Antimicrobial activity: determination of MIC, MBC, and MFC
The data displayed in Table 1 corroborate the results of the positive antimicrobial effects achieved by several researchers listed in the review published by Tiilikkala et al. (2010), and more recently by Souza et al. (2018). Other studies have demonstrated the antibacterial and antifungal properties of WV from varied lignocellulosic sources (Ibrahim et al. 2013; Araújo et al. 2013; Abas et al. 2018, Souza et al. 2018). Besides bacteria, some of the cited works demonstrate the biological effect on fungi, viruses, and protozoans. The experimental results presented here corroborate the results reported by Velmurugan et al. (2009) and Suresh et al. (2019), who assessed the antimicrobial effect of different types of WV both in their original acid form and after neutralization. Both those studies described the antimicrobial effect of WV before and after neutralization and found a decrease in the activity but not the disappearance of the biological effect when pH became neutral.
As can be observed in Table 1, our results demonstrated that as the neutralization increased and the pH approached 7.0, the concentration of WV required for microbial inhibition increased. Even at neutral pH, the WV still had antimicrobial activity, although needing higher concentrations for this purpose. For Pseudomonas aeruginosa, the MIC of WV at pH 2.5 was 3.12% and increased to 50% at neutral pH. When the pH reached 7.5, the MIC returned to the same value of 25% observed for pH 6.5. The same relationship between MIC and pH was determined for Salmonella enteritides. For Candida albicans, the MIC at pH 2.5 was 3.12% and increased to 25% at pH 5.5, after which it remained constant until the pH reached 7.5. The pattern of inhibition as a function of the concentration and pH of the WV is different for each microorganism. As commented by Suresh et al. (2019), the activity of neutralized WV indicates that the antibacterial property of the product is due to its complex chemical composition, and not the presence of acetic acid. According to the same authors, the antibacterial efficiency of the neutralized WV could also be attributed to the antimicrobial effect of ketones, such as furfural and vanillin. Citing other authors, Suresh et al. (2019) highlighted that the inhibition of the WV against microorganisms, especially fungi, is caused by the antioxidative property of the phenolic compounds. In this respect, previous studies have reported that the inhibition of lipid oxidation caused by phenolics is enhanced at acidic pH.
However, for Staphylococcus aureus and Streptococcus agalactiae, a different pattern of inhibition was observed. For the first microorganism, when the pH was equal to 7.0 (neutral), no inhibition in the growth of the culture was observed. When the pH became slightly alkaline (7.5), the MIC was 50%, which is the same concentration required to inhibit completely the culture growth at pH 6.5. In the case of Streptococcus agalactiae, for both pH levels, 7.0 and 7.5, a WV concentration of 50% was not enough to inhibit the microbial growth. Is important to highlight that both microorganisms required higher WV concentration to inhibit growth starting with the original pH, which was 2.5. For the other three microorganisms, the initial MIC value was 3.12%. There are several compounds (phenolics and ketones) in WV’s chemical composition that have antimicrobial activity, so most likely they interact differently with one or another microorganism due to differences in cell wall structure and composition. These differences among microorganisms combined with the degree of response to the action of one or another compound in the chemical composition of WV probably explain why the product does not inhibit the microbial growth with the same inhibition results at the same concentration. In the present study, Staphylococcus aureus and Streptococcus agalactiae were the most resistant species to the inhibitory effect of WV, while Candida albicans was the most sensitive one (Table 1).
