PhD Vermeiren Lieve Compleet - Hogeschool Gent

(Stiles & Holzapfel, 1997). Another development of bacterial taxonomy, polyphasic
taxonomy, aims at the integration of different kinds of data and information (phenotypic,
genotypic and phylogenetic) on LAB (Vandamme et al., 1996).
The classification of LAB remains under investigation but this group is generally restricted to
the genera Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc,
Oenococcus, Pediococcus, Streptococcus, Tetragenococcus, Vagococcus and Weissella.
Representatives of the genera Aerococcus, Alloiococcus, Atopobium, Dolosigranulum,
Gemella, Globicatella, Helcococcus, Melissococcus and Saccharococcus are generally not
considered to belong to the LAB, although they might meet the broad definition of this group
(Vandamme et al., 1996).
Phylogenetically, LAB belong to the so-called Clostridium branch of the Gram-positive
bacteria, a phylum consisting of bacteria with a DNA-base composition of less than 50 mol%
G+C (Kandler & Weiss, 1986; Vandamme et al., 1996).
1.3. Metabolism and energy generation
Metabolically, LAB are at the threshold of anaerobic to aerobic life. They are told to have a
fermentative metabolism and to have a need for a fermentable carbohydrate for growth
because they possess efficient carbohydrate fermentation pathways coupled to substrate level
phosphorylation (Kandler & Weiss, 1986; Vandamme et al., 1996). A second substrate level
phosphorylation site is coupled to arginine fermentation, observed in most of the
heterofermentative lactobacilli. In addition to substrate level phosphorylation, LAB may gain
energy from the proton motive force by lactate efflux. Lactobacilli contain in general no
isoprenoid quinines and no cytochrome systems to perform oxidative phosphorylation.
However, they possess (per)oxidases to carry out the oxidation of NADH2 and O2 as the final
electron acceptor. Furthermore, they are able to perform a Mn-catalysed scavenging of
superoxide although they do not possess dismutase and catalase (Kandler & Weiss, 1986;
Piard & Desmazeaud, 1991).
Lactic acid bacteria, although consisting of a number of diverse genera, are grouped as either
homofermentatives or heterofermentatives based on the end-product(s) of their hexose
fermentation (Figure 1.1). The homofermentatives produce lactic acid as the major endproduct
of glucose fermentation. The heterofermentatives produce a number of products
Chapter 1 – Antagonistic micro-organisms for biopreservation of food products 2