# Lactic Acid Bacteria as Biocontrol Agents against Potato (_Solanum tuberosum_ L.) Pathogens
## 3 Results and Discussion
### Determination of Metabolites Production by Selected Strains of Lactic Acid Bacteria by HPLC  
The antimicrobial efficacy of LAB has previously been well documented in the literature as a result of the production of several compounds with low (<1000 Da) and high (>1000 Da; bacteriocins) molecular mass [40; 44; 45]. The most significant antimicrobial agent produced by LAB in the highest amounts is lactic acid [45]. Although, acetic acid was reported to be far more effective for fungi inhibition than lactic acid in several studies [46; 47]. In our research, lactic acid was determined to be the major fermentation by-product of selected LAB. The highest values in the range from 1680 up to 1750 mg/100 mL were obtained for six strains--_L. acidophilus_ LOCK938, _L. acidophilus_ LOCK927, _L. acidophilus_ LOCK934,  
Figure 1: Antimicrobial activity of selected ten most active lactic acid bacteria strains cultivated on MRS medium (about 10\({}^{8}\) CFU/mL) against potato phytopathogens after 2–5 days of incubation (depending on the pathogen), measured as growth inhibition zones in the agar-plug diffusion method. a–h—results with different letters within the sample are significantly different (Tukey’s test, \(\alpha\) = 0.05).  
_L. plantarum_LOCK861, _L. plantarum_KB2 LAB 03 and _L. plantarum_ BPC2 (Figure 2). The mode of action of lactic acid is associated with cytoplasmic pH reduction of pathogen cells and, consequently, inhibition of metabolic activity. Another hypothesis suggests that organic acids neutralise the electrochemical potential of the plasmonic membrane, which increases its permeability and eventually leads to the death of the susceptible microorganism. These mechanisms were also demonstrated for acetic and propionic acids [44]. The importance of organic acid's role in the antimicrobial spectrum of LAB activity was confirmed in [PERSON] et al. [20] study, where the pH neutralisation treatment resulted in a loss of antimicrobial activity. In addition to lactic acid, the chromatographic assay also revealed a smaller presence of another four organic acids in the tested samples--acetic, propionic, isobutyric, and valeric acid (Figure 2). Acetic acid reached the second highest concentration for eight strains, from 11.15 up to 37.31 mg/mL. For other _L. pentosus_ 03(2)118A LAB and _L. acidophilus_LOCK932, it was propionic acid instead (11.65 and 16.72 mg/mL, respectively). Small amounts of isobutyric acid (3.01-5.32 mg/mL) and valeric acid (1.09-2.45 mg/mL) were produced by all ten or nine selected strains, respectively (Figure 2). Additionally, ethanol (19.51 mg/mL) was observed in the post-culture fluid of _L. plantarum_ BPC2 (Figure 2). Broad antimicrobial activity is often related to other low molecular weight components, such as hydrogen peroxidase, diacetyl, acetaldehyde, reuterin, cyclic dipeptides and 3-hydroxy fatty acids [45; 48]. Bacteriocins, in turn, demonstrate antimicrobial activity mainly against closely related G--positive bacteria, although reports are confirming a wider spectrum of activity for certain bacteriocins [49].  
_Lactiplantiobacillus plantarum_KB2 LAB 03 was selected as the most effective agent against potato pathogens based on antimicrobial activity in the agar-plug diffusion method and metabolites production.