## 5 Lactic Acid Bacteria
### Antimicrobial Compounds Produced by LAB
#### 5.3.4 Other Compounds  
Other compounds produced by LAB also present antimicrobial effects, which intensify the potential of using LAB as a clean label component in food products. Reuterin (3-hydroxypropionaldehyde) is produced by _Lactobacillus reuteri_. This compound shows activity toward foodborne pathogens. Reuterin acts by inducing oxidative stress within microbial cells, interacting with thiol groups present in enzymes and other cellular proteins, and damaging their function. This compound also causes microbial cell membrane disruption, decreases intracellular pH, and damages DNA molecules [17]. One study assessed the antimicrobial potential of reuterin and verified that Gram-positive bacteria were more resistant to reuterin's action than Gram-negative bacteria. In addition, 10 mmol/L of reuterin was enough to completely inhibit the growth of _Penicillium expansum_[94], while the growth of spores of _Fusarium culmorum_ was reduced to 10% by 4 mmol/L, and by 8 mmol/L in the case of _Aspergillus niger_ and _Penicillium expansum_[95].  
Another compound, diacetyl, has antifungal properties against many molds (such as _Penicillium_ spp. and _Fusarium_ spp.) and Gram-positive and Gram-negative bacteria [15]. It works mainly by lowering the internal pH of microbes, which leads to the loss of molecules and eventual cell death [17]. One study evaluated the antimicrobial effects of LAB on pathogens, _Staphylococcus aureus_ and _Listeria monocytogenes_, and the results showed antimicrobial activity resulting from diacetyl production [96]. Diacetyl (at a concentration of 75 \(\upmu\)g/mL) isolated from _Lactiaseiobacillus paracasei_ DGCC2132 inhibited the growth of _Penicillium_ for five days [97], which can be associated with the induction of ROS accumulation, which destroys the membrane structure, resulting in the leakage of cellular materials and cell death [83].  
Hydrogen peroxide also presents antimicrobial activity. Some LAB, such as _Lactobacillus lactis_, _Lactobacillus acidophilus_, and _Lactobacillus delbrueckii_ subsp. _bulgaricus_, produce H\({}_{2}\)O\({}_{2}\) in anaerobic growth conditions due to a lack of cellular catalase, pseudocatalase, or peroxidase. Its antimicrobial action is ascribed to its potent oxidizing capability, which damages cellular components. H\({}_{2}\)O\({}_{2}\) induces bacteriostatic and bactericidal effects; nonetheless, the latter is rarely achieved [98].  
Fatty acids produced by LAB also have strong antifungal activity. Fatty acid from _Lactiplantibacillus plantarum_ also exhibited an antifungal effect against _Aspergillus fumigatus_ and _Bacillus cereus_, with an MIC of 0.21 g/L and 0.25 g/L, respectively [99], while fatty acids from cultures of _Lactobacillus hammesii_ and _Lactiplantibacillus plantarum_ effectively inhibited _Aspergillus niger_ and _Penicillium roqueforti_[100]. [PERSON] et al. [101] found that _Penicillium commune_ inhibition in yogurt was associated with higher amounts of four free fatty acids, including C10:0, C12:0, C16:1, and C18:1. Regarding the antimicrobial mechanism of fatty acids, the lipid bilayers of fungal membranes are partitioned by the acids, increasing membrane fluidity and permeability, with the consequent cytoplasmic disintegration of fungal cells [15].