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Periprosthetic joint infection (PJI) occurring after artificial joint replacement is a major clinical issue requiring multiple surgeries and antibiotic interventions. Staphylococcus aureus is the bacterium most commonly responsible for PJI. Recent in vitro research has shown that staphylococcal strains rapidly form aggregates in the presence of synovial fluid (SF). We hypothesize that these aggregates provide early protection to bacteria entering the wound site, allowing them time to attach to the implant surface, leading to biofilm formation. Thus, understanding the attachment kinetics of these aggregates is critical in understanding their adhesion to various biomaterial surfaces. In this study, the number, size, and surface area coverage of aggregates as well as of single cells of S. aureus were quantified under various conditions on different orthopedic materials relevant to orthopedic surgery: stainless steel (316L), titanium (Ti), hydroxyapatite (HA), and polyethylene (PE). It was observed that, regardless of the material type, SF-induced aggregation resulted in reduced aggregate surface attachment and greater aggregate size than the single-cell populations under various shear stresses. Additionally, the surface area coverage of bacterial aggregates on PE was relatively high compared to that on other materials, which could potentially be due to the rougher surface of PE. Furthermore, increasing shear stress to 78 mPa decreased aggregate attachment to Ti and HA while increasing the aggregates' average size. Therefore, this study demonstrates that SF induced inhibition of aggregate attachment to all materials, suggesting that biofilm formation is initiated by lodging of aggregates on the surface features of implants and host tissues. IMPORTANCE Periprosthetic joint infection occurring after artificial joint replacement is a major clinical issue that require repeated surgeries and antibiotic interventions. Unfortunately, 26% of patients die within 5 years of developing these infections. Staphylococcus aureus is the bacterium most commonly responsible for this problem and can form biofilms to provide protection from antibiotics as well as the immune system. Although biofilms are evident on the infected implants, it is unclear how these are attached to the surface in the first place. Recent in vitro investigations have shown that staphylococcal strains rapidly form aggregates in the presence of synovial fluid and provide protection to bacteria, thus allowing them time to attach to the implant surface, leading to biofilm formation. In this study, we investigated the attachment kinetics of Staphylococcus aureus aggregates on different orthopedic materials. The information presented in this article will be useful in surgical management and implant design., (Copyright © 2020 American Society for Microbiology.)

Pseudomonas aeruginosa is an opportunistic bacterial pathogen and is known to produce biofilms. We previously showed the emergence of colony variants in the presence of tobramycin-loaded calcium sulfate beads. In this study, we characterized the variant colonies, which survived the antibiotic treatment, and identified three distinct phenotypes-classically resistant colonies, viable but nonculturable colonies (VBNC), and phoenix colonies. Phoenix colonies, described here for the first time, grow out of the zone of clearance of antibiotic-loaded beads from lawn biofilms while there are still very high concentrations of antibiotic present, suggesting an antibiotic-resistant phenotype. However, upon subculturing of these isolates, phoenix colonies return to wild-type levels of antibiotic susceptibility. Compared with the wild type, phoenix colonies are morphologically similar aside from a deficiency in green pigmentation. Phoenix colonies do not recapitulate the phenotype of any previously described mechanisms of resistance, tolerance, or persistence and, thus, form a novel group with their own phenotype. Growth under anaerobic conditions suggests that an alternative metabolism could lead to the formation of phoenix colonies. These findings suggest that phoenix colonies could emerge in response to antibiotic therapies and lead to recurrent or persistent infections, particularly within biofilms where microaerobic or anaerobic environments are present., (Copyright © 2020 American Society for Microbiology.)