were identified as Nonomurae salmonae, Streptomyces chromofuscus, and Actinocorallia libanotica using 16S rRNA gene sequence analysis. All the isolates and their crude pigment extracts were screened to assess antifungal activity against five fungal phytopathogens, namely Fusarium oxysporum (F. oxysporum), Fusarium solani (F. solani), Aspergillus flavus (A. flavus), Aspergillus niger (A. niger), and Alternaria alternata (A. alternata), as well as two bacterial phytopathogens, namely Psuedomonas syringae (P. syringae) and Xanthomonas axonopodis (X. axonopodis). Of these, Streptomyces chromofuscus was found to be active against most of the fungal and bacterial phytopathogens tested, followed by Nonomurae salmonae. Actinocorallia libanotica showed little to no activity against the tested microbes. Nonomurae salmonae and Actinocorallia libanotica are rare actinomycetes and the current study is the first to assess their antimicrobial activity against plant pathogens, specifically, plant pathogenic bacteria, i.e., P. syringae and X. axonopodis. The isolation of these species suggests that the chances of the isolation of rare species of microbes, which can serve as promising new sources of bioactive compounds, can be increased by using enhanced techniques for isolation. The results of this preliminary study assessing the antagonistic effect of bioactive pigment-producing bacterial isolates against plant pathogens are encouraging, and suggest a detailed research on the modes of action, optimum working conditions, and active components involved in an antagonism of these bioactive pigment-producing bacteria.