Inhibitory substances produced by Lactobacilli isolated from sourdoughs--a review

Several sourdough lactic acid bacteria (LAB) produce inhibitory substances other than organic acids. Bacteriocins (bavaricin A, and plantaricin ST31), a bacteriocin-like inhibitory substance (BLIS C57), and a new antibiotic (reutericyclin) have been discovered. Maximum antimicrobial production was found in the pH range 4.0-6.0. Temperature optima vary strongly. The substances are resistant to heat and acidity, and inactivated by proteolytic enzymes, except for reutericyclin. Bavaricin A and plantaricin ST31 have been purified to homogeneity. Bavaricin A is classified as a class IIa bacteriocin. Reutericyclin is a new tetramic acid. The mode of action of bavaricin A, BLIS C57, and reutericyclin is bactericidal. Some of these substances are active towards some Bacilli, Staphylococci and Listeria strains. Up to now, only the application potential of purified bavaricin A has been examined. More research should be done to study the production, the activity, and the stability of these inhibitory substances in food systems as these often differ from the broths mostly used in this kind of studies. Furthermore, an extensive screening of the sourdough microflora must be performed, in particular towards Bacilli and fungi. This could lead to the discovery of additional inhibitory substances, although it seems that the frequency of isolating bacteriocin-producing sourdough LAB is low. However, potent antimicrobials towards Bacilli as well as antifungal substances will have to be found using rational screening strategies and novel purification and analytical techniques.