Role of (p)ppGpp in antibiotic resistance, tolerance, persistence and survival in Firmicutes

Abstract

The stringent response and its signalling nucleotides, pppGpp and ppGpp, have been the subject of intense research since the discovery of (p)ppGpp in 1969. Recent studies have revealed that the downstream events that follow (p)ppGpp accumulation vary among species. Consequently, the stringent response as initially characterized in Escherichia coli largely differs from the response in Firmicutes (Bacillota), wherein synthesis and degradation of the messengers (p)ppGpp are orchestrated by the bifunctional Rel enzyme with synthetase and hydrolase activity and the two synthetases SasA/RelP and SasB/RelQ. Here we will summarize recent studies supporting the role of (p)ppGpp in the development of antibiotic resistance and tolerance as well as survival under adverse environmental conditions in Firmicutes. We will also discuss the impact of elevated (p)ppGpp levels on the development of persister cells and the establishment of persistent infections. (p)ppGpp levels are usually tightly controlled to allow optimal growth under non-stressed conditions. Upon the onset of certain 'stringent conditions' the sudden increase in (p)ppGpp levels limits growth while exerting protective effects. In Firmicutes, the (p)ppGpp-mediated restriction of GTP accumulation is one major mechanism of protection and survival under stresses such as antibiotic exposure.

Keywords: (p)ppGpp; antibiotic resistance; antibiotic tolerance; persistence; stress survival; stringent response.

Figure 1.

Activation of the stringent response, synthesis of (p)ppGpp, and main effects of (p)ppGpp on bacterial physiology in Firmicutes. Rel-dependent (p)ppGpp synthesis is induced during amino acid starvation (by sensing of stalled ribosomes by Rel), but in B. subtilis also during lipid starvation, heat shock, and antibiotic stress. RelP and RelQ are activated during cell wall stress by the two-component systems VraRS in S. aureus or WalKR in B. subtilis. RelQ is also activated via the (p)ppGpp-induced release of an inactivating ssRNA. The substrates for synthesis are GTP or GDP and ATP, and the reaction is catalyzed by the synthetase domain of Rel and SAS enzymes (RelP/RelQ). (p)ppGpp degradation occurs solely by the hydrolase domain of Rel. Accumulation of (p)ppGpp modulates then bacterial physiology by directly binding to a variety of enzymes or via lowering of the GTP pools. Low intracellular levels of GTP and BCAA relieve the repression of the transcriptional regulator CodY. In addition, GTP depletion represses the transcription of genes, e.g. rRNA operons, requiring GTP as their initiating nucleotide. Created with BioRender.com.