E. coli Suppresses S. aureus in Joint Implant Biofilms and Changes Antibiotic Response

Periprosthetic joint infections (PJI) affect roughly 2% of the more than one million patients who undergo total joint arthroplasty annually, and they carry high morbidity and mortality. While Staphylococcus aureus accounts for 20–40% of early postoperative PJI, polymicrobial infections — involving two or more organisms — occur in 6–40% of cases and are associated with treatment failure rates exceeding 70%. Despite this clinical burden, how different bacterial species interact within implant-associated biofilms remains poorly understood. This study from Massachusetts General Hospital and Harvard Medical School directly investigated the dynamics of S. aureus and E. coli co-existing in dual-species biofilms on stainless-steel implant surfaces over 48 hours.

Using plate count methods, fluorescent Gram staining, scanning electron microscopy (SEM), and gene expression profiling, the researchers tracked biofilm viability, structure, and molecular adaptation. In co-culture experiments with methicillin-susceptible S. aureus (MSSA), E. coli reduced MSSA biofilm viability by more than 3 logs at both 6 hours (p=0.031) and 24 hours (p=0.0004), with MSSA falling below the detection limit by 48 hours (p=0.0075). For methicillin-resistant S. aureus (MRSA), viability was initially elevated at 6 hours (p=0.013) but dropped below detection by 24 hours (p=0.00076), recovering slightly but remaining significantly suppressed at 48 hours (p=0.022). E. coli viability was unaffected in both co-culture conditions.

Fluorescent microscopy confirmed these dynamics visually: viable MSSA dropped from 78% to 14% of the biofilm population by 48 hours, while E. coli rose from 22% to 86%. With MRSA, viable S. aureus fell from 29% to just 2%, while E. coli climbed from 71% to 98%. SEM imaging revealed morphological changes in both species — S. aureus aggregates became deflated and sparse over time, while E. coli developed chain-like growth and appeared to benefit from the S. aureus extracellular matrix for initial attachment. Notably, S. aureus recovered from dual-species biofilms formed small colony variants (SCVs) on selective agar, a phenotype associated with antibiotic tolerance and chronic infection.

Antibiotic susceptibility testing showed striking strain-specific effects. The minimum biofilm eradication concentration (MBEC) of gentamicin for MSSA dropped dramatically in the presence of E. coli — from 100–500 µg/mL in mono-species biofilms to less than 10–25 µg/mL in dual-species biofilms (p=0.0076 at 6 h; p=0.0067 at 24 h). In contrast, MRSA showed limited change in gentamicin MBEC. Paradoxically, E. coli's gentamicin MBEC increased in dual-species settings — from 25–50 µg/mL in mono-species to up to 500 µg/mL in co-culture with MSSA — suggesting that the polymicrobial environment confers protective resistance to E. coli even as it sensitizes MSSA.

Gene expression profiling revealed molecular-level adaptations in both organisms. S. aureus downregulated adhesion and biofilm-associated genes while upregulating stress response pathways. E. coli showed differential regulation of virulence and biofilm genes in the dual-species context. These findings collectively suggest that inter-species competition reshapes the infection landscape in ways that current treatment protocols do not account for. The suppression of S. aureus by E. coli could theoretically be leveraged therapeutically, but the simultaneous increase in E. coli antibiotic resistance and the emergence of S. aureus SCVs complicate this picture. The authors call for in vivo validation and mechanistic studies to translate these findings into optimized polymicrobial PJI treatment strategies.