In the research of Medeiros and Gasparotto (2021), they found zero effect of neutralized WV on the growth of Escherichia coli and Staphylococcus aureus and attributed the loss of antimicrobial activity to the neutralization of the acetic acid. The authors went further and stated that this organic acid is the only one responsible for the antimicrobial effect of the WV. However, their results were not corroborated by the results of other authors, since, for example, when assessing the antimicrobial activity of neutralized WV on E. coli and S. aureus, Suresh et al. (2019) observed a decrease in the effect of the product after neutralization but not complete disappearance. This trend reported by Suresh et al. (2019) was also identified by us. Still, based on their experimental data, those authors stated categorically that the antimicrobial effect of the WV even after neutralization indicated that the antibacterial property of the product was due to its complex chemical composition rather than the large presence of acetic acid. In their experiments assessing the antimicrobial action on Escherichia coli, Enterobacter aerogenes, Pseudomonas aeruginosa, Listeria monocytogenes, and Enterococcus faecalis, the authors found a loss of activity ranging from 10 to 24%, with the degree of loss varying according to the species.
In Table 1, since the antimicrobial activity occurred even at neutral pH for three of the five microorganisms assessed, the sign “ > ” is placed after some MIC values of Staphylococcus aureus and Streptococcus agalactiae to suggest that perhaps with concentrations higher than 50%, some inhibition might occur. However, another study should be carried out to confirm that hypothesis, which is not unlikely since other authors have found inhibition at concentrations of such order. Also in Table 1, the sign “ > ” is placed after some MBC and MFC values (numbers in italics) to suggest that maybe with WV concentrations higher than 50%, some inhibition might occur. But in Table 1, the sign is placed after the MIC found for Salmonella enteritides (pH value of 7.0), Staphylococcus aureus (6.5, 7.0, and 7.5), and Streptococcus agalactiae (7.0 and 7.5).
Antimicrobial activity: behavior of the absorbances
The pattern of increasing values of absorbance in the graphs in Fig. 1 reflects the intensity of the microbial growth, which means more cells could develop in the medium, making it more turbid and preventing light from passing through (Cunha and Vieira 2014). On the right side of the figures, the different pH levels are identified by colors that are the same as the plotted curves. In Tables 2, 3, 4, 5, and 6, the regression models fitted to explain the effect of the pH of WV on each microorganism according to the concentration are displayed. The models can predict the behavior of the absorbance (Y) as a function of WV concentration (X).
From the curves displayed in Fig. 1, it is possible to verify that at lower pH levels, the concentrations of WV required to achieve the antimicrobial effect were lower, corroborating the experimental data on MIC described previously. As the pH of WV increased, the requirement for higher concentrations of the product to inhibit microbial growth also increased, reflected by higher absorbance values when WV low concentrations were not effective. Another trend that corroborates the behavior of the data displayed in Table 1 is that the absorbances at pH of 7.0 were always higher than those at pH 7.5. That fact indicates that the statement that the antimicrobial properties are not solely related to the effect of acetic acid is correct (Suresh et al. 2019; Pimenta 2021). The acetic acid present in a given aqueous solution is completely converted to sodium acetate at pH 7.0 and remains that way at alkaline pH levels. Consequently, there is no possibility of its being responsible for the antimicrobial effect of WV found in this work, at both pHs of 7.0 and 7.5. The same pattern was found by other authors cited previously.
Therefore, the pH of the WV undoubtedly influences its antimicrobial activity but does not cause it alone. The chemical composition of the WV reported in the literature shows a product composed of around 200 compounds (Schnitzer et al. 2015; Araújo et al. 2017; Pimenta et al. 2018). Among these components, alcohols, furans, ketones, organic acids, phenols, and pyrans are the most representative and abundant (Theapparat et al. 2018; Medeiros et al. 2020). The phenolic compounds are the main group with which antimicrobial properties are closely associated, a fact is long proven by scientific reports (Abas et al. 2018). Thus, the results presented here reinforce the statement that the antimicrobial activities of WV on both bacteria and fungi cannot be associated with the action of a single compound, but rather by the synergistic action of different compounds, as mentioned, for instance, by Yang et al. (2016). Also, Suresh et al. (2019) highlighted that fact and also mentioned that each component has a different mode of action. Because of this, according to the authors, WV is even more interesting for use as an antimicrobial agent, since it is unlikely the microorganisms will develop any mechanism of resistance against all the components of the product at the same time.
