Microbial Stress Response in the Food Environment ◾ 105
some chemicals (LiCl, selenite, bismuth sulfite, or tetrathionate), hydrolytic enzymes (lysozyme or
RNase), antibiotics, dyes (crystal violet or brilliant green), and low-pH and undissociated acids.
They also lose cellular materials, such as K + , peptides, amino acids, and RNA. When exposed
to or maintained in an unfavorable environment, the injured cells die progressively. Also, the
injured cells do not multiply unless the injury has been repaired. In a nonselective and preferably nutritionally balanced medium, the cells are able to repair their injury, which can extend
1–6 hours, depending on the nature of the stress and degree of injury. After an extended repair
phase, the cells regain their normal characteristics and initiate multiplication. The injured cells
in a population can be detected by a suitable nonselective enumeration technique. The results in
Table 10.2 show that normal Esc. coli cells grow almost equally in both the nonselective and selective media, indicating that the cells are not sensitive to the surface-active agent, deoxycholate, in
the medium. Following freezing and thawing, 93.7% of cells die. The survivors form colonies in
the nonselective tryptic soy (TS) agar (they repair and multiply), but 80.1% of the survivors fail to
form colonies in the selective TS-containing deoxycholate (TSD) agar because of their injury and
developed sensitivity to deoxycholate (bile salt). Among the survivors, however, 19.9% are normal
or uninjured cells, inasmuch as they grow equally well both in the TS and TSD agar media and are
no longer sensitive to deoxycholate. Many food systems can reduce both the cell death and injury
incurred from a specific stress.
Bacterial spore injury has been observed following heating, UV and ionizing radiation, treatment by hydrostatic pressure, and some chemicals (e.g., hypochlorite, H 2 O 2 , ethylene oxide, and
probably nitrite) that are important in foods. The injured spores develop sensitivity to NaCl, low
pH, NO 2 , antibiotics, redox potential, gaseous atmosphere, and temperature of incubation. They
also have delayed germination and longer lag for outgrowth, and they develop a need for some
specific nutrients. These manifestations also vary with the nature of stress to which the spores are
exposed.
Sites and Nature of Injury
Altered physiological characteristics of the injured bacterial cells have been used to determine
directly the site of damage in the cellular structural and functional components. There is evidence
table 10.2 effect of Freezing and thawing on Viability Loss and injury
of Esc. coli nCSM
Enumeration
Media a
Media
CFUs/mL
Subpopulations
Before
Freezing
After
Freezing
TS agar
Nonselective
276 × 10 6
17.5 × 10 6 Original population-dead:
276 × 10 6 − 17.5 × 10 6 (93.7%)
TSD agar
Selective
267 × 106 6
3.5 × 10 6
Among survivors-uninjured:
3.5 × 10 6 (19.9%); injured:
17.6 × 10 6 − 3.5 × 10 6 (80.1%)
a Esc. coli cells in water suspension were enumerated before and after freezing (−20ºC
for 16 hours) and thawing simultaneously in TS (tryptic soy) agar and TSD (TS + 0.075%
deoxycholate) agar media by pour plating followed by incubation at 37ºC for 24 hours.
some chemicals (LiCl, selenite, bismuth sulfite, or tetrathionate), hydrolytic enzymes (lysozyme or
RNase), antibiotics, dyes (crystal violet or brilliant green), and low-pH and undissociated acids.
They also lose cellular materials, such as K + , peptides, amino acids, and RNA. When exposed
to or maintained in an unfavorable environment, the injured cells die progressively. Also, the
injured cells do not multiply unless the injury has been repaired. In a nonselective and preferably nutritionally balanced medium, the cells are able to repair their injury, which can extend
1–6 hours, depending on the nature of the stress and degree of injury. After an extended repair
phase, the cells regain their normal characteristics and initiate multiplication. The injured cells
in a population can be detected by a suitable nonselective enumeration technique. The results in
Table 10.2 show that normal Esc. coli cells grow almost equally in both the nonselective and selective media, indicating that the cells are not sensitive to the surface-active agent, deoxycholate, in
the medium. Following freezing and thawing, 93.7% of cells die. The survivors form colonies in
the nonselective tryptic soy (TS) agar (they repair and multiply), but 80.1% of the survivors fail to
form colonies in the selective TS-containing deoxycholate (TSD) agar because of their injury and
developed sensitivity to deoxycholate (bile salt). Among the survivors, however, 19.9% are normal
or uninjured cells, inasmuch as they grow equally well both in the TS and TSD agar media and are
no longer sensitive to deoxycholate. Many food systems can reduce both the cell death and injury
incurred from a specific stress.
Bacterial spore injury has been observed following heating, UV and ionizing radiation, treatment by hydrostatic pressure, and some chemicals (e.g., hypochlorite, H 2 O 2 , ethylene oxide, and
probably nitrite) that are important in foods. The injured spores develop sensitivity to NaCl, low
pH, NO 2 , antibiotics, redox potential, gaseous atmosphere, and temperature of incubation. They
also have delayed germination and longer lag for outgrowth, and they develop a need for some
specific nutrients. These manifestations also vary with the nature of stress to which the spores are
exposed.
Sites and Nature of Injury
Altered physiological characteristics of the injured bacterial cells have been used to determine
directly the site of damage in the cellular structural and functional components. There is evidence
table 10.2 effect of Freezing and thawing on Viability Loss and injury
of Esc. coli nCSM
Enumeration
Media a
Media
CFUs/mL
Subpopulations
Before
Freezing
After
Freezing
TS agar
Nonselective
276 × 10 6
17.5 × 10 6 Original population-dead:
276 × 10 6 − 17.5 × 10 6 (93.7%)
TSD agar
Selective
267 × 106 6
3.5 × 10 6
Among survivors-uninjured:
3.5 × 10 6 (19.9%); injured:
17.6 × 10 6 − 3.5 × 10 6 (80.1%)
a Esc. coli cells in water suspension were enumerated before and after freezing (−20ºC
for 16 hours) and thawing simultaneously in TS (tryptic soy) agar and TSD (TS + 0.075%
deoxycholate) agar media by pour plating followed by incubation at 37ºC for 24 hours.