What is the mechanism for persistent coexistence of drug-susceptible and drug-resistant strains of Streptococcus pneumoniae?
The rise of antimicrobial resistance in many pathogens presents a major challenge to the treatment and control of infectious diseases. Furthermore, the observation that drug-resistant strains have risen to substantial prevalence but have not replaced drug-susceptible strains despite continuing (and even growing) selective pressure by antimicrobial use presents an important problem for those who study the dynamics of infectious diseases. While simple competition models predict the exclusion of one strain in favour of whichever is 'fitter', or has a higher reproduction number, we argue that in the case of Streptococcus pneumoniae there has been persistent coexistence of drug-sensitive and drug-resistant strains, with neither approaching 100 per cent prevalence. We have previously proposed that models seeking to understand the origins of coexistence should not incorporate implicit mechanisms that build in stable coexistence 'for free'. Here, we construct a series of such 'structurally neutral' models that incorporate various features of bacterial spread and host heterogeneity that have been proposed as mechanisms that may promote coexistence. We ask to what extent coexistence is a typical outcome in each. We find that while coexistence is possible in each of the models we consider, it is relatively rare, with two exceptions: (i) allowing simultaneous dual transmission of sensitive and resistant strains lets coexistence become a typical outcome, as does (ii) modelling each strain as competing more strongly with itself than with the other strain, i.e. self-immunity greater than cross-immunity. We conclude that while treatment and contact heterogeneity can promote coexistence to some extent, the in-host interactions between strains, particularly the interplay between coinfection, multiple infection and immunity, play a crucial role in the long-term population dynamics of pathogens with drug resistance.
Colijn
Caroline
C
Department of Engineering Mathematics, University of Bristol, Bristol, UK. ccolijn@gmail.com
Cohen
Ted
T
Fraser
Christophe
C
Hanage
William
W
Goldstein
Edward
E
Givon-Lavi
Noga
N
Dagan
Ron
R
Lipsitch
Marc
M
D001419:Bacteria / Q000187:drug effects / Q000235:genetics; D003141:Communicable Diseases / Q000188:drug therapy / Q000235:genetics; D004198:Disease Susceptibility; D006801:Humans; D007239:Infections / Q000188:drug therapy / Q000235:genetics; D011157:Population Dynamics; D015995:Prevalence; D013296:Streptococcus pneumoniae / Q000187:drug effects* / Q000235:genetics