ABSTRACT
INTRODUCTION Fungi can be found in a wide variety of environments, such as in seeds, plants, soil, water, insects, food and food products, and animal products. Phytopathogenic fungi cause damage to the living crops upon storage as Colletotrichum that causes anthracnose disease in several fruits and vegetables such as banana, avocado, papaya, and tomato, decreasing their commercial values. Fungal infection of grain, nuts and fruits is often preceded by physical damage caused by insect invasion or mechanical injury during harvest. Fungal growth reduces the nutritional value of storage grains and animal feed and can result in the production of mycotoxins (D'Mello and MacDonald, 1997). Mycotoxins are poisonous, often carcinogenic secondary metabolites of fungi, which are associated with certain disorders in animals and humans (for Fusarium on grain see for instance, D'Mello et al., 1998; Reid et al., 1999). Food products also become contaminated during processing and handling operations. Processed food can be considered as a complex often plant-based medium that fungi colonise and spoil. Fungal species associated with particular foods correlate with the characteristics and properties of the product (Dijksterhuis and Samson, 2002; Filtenborg et al., 2004). The primary cause for the deterioration of rye bread for example are the fungi Penicillium roqueforti, P. paneum, P. carneum and Paecilomyces variotii. Contaminated commodities, such as cereals,
can deteriorate during storage, resulting in enhanced contamination levels of whole wheat flour (Weidenborner et al., 2000). In food products, the issue of mycotoxins requires continuous attention, but more recently fungal spores are also increasingly recognized as aeroallergen sources (Green et al., 2005). Fungal contamination and the toxic metabolites it forms cause massive economic losses of food. There is a great interest among agricultural, food industrial and medical disciplines to prevent or control fungal contamination. These include different techniques that manipulate the physical environment of the fungus including acidification, increase of the osmotic potential, drying, cooled storage, pasteurisation and the use of modified atmospheres. Some fungal species are able to grow at such adverse conditions and are able to thrive at situations that are meant to be free of spoilage. Contamination and colonisation of the food products is often by means of survival vehicles including airborne spores. Fungi are known for their capability to produce sexual and/or asexual spores as agents of reproduction, dispersal and survival. Some fungal species predominantly form sexual spores as Talaromyces species even without the need of different mating types (homothallic) and ascospores are produced in high numbers, while there is only restricted production of asexual spores. Alternatively, many fungal species do not have a well recognised sexual stage and are designated as the Deuteromycetes (mitosporic fungi). This group includes many members of genera as Aspergillus, Penicillium and Fusarium,
which are very relevant fungi for food situations (Dijksterhuis and Samson, 2002). Spores play an important role in the life cycle of fungi acting as dispersal or survival spores. Dispersal spores are separated completely from the parent mycelium by different factors to facilitate migration to a new site. They have a moderate capacity for survival in a resting state (dormancy). They are also capable to germinate readily in the presence of nutrients or favourable environmental conditions (Griffin, 1994). In case of Aspergillus and Penicillium, conidia are formed in chains on specialised sporeforming cells (phialides). Mature conidia have to survive in dry conditions during dispersion through the air current (Dijksterhuis and Samson, 2002). In contrast, survival spores are often produced in lower numbers and may not be separated from the parent mycelium (Carlisle et al., 1994). As an example, thick-walled chlamydospores are produced by, e.g., Mucor racemosus, F. culmorum and Paecilomyces variotii and typically produced between hyphal cells. Besides, many ascospores are formed inside closed or open fruit bodies (ascomata) that reside within the mycelium and not on specialised structures (conidiophores) that enable the spores to be distributed by air-or water currents. Many fungal species are able to produce different types of spores within one colony as is the case with for example Fusarium species (microconidia, macroconidia and chlamydospores) and Eurotium species (conidia and ascospores) (Samson et al., 2004). As is stated above, fungal contamination of foods and food products and colonisation and infection of plants and animals is usually initiated by contact of the host with spores (conidia). Contamination by the external environment, e.g., air, water, walls and floors for instance is considered to be the main source of contamination of beef carcasses with Penicillium, Aspergillus, Mucor and Cladosporium species (Ismail et al., 1995). Additionally spores can be brought on the crop or food product via an encounter with organisms (insects, mites). The germination process is the beginning of fungal colonisation into food and on plants or ani-
mals. It involves the initiation of biochemical activities, with an increase of the metabolic rates and induction of morphological changes (Griffin, 1994; D'Enfert and Fontaine, 1997). A better understanding of spore survival and the different processes of spore germination could lead to novel techniques to prevent food spoilage. This chapter describes the germination process of fungal spores and the relation between germination and fungal contamination, mycotoxin production, control methods and the mode of action of antifungal agents. The problem of fungal contamination can be partially confronted with the use of fungal inhibitors of germination and hyphal growth, but spores are less sensitive to different compounds. It is here, that the terms fungistatic and fungicidal have a different meaning. Germination of a spore includes a continuous change from a “stasis”-like situation towards a vegetatively growing hyphal cell expressing processes as active metabolism, expanding cell mass and nuclear division. LANDING, ADHESION AND WETTING OF THE CONIDIA The first events of fungal colonisation are the landing of the spores on the substratum and subsequent hydration. Airborne spores are cells that have to deal with drying and rewetting and certainly will possess mechanisms that address the redistribution of cell components that accompany these changes. In Magnaporthe grisea, the conidia that are transported through the air have a collapsed appearance as a result of dehydration and this stage is regarded as a normal part of the life cycle of the cell and not as an artefact due to preparation of the cells. After rewetting these conidia retained their turgid shape (Howard, 1993). A similar feature is visible with dry rust spores immediately after contact with the leaf surface (Deising et al., 1992). Upon landing, attachment of the spore is important especially in case of the colonisation of plant surfaces, which often have a hydrophobic nature.
