Infectious Dose Dictates the Host Response during Staphylococcus aureus Orthopedic-Implant Biofilm Infection

Abstract

Staphylococcus aureus is a leading cause of prosthetic joint infections (PJIs) that are typified by biofilm formation. Given the diversity of S. aureus strains and their propensity to cause community-or hospital-acquired infections, we investigated whether the immune response and biofilm growth during PJI were conserved among distinct S. aureus clinical isolates. Three S. aureus strains representing USA200 (UAMS-1), USA300 (LAC), and USA400 (MW2) lineages were equally effective at biofilm formation in a mouse model of PJI and elicited similar leukocyte infiltrates and cytokine/chemokine profiles. Another factor that may influence the course of PJI is infectious dose. In particular, higher bacterial inocula could accelerate biofilm formation and alter the immune response, making it difficult to discern underlying pathophysiological mechanisms. To address this issue, we compared the effects of two bacterial doses (10 3 or 10 5 CFU) on inflammatory responses in interleukin-12p40 (IL-12p40) knockout mice that were previously shown to have reduced myeloid-derived suppressor cell recruitment concomitant with bacterial clearance after low-dose challenge (10 3 CFU). Increasing the infectious dose of LAC to 10 5 CFU negated these differences in IL-12p40 knockout animals, demonstrating the importance of bacterial inoculum on infection outcome. Collectively, these observations highlight the importance of considering infectious dose when assessing immune responsiveness, whereas biofilm formation during PJI is conserved among clinical isolates commonly used in mouse S. aureus infection models.

Influence of fracture stability on Staphylococcus epidermidis and Staphylococcus aureus infection in a murine femoral fracture model

“…epidermidis infection resolved in many animals, which was similar to previous implant-associated infection models for this organism (Lankinen et al, 2012;Laure et al, 2008;Lovati et al, 2016b). Higher dose inocula may guarantee a persistent infection; however, as reported recently for S. aureus, infectious dose might alter immune responses and has to be considered carefully (Vidlak and Kielian, 2016). Considering our experimental hypothesis that fracture instability predisposes to infection, a low dose inoculum enabled us to differentiate between the development of infection (or not) rather than merely looking for changes in CFU counts, which would be less indicative of an effect.…”

Staphylococcal Biofilms and Immune Polarization During Prosthetic Joint Infection

“…These experiments demonstrate that S. aureus biofilms differentially modify their gene expression patterns depending on the leukocyte subset encountered. The finding that biofilms were more responsive following macrophage addition rather than neutrophils is in agreement with the preferential recruitment of macrophages but minimal neutrophils during biofilm formation [13–16,21,25–27]. While the anti-inflammatory response appears to be driven, in part, by biofilm products, the specific effectors and their mechanism(s) of action have yet to be fully elucidated.…”

“…This alternatively-activated response induces robust fibrosis surrounding the biofilm, effectively preventing macrophage invasion and phagocytosis of biofilm-associated bacteria, favoring biofilm persistence [15,25,26]. Although proinflammatory cytokine production is detected during S. aureus biofilm infections, this response is clearly not sufficient to mitigate biofilm growth or survival [13–15,27]. This suggests a primary defect in the phagocytes that normally clear bacteria, which is supported by the preferential recruitment of MDSCs into staphylococcal biofilms that possess anti-inflammatory properties by preventing macrophage proinflammatory activity and T cell activation.…”

“…These discrepancies may be explained by the distinct model systems employed; namely the use of a tibial implant pre-coated in vitro with an established biofilm compared to our approach where a sterile orthopedic implant is infected in vivo using a 200-fold lower bacterial dose. Indeed, we recently reported that increasing the infectious inoculum from 10 3 to 10 5 CFU in the mouse PJI model altered leukocyte recruitment and inflammatory mediator production as well as biofilm growth/clearance [27]. This highlights the need to carefully consider infectious doses when examining inflammatory attributes of biofilm infection, in particular to their relevance in terms of modeling events that might occur during native PJI in humans.…”

Titanium surfaces immobilized with the major antimicrobial fragment FK-16 of human cathelicidin LL-37 are potent against multiple antibiotic-resistant bacteria

“…Interestingly, the same 10 3 inoculum is reported to be critical to evaluate the immune response to various S. aureus strains in a mouse orthopedic-implant biofilm infection model. This is because all the differences disappeared at a higher CFU (Vidlak and Kielian 2016). Thus, our use of 10 3 CFU could be medically relevant.…”