Supplement bij veertiende jaargang, april 2006 - NVMM

with the purpose of supplying a surplus of ribosomes in
an environment with fluctuating nutrient concentrations,
enabling cells to start growing quickly when nutrient
concentrations increase. In contrast, our models suggest
that the experimentally observed regulation is optimal with
respect to growth rate.
The second phenomenon we studied was the use of
metabolically efficient and inefficient pathways in microorganisms.
When given the choice of two alternative
metabolic pathways, one with high metabolic efficiency
but using many enzymes, and one with low metabolic
efficiency but using less enzymes, our models predict
that it is optimal to use the metabolically efficient pathway
at low growth rates and the inefficient pathway at high
growth rates. Such behaviour is observed, for example, in
lactic acid bacteria that use mixed acid fermentation at low
growth rates and homolactic fermentation at high growth
rates, or in yeast that start producing alcohol aerobically at
high growth rates.
Our models are characterized by a highly simplified
description of a complete growing system and include some
known physical and biochemical constraints. We show that
such simple models can help in understanding growth-rate
dependent regulation. They may be used as a starting point
for more detailed descriptions in systems biology.
06.22
Unravelling the complexity of flux regulation
S. Rossell
Free University, Amsterdam
An important question is to what extent metabolic fluxes
are regulated by gene-expression or by metabolic regulation.
There are two distinct aspects to this question: (i) the local
regulation of the fluxes through the individual steps in
the pathway, and (ii) the influence of such local regulation
upon the pathway’s flux. We developed regulation analysis
so as to address the former aspect, for all steps in a
pathway. We demonstrate the new method for the issue of
how Saccharomyces cerevisiae regulates the fluxes through
its individual glycolytic and fermentative enzymes when
confronted with nutrient starvation. Regulation was
dissected quantitatively into: (i) changes in maximum
enzyme activity (V max – called hierarchical regulation),
and (ii) changes in the interaction of the enzyme with the
rest of metabolism (called metabolic regulation). Within
a single pathway, the regulation of the fluxes through
individual steps varied from fully hierarchical to exclusively
metabolic. Existing paradigms of flux regulation (such as
single- and multi-site modulation, and exclusively metabolic
regulation) were tested for the first time for a complete
pathway, and falsified for a major pathway in an important
model organism. We propose a subtler mechanism of flux
Ned Tijdschr Med Microbiol 2006; 4:Supplement
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regulation, with different roles for different enzymes, i.e.
‘leader’, ‘follower’ or ‘conservative’, the latter attempting to
hold back the change in flux. This study makes the subtlety,
so typical for biological systems, tractable experimentally,
and invites reformulation of the questions concerning the
drives and constraints governing metabolic flux regulation.
07.01
In vitro susceptibility of Biofilm growing Staphylococcus
aureus bovine mastitis isolates
M.B. Melchior1 , J. Fink-Gremmels 1 , W. Gaastra2 1
University Utrecht, Veterinary Pharmacology, Utrecht,
2University Utrecht, Bacteriology, Utrecht
Biofilm growing Staphylococcus aureus are related to chronic
infections in human medicine since the 1990s, while in
bovine mastitis S. aureus is well known for its therapy
resistance in chronic infections. Biofilms are a structured
community of bacterial cells enclosed in a self-produced
polymeric matrix and adherent to an inert or living surface.
Several in vitro studies have shown that bacteria growing
in biofim can become 10-1000 times more resistant to the
effects of antimicrobial agents compared to planktonic,
free floating bacteria of the same strain. Current antibiotic
susceptibility testing according to CLSI guidelines show
that, in general, S. aureus mastitis isolates have good susceptibility
for most therapeutic agents, however therapy results,
especially in more chronic cases, are often disappointing.
Objective of this research was to test susceptibility of
several S. aureus mastitis strains growing in Biofilm, for
a panel of 10 antimicrobials in cation adjusted Mueller-
Hinton broth and milk, in order to compare these results
with outcomes of clinical trials.
Results show that several several S. aureus therapeutics
have very poor efficacy against Biofilm growing bacteria,
despite their good efficacy against planktonic bacteria.
07.02
growth condition dependent esp expression and Biofilm
formation of Enterococcus faecium (efm)
W.J.B. van Wamel, M.J.M. Bonten, J. Top, R.J.L. Willems
UMC Utrecht, Medical Microbiology, Utrecht
Background: Hospital outbreaks of vancomycin-resistant
Efm are primarily caused by strains belonging to MLSTbased
clonal complex 17 (CC17), which is characterized by
the presence the Efm variant of enterococcal surface protein
(Esp). Esp of Enterococcus faecalis (homologous up to 90%
with Efm Esp) is involved in initial adherence and biofilm
formation. To explore the role of Esp in the pathogenesis of
Efm, Esp expression was studied under different growth
conditions and the role of cell surface associated Esp in
primary attachment and biofilm formation was assessed.