Role of oxidative stress in persister tolerance.

Persisters are dormant phenotypic variants of regular cells that are tolerant to antibiotics and play an important role in recalcitrance of chronic infections to therapy. Persisters can be produced stochastically in a population untreated with antibiotics. At the same time, a deterministic component of persister formation has also been documented in a population of cells with DNA damaged by fluoroquinolone treatment. Expression of the SOS response under these conditions induces formation of persisters by increasing expression of the TisB toxin. This suggests that other stress responses may also contribute to persister formation. Of particular interest is oxidative stress that pathogens encounter during infection. Activated macrophages produce reactive oxygen and nitrogen species which induce the SoxRS and OxyR regulons. Genes controlled by these regulons deactivate the oxidants and promote repair. We examined the ability of oxidative stress induced by paraquat (PQ) to affect persister formation. Preincubation of cells with PQ produced a dramatic increase in the number of persisters surviving challenge with fluoroquinolone antibiotics. PQ did not affect killing by kanamycin or ampicillin. Persisters in a culture treated with PQ that survived a challenge with a fluoroquinolone were also highly tolerant to other antibiotics. PQ induces SoxRS, which in turn induces expression of the AcrAB-TolC multidrug-resistant (MDR) pump. Fluoroquinolones are extruded by this MDR pump, and the effect of PQ on antibiotic tolerance was largely abolished in a mutant that was defective in the pump. It appears that PQ, acting through AcrAB-TolC, reduces the concentration of fluoroquinolones in the cells. This allows a larger fraction of cells to become persisters in the presence of a fluoroquinolone. Analysis of a lexA3 mutant indeed showed a dependence of persister induction under these conditions on SOS. These findings show that induction of a classical resistance mechanism, MDR efflux, by oxidative stress leads to an increase in multidrug-tolerant persister cells.

Persisters: Methods for Isolation and Identifying Contributing Factors--A Review.

Persister cells are phenotypic variants surviving a lethal dose of antibiotic, sufficient to kill the bulk of an exponential phase population. In this chapter we summarize current techniques to isolate persisters and discuss limitations associated with identifying mechanisms of persister formation.

High Persister Mutants in Mycobacterium tuberculosis.

Mycobacterium tuberculosis forms drug-tolerant persister cells that are the probable cause of its recalcitrance to antibiotic therapy. While genetically identical to the rest of the population, persisters are dormant, which protects them from killing by bactericidal antibiotics. The mechanism of persister formation in M. tuberculosis is not well understood. In this study, we selected for high persister (hip) mutants and characterized them by whole genome sequencing and transcriptome analysis. In parallel, we identified and characterized clinical isolates that naturally produce high levels of persisters. We compared the hip mutants obtained in vitro with clinical isolates to identify candidate persister genes. Genes involved in lipid biosynthesis, carbon metabolism, toxin-antitoxin systems, and transcriptional regulators were among those identified. We also found that clinical hip isolates exhibited greater ex vivo survival than the low persister isolates. Our data suggest that M. tuberculosis persister formation involves multiple pathways, and hip mutants may contribute to the recalcitrance of the infection.

Persister cells and tolerance to antimicrobials.

Bacterial populations produce persister cells that neither grow nor die in the presence of microbicidal antibiotics. Persisters are largely responsible for high levels of biofilm tolerance to antimicrobials, but virtually nothing was known about their biology. Tolerance of Escherichia coli to ampicillin and ofloxacin was tested at different growth stages to gain insight into the nature of persisters. The number of persisters did not change in lag or early exponential phase, and increased dramatically in mid-exponential phase. Similar dynamics were observed with Pseudomonas aeruginosa (ofloxacin) and Staphylococcus aureus (ciprofloxacin and penicillin). This shows that production of persisters depends on growth stage. Maintaining a culture of E. coli at early exponential phase by reinoculation eliminated persisters. This suggests that persisters are not at a particular stage in the cell cycle, neither are they defective cells nor cells created in response to antibiotics. Our data indicate that persisters are specialized survivor cells.

The Spectrum of Drug Susceptibility in Mycobacteria.

A major factor complicating efforts to control the tuberculosis epidemic is the long duration of treatment required to successfully clear the infection. One reason that long courses of treatment are required may be the fact that mycobacterial cells arise during the course of infection that are less susceptible to antibiotics. Here we describe the paradigms of phenotypic drug tolerance and resistance as they apply to mycobacteria. We then discuss the mechanisms by which phenotypically drug-tolerant and -resistant cells arise both at a population level and in specialized subpopulations of cells that may be especially important in allowing the bacterium to survive in the face of treatment. These include general mechanisms that have been shown to alter the susceptibility of mycobacteria to antibiotics including growth arrest, efflux pump induction, and biofilm formation. In addition, we discuss emerging data from single-cell studies of mycobacteria that have identified unique ways in which specialized subpopulations of cells arise that vary in their frequency, in their susceptibility to drug, and in their stability over time.

Killing by bactericidal antibiotics does not depend on reactive oxygen species.

