PhD Vermeiren Lieve Compleet - Hogeschool Gent

challenge experiments with L. monocytogenes in the CFS, obtained from 10A’s growth in the
media with the two different nutrient levels, allowed distinction between the different
antagonistic effects (pH-reduction, lactic acid production and nutrient competition). In the
nutrient-poor broth, growth inhibition was exclusively caused by nutrient competition and
competition for glucose was, at least partly, involved. In the nutrient-rich broth, growth
inhibition was caused by a combination of several factors: the antimicrobial effect of the
produced lactic acid/lactate, nutrient competition and pH-reduction as a consequence of lactic
acid formation.
In a final experiment, no effect of supplementation with vitamins and minerals on the
inhibition phenomena could be observed.
Chapter 8 reports on the use of bacteriophage P100 to prevent proliferation of postprocessing
contaminating L. monocytogenes cells on vacuum packaged sliced cooked meat
products.
At first, broth experiments revealed that the three L. monocytogenes strains, used in chapters 5
and 6 of this PhD-work, were each susceptible to the action of bacteriophage P100 and this at
30°C as well as at 7°C. However, at 30°C the susceptibility towards P100 was straindependent
since the time, at which the OD(600 nm) of the growth medium containing
L. monocytogenes and P100 started to increase, differed among the three strains. Therefore
subsequent application trials on cooked meat products were making use of a cocktail of the
three L. monocytogenes strains.
In a preliminary application test on a sliced, cooked poultry product the presence of phage
P100 resulted in a reduction of the L. monocytogenes count with 3.32 log10(cfu/g) compared
to the untreated control after 21 days of storage (7°C, vacuum). A more elaborate application
test on sliced cooked ham confirmed the antilisterial effect of P100 on CMP during storage at
7°C under vacuum packaged conditions. In the latter experiment, treatment with P100 at a
level of 1×10 7 pfu/cm 2 or 5×10 6 pfu/cm 2 reduced the population of L. monocytogenes after 10
days of storage (7°C, vacuum) with 0.97 and 0.61 log10(cfu/g), respectively compared to the
untreated control. However, the difference in antilisterial effect between the two different
phage doses was shown to be not significant.
In conclusion, this chapter provided evidence on the usefulness of bacteriophage P100 to
control the growth of L. monocytogenes on sliced, cooked meat products during anaerobic
storage at 7°C.
Summary
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