Isolation of a New Mexican Strain of Bacillus subtilis with Antifungal and Antibacterial Activities

1. Introduction

Data published by the Food and Agriculture Organization [1] indicated that Mexico is among the tenth largest producers of strawberries worldwide. In Mexico, the State of Guanajuato is one of the major producers of this crop, but during the past few years different factors, including the “secadera” disease, have led to an ~50% decline in productivity [2]. The etiologic agents of “secadera” are fungi, primarily Rhizoctonia sp., Fusarium sp., Verticillium sp., and Phytophthora sp. [3].

In Mexico, chemical pesticides are used to control agricultural pests, but their long-term effects on animals and the environment have stimulated much concern [4]. To circumvent known and proposed detrimental effects related to widespread use of these pesticides, much interest is focused in identifying and developing biological agents to control insects and phytopathogenic microbes [5]. In particular, the use of selected nonpathogenic microorganisms which are ubiquitous in the soil and has potential applied use if they naturally produce antagonistic metabolites that kill bacterial or fungal pathogens or significantly inhibit their proliferation. For example, it has been demonstrated that an antibiotic isolated from Bacillus cereus inhibited mycelial growth of Fusarium roseum, and chitinases synthesized by Serratia marcescens, Streptomyces spp. Bacillus circulans, and Trichoderma harzianum are involved in the degradation of the host fungus cellular walls [6].

Volatile antimicrobials compounds produced by the endophytic fungus Muscodor albus are also known to kill a broad range of fungi and bacteria that are pathogenic to plants and humans [7]. Indeed, both diffusible and volatile compounds, bacteriocins, siderophores, chitinases, cellulases, amylases, lipopeptide antibiotics (e.g., fengycin, surfactin, iturin), and nonlipopeptide antibiotics (e.g., sublacin, subtilin, subtilosin) are examples of other molecules produced by microbes that interfere with the establishment and survival of microbial ecological communities [8, 9].

Interestingly, wild type and recombinant strains of Bacillus subtilis are known to not only promote plant growth but also synthesize different metabolites with antibacterial and antifungal activities [9]. As such, natural strains of probiotic B. subtilis could be beneficial as biocontrol agent in the strawberry industry. However, though many different environmental isolates and strains of B. subtilis have been described, to date there is no report regarding the identification of native B. subtilis isolates from strawberries in Mexico that elicit significant antagonistic activity against...