Effects of Sublethal Temperature Stress on the Growth, Survival and Culturability of Listeria Monocytogenes
Survival and growth of Listeria monocytogenes L56 (JMR isolate) was studied in trypticase soy broth grown at 37°C before being subjected to three selected sublethal stress of temperatures (55°C, 28°C and 4°C) using log and stationary phase as inoculums using two-plating systems; TSA with and with...
Saved in:
| Main Author: | |
|---|---|
| Format: | Thesis |
| Language: | English English |
| Published: |
2002
|
| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/8493/1/FSMB_2002_28_IR.pdf http://psasir.upm.edu.my/id/eprint/8493/ |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| Summary: | Survival and growth of Listeria monocytogenes L56 (JMR isolate) was studied in
trypticase soy broth grown at 37°C before being subjected to three selected sublethal
stress of temperatures (55°C, 28°C and 4°C) using log and stationary phase as inoculums
using two-plating systems; TSA with and without 4% NaCl (TSAS). The influence of
morphological changes and listerial motility as affected by sublethal stress of
temperatures were also determined using Scanning Electron Microscopy (SEM) and
motility media, respectively. A standard growth curve of L. monocytogenes at 37°C was
established using plate counts showed that the log and stationary phase of the organism
were achieved after 12 and 19 hours, respectively. It was observed that viable bacterial
population (CFU/ml) after log and stationary phase were 108 and 109, respectively. From
the growth curve, the generation time of L. monocytogenes at 37°C was 60 min. The bacterial growth rates obtained from culturability on culture plates assessed using
two-system media, TSA with and without 4% NaCl concentration (TSAS) were assessed
by their generation time. Cells of L. monocytogenes grown in exponential phase cultures
demonstrated biphasic survival curves at 55°C and 4°C in both media. In contrast,
survival curves at 28°C were not biphasic. The growth rates of L. monocytogenes grown
in stationary phase cultures were also assessed by their generation time. The addition of
sodium chloride enhanced heat resistance of microorganism.
It has been proven that biphasic curve and tailing with/without shoulder from thennal
inactivation curves in this study were associated with the occurrence of microbial injury.
During the exponential phase of L. monocytogenes, the percentage injury at 55°C, 28°C
and 4°C were ranged between 3.21% to 28.49%, 2.47% to 4.38%, and 4.34% to 8.61%,
respectively. Whilst, during the stationary phase of L. monocytogenes, the percentage
injury at 55°C, 28°C and 4°C were ranged between 2.05% to 4.15%, 1.44% to 3.06%,
and 1.07 to 4.25%, respectively.
L. monocytogenes cells were able to survive throughout the sublethal stress of
temperatures and undergone morphological changes to adapt to new temperatures. In
this study, results from Scanning Electron Microscopy (SEM) revealed three different
analyses of temperature-stressed cells which were average mean of cells length,
distribution of cells length, and minimum versus maximum cells length. The study
demonstrated cells of both log and stationary phase showed a significant variation of
morphology. Cells of log phase became elongated only at 55°C, not at 28°C and 4°C
whereas cells of stationary phase were shorter and more coccoidal rather than elongated as in log phase cells. However, cells at 28°C were more intact than cells at other
temperatures for most of the observations.
In conjunction with SEM results, the variation of listerial morphology and the effect of
listerial motility would be a part of microbial adaptation towards sublethal stress of
temperatures. The loss of motility in stationary phase cells strongly suggested that
listerial motility play a role in survival of the organism under temperature stress. The
stationary phase cells of L. monocytogenes were more resistant than exponential phase
cells exhibited by increased of generation times, lower percentage injury and most of the
cells became coccoid. |
|---|