A brand new area of antibiotic drug development is on the horizon as researchers recently unlocked the molecular mechanism controlling the mysterious process of bacterial hibernation.
A mechanism of resistance that has baffled researchers for years is that of dormant cells or persister cells. These types of bacteria do not mutate to survive antibiotic treatment but instead shift their cellular processes into a dormant or hibernation-like state, they physically shrink in size and have almost undetectable metabolic processes. This subpopulation of bacterial have no detectable genetic mutations and simply “wait out” the antibiotic storm in this dormant state. Once the antibiotic pressure is removed the cells shift back into a fully metabolically active state.
By studying E. coli isolates from urinary tract infections researchers were able identify the cellular processes involved in persister states. Previous findings led these researchers to focus on the HipA protein that when overexpressed causes a toxic phenotype of growth arrest by halting translation and inducing persistence through what is called the stringent response. The stringent response is a series of molecular events initiated during stress that activate survival functions in the cell including dormancy. Another protein, HipA7, a mutant of HipA was also previously shown to shift cells into a dormant state but with no toxic effect. Together these previous findings guided the researchers in uncovering the molecular players involved in persistence.
Using a series of carefully designed whole cell experiments, antibiotics to trigger dormant or persister cell formation and various genetic tools a total of 230 unique proteins were identified as HipA phosphorylation targets. Interestingly, proteins involved in regulation of DNA transcription and translation were identified which may play a role in persister cell formation.
Intriguingly identical experiments performed using HipA7, the mutant of HipA, revealed only one protein that was phosphorylated, glutamate-tRNA synthetase, also found as a hit in HipA experiments. While HipA7 was capable of shifting cells into a persistant state it had dramatically less downstream modifications to proteins compared to HipA, leading to less bacterial toxicity. Another curious finding was revealed when the expression of HipA7 was increased. They identified new overexpressed downstream targets that were unique to HipA7. What’s more exciting is that these targets were identified as stress-induced chaperones with functionality in protein folding and protein degradation. This suggests that HipA7 may be recruiting proteins that help to maintain a non-toxic cellular environment while simultaneously shifting those cells into a dormant state.
These new genes implicated in persister cell formation represent potential targets for new classes of antimicrobial drugs that can inhibit cell dormancy or maybe trigger toxic products in the cell leading to cell death. These researchers hope to perform follow up studies to clearly delineate the connection between the HipA proteins and targets identified in this study. With more research this area of bacterial drug resistance can be exploited to identify new targets in the hunt for new classes of antibiotics.
Dr. Dinesh Fernando