Fundamental Food Microbiology, Third Edition - Fuad Fathir

CHARACTERISTICS OF PREDOMINANT MICROORGANISMS IN FOOD 19
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B. Bacterial Cells
Bacteria are unicellular, most ca. 0.5–1.0 ¥ 2.0–10 mm in size, and have three
morphological forms: spherical (cocci), rod shaped (bacilli), and curved (comma)
(Figure 2.1). 7 They can form associations such as clusters, chains (two or more
cells), or tetrads. They can be motile or nonmotile. Cytoplasmic materials are
enclosed in a rigid wall on the surface and a membrane beneath the wall. Nutrients
in molecular and ionic form are transported from the environment through the
membrane by several but specific mechanisms. The membrane also contains energygenerating
components. It also forms intrusions in the cytoplasm (mesosomes). The
cytoplasmic material is immobile and does not contain organelles enclosed in a
separate membrane. The ribosomes are 70S type and are dispersed in the cytoplasm.
The genetic materials (structural and plasmid DNA) are circular, not enclosed in
nuclear membrane, and do not contain basic proteins such as histones. Both gene
transfer and genetic recombination occur, but do not involve gamete or zygote
formation. Cell division is by binary fission. Procaryotic cells can also have flagella,
capsules, surface layer proteins, and pili for specific functions. Some also form
endospores (one per cell).
On the basis of Gram-stain behavior, bacterial cells are grouped as Gram-negative
or Gram-positive. Gram-negative cells have a complex cell wall containing an outer
membrane (OM) and a middle membrane (MM) (Figure 2.2). The OM is composed
of lipopolysaccharides (LPS), lipoprotein (LP), and phospholipids. Phospholipid
molecules are arranged in a bilayer, with the hydrophobic part (fatty acids) inside
and hydrophilic part (glycerol and phosphate) outside. LPS and LP molecules are
embedded in the phospholipid layer. The OM has limited transport and barrier
functions. The resistance of Gram-negative bacteria to many enzymes (lysozyme,
which hydrolyzes mucopeptide), hydrophobic molecules (SDS and bile salts), and
antibiotics (penicillin) is due to the barrier property of the OM. LPS molecules also
have antigenic properties. Beneath the OM is the MM, composed of a thin layer of
peptidoglycan or mucopeptide embedded in the periplasmic materials that contain
several types of proteins. Beneath the periplasmic materials is the plasma or inner
membrane (IM), composed of a phospholipid bilayer in which many types of proteins
are embedded.
Gram-positive cells have a thick cell wall composed of several layers of
mucopeptide (responsible for thick rigid structure) and two types of teichoic acids
(Figure 2.2). Some species also have a layer over the cell surface, called surface
layer protein (SLP). The wall teichoic acid molecules are linked to mucopeptide
layers, and the lipoteichoic acid molecules are linked to both mucopeptide and
cytoplasmic membrane. Teichoic acids are negatively charged (because of phosphate
groups) and may bind to or regulate the movement of cationic molecules in and out
of the cell. Teichoic acids have antigenic properties and can be used to identify
Gram-positive bacteria serologically. Because of the complexity in the chemical
composition of the cell wall, Gram-positive bacteria are considered to have evolved
before Gram-negative bacteria.