A Population-level Strain Genotyping Method to Study Pathogen Strain Dynamics in Human Infections

Abstract

A hallmark of chronic bacterial infections is the long-term persistence of one or more pathogen species at the compromised site. Repeated detection of the same bacterial species can suggest that a single strain or lineage is continually present. However, infection with multiple strains of a given species, strain acquisition and loss, and changes in strain relative abundance can occur. Detecting strain-level changes and their effects on disease is challenging as most methods require labor intensive isolate-by-isolate analyses, thus, only a few cells from large infecting populations can be examined. Here we present a population-level method for enumerating and measuring the relative abundance of strains called "PopMLST". The method exploits PCR amplification of strain-identifying polymorphic loci, next-generation sequencing to measure allelic variants, and informatic methods to determine whether variants arise from sequencing errors or low abundance strains. These features enable PopMLST to simultaneously interrogate hundreds of bacterial cells that are either cultured en masse from patient samples, or are present in DNA directly extracted from clinical specimens without ex vivo culture. This method could be used to detect epidemic or super-infecting strains, facilitate understanding of strain dynamics during chronic infections, and enable studies that link strain changes to clinical outcomes.