**1. Introduction**

Invasion of food, feed and agricultural crops with mycotoxigenic fungi from the genera *Asper‐ gillus*, *Fusarium* and *Penicillium* is an economic problem that is not yet under adequate con‐ trol despite modern food production technologies and the wide range of preservation techniques available (Bennett & Klich, 2003). A small number of characterized fungi are as im‐ portant as the genus *Aspergillus*, a taxonomic group which encompasses members with patho‐ genic, agricultural, industrial and pharmaceutical importance (Jamali et al., 2012). Nearly all fungi that produce aflatoxins, the most potent naturally occurring hepatocarcinogens, are members of the genus *Aspergillus* classified into the section *Flavi*. Among 22 closely related spe‐ cies in *Aspergillus* section *Flavi*, the members frequently encountered in agricultural prod‐ ucts i.e. *Aspergillus flavus* and *A. parasiticus* are responsible for the majority of aflatoxin (AF) contamination events, with *A. flavus* being by far the most common (Varga et al., 2011). Afla‐ toxigenic fungi are common soil habitants all over the world and they frequently contami‐ nate agricultural crops, such as peanuts, cottonseed, maize, and tree nuts (Bennett & Klich, 2003; Hedayati et al., 2007; Razzaghi-Abyaneh et al., 2006; Sepahvand et al., 2011). The fun‐ gal community structure composed of several players, species, strains, isolates and vegeta‐ tive compatibility groups (VCGs), in the soil and on the crop determines the final AF concentration (Jamali et al., 2012; Razzaghi-Abyaneh et al., 2006). The life cycle of *A. flavus* in a pistachio orchard is shown in Fig. 1. AF contamination of agricultural crops is a major con‐ cern due to economical losses resulting from inferior crop quality reduced animal productiv‐ ity and impacts on trade and public health. In a global context, AF contamination is an everlasting concern between the 35N and 35S latitude. Most of the countries in the belt of con‐ cern are developing countries and this makes the situation even worse because in those coun‐ tries people frequently rely on highly susceptible crops for their daily nutrition and income. It has also been evident that AF more and more becomes a problem in countries that previous‐ ly did not have to worry about AF contamination.

© 2013 Shams-Ghahfarokhi et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Shams-Ghahfarokhi et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of efficiently inhibit toxigenic fungus growth and AF production. They mainly belong to the genera *Bacillus*, *Pseudomonas*, *Agrobacterium* and *Streptomyces* which have worldwide distribu‐ tion (Holmes et al., 2008; Ongena & Jacques, 2007; Razzaghi-Abyaneh et al., 2011; Stein, 2005). Metabolites from *Bacillus subtilis* (Fengycins A and B, plipastatins A and B, iturin A, mycosub‐ tilin, bacillomycin D), *Streptomyces* spp. (dioctatin A, aflastatin A, blasticidin A), and *Achromo‐ bacter xylosoxidans* [cyclo (L-leucyl-L-propyl)] are good examples of potent inhibitors of AF biosynthesis in laboratory conditions, crop model systems and also in the field (For review, see Razzaghi-Abyaneh et al., 2011). Since production of antifungal metabolites in bacteria is quite dependent to the strain and species, ongoing search on finding strange bacteria within the ex‐ isting biodiversity to increase the chance of finding novel antifungals is currently done all over

Terrestrial Bacteria from Agricultural Soils: Versatile Weapons against Aflatoxigenic Fungi

http://dx.doi.org/10.5772/45918

25

This chapter highlights comprehensive data on antagonistic bacteria isolated from agricul‐ tural soils of pistachio, peanuts and maize fields with an emphasis on their ability for inhib‐ iting growth of aflatoxigenic fungi and AF production. We first describe how we can isolate and identify a large number of soil bacteria with antagonistic activity against toxigenic *A. parasiticus* by simple, efficient and low-cost screening methods. Next to be addressed will be a practical approach to isolation, purification and identification of antifungal metabolites from antagonistic bacteria by a combination of traditional and recent advanced technologies.

Biological control is defined as i) a method of managing pests by using natural enemies ii) an ecological method designed by man to lower a pest or parasite population to acceptable subclinical densities or iii) to keep parasite populations at a non-harmful level using natural liv‐ ing antagonists (Baker, 1987). The history of biological control dates back to an outstanding successful story, the biocontrol of the cottony-cushion scale (*Icerya purchasi*) on *Citrus* plant in California (Debach & Rosen, 1991). Biological control agents act against plant pathogens through different modes of action. Antagonistic interactions that can lead to biological control include antibiosis, competition and hyperparasitism (Bloom et al., 2003; Bull et al., 2002; Cook, 1993; Hoitink & Boehm, 1999). Competition occurs when two or more microorganisms re‐ quire the same resources in excess of their supply. These resources can include space, nu‐ trients, and oxygen. In a biological control system, the more efficient competitor, i.e., the biological control agent out-competes the less efficient one, i.e., the pathogen. Antibiosis oc‐ curs when antibiotics or toxic metabolites produced by one microorganism have direct inhibi‐ tory effect on another. Hyperparasitism or predation results from biotrophic or necrotrophic interactions that lead to parasitism of the plant pathogen by the biological control agent. Some microorganisms, particularly those in soil, can reduce damage from diseases by promoting plant growth or by inducing host resistance against a myriad of pathogens. Nowadays, atoxi‐ genic *A. flavus* strains, biocompetitive bacteria and antagonistic yeasts has been effectively used to reduce AF contamination in field and laboratory conditions (Brown et al., 1991; Dorn‐ er et al., 1998, 1999; Hua et al., 1999; Palumbo et al., 2006). Commercial products from atoxigen‐ ic *A. flavus* under the names of AF36, AflaSafe and AflaGuard have been successfully used for biocontrol of aflatoxigenic fungi in maize, peanuts, cottonseed and pistachio fields in South‐ ern US, Northern Mexico, Nigeria and West Africa (Atehnkeng et al., 2008; Donner et al., 2010).

