# The Role of Microorganisms and Their Antibacterial Compounds in Food Biopreservation
## 3 Antimicrobial Compounds Produced by Bacteria
### Organic Acids and Other Small Molecules  
The antibacterial activity of organic acids is due to their passive diffusion across the membrane into the cell, which leads to a decrease in intracellular pH (pH). The acidification of the cytosol significantly reduces enzyme activity, is genotoxic, and causes protein denaturation, leading to cell death [12]. Organic acid comes from the fermentative metabolism. Thea are also available as commercial products that can be added to food. Other LAB biologically active byproducts are ethanol, enzymes, diacetyl, exopolysaccharideaccharides (EPS), carbon dioxide, and hydrogen peroxide [3; 13].  
Lactic acid is a commonly used chemical preservative that inhibits the growth of both Gram-positive and Gram-negative bacteria. Lactic acid bacteriostatic properties are present even at concentrations of 1-2% and at pH > 5. Lactic acid was discovered in sour milk by [PERSON] in 1780. On the European Food Safety Authority (EFSA) food additives/preservatives list, lactic acid has the number E270 [14; 15; 16].  
Acetic acid is used as a preservative in a variety of food products. Acetic acid effectively suppresses the growth and decreases the viability of Gram-positive and Gram-negative bacteria, as well as yeasts and molds. Bactericidal activity occurs at concentrations >0.3% while bacteriostatic can occur at concentrations 0.2%. On the EFSA food additives/preservatives list, acetic acid has the number E260 [14; 15].  
Propionic acid is a relatively strong acid. Small quantities (0.1-0.2%) inhibit the growth of pathogenic bacteria, fungi, molds, and yeast. Propionic acid reduces the presence of pathogenic microorganisms in dairy and grain products, soft or low-alcohol drinks, and some fresh fruits and vegetables. Added to poultry feed products, it reduces contamination by _Salmonella_ spp. and mold formation. On the EFSA food additives/preservatives list, propionic acid has the number E280 [14; 17].  
Diacetyl effectively suppresses Gram-negative bacteria, yeasts, and, in higher concentrations, Gram-positive bacteria. For years, it has been known as a flavoring compound with the status of GRAS (Generally Recognized as Safe). Its antimicrobial activity was discovered in 1927 [18; 19]. Diacetyl is non-toxic; however, it should be noted that volatile diacetyl, when inhaled by humans, can be toxic [20].  
Hydrogen peroxide is a by-product of some anaerobic LAB strains. The antimicrobial toxic effect on bacteria, molds, and viruses is through peroxides and free radicals called reactive oxygen species (ROS) containing at least one oxygen atom and one or more unpaired electrons [21].  
Carbon dioxide produced by LAB has been shown to inhibit the growth of Gram-positive bacteria and Gram-negative including _Enterobacteriaceae_ and _Listeria monocytogenes_. On the EFSA food additives/preservatives list, carbon dioxide has the number E290 [14; 22].  
Exopolysaccharides (EPS) are high-molecular-weight biopolymers synthetized by LAB. They are classified into two groups: homopolysaccharides (HoPS) and heteropolysaccharides (HePS). LAB produces mostly HePS [23]. EPS are used in the food industry as emulsifiers, stabilizers, thickeners, and gelling agents. Some LAB-derived EPS demonstrated antibacterial activity against _Salmonella enterica_ serovar _Typhimuriumum_, _Escherichia coli_, _Vibrio parahaemolyticus_, _Salmonella typhimurium_, _Staphylococcus aureus_, and _Bacillus cereus_[24; 25; 26; 27].