Beauveria bassiana: Pathogenic Pesticide

by NH

Pesticides. Toxic, unhealthy, chemicals. The word is increasingly becoming associated with concerns regarding chemical residues on produce as well as toxic runoff and groundwater contamination. Although chemical pest control has proven to be effective, it is clear that the side effects of widespread chemical treatments have negative impacts on both the environment and human health. Current legislation does not even require potential toxins to be listed on pesticide labels, and agricultural runoff is vastly unregulated (1). Acquired resistance of insects to these chemicals additionally undermines their applicability as sustainable pesticides. However, there are safer alternatives to chemical pesticides; one such option being the fungus Beauveria bassiana. That’s right. A fungus used as a pesticide, specifically called a microbial pesticide. Microbial pesticides are a type of biopesticide that utilize living organisms to control pests. Microbial pesticides present significant benefits over chemical pesticides as they are nontoxic to humans and wildlife and are specific to a particular group of insects (2).

Beauveria bassiana is a species of ubiquitous soil fungus and insect pathogen capable of growing in a variety of climates (3). Its multifaceted life cycle allows B. bassiana to transition between these lifestyles (3). One stage of this life cycle allows the fungi to produce spores capable of infecting insects, causing white muscardine disease (3). This disease is characterized by a white, spore-covered mold that grows from inside the body of the infected insect. B. bassiana is also found in a wide range of geographical locations, likely due to its high natural genetic variability between populations (4). It is thought that this variability also contributes to the diversity of insects that strains of B. bassiana are able to infect. The variety of potential species-specific isolates makes this fungus an attractive candidate to control a variety of pests.

B. bassiana’s mode of infection also contributes to its potential for large scale use as a biopesticide. Unlike common chemical pesticides, which must be directly consumed by insects, B. bassiana can infect insects simply by coming into contact with them. The ease of insect infection by B. bassiana presents a benefit of using the fungus as an alternate means of insect control. During the infectious stage of its life cycle, the fungus produces spores that adhere to and grow on the bodies of insects (3). Once mature, the fungi can enter the insect using a combination of mechanical pressure and secretion of specialized enzymes to degrade the outer layer of the insect (3). As previously mentioned, the enzymes secreted vary based on the species of insect infected, highlighting the versatility of B. bassiana as a microbial pesticide. Once inside the insect, B. bassiana switches its growth to single celled structures called blastospores (3). These blastospores are able to circulate through the body of the insect, utilize its nutrients, and secrete toxins that kill the host (3). Interestingly, the blastospores are able to shed components of their cell surfaces to evade detection by the insect’s immune system (8). The components shed include receptors which likely allow for recognition by the host immune system (8). After the death of the insect, its body is again ruptured so the fungus can release spores to initiate a new infection (3).

 B. bassiana has a host range of over 700 insect species including crop pests, ecologically hazardous pests, and insect disease vectors such as mosquitos and ticks (3, 5). A vector is an organism that spreads infection by transmitting pathogens between hosts. Although B. bassiana has a wide host range, the capability of the fungus to infect different hosts depends on the specific strain. To target a specific pest using B. bassiana as a microbial pesticide, it is possible to isolate a strain of the fungus capable of infecting that particular pest. The ability of strains of this fungus to infect such a wide variety of insects is likely due to host-specific gene expression. In fact, it has been found that approximately 2500 genes are differentially expressed by B. bassiana based on the environment in which it is grown (6). Recently, a group of researchers grew B. bassiana on samples of media containing extracts of different crop pests. They determined that the enzymes produced by B. bassiana differed depending on the media on which they were grown (6). These findings are important because they show that B. bassiana can regulate its gene expression to live in different environments and infect different hosts, making it a marketable microbial pesticide. Determining the genes involved in this differential expression can allow us to elucidate virulent strains of B. bassiana that are specific to particular species of insects. A virulent organism is one that is capable of causing disease in another organism. The more virulent the strain, the more effective it will be as a microbial pesticide.

The wide host range of B. bassiana strains also allows the fungus to infect human disease vectors, providing a potential solution to chemical pesticide-resistant insects. Over time, mutations in the DNA of the insects can result in individuals that become resistant to a particular chemical insecticide. These individuals survive the toxic chemicals, and are able to mate and expand the population of resistant insects. At this point, a new chemical has to be developed to inhibit the resistant populations of insects. Microbial pesticides offer an alternative to the multitude of chemicals to which insects are already resistant. It was determined that B. bassiana is capable of infecting and killing species of insecticide-resistant Anopheles arabienses mosquitos (7). A. arabienses are the mosquitos that infect humans with the bacteria that cause malaria. These mosquitos become difficult to control as they develop resistance to chemical insecticides. This finding eliminates the possibility that resistance to insecticides also confers resistance to fungal pathogens (7).

