Decomposing virulence to understand bacterial clearance in persistent infections
Authors: Hidalgo Beatriz Acuña , Silva Luís M. , Franz Mathias , Regoes Roland R. , Armitage Sophie A.O. 
Published: March 2021

Hosts are not always successful at controlling and eliminating a pathogen and the factors causing variation in pathogen clearance are not well understood. To address this problem, we usedDrosophila melanogasterto investigate how infections with different bacterial pathogens affects virulence, clearance and persistence. In this context we developed novel hypotheses that focus on how variation in clearance should be related to variation in different components of virulence, where virulence is the infection-related reduction in host fitness. To achieve this, virulence was decomposed into exploitation, i.e., how well bacteria can replicate inside the host, and per parasite pathogenicity (PPP), i.e., the amount of damage per parasite inflicted on the host. We used four bacterial species:Enterobacter cloacae,Providencia burhodogranariea, Lactococcus lactisandPseudomonas entomophila. The injection doses spanned four orders of magnitude, and survival was followed to estimate virulence. Bacterial load was quantified in live flies during the acute (1-4 days) and chronic (7-35 days) phases of infection, and we tested infection status of flies that had died up to ten weeks post infection. We show that sustained persistent infection and clearance are both possible outcomes for bacterial species across a range of virulence. Bacteria of all species could persist inside the host for at least 75 days, and injection dose partly predicted within-species variation in clearance. Our decomposition of virulence showed that species differences in bacterial virulence could be explained by a combination of variation in both exploitation and PPP. In addition, we found that that higher exploitation leads to lower bacterial clearance, whereas we could not detect any effect of PPP on clearance. The differing effects of exploitation and PPP imply that there can be different means by which variation in virulence is related to clearance, which could critically affect pathogen transmission and the evolution of pathogen virulence.

Author summary

Following an infection, hosts are not always able to quickly clear the pathogen, and they instead either die or survive with a persistent infection. Such variation is ecologically and evolutionarily important, because it can affect infection prevalence and transmission, and also virulence evolution. But what causes variation in infection outcomes? Here we contribute towards answering this question by investigating infection dynamics in flies infected with one of four bacterial species. We first establish that the bacterial species differ in virulence,i.e., the host death rate after infection. We find that variation in virulence arises because the bacteria differ in the two components of virulence: bacterial growth inside the host (exploitation), and the amount of damage caused per bacterium (per parasite pathogenicity).

Furthermore, as early-phase exploitation increases, bacterial clearance later in the infection decreases. This finding can be explained by increasing exploitation leading to increasing clearance costs for the host. Taken together we demonstrate that variation in infection outcomes can be partly explained by how virulence, and its components, relate to the rate of pathogen clearance. We propose that the decomposition of virulence is valuable for understanding variation in infection outcomes – potentially also beyond the interrelation between virulence and clearance.