In the present work, what varies regarding a particular microorganism are the pH of the WV and the concentration of this product. Thus, for each microorganism, regression models were fitted, one model for each pH level, where the independent variable is the absorbance and the dependent one is the WV concentration. This way, for each microorganism subjected to the action of WV, the models fitted for each pH could be compared to detect differences in the antimicrobial action depending on pH. The importance of this comparison is not only to determine the effectiveness of the WV itself at each pH level but also to provide information about the interaction between the microorganism and the WV at a given pH when the concentration is varied.
Then, as displayed in Table 7 (A), the quality of WV changed as pH increased, since the comparison among the models was different from one pH to another. In other words, for each microorganism, the variation of pH could result in a different effect of WV and concomitantly a specific type of interaction. Something in the WV was decreasing and making it weaker, so as the pH of the WV increased, higher concentrations of the product were required to continue inhibiting the growth of the five microorganisms in the culture medium. This pattern corroborates the results of Setiawati et al. (2019), cited previously, who observed that changes in the composition occur with different pH levels. A slightly different pattern was observed in Table 8 (B), where the identity test, when applied to the models fitted for Candida albicans, determined that the effect of WV at pH of 3.0 and 3.5 was equal to that at pH 4.0, probably meaning the presence of an interaction of the inhibitory product with this microorganism. Nevertheless, for all other pH levels, the regression models were different from each other.
As commented in previous work (Pimenta 2021), the partial loss of the antimicrobial activity can probably be attributed to the reaction of the sodium hydroxide with the phenolic compounds, turning them into salts, a type of chemical change that deactivates their hydroxyl groups, which are responsible for the antiseptic properties presented by most of them in the pristine form (Aldred et al. 2009). Phenols are acids with pKa around 10.0, so they are weak acids. There are at least 20 types of phenols in WV’s chemical composition (Araújo et al. 2017; Pimenta et al., 2018), and since each compound is different, as the pH increases, their hydroxyl groups are not equally neutralized because of the substituent groups that are present in the aromatic ring. This way as the pH increases, most likely different chemical species are generated in aqueous media, ones more available than others to exert an antimicrobial effect.
The results of Setiawati et al. (2019) and the results of the present work corroborate the points raised in the previous paragraph. The cited authors, when evaluating neutralized WV, found some change in the percentage of phenolic compounds in the chemical composition of the product obtained from durian wood. According to them, in the acidic version, the main compound was guaiacol, while in the neutralized product, pyrocatechol was the prevailing substance. The explanation presented by the authors was that in the neutralized form, alkyl groups in the para position (carbon 4) of phenolics accept electrons and that behavior decreases the ionization of the compounds due to the addition of NaOH in WV. The changes in the proportion of phenolic compounds in the neutral version of WV explain why the product becomes less effective when compared to the acidic versions in terms of the power to inhibit the growth of microorganisms. These results were expected to a certain extent because according to Brown et al. (1997), acid and basic solutions can differ greatly from each other in their chemical properties so products obtained from the reaction after neutralization do not have the same characteristics as the original solution. However, since WV is a solution containing many kinds of compounds, even with its acid fraction completely neutralized, there are still other compounds preserving the bioactive characteristic of the product.
Further research should be performed to understand the specific chemical species of WV that prevail as inhibitors at each pH level when increasing neutralization as carried out. This could enable predicting the behavior of the product when employed in varied applications as a natural antibiotic or parasiticide. For example, if the product is employed as an additive for animal feed, the WV after being swallowed will find strong acidic conditions in the digestive tract of poultry and swine. In this condition, the inhibitory power of WV on microorganisms is maximized. However, if the intention is to use the product to compose drug formulations for external uses such as ointments and creams, or to deter parasites like ticks, the importance of the pH in the final use may be important to maximize the action of the product.