Bactericidal antibiotics kill by modulating their respective targets. This traditional view has been challenged by studies that propose an alternative, unified mechanism of killing, whereby toxic reactive oxygen species (ROS) are produced in the presence of antibiotics. We found no correlation between an individual cell's probability of survival in the presence of antibiotic and its level of ROS. An ROS quencher, thiourea, protected cells from antibiotics present at low concentrations, but the effect was observed under anaerobic conditions as well. There was essentially no difference in survival of bacteria treated with various antibiotics under aerobic or anaerobic conditions. This suggests that ROS do not play a role in killing of bacterial pathogens by antibiotics.

Persister eradication: lessons from the world of natural products.

Persisters are specialized survivor cells that protect bacterial populations from killing by antibiotics. Persisters are dormant phenotypic variants of regular cells rather than mutants. Bactericidal antibiotics kill by corrupting their targets into producing toxic products; tolerance to antibiotics follows when targets are inactive. Transcriptome analysis of isolated persisters points to toxin/antitoxin modules as a principle component of persister formation. Mechanisms of persister formation are redundant, making it difficult to eradicate these cells. In Escherichia coli, toxins RelE and MazF cause dormancy by degrading mRNA; HipA inhibits translation by phosphorylating Ef-Tu; and TisB forms an anion channel in the membrane, leading to a decrease in pmf and ATP levels. Prolonged treatment of chronic infections with antibiotics selects for hip mutants that produce more persister cells. Eradication of tolerant persisters is a serious challenge. Some of the existing antibiotics are capable of killing persisters, pointing to ways of developing therapeutics to treat chronic infections. Mitomycin is a prodrug which is converted into a reactive compound forming adducts with DNA upon entering the cell. Prolonged treatment with aminoglycosides that cause mistranslation leading to misfolded peptides can sterilize a stationary culture of Pseudomonas aeruginosa, a pathogen responsible for chronic, highly tolerant infections of cystic fibrosis patients. Finally, one of the best bactericidal agents is rifampin, an inhibitor of RNA polymerase, and we suggest that it "kills" by preventing persister resuscitation.

Efferocytosis is an innate antibacterial mechanism.

Mycobacterium tuberculosis persists within macrophages in an arrested phagosome and depends upon necrosis to elude immunity and disseminate. Although apoptosis of M. tuberculosis-infected macrophages is associated with reduced bacterial growth, the bacteria are relatively resistant to other forms of death, leaving the mechanism underlying this observation unresolved. We find that after apoptosis, M. tuberculosis-infected macrophages are rapidly taken up by uninfected macrophages through efferocytosis, a dedicated apoptotic cell engulfment process. Efferocytosis of M. tuberculosis sequestered within an apoptotic macrophage further compartmentalizes the bacterium and delivers it along with the apoptotic cell debris to the lysosomal compartment. M. tuberculosis is killed only after efferocytosis, indicating that apoptosis itself is not intrinsically bactericidal but requires subsequent phagocytic uptake and lysosomal fusion of the apoptotic body harboring the bacterium. While efferocytosis is recognized as a constitutive housekeeping function of macrophages, these data indicate that it can also function as an antimicrobial effector mechanism.

Characterization and transcriptome analysis of Mycobacterium tuberculosis persisters.

Tuberculosis continues to be a major public health problem in many parts of the world. Significant obstacles in controlling the epidemic are the length of treatment and the large reservoir of latently infected people. Bacteria form dormant, drug-tolerant persister cells, which may be responsible for the difficulty in treating both acute and latent infections. We find that in Mycobacterium tuberculosis, low numbers of drug-tolerant persisters are present in lag and early exponential phases, increasing sharply at late exponential and stationary phases to make up ~1% of the population. This suggests that persister formation is governed by both stochastic and deterministic mechanisms. In order to isolate persisters, an exponentially growing population was treated with d-cycloserine, and cells surviving lysis were collected by centrifugation. A transcriptome of persisters was obtained by using hybridization to an Affymetrix array. The transcriptome shows downregulation of metabolic and biosynthetic pathways, consistent with a certain degree of dormancy. A set of genes was upregulated in persisters, and these are likely involved in persister formation and maintenance. A comparison of the persister transcriptome with transcriptomes obtained for several in vitro dormancy models identified a small number of genes upregulated in all cases, which may represent a core dormancy response.

Specialized persister cells and the mechanism of multidrug tolerance in Escherichia coli.

Bacterial populations produce persisters, cells that neither grow nor die in the presence of bactericidal agents, and thus exhibit multidrug tolerance (MDT). The mechanisms of MDT and the nature of persisters have remained elusive. Our previous research has shown that persisters are largely responsible for the recalcitrance of biofilm infections. A general method for isolating persisters was developed, based on lysis of regular cells by ampicillin. A gene expression profile of persisters contained toxin-antitoxin (TA) modules and other genes that can block important cellular functions such as translation. Bactericidal antibiotics kill cells by corrupting the target function (for example, aminoglycosides interrupt translation, producing toxic peptides). We reasoned that inhibition of translation will lead to a shutdown of cellular functions, preventing antibiotics from corrupting their targets, giving rise to MDT persister cells. Overproduction of the RelE toxin, an inhibitor of translation, caused a sharp increase in persisters. Functional expression of a putative HipA toxin also increased persisters, while deletion of the hipBA module caused a sharp decrease in persisters in both stationary and biofilm populations. HipA is thus the first validated persister-MDT gene. We suggest that random fluctuation in the levels of MDT proteins leads to the formation of rare persister cells. The function of these specialized dormant cells is to ensure the survival of the population in the presence of lethal factors.