Microorganisms

Microbes can be essentially divided into two categories: the prokaryotes and the eukaryotes. The former, which embrace the bacteria, are substantially the simpler, in that they essentially comprise a protective cell wall, surrounding a plasma membrane, within which is a nuclear region immersed in cytoplasm (Fig. 1.1). This is a somewhat simplistic description, but suitable for our needs. The nuclear material (deoxyribonucleic acid, DNA), of course, figures as the genetic blueprint of the cell. The cytoplasm contains the enzymes that catalyse the reactions necessary for growth, survival and reproduction of the organisms (the sum total of reactions, of course, being referred to as metabolism). The membrane regulates the entry and exit of materials into and from the cell.

The eukaryotic cell (of which baker's or brewer's yeast, Saccharomyces cerevisiae, a unicellular fungus, is the model organism) is substantially more complex (Fig. 1.2). It is divided into organelles, the intracellular equivalent

Fig. 1.1 A simple representation of a prokaryotic cell. The major differences between Grampositive and Gram-negative cells concern their outer layers, with the latter having an additional membrane outwith the wall in addition to a different composition in the wall itself.

Fig. 1.1 A simple representation of a prokaryotic cell. The major differences between Grampositive and Gram-negative cells concern their outer layers, with the latter having an additional membrane outwith the wall in addition to a different composition in the wall itself.

Nucleoid

Nucleoid

Vacuole

Vacuole

Golgi apparatus

Fig. 1.2 A simple representation of a eukaryotic cell.

Golgi apparatus

Fig. 1.2 A simple representation of a eukaryotic cell.

of our bodily organs. Each has its own function. Thus, the DNA is located in the nucleus which, like all the organelles, is bounded by a membrane. All the membranes in the eukaryotes (and the prokaryotes) comprise lipid and protein. Other major organelles in eukaryotes are the mitochondria, wherein energy is generated, and the endoplasmic reticulum. The latter is an interconnected network of tubules, vesicles and sacs with various functions including protein and sterol synthesis, sequestration of calcium, production of the storage polysaccharide glycogen and insertion of proteins into membranes. Both prokaryotes and eukaryotes have polymeric storage materials located in their cytoplasm.

Table 1.1 lists some of the organisms that are mentioned in this book. Some of the relevant fungi are unicellular, for example, Saccharomyces. However, the major class of fungi, namely the filamentous fungi with their hyphae (moulds), are of significance for a number of the foodstuffs, notably those Asian products involving solid-state fermentations, for example, sake and miso, as well as the only successful and sustained single-cell protein operation (see Chapter 17).

Table 1.1 Some micro-organisms involved in food fermentation processes.

Bacteria

Fungi

Yeasts and non-

Gram negativea

Gram positivea

Filamentous

filamentous fungi

Acetobacter

Arthrobacter

Aspergillus

Brettanomyces

Acinetobacter

Bacillus

Aureobasidium

Candida

Alcaligenes

Bifidobacterium

Fusarium

Cryptococcus

Escherichia

Cellulomonas

Mucor

Debaromyces

Flavobacterium

Corynebacter

Neurospora

Endomycopsis

Lactobacillus

Penicillium

Geotrichum

Gluconobacter

Lactococcus

Rhizomucor

Hanseniaspora (Kloeckera)

Klebsiella

Leuconostoc

Rhizopus

Hansenula

Methylococcus

Micrococcus

Trichoderma

Kluyveromyces

Methylomonas

Mycoderma

Monascus

Propionibacter

Staphylococcus

Pichia

Pseudomonas

Streptococcus

Rhodotorula

Thermoanaerobium

Streptomyces

Saccharomyces

Xanthomonas

Saccharomycopsis

Zymomonas

Schizosaccharomyces Torulopsis Trichosporon Yarrowia

Zygosaccharomyces

aDanish microbiologist Hans Christian Gram (1853-1928) developed a staining technique used to classify bacteria. A basic dye (crystal violet or gentian violet) is taken up by both Gram-positive and Gram-negative bacteria. However, the dye can be washed out of Gram-negative organisms by alcohol, such organisms being counterstained by safranin or fuchsin. The latter stain is taken up by both Gram-positive and Gram-negative organisms, but does not change the colour of Gram-positive organisms, which retain their violet hue.

aDanish microbiologist Hans Christian Gram (1853-1928) developed a staining technique used to classify bacteria. A basic dye (crystal violet or gentian violet) is taken up by both Gram-positive and Gram-negative bacteria. However, the dye can be washed out of Gram-negative organisms by alcohol, such organisms being counterstained by safranin or fuchsin. The latter stain is taken up by both Gram-positive and Gram-negative organisms, but does not change the colour of Gram-positive organisms, which retain their violet hue.

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