Persistence: another blow for antibiotics?
Under various stress conditions such as antibiotic exposure, a subpopulation of bacteria exhibit the ability to transiently withstand the stress and revive once conditions return to normal. This phenomenon is known as antibiotic persistence and should not be confused with antibiotic resistance where cells pass on to their progeny their ability to grow in the presence of an antibiotic. Persistence highlights the phenotypic heterogeneity in a bacterial culture susceptible to the antibiotic and is responsible for infection relapse.
Novel methods for studying persistent bacteria
Antibiotic persistence has major clinical consequences, including long treatment times for tuberculosis, recurrence of disease, and emergence of resistance. However, little is known about the mechanisms underlying the phenomenon of persistence. The scope of the COMBATTB project was to functionally characterise the molecular mechanisms persistent bacteria engage to survive exposure to lethal concentrations of antibiotics. Research was undertaken with the support of the Marie Skłodowska-Curie Actions (MSCA) programme and facilitated the development of methods for the identification and isolation of persistent bacteria. “A key issue in the identification of persistent bacteria is that standard clinical antibiotic susceptibility assays do not assess antibiotic persistence,” explains the MSCA research fellow Helene Botella. Persistent bacteria are also difficult to study because available experimental procedures also become enriched with resistant cells which grow in the presence of an antibiotic and quickly outnumber any non-growing survivors. The scientific team developed the method ‘recombination-mediated isolation of non-dividers’ (ReMIND) that allows for efficient separation of persistent bacteria from resistant ones. It is based on the ability of the former population to retain certain phenotypic traits, unlike the latter, when exposed to an antibiotic.
Implementation of ReMIND for tuberculosis
Botella validated ReMIND in Mycobacterium tuberculosis, the etiologic agent of tuberculosis, as a powerful tool to inform the molecular mechanisms by which persistent bacteria form during infection. “The development of this method for the specific isolation of Mycobacterium tuberculosis persisters in vitro and in physiologic biological context is a major achievement,” outlines Botella. Blocking pathways that foster the formation of persistent cells has the potential to reduce treatment time, which is particularly important in the context of tuberculosis. Treatment usually encompasses a complex cocktail of drugs and requires a minimum of 6 months of administration, while relapse is not unusual. The fellow reveals that the plan is to further use ReMIND to also elucidate the host pathways that are implicated in the phenomenon of antibiotic persistence. In particular, Botella is interested to understand how host immunity, including macrophages, which are the preferred niche of Mycobacterium tuberculosis, contributes to Mycobacterium tuberculosis antibiotic persistence. According to Botella: “Antimicrobial resistance not only hampers treatment for tuberculosis but for other infections as well, making it a threat to human health, with obvious socio-economic implications.” Based on recent evidence, antimicrobial resistance has claimed the lives of nearly 1.3 million people in 2019. There is no evidence on how antibiotic persistence further contributes to this toll. COMBATTB findings offer fundamental knowledge on the emergence of persistence in Mycobacterium tuberculosis and can pave the way towards the identification of drugs that kill persistent bacteria. Moreover, ReMIND can assist the development of diagnostic tools to prevent the emergence of genetic resistance in tuberculosis, thereby offering better husbandry of current treatment regimens.
Keywords
COMBATTB, tuberculosis, persistent bacteria, antibiotic persistence, Mycobacterium tuberculosis, ReMIND, macrophages