Modifying TIMER to generate a slow-folding DsRed derivative for optimal use in quickly-dividing bacteria

It is now well appreciated that members of pathogenic bacterial populations exhibit heterogeneity in growth rates and metabolic activity, and it is known this can impact the ability to eliminate all members of the bacterial population during antibiotic treatment. It remains unclear which pathways promote slowed bacterial growth within host tissues, primarily because it has been difficult to identify and isolate slow growing bacteria from host tissues for downstream analyses. To overcome this limitation, we have developed a novel variant of TIMER, a slow-folding fluorescent protein, named DsRed 42 , to identify subsets of slowly dividing bacteria within host tissues. The original TIMER folds too slowly for fluorescence accumulation in quickly replicating bacterial species ( Escherichia coli , Yersinia pseudotuberculosis ), however DsRed 42 accumulates red fluorescence in late stationary phase cultures of E . coli and Y . pseudotuberculosis . We show DsRed 42 signal also accumulates during exposure to sources of nitric oxide (NO), suggesting DsRed 42 signal detects growth-arrested bacterial cells. In a mouse model of Y . pseudotuberculosis deep tissue infection, DsRed 42 signal was detected, and primarily accumulates in bacteria expressing markers of stationary phase growth. There was no significant overlap between DsRed 42 signal and NO-exposed subpopulations of bacteria within host tissues, suggesting NO stress was transient, allowing bacteria to recover from this stress and resume replication. This novel DsRed 42 variant represents a tool that will enable additional studies of slow-growing subpopulations of bacteria, specifically within bacterial species that quickly divide.