lactate and acetate. They include some species important in food fermen-
tation such asLb. plantarum,Lb. casei,andLb. sake. Obligate heteroferm-
enters which include Lb. brevis, Lb. fermentum and Lb. kefir use the
phosphoketolase pathway for hexose fermentation.
Leuconostocis treated as a separate genus on morphological grounds as
its members are typically irregular cocci. This is not entirely satisfactory
since the vexed question, ‘When does a short rod become a coccus?’ often
arises; for example,Lactobacillus confususwas originally classed as a
Leuconostoc. It is possible to distinguish leuconostocs from most hetero-
fermentative lactobacilli by two phenetic characteristics: their production
of onlyD-lactate and inability to produce ammonia from arginine.
The genusPediococcusalso includes species of importance in food
fermentations such asP. pentosaceusand, until fairly recently,P. halo-
philusnow in a genus of its own asTetragenococcus halophilus.
Nucleic acid studies of the streptococci have shown that they comprise
three distinct groups worthy of genus status. The enterococci now form
the genus Enterococcus although the faecal strains of S. bovis and
S. equinuswhich also react with the group D antisera used in Lancefield’s
classical serological classification scheme are not included. What were
known as Lancefield’s group N streptococci, the lactic or dairy strepto-
cocci, are now members of the genusLactococcusand a number of these
which were considered distinctStreptococcusspecies are now classified as
subspecies of Lactococcus lactis. The yoghurt starter Streptococcus
thermophilusdoes not possess the Group N antigen and remains in the
genusStreptococcus.
Some authors also includeBifidobacteriumamong the lactic acid bac-
teria although this has less justification as they are quite distinct both
phylogenetically and biochemically. For example, hexose fermentation by
bifidobacteria follows neither the EMP glycolytic pathway nor the phos-
phoketolase pathway but produces a mixture of acetic and lactic acids.
9.4 Activities of Lactic Acid Bacteria in Foods
9.4.1 Antimicrobial Activity of Lactic Acid Bacteria
Lactic acid bacteria are often inhibitory to other micro-organisms and
this is the basis of their ability to improve the keeping quality and safety
of many food products. The principal factors which contribute to this
inhibition are presented in Table 9.5. By far the most important are the
production of lactic and acetic acids and the consequent decrease in pH.
Just how organic acids and low pH inhibit microbial growth and survival
is discussed in Section 3.2.2 and will not be repeated here.
Bacteriocins are bactericidal peptides or proteins which are usually
active against species closely related to the producing organism.
Chapter 9 317