Natural Antimicrobials for Food Biopreservation - Food Biopreservation - page 6

decanoic acid and 3-hydroxy-5-dodecenoic acid (Ryu et al. 2014 ) or 3,6-bis(2-
methylpropyl)-2,5-piperazinedion (Yang and Chang 2010 ) have been identifi ed.
Among the cyclic dipeptides described in cell-free supernatants of antifungal LAB
strains are cyclo (Gly-LLeu) (Niku-Paavola et al. 1999 ), cyclo (L-Leu-L-Pro) and
cyclo (L-Phe-L-Pro) (Dal Bello et al. 2007 ), cyclo (L-Phe-l-Pro) and cyclo (L-Phe-
trans-4-OH-L-Pro) (Ström et al. 2002 ). Some of them require high concentrations
in order to be effective, while others have demonstrated potential for biopreserva-
tion in certain foods such as bread (Dal Bello et al. 2007 ).

2.1.3 Bacteriocins

Bacteriocins can be defi ned as ribosomally synthesized antimicrobial peptides or
proteins, which can be posttranslationally modifi ed or not (Jack et al 1995 ).
Bacteriocins from Gram-positive bacteria are generally classifi ed according to size,
structure, and modifi cations. Klaenhammer ( 1993 ) defi ned four classes of bacterio-
cins produced by LAB. Class I bacteriocins or 'lantibiotics' are small, ribosomally
synthesized peptides that undergo extensive post-translational modifi cation. They
contain lanthionine and b-methyl lanthionine residues, as well as dehydrated amino
acids. Class II bacteriocins are small (4-6 kDa), heat-stable, ribosomally synthe-
sized peptides which were differentiated from lantibiotics because they do not
undergo extensive post-translational modifi cation, except for cleavage of a leader
peptide (when present) during transport out of the cell. Nevertheless, some excep-
tions to this rule have been reported recently, illustrated by the n-terminal formylated
two-peptide bacteriocin from E. faecalis 710C (Liu et al. 2011 ) and enterocin
BacFL31 which contains hydroxyproline residues (Chakchouk-Mtibaa et al. 2014 ).
Nes et al. ( 1996 ) regrouped the class II bacteriocins, retaining class IIa and IIb but
changing class IIc to include bacteriocins that contain a typical signal peptide and
that are secreted by the general translocase ( sec ) pathway of the cell. Cotter et al.
( 2005 ) suggested to divide class II bacteriocins into several subclasses: class IIa
(pediocin-like bacteriocins), class IIb (two-peptide bacteriocins), and class IIc (cir-
cular bacteriocins) Cotter et al. ( 2005 ). However, circular bacteriocins may also be
considered as a separate class (Franz et al. 2007 ; van Belkum et al. 2011 ).
Nonbacteriocin lytic proteins, termed bacteriolysins (also referred to as class III
bacteriocins), are large and heat-labile proteins with a distinct mechanism of action
from other Gram-positive bacteriocins (Cotter et al. 2005 ). Specifi c classifi cation
schemes were also proposed for bacteriocins from genus Enterococcus (Franz et al.
2007 ) and genus Bacillus (Abriouel et al. 2011 ) . This last one generates a wide
variety of peptide structures containing modifi ed amino acid residues other than the
classical ones found in, for example, nisin. An updated classifi cation of bacteriocins
was proposed by Rea et al. ( 2011 ) including two additional subclasses for the lanti-
biotics and one additional subclass for the non-modifi ed peptides. An orientative
summary on the diversity of bacteriocins from Gram-positive bacteria is presented
in Table 2.1 .