Microbes and food 2. Microbe facts
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Fungal growth photo
Picture 2.3a A fungal growth in a petri dish.
2.3 Fungi (singular: fungus)
Fungi are a large and diverse group of organisms. Their main characteristics are:
  • their cells have membrane-bound nuclei (we call them eukaryotic)
  • they do not use photosynthesis
  • they form spores
  • they have rigid cell walls
  • respiration takes place in bodies called mitochondria in the cytoplasm.

Fungal cells have an elaborate arrangement of internal membranes. Fungi can be divided into two broad groups: filamentous fungi (including moulds and mushrooms) and yeasts.

How do they reproduce?

Fungi reproduce by sexual and asexual means. Most produce spores which in some types are borne on bodies called sporangia. Both spores and sporangia vary widely in size and form, depending on how they are spread – by wind, water, mechanical means or vectors.

Macrofungi produce large fruiting bodies which are familiar to us as mushrooms and toadstools. These produce spores in huge numbers and disperse them into the environment. In favourable conditions, these spores germinate and produce hyphae.

Potato blight photomicrograph
Picture 2.3b Potato blight is caused by a mould growing on the leaves of the plant.
Moulds

Moulds are filamentous (thread-like) fungi. A single filament is called a hypha. The hyphae branch as they grow forming a network called a mycelium.

Each hypha grows from the tip and divides repeatedly along its length. The hyphae penetrate their food source (usually dead, but sometimes living, plant and animal matter). They excrete enzymes which break down the complex organic molecules into simpler substances. The soluble nutrients pass through the cell wall and membrane, enabling the fungus to grow.

In most moulds the hyphae are divided by cross walls called septa which help to make filaments rigid but also control nutrient flow.

Moulds can grow in dry and acid conditions and can tolerate a wide range of temperatures. These fungi produce airborne spores.

Examples of moulds are: Penicillium, Mucor, Aspergillus.

Picture 2.3c yeast multiplying by budding.
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Yeasts

These are microscopic, single celled fungi that are usually round or oval in shape. Most reproduce by budding. When yeasts respire anaerobically they convert sugars into ethanol and carbon dioxide by a process known as fermentation.

They are mainly used to make fermented foods such as beer, wine or bread, but the biochemical activities of yeasts can have unwanted effects in some food products. Yeasts can tolerate dry and acid conditions.

Examples of yeasts include: Saccharomyces cerevisiae, Candida.