Fundamental Food Microbiology, Third Edition - Fuad Fathir

20 FUNDAMENTAL FOOD MICROBIOLOGY
Figure 2.2 Schematic representations of cell envelopes of bacteria. (A) Gram-positive bacteria:
SL: surface layer proteins with protein subunits (1); CW: cell wall showing
thick mucopeptide backbone layers (2) covalently linked to peptides (4), wall
teichoic acids (or teichouronic acid); (3), lipoteichoic acids (anchored to cytoplasmic
membrane; (5); CM: cytoplasmic membrane with lipid bilayers containing
phospholipids (7), glycolipids (6) and embedded proteins (8). (B) Gram-negative
bacteria: OM: outer membrane containing lipopolysaccharide molecules, stabilized
by divalent cations (1), phospholipids and proteins; MM, middle membrane containing
thin mucopeptide layers (4) covalently liked to peptides (3) and lipoproteins
(2); IM: inner membrane with phospholipid bilayers (5) and proteins (6).
C. Viruses
Viruses are regarded as noncellular entities. Bacterial viruses (bacteriophages)
important in food microbiology are widely distributed in nature. 8–10 They are composed
of nucleic acids (DNA or RNA) and several proteins. The proteins form the
head (surrounding the nucleic acid) and tail.
A bacteriophage attaches itself to the surface of a host bacterial cell and inoculates
its nucleic acid into the host cell. Subsequently, many phages form inside a
host cell and are released outside following lysis of the cell. This is discussed in
Chapter 13.