Pyroligneous acid and antibacterial activity: criticism of a paper by Araujo et al. (2018)

Aims A paper by Araujo et al. (2018) claims that a variety of compounds present in pyroligneous acid (guaiacol, phenols and furfural) are responsible for the observed antimicrobial activity. We show, on the other hand, that the low pH due to acetic acid present in pyroligneous acid is the real cause of the activity. Methods and Results Pyroligneous acid (PA) was synthesized according to a previous method (Medeiros et al. 2019) with its inhibition activity tested on Escherichia coli and Staphylococcus aureus via the agar diffusion method. The activity of acetic acid at different concentrations was also evaluated for comparison. As expected, crude PA (pH 3.0) and acetic acid produced inhibition halos whose diameters varied according to their employed concentration. However, any PA inhibitory activity completely vanished upon neutralization (pH 7.0), a behaviour also observed for neutralized acetic acid. Conclusions The claim that guaiacol, phenols and furfural are responsible for any inhibitory activity is unsubstantiated. The authors should have neutralized the pyroligneous acid if any activity was to be discovered. Significance and Impact of Study To increase awareness that interfering species may play a detrimental role on the interpretation of results. In this case, the action of acetic acid is vastly more important for the inhibitory activity than any other compound present in PA.