Metabolic aspects of bacterial persisters

Abstract

Persister cells form a multi-drug tolerant subpopulation within an isogenic culture of bacteria that are genetically susceptible to antibiotics. Studies with different Gram negative and Gram positive bacteria have identified a large number of genes associated with the persister state. In contrast, the revelation of persister metabolism has only been addressed recently. We here summarize metabolic aspects of persisters, which includes an overview about the bifunctional role of selected carbohydrates as both triggers for the exit from the drug tolerant state and metabolites which persisters feed on. Also alarmones as indicators for starvation have been shown to influence persister levels via different signaling cascades involving the activation of toxin-antitoxin systems and other regulatory factors. Finally, recent data obtained by (13)C-isotopolog profiling demonstrated an active amino acid anabolism in Staphylococcus aureus cultures challenged with high drug concentrations. Understanding the metabolism of persister cells poses challenges but also paves the way for the development of anti-persister compounds.

Keywords: Staphylococcus aureus; biofilm; metabolism; persisters; ppGpp; toxin-antitoxin system.

Figure 1

Schematic overview of metabolic aspects associated with the persister state. ①, ② Toxin/antitoxin-systems: In a number of bacteria, Lon or Clp proteases, activated in response to starvation, degrade antitoxin proteins ①. Liberated toxins (e.g., RelE) can cleave mRNA or employ a ppGpp-dependent signal transduction ② to induce growth arrest. ③ Uncharged tRNAs due to amino acid starvation lead to the synthesis of ppGpp via RelA. ④ Nutrient limitation favors the synthesis of the second messenger cAMP by adenylate cyclase. cAMP binds to the cAMP receptor protein (Crp) and the cAMP/Crp complex activates the expression of both relA and cspD. ⑤ Inhibition of DNA replication by CspD. ⑥ Modulation of RNA polymerase (RNAP) activity by the DksA/ppGpp complex. ⑦ Metabolic flux alterations result in a decreased TCA cycle activity and increased persistence. Synthesis of methylglyoxal leads to growth inhibition. Acetoin and triglyceride synthesis represent alternative pathways for the deprivation of pyruvate and acetyl-CoA from the TCA cycle. ⑧ Different branches of metabolism can produce reactive oxygen species (ROS) as hazardous side products, impairing persister formation. Enzymes counteracting ROS activity (e.g., KatA) are upregulated in persister cells. ⑨ Biofilms containing protein- and/or aminosugar-polymer structures (blue meshwork) may represent environments of low supply of nutrients, such as glucose (glc), which favors persister formation.