Progress is currently being made to increase the applicability of B. bassiana as a viable biocontrol agent. To use the fungus as an insecticide, labs develop and collect the infectious spores to incorporate into a formula to be applied to plants. Solid-state fermentation has been identified as the most efficient way to produce these spores. Using this method, the fungi are grown on media saturated with water and plant byproducts (9). Solid-state fermentation is advantageous over other methods in that it is relatively simple, cost effective, energy-efficient, and uses less water (9). Production methods that use less water yield spores with higher shelf lives (9). Ideally, streamlining the industrial production methods of B. bassiana as a microbial pesticide will help increase its appeal for use in large scale insect control.

Although microbial pesticides offer a nontoxic, species-specific approach to insect control, there are potential issues to using living organisms to control pests. The majority of these concerns involve manufacturing and storage procedures of the pesticides, which are currently being refined. Microbial pesticides are also typically more susceptible to heat and desiccation, and therefore proper timing and application procedures are especially important (2). Additionally, the species specificity of microbial pesticides often results in the need to utilize several different pesticides to control additional pests in the area (2). The insect selectivity of microbial pesticides also has the potential to limit the market for their use as biocontrol agents (2).

In all, the fungus Beauveria bassiana expresses many characteristics that make it an appealing alternative to potentially harmful chemical pesticides currently used to control insects. A variety of species-specific strains found in a multitude of climates make isolates of B. bassiana suitable candidates for use against both agricultural pests and human disease vectors. An efficient and sustainable method of microbial pesticide production has been determined. Genes associated with pathogenesis are being studied to identify particularly virulent strains of B. bassiana. The research presented indicates that the widespread success of this fungus as an insect pathogen can translate into its potential as an effective biopesticide.

Citations

1.     Joe Magner. “Is Water Getting Cleaner? An Introduction to Water Quality.” Department of Biosystems & Bioproducts Engineering. 2016. Web.

2.     Canan Usta. “Microorganisms in Biological Pest Control — A Review (Bacterial Toxin Application and Effect of Environmental Factors).” Current Progress in Biological Research. N.p., 2013. Web.

3.     Claudio A. Valero-Jimenez et al. “Genes Involved in Virulence of the Entomopathogenic Fungus Beauveria bassiana.” Journal of Invertebrate Pathology 133 (2016): 41–49. Web.

4.     P. Maurer et al. “Genetic Diversity of Beauveria bassiana and Relatedness to Host Insect Range.” Mycological Research 101.2 (1997): 159–164. Web.

5.     Sandhya Galidevara, Anette Reineke, and Uma Devi Koduru. “In Vivo Expression of Genes in the Entomopathogenic Fungus Beauveria Bassiana during Infection of Lepidopteran Larvae.” Journal of Invertebrate Pathology 136 (2016): 32–34. Web.

6.     Akbar Ali Pathan et al. “Analysis of Differential Gene Expression in the Generalist Entomopathogenic Fungus Beauveria Bassiana (Bals.) Vuillemin Grown on Different Insect Cuticular Extracts and Synthetic Medium through cDNA-AFLPs.” Fungal Genetics and Biology 44.12 (2007): 1231–1241. Web.

7.     Christophe K Kikankie et al. “The Infectivity of the Entomopathogenic Fungus Beauveria Bassiana to Insecticide-Resistant and Susceptible Anopheles Arabiensis Mosquitoes at Two Different Temperatures.” Malaria Journal 9.71 (2010): n. pag. Web.

8.     J. C. Pendland, S. Y. Hung, and D. G. Boucias. “Evasion of Host Defense by In Vivo-Produced Protoplast-Like Cells of the Insect Mycopathogen Beauveria Bassiana.” Journal of Bacteriology 175.18 (1993): 5962–5969. Web.

9.     Pham, Tuan Anh, Jeong Jun Kim, and Keun Kim. “Optimization of Solid-State Fermentation for Improved Conidia Production of Beauveria Bassiana as a Mycoinsecticide.” Mycobiology 38.2 (2010): 137–143. PMC. Web.