Morphology and Structure of Microorganisms in Foods Yeasts and Molds

Both yeasts and molds are eukaryotic, but yeasts are unicellular, whereas molds are multicellular.5,6 Eukaryotic cells are generally much larger (20–100 μm) than prokaryotic cells (1–10 μm). Eukaryotic cells have rigid cell walls and thin plasma membranes. The cell wall does not have peptidoglycan, is rigid, and is composed of carbohydrates.

The plasma membrane contains sterol. The cytoplasm is mobile (streaming) and contains organelles (mitochondria, vacuoles) that are membrane bound. Ribosomes are 80S type and attached to the endoplasmic reticulum. The DNA is linear (chromosomes), contains histones, and is enclosed in a nuclear membrane. Cell division is by mitosis (i.e., asexual reproduction); sexual reproduction, when it occurs, is by meiosis. Molds are nonmotile, filamentous, and branched (Figure 2.2).

The cell wall is composed of cellulose, chitin, or both. A mold (thallus) is composed of large numbers of filaments called hyphae. Figure 2.2 Photograph of microbial morphology. (a) Molds: conidial head of Penicillium sp. showing conidiophore (stalk) and conidia. (b) Yeasts: Saccharomyces cerevisiae, some carrying buds. (c) Rod-shaped bacteria: Bacillus sp., single and chain. (d) Spherical-shaped bacteria: Streptococcus sp., chain. (e) Spherical-shaped bacteria: tetrads.

(f) Bacillus cells carrying spores, center and off-center. (g) Clostridium cells, some carrying terminal spores (drumstick appearance). (h) Motile rod-shaped bacterium (Clostridium sp.) showing peretrichous flagella. An aggregate of hyphae is called mycelium. A hypha can be nonseptate, septate-uninucleate, or septate-multinucleate. A hypha can be vegetative or reproductive. The reproductive hypha usually extends in the air and forms exospores, either free (conidia) or in a sack (sporangium).

Shape, size, and color of spores are used for taxonomic classification. Yeasts are widely distributed in nature. The cells are oval, spherical, or elongated, about 5–30 × 2–10 μm in size (Figure 2.2).4 They are nonmotile.

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The cell wall contains polysaccharides (glycans), proteins, and lipids. The wall can have scars, indicating the sites of budding. The membrane is beneath the wall. The cytoplasm has a fine granular appearance for ribosomes and organelles. The nucleus is well defined with a nuclear membrane.

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