Fundamental Food Microbiology, Third Edition - Fuad Fathir

112 FUNDAMENTAL FOOD MICROBIOLOGY
Before Stress Stress
8
6
4
Log
cfu/ml
2
0
Dead
Injured
Nonselective Selective
Survivors
Repair
Min at 30�C
Growth
No Repair (4�C)
0 30 60 90 120 150
Figure 9.3 A hypothetical repair curve of injured bacteria. Repair is indicated by an increase
in counts only in selective agar media during incubation in nonselective repair
broth. Cell multiplication is indicated by a simultaneous increase in counts on both
selective and nonselective agar media.
(to destroy H 2O 2 produced by the cells) also enhances repair and increases the
number of repaired cells. It has been suggested that during the rapid repair process,
many cells generate H 2O 2 but fail to hydrolyze it because of an injured peroxidase
system. Accumulated H 2O 2 then causes cell death. A simple medium with a suitable
energy source can also enable some cells with low level of injury to repair damage
in the OM (or cell wall). Depending on the sublethal stress, complete repair can be
achieved in 1 to 6 h at 25 to 37�C. For freezing and drying injuries, the rate is very
rapid; for heat injuries, the rate can be slow. Specific studies show that the metabolic
processes during repair vary with the nature of a stress and involve synthesis of ATP,
RNA, DNA, and mucopeptide. Reorganization of the existing macromolecules can
also be an important event during the repair process. The cell wall (or OM) regains
the ability to prevent entrance of many chemicals to the cells; the cytoplasmic
membrane (or IM) regains its permeability barrier function as well as the enzymes;
and RNA and DNA regain their original characteristics. Finally, the cells regain their
ability to multiply. Direct or indirect studies have not been conducted to show that
the repair process is associated with activation or inactivation of genetic material.
Repair conditions for injured spores vary with the type (aerobic or anaerobic
species) of spores. The composition of media is very important; in addition to good
carbon and nitrogen sources, addition of special compounds, such as starch, a
reducing compound (such as cysteine), lysozyme, and divalent cations, may be