Biomedical Engineering Reference
In-Depth Information
media, several authors also described variations of the original MRS medium
formulation for isolation of sourdough LAB. Vogel and co-workers [
203
] proposed
a modified MRS medium, referred to as MRS “Vogel”, with higher pH value (6.3),
whereas the MRS5 medium [
185
] contains the three major carbohydrates present in
the sourdough ecosystem (i.e., maltose, fructose, and glucose) in addition to cystein
and a vitamin mixture. In subsequent studies, the MRS5 medium has been success-
fully used for the isolation of several novel
Lactobacillus
species from sourdough
such as
Lb. spicheri
[
80
] ,
Lb. namurensis
[
87
] , and
Lb. crustorum
[
86
] . From these
recent descriptions, it thus appears that the use of a modified MRS formulation with
a lowered pH (<6.0) and supplemented with an additional carbon source such as
maltose and/or fructose as well as with amino acids and vitamins under anaerobic
conditions is one of the most successful strategies for the isolation of (new) sour-
dough LAB species. In a recent study, the qualitative and quantitative performance
of 11 elective and selective culture media was compared for isolation of lactobacilli
from type I sourdoughs [
204
]. On the basis of the identification results obtained
with protein profiling, the largest species diversity was recovered on maltose-con-
taining MRS medium. However, the fact that MRS5 medium allowed the isolation
of a specific (but unidentified) subpopulation only found on this medium indicates
that there is no single efficient medium for the recovery of all lactobacilli from type
I sourdoughs.
5.4
Identi fi cation of Sourdough Yeasts and Lactic Acid
Bacteria
Traditionally, identification of sourdough microorganisms relied on (selective) cul-
turing, selection and purification of a limited number of isolates, and identification
of purified isolates with phenotypic and/or genotypic methods. Although this
approach has significantly contributed to our current knowledge of the sourdough-
associated yeast and LAB species diversity, the use of culture media holds a number
of intrinsic limitations. In a culture-based approach, species with very specific nutri-
ent and growth conditions may only sporadically or even not be recovered which
leads to an underestimation of the actual microbial species diversity present in the
complex sourdough ecosystem [
68
]. In contrast, culture-independent techniques
that are based on phylogenetic dissection of the metagenomic DNA extracted
directly from the sample allow one to unravel the species diversity and dynamics of
sourdough yeasts and LAB without the need to isolate and culture its single compo-
nents. However, also these DNA-based methods have a number of limitations,
including poor detection capacity of subdominant species and inadequate taxonomic
resolution between phylogenetically closely related species. Depending on the aim
of the study, conventional culturing and molecular methods are therefore often com-
bined to obtain a more complete picture of the microbial species diversity of sour-
dough ecosystems [
41,
106
] .
