3.2. Effect of magnesium addition on Salmonella spp. growth with
TV
BPW þ V medium containing TV was supplemented with
increasing concentration of MgCl2 (1e15 g/l) and inoculated with
low levels of Salmonella. Increase of Salmonella signals (growth and
TV reduction) was observed (data not shown) with increasing
magnesium concentration, maximizing at 5 g/l. Higher concentrations
led to decrease in signals which was not due to pH variation
but may result from osmotic stress on the bacterial structure.
Fig. 3 illustrates the impact of addition of 5 g/l of Mg2þ to the
medium inoculated with Salmonella serotype Enteritidis, containing
a high concentration of TV (1 g/l). Addition of Mg2þ in the
culture medium led to an improvement of apparent growth rate
and to a reduction of apparent lag. We also observed an important
increase of the final optical density signal with magnesium addition.
All these results seem to validate hypothesis of the protective
effect of magnesium on the bacterial growth. Indeed, apparent
growth rate improvement, reducing lag and final OD increase
translate into better bacterial growth and formazan production.
As seen in paragraph 3.1, the toxic effect of TV was mainly
observed on m* and we investigated the effect of magnesium
addition on the same Salmonella strains. Kinetic signals were
fitted, and Fig. 2B shows the evolution of m* depending on the TV
concentration in presence of 5 g/l of MgCl2. We observed an
important difference between results obtained in the absence of
magnesium (Fig. 2A) and results obtained in presence of magnesium
(Fig. 2B). In the absence of magnesium m* decreased with
increasing concentration of TV with total inhibition occurring at 1.2 g/l for all Salmonella strains tested. Magnesium addition
allowed stabilization and in some cases an increase of m*. Salmonella
London which was incapable of growth at a TV concentration
of 0.4 g/l in the absence of magnesium grew in a magnesium
supplemented medium containing up to 0.8 g/l of TV. A protective
effect of magnesium against the inhibitory effect of TV for the
Salmonella strains tested has been clearly demonstrated in this
study. This effect can be due to an improvement in permeability
regulation of the bacterial membrane as described by Strange
(1964) which can decrease the osmotic stress due to high TV
extra-cellular concentration. Moreover, magnesium-induced stabilization
of the membrane can strengthen the cell resistance to
intra-cellular formazan crystal accumulation and allow signal
enhancement.