Nine-month-old Eva had 12 clinic visits over a seven-month period due to runny nose and/or ear infections. During each episode, nasopharyngeal swabs were obtained for bacterial culture and Streptococcus pneumoniae was obtained. Although all the infections were resolved with antibiotics, scientists gained insight into pathogen evolution by seeing how the isolates compared. Each isolate was first analyzed for seven genes that can indicate strain variability, a process called multilocus sequence typing (MLST). The lab found two MLST types, indicating that the patient was infected with two divergent strains of S. pneumoniae. However, when the complete genomes of several of the isolates were sequenced and compared, four different genotypic strains were identified, all of which evolved as a result of 16 distinct recombination events between strains. Curiously, the technicians noticed that some recombined sequences did not seem to come from either of the two primary coinfecting strains. It appeared that this new DNA came from uncultured and undetected strains of S. pneumoniae that had colonized Eva’s throat at one time or another. Horizontal gene transfers (transformation) between the undetected and detected isolates appear to have produced the new recombined strains of S. pneumoniae. The genome sequence data clearly support the idea that S. pneumoniae evolution is characterized by high rates of horizontal gene transfer and recombination. The scientists concluded that this and other related bacterial species use horizontal gene transfer to defeat the adaptive immunity of individual hosts, possibly explaining Eva’s recurring infections. These types of bioinformatic comparisons suggest that horizontal gene transfers that produce most of the genetic variation pathogens develop while persisting within a host.

Could antibiotic resistance from one strain of Streptococcus pneumoniae to another occur by transformation?