Coagulase-negative staphylococci in prosthetic hip infections

Abstract: More than 11,000 primary total hip replacements were performed in Sweden in the year 2000, corresponding to 125 primary total hip replacements per 100,000 inhabitants, according to The Swedish Total Hip Replacement Register. In general, this procedure provides highly satisfactory results. The most common complications associated with prosthetic hip joints are aseptic biomechanical failures and infections. Delayed low-grade infections occur most often, and they are also the most difficult to distinguish from aseptic mechanical failures because of similar symptoms.During the period 1994 to 2001, a prospective study was conducted to compare inflammatory markers in blood, synovial fluid, and histopathological specimens in patients diagnosed with aseptic or septic loosening of hip prostheses. Coagulase-negative staphylococci (CoNS) were found to be the most common pathogens in the patients with prosthetic hip joint infections.Further characterisation of the CoNS revealed that patients were co-infected with the following: (i) CoNS and other bacterial species, (ii) various CoNS species, and (iii) different S. epidermidis clones. Expression of the icaADB gene complex, which is important for the biofilm mode-of-growth characteristic of S. epidermidis, was not necessary to allow S. epidermidis to infect orthopaedic prostheses. The majority of the S. epidermidis isolates, both from prostheses and normal bacteria flora, were able to bind to at least one of the extracellular matrix proteins we tested, this adhesion ability is a probable virulence factor. Histologically, the extent of cell infiltration differed between aseptic and septic loosening of prostheses, and neutrophils in tissue at a rate of ≥ 5 cells/high power field greatly favoured infection. Periprosthetic tissue contained the cells that are required not only for an innate, but also a specific, immune response, which supports the theory that the limited systemic inflammatory response in infections of hip prostheses is not due to failure of inflammatory cells to reach the infected area, but is more likely caused by an inadequate response to the infecting organisms.Therefore, we studied the interactions between neutrophils and S. epidermidis (the most common CoNS species in hip prosthetic infections). Neutrophils phagocytosed both surfaceadherent and planktonic S. epidermidis, but they ingested adherent S. aureus isolates more readily than they consumed adherent S. epidermidis. The reduced phagocytosis of S. epidermidis by neutrophils is viewed as a virulence factor, together with the lack of an ability to prime or sufficiently activate the neutrophil oxidase, and thereby evade adequate killing by neutrophils. The weak oxidative activation agrees with the low inflammatory response seen in patients with S. epidermidis infections related to implanted devices. Furthermore, both planktonic and adherent S. epidermidis delayed neutrophil apoptosis, and we suggest that this leads to the accumulation of neutrophils at the site of inflammation. We propose that the complex interplay between the S. epidermidis-induced delay in apoptosis in neutrophils and the interaction of S. epidermidis-containing neutrophils with macrophages in periprosthetic tissue has negative impact on the outcome in patients with prosthetic hip joint infections, resulting in low grade inflammation, tissue damage, and finally loosening of the prosthesis.