Concerns about the effect of antimicrobials on the increasing resistance to antibiotics of bacteria were raised more than 50 years ago ^[1][48]. Giuliano and Rybak ^[2][44] showed that there is a potential link between triclosan and antibiotic resistance. Lu et al. ^[3][49] report bacterial mutants resistant to quinolone and mupirocin that have decreased susceptibility to triclosan. Exposure to benzalkonium chloride and triclosan resulted in increased resistance to erythromycin and ciprofloxacin strains of Campylobacter jejuni and Campylobacter coli ^[4][50]. Triclosan exposure in strains of E. coli and P. aeruginosa increased ten times the resistance to chloramphenicol and tetracycline ^[5][51]. However, there is still a lack of evidence on the role of EMPs and their direct effect on antibiotic resistance.

Triclosan (TCS) is a common antimicrobial chemical in numerous PCPs (soaps, sanitizers, and toothpaste) ^[6][52] and is widely detected in aquatic environments at μg/L ^[7][8][53,54] to mg/L level ^[9][55]. It can serve as an external pressure to co-select for triclosan resistance and antibiotic resistance in many bacteria ^[10][11][12][56,57,58]. Triclosan induces oxidative stress, causing genetic mutations in a few genes, such as marR, frdD, fabI, acrR, and soxR ^[13][14][15][59,60,61]. The gene fabI is an acyl carrier protein reductase gene, a key enzyme in fatty acid synthesis in bacteria ^[16][62]. The interference with fatty acid synthesis results in modifications of the membrane structure, which causes less antibiotic uptake ^[17][63]. Subinhibitory concentrations of triclosan decrease the susceptibility of E. coli to ciprofloxacin, kanamycin, and gentamicin due to alteration of the membrane structure and biofilm formation ^[18][64]. Likewise, Stenotrophomonas maltophilia exposed to triclosan resulted in overexpression of the multidrug efflux pump SmeDEF and reportedly reduced the susceptibility to chloramphenicol, tetracycline, and ciprofloxacin ^[19][65]. Similarly, Salmonella enterica exposed to triclosan with increasing concentrations showed overexpression of the AcrAB efflux pump and reportedly reduced susceptibility to chloramphenicol, tetracycline, and ampicillin ^[20][66]. An efflux pump overexpression suggests the co-selective potential for more antimicrobial chemicals. In addition, E. coli exposed to triclosan also resulted in overexpression of the multidrug efflux pump and a transcription of genes encoding beta-lactamases. In contrast, the expression of genes related to membrane permeability decreases ^[15][61]. Hartmann et al. ^[21][67] documented that exposure to antimicrobials methyl-, ethyl-, propyl-, butylparaben, triclocarban, and triclosan increases ARGs in the microbiome.

Parabens are widely used as preservatives in many pharmaceutical, food, and cosmetic products due to their low toxicity ^[22][68]. They are often used in small amounts and primarily prevent bacteria growth and prolong shelf life. The mode of action against microorganisms mostly interferes with cellular membrane transfer processes. The effectivity of the paraben is correlated with the size of the chemical; propylparaben is considered more active against most bacteria than methylparaben ^[23][69]. Propylparaben specifically induces the permeabilization of bacterial membranes, causing the release of potassium ^[24][25][70,71]. The mechanisms of microbial resistance to parabens need to be better understood. Parabens are less active toward Gram-negative than Gram-positive bacteria ^[26][27][72,73]. Paraben resistance has been linked to the cell wall characteristics and non-specific efflux systems ^[28][74]. However, so far, only a few cases of resistance to parabens have been reported, occurring in the specific strains of P. aeruginosa, Burkholderia cepacia, and Cladosporium resinae ^[22][68]. Wu et al. ^[29][75] reported biodegradation of methyl- and propylparaben under aerobic and anoxic conditions, and benzoic acid was identified as one of the significant degradation products, thus reducing the efficacy of the compound.

Tonalide and galaxolide are the most used synthetic fragrance compounds in various PCPs, such as detergents, perfumes, deodorants, skin creams, and soaps ^[30][76]. A tonalide concentration has been detected in wastewater treatment plants (WWTP), ranging between 0.086 and 12.5 μg/L and 0.043 and 16.6 μg/L for galaxolide ^[31][77]. They are moderately soluble in water and thus increase the possibility of accumulation in the environment ^[32][78]. Tonalide and galaxolide are volatile lipophilic compounds and can, therefore, relatively easily penetrate through the cell wall of microorganisms. They are more active against Gram-positive than Gram-negative bacteria ^[33][79]. Tonalide and galaxolide disrupt and damage the structure of the membrane, resulting in a loss of ions, collapse of the proton pump and cytoplasm leakage, enzyme inhibition, and proton exchange disruption ^[34][80]. The metabolic pathway of hydroxylation is mainly causing the biotransformation of galaxolide, which is attributed to the existence of the cytochrome P-450 enzymes, which are linked to the inactivation of antibiotics ^[35][81].

Oxybenzone and octocrylene are filters that absorb U.V. radiation between 280 and 400 nm. U.V. filters are the main components of sunscreen due to their absorbing properties, but they are also found in other industrial products such as plastics and paints. Lozano et al. ^[36][82] were the first to analyze the effect of these compounds on microbes, reporting that they affected only gram-negative microbes. A correlation has been detected between genome size and the appearance of resistance mechanisms ^[37][83].

Quaternary ammonium compounds (QACs) are a group of chemicals found in most household cleaning products because of their different functions. They can act as disinfectants, surfactants, or preservatives ^[38][84]. Several genes, such as qacE, qacE11, qacF, qacG, and qacH, have been reported to confer resistance to QACs in Gram-negative bacteria, with qacE11 being the most widespread ^[39][85]. These genes belong to the small multidrug resistance (SMR) family ^[40][86], and their resistance to QACs is efflux-mediated ^[41][87]. The use of QACs drives the spread of class I integrons, responsible for a significant part of antimicrobial resistance in Gram-negative bacteria ^[42][43][88,89]. Benzalkonium chlorides (BAC) are the most commonly used QACs ^[44][90]. Isolates of P. aeruginosa exposed to increasing concentrations of BAC caused mutations in the pmrB gene and physiological adaptations that contributed to a higher tolerance to antibiotics ^[45][91]. Additionally, Guerin et al. ^[46][92] report the susceptibility of Listeria monocytogene to various antibiotics such as ciprofloxacin, gentamicin, or kanamycin after exposure to BAC. Efflux pump expression most likely causes antibiotic resistance to BAC, accompanied by the minor role of reduced membrane permeability ^[47][93]. Efflux genes such as qacG, acrA, qacH, and acrB have been identified in bacteria resistant to BAC ^[48][94].

Table 12 summarizes the EMPs their effect on bacteria and the defense mechanism against EMPs. In summary, non-antibiotic chemicals induce antibiotic resistance, and the cell wall is the first encounter between the bacteria and the chemical and is, thus, an essential mechanism of resistance.