# Strategies to Combat Multidrug-Resistant and Persistent Infectious Diseases
## 1 Introduction
Persister cells can survive in immune-compromised patients but also in patients in whom antibiotics do not effectively kill pathogenic bacteria via immune-evasion strategies [16]. There is currently strong evidence suggesting that the ability of bacteria to live inside some cells (such as macrophages) and the formation of biofilms are both associated with persistent infections [18]. Furthermore, clinical isolates from chronic infections caused by _Candida albicans_[19], _P. aeruginosa_[20], or uropathogenic _E. coli_[21] exposed to antibiotic pressure during a long period of time are also associated with persistent infections, exhibiting increased persister levels relative to isolates from acute or early-stage infections. The presence of persisters in common bacterial infections is reported in patients and linked to relapses of infection. [PERSON] and colleagues isolated clinical strains of _P. aeruginosa_ from the lungs of cystic fibrosis patients and observed increasing persister levels during antibiotic treatment [20]. [PERSON] and colleagues showed that high-persister _hipA_ mutations in _E. coli_ were selected in recurrent urinary infections over time and also observed the importance of the _hipA7_ mutation in persister formation in vitro [22]. Tolerant bacteria also exhibit an antibiotic-resistant phenotype. Tolerance is defined as the ability to survive transient exposure to high concentrations of antibiotics, and it can be inherited or not [9]. Unlike persistent subpopulations, tolerant bacteria are metabolically active, although their vital processes were shown to slow down [23]. Both tolerant and persistent cells are currently underestimated by the scientific community, despite evidence showing that they promote the evolution of resistance in bacteria. As early as 1988, [PERSON] and [PERSON] demonstrated that cyclic exposure of pneumococcus (_Streptococcus pneumoniae_) to high concentrations of penicillin selects for tolerant mutants, while resistant mutants evolve during exposure to low (but sustained) concentrations of penicillin [24]. One decade later, [PERSON] and colleagues established that clinical isolates of vancomycin-tolerant pneumococcus had mutations in the _vncS_ gene and that streptomycin- or penicillin-tolerant _S. pneumoniae_ mutant showed greater efficiency in transformation with DNA compared to a WT _S. pneumoniae_ strain [25]. This finding consolidated the idea of tolerant bacteria facilitating the acquisition of resistance genes via horizontal transfer, e.g., via transformation. In 2017, two important studies shed some light on the mechanisms driving the evolution of resistance by persisters and tolerant bacteria. Firstly, in studies involving long exposure of _Mycobacterium tuberculosis_ to rifamppin, [PERSON] and colleagues concluded that persisters are a source of de novo resistant mutants [26]; secondly, [PERSON] and colleagues observed that the evolution of increased tolerance or persistence confer mutations in _E. coli_ populations under intermittent ampicillin exposure [27]. In 2019, [PERSON] and colleagues discovered a strong, positive correlation between persister levels and the probability of resistance evolving in natural isolates and laboratory strains of _E. coli_[28]. Although different types of antibiotics and different experimental conditions were used in these three studies, the results are consistent, strongly suggesting a widespread link between persistence/tolerance to antibiotics and the evolution of resistance to these drugs. Considerable efforts should hereafter be devoted to the development of strategies that can eliminate tolerant and persistent cells, which potentially favor the evolution of resistance [28]. Given the immense number of possible strategies for treating MDR and persistent bacteria, it is time to examine what was achieved so far by the different therapies available.