Spora Kapang, Khamir dan Bakteri

Spora Kapang, Khamir dan Bakteri

MOLD SPORES

Molds form spores by both asexual and sexual reproduction and on this basis are classified as perfect or imperfect molds, respectively. Molds form large numbers of asexual spores and, depending on the type, can form conidia, sporangiaspores, and arthrospores. Conidia are produced on special fertile hyphae called conidiophores (Figure 8.1). Among the important molds in food, Aspergillus and Penicillium species form conidia. Sporangiospores are formed in a sack (sporangium) at the tip of a fertile hypha (sporangiophores). Mucor and Rhizopus species are examples of molds that form sporangiospores. Arthrospores, formed by the segmentation of a hypha, are produced by Geotrichum. An asexual spore in a suitable environment germinates to form a hypha and resumes growth to produce the thallus. Sexual spores form from the union of the tips of two hyphae, two gametes, or two cells. However, among the molds important in food, sexual reproduction is rarely observed. Some examples include Mucor and Neurospora.

YEAST SPORES

On the basis of sporeforming ability, yeasts important in food are divided into two groups: those that can produce sexual ascospores are designated as Ascomycetes (true yeasts), and those that do not form spores are called false yeasts. Examples of some yeasts important in food that form ascospores are Saccharomyces, Kluyveromyces, Pichia, and Hansenula. Species in the genera Candida, Torulopsis, and Rhodotorula do not form spores. Ascospores form by the conjugation of two yeast cells; in some cases, this can result from the union of the mother cell and a bud (daughter cell) (Figure 8.1). The number of spores developed in an ascus varies with species. In a suitable environment, each spore develops into a yeast cell.

 

BACTERIAL SPORES

The ability to form spores is confined to only a few bacterial genera, namely the Gram-positive Bacillus, Alicyclobacillus, Clostridium, Sporolactobacillus, and Sporosarcina and the Gram-negative Desulfotomaculum species. Among these, Bacillus, Alicyclobacillus, Clostridium, and Desulfotomaculum are of considerable interest in food, because they include species implicated in food spoilage and foodborne diseases. Several Bacillus and Clostridium species are used to produce enzymes important in food bioprocessing. In contrast to mold and yeast spores, bacterial cells produce endospores (inside a cell) and one spore per cell. During sporulation and until a spore emerges following cell lysis, a spore can be located terminal, central, or off-center, causing bulging of the cell. Under a phase-contrast microscope, spores appear as refractile spheroid or oval structures. The surface of a spore is negatively charged and hydrophobic. Spores, as compared with vegetative cells, are much more resistant to physical and chemical antimicrobial treatments, many of which are employed in the processing and preservation of food. This is because the specific structure of bacterial spores is quite different from that of vegetative cells from which they are formed. From inside to outside, a spore has the following structures (Figure 8.2): a protoplasmic core containing important cellular components such as DNA, RNA, enzymes, dipicolinic acid (DPN), divalent cations, and very little water; an inner membrane, which is the forerunner of the cell cytoplasmic membrane; the germ cell wall, which surrounds this membrane, and is the forerunner of the cell wall in the emerging vegetative cell; the cortex, around the cell wall, composed of peptides, glycan, and an outer forespore membrane; and the spore coats, outside the cortex and membrane, composed of layers of proteins that provide resistance to the spores. Spores of some species can have a structure called exosporium outside the coat. During germination and outgrowth, the cortex is hydrolyzed, and outer forespore membrane and spore coats are removed by the emerging cell. The spores are metabolically inactive or dormant, can remain in dormant form for years, but are capable of emerging as vegetative cells (one cell per spore) in a suitable environment. As opposed to nonsporeforming bacteria, the life cycle of sporeforming bacteria has a vegetative cycle (by binary fission) and a spore cycle (Figure 8.3). The spore cycle also goes through several stages in sequence, during which a cell sporulates and a vegetative cell emerges from a spore. These stages are genetically controlled and influenced by different environmental parameters and biochemical events, which are briefly discussed here.

   

 

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Third Edition FUNDAMENTAL FOOD MICROBIOLOGY