Microbial toxins in the green world


Toxins are defined as poisonous substances, proteins or secondary metabolites, which can be produced by prokaryotic or eukaryotic organisms and which can cause disease or even have lethal effects on other organisms after contact or uptake. Toxins of microbial origin serve different functions in various environments. Terrestrial and marine predators may use toxins to kill their prey. Bacteria and fungi may produce antimicrobial compounds to compete for nutrients or to occupy particular ecological niches. Necrotrophic plant pathogens often produce toxins to kill plant tissues in advance of colonizing them, whereas obligate biotrophic pathogens are dependent on living host cells and have often lost the ability to produce toxins.

However, of many toxins that are lethal to animals and humans, we do not know their natural function. Mycotoxins can contaminate feed and food and can be extremely toxic to mammals, while their intrinsic functions in their natural habitat are often not known yet. Furthermore, the origin of genes encoding enzymes for toxin synthesis frequently remains unknown. Such genes tend to be clustered and coregulated. Horizontal gene transfer or even horizontal chromosome transfer has been proposed in the literature as a mechanism for organisms to become toxin producers. As recently, many genomes from diverse organisms producing toxins have been sequenced; comparative genomics enable the dissection of biosynthetic pathways involved in toxin production and regulation.

In this Thematic Issue, various aspects of proteinaceous and secondary metabolite toxins produced by bacteria, cyanobacteria and fungi are reviewed. The first review covers the mechanisms of action of Bt toxins. Fifty years ago, the marine biologist Rachel Carson wrote ‘Silent Spring’, a highly influential book on the widespread use of poisonous chemicals in the environment. Her movement stimulated the development of biocontrol agents, including Bacillus thuringiensis (Bt), the producer of Bt toxin, which is now widely used as a spray insecticide or in Bt-transgenic crop plants like cotton or corn. The concern is that insects will eventually develop resistance against Bt toxins. Fortunately, there are many classes of Bt toxins that have different specificities and activities. The contribution by Pardo-López et al. (2012) focuses on activities of 3-domain Cry toxins and discusses various mechanisms that insects can develop to become resistant against Bt toxins.

In a second contribution, Dittmann et al. (2012) give an update on cyanobacterial toxins which are produced by various species of cyanobacteria and which cause recurrent harmful algal blooms each summer, resulting in severe health problems or even death in humans. The biosynthetic pathways of various cyanobacterial toxins are described, including those of nonribosomal peptides, alkaloids and other secondary metabolites.

The last three reviews cover fungal toxins. The first review by Tsuge et al. (2012) describes the production of host-selective toxins (HSTs) produced by the plant pathogenic fungus Alternaria alternata. HSTs are low-molecular-weight secondary metabolites with a diverse range of structures that function as effectors controlling pathogenicity or virulence in particular plant–pathogen interactions. Studies on the molecular genetics of HST production have identified supernumerary chromosomes encoding HST gene clusters and have provided new insights into the evolution of A. alternata pathotypes.

The next review by Stergiopoulos et al. (2012) is on phytotoxic secondary metabolites and peptides produced by plant pathogenic Dothideomycete fungi. Many necrotrophic plant pathogenic fungi belonging to the class of Dothideomycetes synthesize such toxins as virulence factors. Dothideomycetes are remarkable because they are the only fungal species known so far to produce these toxins. The synthesis, regulation and mechanisms of action of the most important HSTs and non-HSTs are reviewed. Programmed cell death, which is often induced by HSTs, and the plant hypersensitive response, which results from recognition of effectors produced by biotrophic pathogens, often share common pathways.

The last review by Woloshuk and Shim (2012) is on aflatoxins, fumonisins and trichothecenes. Many Aspergillus and Fusarium species produce these mycotoxins while infecting seeds of food and feed crops including corn, wheat and barley, thereby threatening health and food security worldwide. The authors give an update on the biosynthesis and regulation of these three important mycotoxins.