Growth feedback as a basis for persister bistability

Abstract

A small fraction of cells in many bacterial populations, called persisters, are much less sensitive to antibiotic treatment than the majority. Persisters are in a dormant metabolic state, even while remaining genetically identical to the actively growing cells. Toxin and antitoxin modules in bacteria are believed to be one possible cause of persistence. A two-gene operon, HipBA, is one of many chromosomally encoded toxin and antitoxin modules in Escherichia coli and the HipA7 allelic variant was the first validated high-persistence mutant. Here, we present a stochastic model that can generate bistability of the HipBA system, via the reciprocal coupling of free HipA to the cellular growth rate. The actively growing state and the dormant state each correspond to a stable state of this model. Fluctuations enable transitions from one to the other. This model is fully in agreement with experimental data obtained with synthetic promoter constructs.A s far back as the 1940s, it was known that a small fraction of a bacterial population can survive even when exposed to prolonged antibiotic treatment (1, 2). This phenomenon is termed persistence and members of the surviving subpopulation are called persisters. It has been estimated that the frequency of persisters in normal wild-type populations is extremely small, perhaps of order 10 −5 ∽10 −6 (3). Although the number of persisters is tiny, they are often the main obstacle to attempts to completely eradicate infection.Remarkably, there is no apparent change in the persisters' DNA sequence; i.e., their survival is not due to mutation (4). Already in 1944, Bigger suggested that persisters are phenotypically different, in a dormant state instead of an actively growing state (1). The dormant state is presumably better able to deal with common antibiotics, which typically target only actively growing cells. Bigger's assumption was confirmed by a later study (3). In this study, Balaban et al. investigated the persistence of a single cell of Escherichia coli by using a microfluidic device. They showed that individual persisters do not always remain in the dormant state. Instead, they stochastically transit into an actively growing state and these newly transited cells are indistinguishable from other normally growing cells. Conversely, normal cells can transit into the persistent state. Thus, bacterial persistence at the population level is governed by a single-cell "phenotypic switch." The precise workings of this switch have to date remained unclear.In the 1980s, Moyed and Bertrand identified the first highpersistence mutant, HipA7, having a persister frequency that is near 10 −2 (4). The discovery of HipA7 facilitated the study of bacterial persistence due to its relatively high proportion of persisters. It was found that HipA7 is formed by a two-residue substitution in the HipA protein. This protein acts as a toxin in a toxin-antitoxin (TA) module (5, 6), where the hipB gene is coexpressed with hipA and the corresponding protein binds to and neutralizes HipA toxicity. To da...

Biofilm-Related Infections: Bridging the Gap between Clinical Management and Fundamental Aspects of Recalcitrance toward Antibiotics

“…Panel B was inspired by previous reports (13,31,94). slow growth (stationary-phase cultures) (106). HipA was first thought to phosphorylate the translation factor EF-Tu, leading to persistence via cell stasis (107).…”

The Alternative Sigma Factors SigE and SigB Are Involved in Tolerance and Persistence to Antitubercular Drugs

“…It is well-known that, due to their nonhomogeneous nature, bacterial populations often include bacteria transiently resistant to drugs (persisters), and these bacteria usually make up a small subpopulation (probably less than 1% of the total population of bacteria) with a temporary phenotype, which makes them challenging to study in an in vivo model of infection (5,6). This phenomenon is hypothesized to be at least in part due to the presence in bacterial genomes of bistable switches which stochastically regulate fundamental aspects of bacterial physiology (7).…”

Interpreting phenotypic antibiotic tolerance and persister cells as evolution via epigenetic inheritance

“…The model explored here assumes that the expression level is free to change in an unbiased fashion, aside from homeostasis tending to push it towards a set point value. There is some suggestion in the literature that, in fact, a positive feedback mechanism might operate that tends to hold persister cells in their dormant state as well (Lou et al 2008;Feng et al 2014). In effect, a continuous variable like expression level somehow underlies a bistable cellular 'switch' between the persister and nonpersister states (Satory et al 2011).…”