###### Abstract  
It is known that many bacteria are susceptible to low pH, but it is less clear how the acid nature influences this susceptibility. The objective of this work was to evaluate the susceptibility of selected bacteria to different low pH conditions and to understand if their tolerance is affected by the type of acid (organic vs inorganic) in the media acidification. Strains of different species of human pathogenic bacteria were cultivated at different acidic conditions, using organic or inorganic acids. Overall, we verified that tolerance to acidic conditions is dependent on the type of acid used to acidify the growing medium, organic or inorganic. The strains analysed, although having different tolerance to low pH, were shown to be more susceptible to organic than inorganic acids. This result shows that beyond the pH of the media, the nature of the acid affects bacterial growth. Such a result has consequences in the evaluation of the efficiency of an antibacterial extract, which should take into consideration the differential susceptibility of bacteria to low pHs obtained using organic or inorganic acids.  
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Under acidic environments, the destabilization of the proton motive force leads to the leakage of ions, including protons, which eventually reduces the cytoplasmic pH [3,5]. This event triggers different harmful effects in bacteria [6], and ultimately, the failure to restore physiological pH will damage cellular structures, leading, for instance, to the disintegration of the cytoplasmic membrane [2,3,5,6]. This has led Gram-positive and Gram-negative neutralophilic bacteria to develop different mechanisms to mitigate acid-induced damage [2,3,5,7,8, 5, 6]. In Gram-negative bacteria, the outer membrane is not a very efficient barrier to the movement of protons caused by acidic conditions because porins are large enough to allow ion passages [5]. In contrast, the cytoplasmic membrane is a better barrier to proton influx and cyclopropane fatty acids (CFAs) and seems to be very relevant in this protection. This is supported by the evidence of _CFA_ gene upregulation and augmented CFAs levels correlating with acidic resistance [9,10]. Nevertheless, the ability of different bacteria to survive in acidic environments usually involves a synergistic combination of different regulatory circuits of acid resistance/tolerance, as suggested by studies on the activation and/or mutation of different genes [8,11,12] (Supplementary Figure S1). In _Escherichia coli_, the transcriptomic analysis of two strains exposed to organic and inorganic acids revealed acidulant- and strain-specific acid tolerance responses [13]. In turn, citric acid treatment of burn infections was found to be more effective in the control of _Staphylococcus aureus_ than conventional antibiotic therapy [14]. Overall, knowledge about the mechanisms that bacteria use to adapt to surrounding acidic environments is very relevant for the evaluation of antibacterial activity under different conditions and may have important applications, namely in clinical and food safety fields. In food research, for instance, substitution of artificial conservants by natural compounds able to mitigate food spoilage is highly desirable and requested by the consumers. On the other hand, interest and industrial applications for the use of probiotic organisms with health-protective effects in human and animal feeding is increasing exponentially. In this context, the objective of this work was to evaluate the susceptibility of different bacteria, _S. aureus_, _E. coli_, _Klebsiella pneumoniae_, and _Acinetobacter_ sp., to different acidic conditions and verify if the bacterial viability was dependent on the type of acid used to obtain such conditions.