Intracellular efflux pumps have been largely the research focus in multidrug-resistant (MDR) Gram-positive and Gram-negative microorganisms and parasites including cancers. However, drug efflux mechanisms other than pumps per se have been observed, supported by the effects of isomeric, non-antibiotic depressant (DPR), phenothiazines and thixenes, and antidepressant (ADPR) phenylpiperidine neurotropic drugs, alone or in combination with classical antimicrobials on MDR Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Streptococcus pyogenes and Streptococcus pneumoniae. Of the non-antibiotics we investigated, the DPR l-thioridazine, trans-clopenthixol and isomers of phenylpiperidines NNC 20-4962 (isomer of femoxetine) and NNC 20-7052 (isomer of paroxetine) were potent antimicrobials with the least neurotropic activity, pointing to a possible general isomeric structure-activity relationship. These compounds may be regarded as new efflux inhibitors. Moreover, these isomers have considerably reduced, in some cases absent, neurotropism and reduced mammalian toxicity. This may alleviate concerns about adverse effects and therapeutic safety for infected patients in life-threatening situations where the non-antibiotic dosage would be in the lower, non-chronic dosage ranges generally prescribed for individuals with mild mental health problems. The results point to the prokaryotic and eukaryotic microorganisms' phospholipid/protein domain involvement of the cationic, amphiphilic, non-antibiotic DPR and ADPR, with the phospholipids being the initial sites attracting and concentrating the neurotropes to induce a form of suspended animation, followed by gross changes of cell wall and membrane structure, and thereby potentiating their destructive or immobilizing effects on various as yet only hinted at resistance and efflux mechanisms. Combination of appropriate isomeric non-antibiotic DPR and ADPR of low neurotropism and toxicity with conventional and classical antimicrobials promises early, new therapeutic strategies salutary against microbial resistance, resistance development, pathogenicity and virulence.

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