the world (Ranjbarian et al., 2011; Stein, 2005).

**2. Biological control: a powerful management strategy**

**Figure 1.** The life cycle of *A. flavus* is shown in a pistachio orchard. Infection of fruits with air-borne conidia occurs during Spring/Summer, while the fungus will survive by resistant structures named "sclerotia" during Autumn/Winter.

To ensure global safety on food and feed supplies, extensive researches have been carried out to effectively control and manage AF contamination of crops. The strategies for preventing AF contamination are generally divided into two categories including pre- and post-harvest con‐ trols (Kabak et al., 2006). Pre-harvest control strategies include appropriate field management practices (crop rotation, irrigation, soil cultivation, etc.), enhancing host resistance (transgenic or genetically modified crops), biological (application of antagonistic fungi and bacteria) and chemical control (fungicides, insecticides). Respect to biocontrol approaches, the rapid expan‐ sion in our knowledge about the role of microorganisms in inhibiting AF biosynthesis has en‐ abled us to utilize them as potential AF biocontrol agents (Holmes et al., 2008; Raaijmakers et al., 2002). A large number of plants, mushrooms, bacteria, microalgae, fungi and actinomy‐ cetes have now been screened for the ability to inhibit toxigenic fungal growth and/or AF pro‐ duction (Alinezhad et al., 2011, Bagheri-Gavkosh et al., 2009; Ongena & Jacques, 2007; Razzaghi-Abyaneh & Shams-Ghahfarokhi, 2011; Razzaghi-Abyaneh et al., 2005, 2007, 2008, 2009, 2010, 2011). Substantial efforts have been carried out in identifying organisms inhibitory to AF biosynthesis through co-culture with aflatoxigenic fungi with the aim of finding poten‐ tial biocontrol agents as well as novel inhibitory metabolites. The use of beneficial microorgan‐ isms is one of the most promising methods to the development of environmentally friendly alternatives to chemical pesticides in preventing the growth of aflatoxigenic fungi and subse‐ quent AF contamination of susceptible crops. Among beneficial microorganisms, antagonis‐ tic bacteria are in the first line of investigation because of a much greater diversity than that of any other organism and possessing valuable pharmaceutically active molecules (Ongena & Jacques, 2007; Stein, 2005). Recent advances in analytical methods and enormous expanding of natural products libraries, cloning, and genetic engineering have provided a unique opportu‐ nity for isolation and structural elucidation of novel bioactive antifungal compounds from bac‐ terial communities all over the world. It has been reported that, on average, two or three antibiotics derived from bacteria break into the market each year (Clark, 1996). Among an esti‐ mated number of 1.5 million bacterial species exists on our planet, only a little portion (less than 1%) has been identified yet of which a more little have tested for bioactive antifungal me‐ tabolites. Terrestrial bacteria are an interesting group of antagonistic microorganisms capable of efficiently inhibit toxigenic fungus growth and AF production. They mainly belong to the genera *Bacillus*, *Pseudomonas*, *Agrobacterium* and *Streptomyces* which have worldwide distribu‐ tion (Holmes et al., 2008; Ongena & Jacques, 2007; Razzaghi-Abyaneh et al., 2011; Stein, 2005). Metabolites from *Bacillus subtilis* (Fengycins A and B, plipastatins A and B, iturin A, mycosub‐ tilin, bacillomycin D), *Streptomyces* spp. (dioctatin A, aflastatin A, blasticidin A), and *Achromo‐ bacter xylosoxidans* [cyclo (L-leucyl-L-propyl)] are good examples of potent inhibitors of AF biosynthesis in laboratory conditions, crop model systems and also in the field (For review, see Razzaghi-Abyaneh et al., 2011). Since production of antifungal metabolites in bacteria is quite dependent to the strain and species, ongoing search on finding strange bacteria within the ex‐ isting biodiversity to increase the chance of finding novel antifungals is currently done all over the world (Ranjbarian et al., 2011; Stein, 2005).

This chapter highlights comprehensive data on antagonistic bacteria isolated from agricul‐ tural soils of pistachio, peanuts and maize fields with an emphasis on their ability for inhib‐ iting growth of aflatoxigenic fungi and AF production. We first describe how we can isolate and identify a large number of soil bacteria with antagonistic activity against toxigenic *A. parasiticus* by simple, efficient and low-cost screening methods. Next to be addressed will be a practical approach to isolation, purification and identification of antifungal metabolites from antagonistic bacteria by a combination of traditional and recent advanced technologies.
