The use of biological molecules produced by microorganisms – such as lactic bacteria – is of special interest in food industry. Such bacteria, indeed, are able to carry out an antagonistic action against unwanted microorganisms, by producing antimicrobial and antifungal metabolites which represent a possible alternative to the use of chemical compounds (Pawlowska et al 2012; Tropcheva et al 2014; Varsha et Nampoothiri, 2016). These molecules can also have an antifungal effect thanks to the production of bioactive molecules, among which organic acids (lactic, acetic and propionic), phenylacetic acid and other carboxylic acids (for ex. D-glucoronic acid, salicylic acid, benzoic and cinnamic acids). In this context, even cyclic dipeptides, fatty acids, hydrogen peroxide, ethanol, nucleosides, lactones and protein compounds such as bacteriocins play a key role in influencing the microbial balances. For several years, they have been examined in both human and veterinary clinical field, especially for their potential use as valid alternative to antibiotics, enabling the limitation of antibiotic resistance, a phenomenon in strong increase.
Bacteriocins are protein substances with bactericidal action against strains taxonomically related with the producers’ ones. The first information about the nature and action modes of bactericidal substances came from extensive and numerous studies conducted on Gram-negative bacteria, mainly coliforms. The areas of research have then expanded, also covering Gram-positive bacteria, among which – more recently – lactic bacteria. At first, essentially referring to colicins, with the term bacteriocins we used to indicate lethal proteins with specific activity and able to react with specific receptors. Later, analogous but more defined criteria have been used to characterise other bacteriocins, especially:
- Narrow action spectrum towards homologous species;
- Bactericidal action;
- An action on specific receptor sites;
- A genetic codification related to plasmid DNA, corresponding to a producer strain immunity.
The bacteriocins produced by Gram-positive bacteria not always correspond to these definitions. For instance, there are wider action spectra, also towards different species, and a producer strain less immunity against homologous bacteriocins. For this reason, we prefer to indicate proteins with the features described above, with the more general term of substances similar to bacteriocins. In literature, we can find that many of the antibiotics produced by bacteria show properties similar to substances described as bacteriocins. For example, the nisin, which was considered an antibiotic produced by the Str. Lactis and Subsp. Lactis strains, is now classified as bacteriocin by English authors. Similarly, lytic enzymes produced by bacteria can provide inhibitions similar to the ones provided by bacteriocins. In facts, staphylococci cultures are able to produce enzymes, such as virolysin, lysostaphin, lysozyme, glucosaminidase and nisin.
BACTERIOCINS: PRODUCTION AND CHARACTERISTICS
The bacteriocins are extremely heterogeneous substances, but their common characteristic is their protein nature. The chemical analysis show that they are often complex proteins associated to lipids and glucose components, as in the case of lactobacilli. As far as their stability to external agents is concerned, we can claim that they present a high resistance to heat treatments, have greater tolerance towards low pH values compared to other values and the increase in the degree of purity decreases their stability. The production of bacteriocins can be influenced by several factors, for example the culture medium composition or the presence of various growth factors in the medium, that can ease or increase their production. Furthermore, the incubation conditions, such as temperature, pH, presence of oxygen, can promote or inhibit the production of bacteriocins. For this reason, it is best to find the optimal requirements in the producer strain, which will allow a greater production of bacteriocins (or similar), that will be used in feed to improve their organoleptic and functional properties (Varsha et Nampoothiri, 2016)