Optimised chronic infection models demonstrate that siderophore ‘cheating’ in Pseudomonas aeruginosa is context specific

Abstract

The potential for siderophore mutants of Pseudomonas aeruginosa to attenuate virulence during infection, and the possibility of exploiting this for clinical ends, have attracted much discussion. This has largely been based on the results of in vitro experiments conducted in iron-limited growth medium, in which siderophore mutants act as social ‘cheats:’ increasing in frequency at the expense of the wild type to result in low-productivity, low-virulence populations dominated by mutants. We show that insights from in vitro experiments cannot necessarily be transferred to infection contexts. First, most published experiments use an undefined siderophore mutant. Whole-genome sequencing of this strain revealed a range of mutations affecting phenotypes other than siderophore production. Second, iron-limited medium provides a very different environment from that encountered in chronic infections. We conducted cheating assays using defined siderophore deletion mutants, in conditions designed to model infected fluids and tissue in cystic fibrosis lung infection and non-healing wounds. Depending on the environment, siderophore loss led to cheating, simple fitness defects, or no fitness effect at all. Our results show that it is crucial to develop defined in vitro models in order to predict whether siderophores are social, cheatable and suitable for clinical exploitation in specific infection contexts.

Susceptibility of monomicrobial or polymicrobial biofilms derived from infected diabetic foot ulcers to topical or systemic antibiotics in vitro

“…Models of infection and biofilms are limited, particularly in vitro models [38,60]. The model presented in this study incorporates type 1 collagen, HA, ECM and fibroblasts in order to create a matrix that represents the human dermis as a substrate for biofilm formation.…”

“…However, clinical isolates often behave differently to lab strains and there is evidence that polymicrobial biofilms have altered tolerance to antibiotics and form more robust biofilms with different architecture because of symbiotic interactions between the different species [35,37]. Increasingly there is awareness that laboratory biofilm models that use abiotic surfaces as the substrate for biofilm growth do not reflect environmental or in vivo biofilms and additionally that factors produced in vivo such as growth factors, proteases and serum proteins are absent in traditional microbiological media [38].…”

Siderophore-Mediated Interactions Determine the Disease Suppressiveness of Microbial Consortia

“…Our results suggest that inoculated strains can either inhibit or promote the pathogen, and the resulting disease outbreaks potentially depend on the compatibility of siderophores produced by each strain. Other studies have reported that incompatible siderophores could potentially constrain pathogen infection by reducing available iron in the environment (48)(49)(50)(51). Although bacteria belonging to the same genus may share siderophores as public goods, e.g., Pseudomonas (45), strain-specific siderophores may be produced to avoid intraspecific competition and exploitation by social cheats (52,53).…”

Cheating on cheaters dramatically affects social interactions inPseudomonas aeruginosa

“…The simplicity of such an approach has allowed to substantially increase our understanding of the dynamics of cooperative interactions and revealed several mechanisms involved in the maintenance of cooperation [2,4,47]. In particular, the ability of lasR or pvdS mutants to behave as cheaters individually has been extensively documented [9,11,25–27,34,35,64,67,70–74], and these mutants are commonly isolated from bacterial populations colonizing CF lungs [41,45].…”