Changes in Fourier transform infrared spectra of Salmonella enterica serovars Typhimurium and Enteritidis after adaptation to stressful growth conditions
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Abstract
The effects of growth conditions (temperature in the range 10-45 degrees C, sodium
chloride concentration in the range 0-4%, aerobic vs. anaerobic growth and acidification
of the growth medium, up to pH 4.5) on the Fourier transform infrared (FT-IR) spectra
of Salmonella enterica serovars Typhimurium and Enteritidis were studied using multivariate
statistical methods (Hierarchical Cluster Analysis and Factor Analysis). Although
all environmental factors tested affected S. Typhimurium and S. Enteritidis FT-IR
spectra to some extent, growth temperature was the most influential factor within
the five spectral regions. The w(4) spectral region (1200 to 900 cm(-1)) was the most
variable region, suggesting that S. Typhimurium and S. Enteritidis modulate their
cell wall and cell membrane composition in response to shifts in growth temperature.
Changes in membrane fluidity were determined by monitoring the vibrational modes of
the acyl chain v(s)CH(2) symmetric stretching band by FT-IR spectroscopy. For cells
grown in unsupplemented media an increase in growth temperature was linked to a decrease
in membrane fluidity. Even though the effect of NaCl concentration, pH and atmosphere
was considered of less importance, cells grown in acidified media also showed a reduction
in their membrane fluidity, and the addition of sodium chloride to the culture medium
was associated with an increase in the bacterial membrane fluidity. These findings
can help interpret how important adaptive mechanisms for the survival of pathogenic
bacteria in foods are, and show that FT-IR spectroscopy is a useful tool to understand
how environmental conditions mimicking those in certain food products affect the cell.
Also, FT-IR can be used to perform a rapid discrimination between bacterial phenotypes
with different adaptive tolerance responses to environmental stress.
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