## 3 Results and Discussion
pneumoniae_, _Acinetobacter_ sp. belongs to a different order and family [23]. Moreover, _E. coli_ and _K. pneumoniae_ are enteric bacteria and as such, they must survive the passage through the stomach [11], where HA, an inorganic/strong acid, is present. It was therefore expected that _E. coli_ and _K. pneumoniae_ would be more tolerant to acidic challenges than _Acinetobacter_ sp. Furthermore, _E. coli_ and _K. pneumoniae_ were found the most resistant to pH, being able to tolerate pH 3.5 obtained using inorganic acids. Moreover, the differential sensitivity to organic and inorganic acids is again observed in both species, being more pronounced in _K. pneumoniae_, which does not tolerate organic acids below pH 5.0. As already referred, the outer membrane of Gram-negative bacteria is relatively permeable to ions [5] and thus the membrane remodelling, particularly in its composition in CFAs, is an important mechanism of protection against acidic stress and preservation of membrane integrity [9; 10]. This was demonstrated in the correlation between the levels of CFAs and resistance to acidic environments observed in _E. coli_, as well as in the increased sensitivity to low pH values exhibited by _cfa_ mutants [9; 10]. _S. aureus_ can grow in an optimum pH range of 6-7 but is killed rapidly at pH 2 [24]. Between these limits, _S. aureus_ growth is dependent on the combination of optimum levels of different parameters, such as nutrient availability and temperature [24]. At the molecular levels, the resistance of _S. aureus_ to pH relies on the maintenance of the intracellular pH (pH) by proton pumps, the production of intracellular chaperones, and regulators. In fact, [PERSON] and [PERSON] have sown that \(\sigma^{\text{B}}\) is directly involved in the survival of _S. aureus_ in acidic environments [7; 25]. This mechanism seems to be conserved in other Gram-positive bacteria. _L. monocytogenes_, for instance, was reported to be more tolerant to acidic conditions caused by hydrochloric and citric acids than acetic and lactic acids. As in _S. aureus_, this differential resistance has been associated with the ability of _L. monocytogenes_ to maintain pH homeostasis [26]. All evidence points that cell membrane plays a role in the protective mechanisms necessary to maintain the cell integrity and reduce proton influx in such hard conditions [5]. Overall, taking into consideration the selected bacteria and acidic environments assayed, the results obtained support previous suggestions that permeant weak organic acids, as the acetic acid here assayed (AA), are more effective against bacteria than inorganic strong acids (e.g., HCl, referred as HA) [27]. This is certainly the main reason why media acidification is used to improve food preservation, particularly fresh and unprocessed foods. However, the results obtained evidence that such acidification should be performed using weak organic acids instead of inorganic strong acids. In this context, it is noteworthy to refer to different reports pointing to the effectiveness of weak acids against bacterial growth and spoilage. [PERSON] et al. [28; 29; 30], for instance, proposed gaseous acetic acid treatment to reduce the risk of foodborne illness caused by the consumption of contaminated fresh sprouts and spices. This strategy has a great potential for many other applications, particularly those involving fresh fruits and vegetables [31]. In turn, [PERSON], et al. [32] has modelled the effect of temperature, pH, and acetic acid concentrations on the control of the acid-tolerant _Bacillus_ spores, showing a synergistic effect of decreased pH and the presence of acetic and lactic acids. This solution offers an alternative for the ambient storage for low-acid pasteurized sauces without involving the use of preservatives such as benzoic and sorbic acids. More recently, [PERSON], et al. [33] reported that the combination of ultrasound and acetic acid presented equal to or greater efficiency than chlorinated compounds in the decontamination of kale.