Intracellular Staphylococcus aureus and antibiotic resistance: implications for treatment of staphylococcal osteomyelitis

Diagnosis and management of chronic infection

High-energy penetrating extremity injuries are often associated with severe open fractures that have varying degrees of soft-tissue contamination and tenuous soft-tissue coverage. The result is a relatively high prevalence of chronic osteomyelitis compared with that in civilian trauma patients. Diagnosing chronic osteomyelitis requires a careful history and thorough physical and radiographic examinations. Cross-sectional imaging can help delineate the extent of bony involvement, and scintigraphy can be used as a diagnostic tool and to gauge response to treatment. Clinical staging also directs surgical management. Adequacy of débridement remains the most important clinical predictor of success; thus, adopting an oncologic approach to complete (ie, wide) excision is important. Reconstruction can be safely performed by a variety of methods; however, proper staging and patient selection remain critical to a successful outcome. Although systemic and depot delivery of antibiotics plays a supporting role in the treatment of chronic osteomyelitis, the ideal dosing regimens, and the duration of treatment, remain controversial.

The role of increased frequency of treg cells in patients with chronic osteomyelitis

The aim of this study is to determine whether regulatory T cells (Treg cells) are increased in patients with chronic osteomyelitis and whether they suppress cellular immune responses to the bacteria. The frequency of circulating CD4(+)CD25(+) and CD4(+)CD25(+)FoxP3(+) T cells in 30 chronic osteomyelitis patients were compared with 30 healthy donors. Treg-depleted PBMCs from the patients were cultured together with autologous antigen, unfractioned PBMCs used as the control. The cell proliferation and production of IL-10 and IFN-γ were compared with those of the control. The results demonstrated that frequencies of CD4(+)CD25(+) (10.85±2.82% vs 6.08±1.62%, P<.001) and CD4(+)CD25(+)FoxP3(+) T cells (2.06±0.83% vs 1.43%±0.51%, P<.001) in blood from chronic osteomyelitis patients were significantly higher than in healthy donors. The level of IL-10 (117±91 pg/ml vs 323±189 pg/ml, P<.001) in supernatants of Treg-depleted PBMCs was decreased. Cell proliferation (4489±11876 cpm vs 3547±1517 cpm, P<.05) and IFN-γ (875±203 pg/ml vs 405±129 pg/ml, P<.001) production by CD4(+)CD25(+) T cell in response to antigen was significantly inhibited by CD4(+)CD25(+) T cells. These results indicate that specific Tregs can depress the T cell mediated immune responses to bacteria in chronic osteomyelitis, and may play an important role in the persistence of bacteria.

Rifampin combination therapy for nonmycobacterial infections

The increasing emergence of antimicrobial-resistant organisms, especially methicillin-resistant Staphylococcus aureus (MRSA), has resulted in the increased use of rifampin combination therapy. The data supporting rifampin combination therapy in nonmycobacterial infections are limited by a lack of significantly controlled clinical studies. Therefore, its current use is based upon in vitro or in vivo data or retrospective case series, all with major limitations. A prominent observation from this review is that rifampin combination therapy appears to have improved treatment outcomes in cases in which there is a low organism burden, such as biofilm infections, but is less effective when effective surgery to obtain source control is not performed. The clinical data support rifampin combination therapy for the treatment of prosthetic joint infections due to methicillin-sensitive S. aureus (MSSA) after extensive debridement and for the treatment of prosthetic heart valve infections due to coagulase-negative staphylococci. Importantly, rifampin-vancomycin combination therapy has not shown any benefit over vancomycin monotherapy against MRSA infections either clinically or experimentally. Rifampin combination therapy with daptomycin, fusidic acid, and linezolid needs further exploration for these severe MRSA infections. Lastly, an assessment of the risk-benefits is needed before the addition of rifampin to other antimicrobials is considered to avoid drug interactions or other drug toxicities.

Oral antibiotics are effective for highly resistant hip arthroplasty infections

Infected arthroplasties reportedly have a lower eradication rate when caused by highly resistant and/or polymicrobial isolates and in these patients most authors recommend intravenous antibiotics. We asked whether two-stage revision with interim oral antibiotics could eradicate these infections. We prospectively followed 36 patients (mean age, 71.8 years) with late hip arthroplasty infections. Combinations of oral antibiotics were prescribed according to cultures, biofilm, and intracellular effectiveness. The minimum followup was 1 year (mean, 4.4 years; range, 1-12 years). We presumed eradication in the absence of clinical, serologic, and radiographic signs of infection. Infection was eradicated in all 13 patients with highly resistant bacteria who completed a two-stage protocol (10 with methicillin-resistant Staphylococci) and in eight of 11 patients treated with only the first stage (and six of nine with methicillin-resistant Staphylococci). Infection was eradicated in six of six patients with polymicrobial isolates (of sensitive and/or resistant bacteria) who completed a two-stage protocol and in five of seven with polymicrobial isolates treated with only the first surgery. The Harris hip score averaged 88.1 (range, 70-98) in patients who underwent reimplantation and 56.8 (range, 32-76) in patients who underwent resection arthroplasty. Long cycles of combined oral antibiotics plus a two-stage surgical exchange appear a promising alternative for infections by highly resistant bacteria, methicillin-resistant Staphylococci, and polymicrobial infections.

Encapsulated Staphylococcus aureus strains vary in adhesiveness assessed by atomic force microscopy

Staphylococcus aureus capsular polysaccharides are believed to play a role in adhesion to surfaces and may contribute to their antimicrobial resistance, thereby increasing the rates and severity of associated infections. The purpose of this study was to compare the adhesiveness of distinct S. aureus capsular polysaccharides to determine whether adhesiveness was a general or specific feature across different S. aureus strains. Atomic force microscopy was used to confirm the presence or absence of capsular polysaccharides and to measure adhesive forces on a noncapsulated, serotype 8, and serotype 2 strain of S. aureus. Serotype 8 displayed a larger range of adhesive forces (1-19 nN) than the noncapsulated (0-4 nN) and serotype 2 (0-4 nN) strain. The majority of adhesive forces for serotype 8 were in the 10-15 nN range. Removal of capsular polysaccharides gave a marked decrease in adhesive forces measured for serotype 8 and, to a lesser extent, a decrease for serotype 2. Noncapsulated, serotype 8, and serotype 2 S. aureus had water contact angles of 23.8 (+/-8.9), 34.4 (+/-2.5), and 56.7 (+/-11.2) degrees (mean +/- standard deviation), respectively. For the first time, capsular polysaccharides from serotype 8 (clinically common) and serotype 2 (clinically rare) were demonstrated to have different physical properties, which may account for variations in studies in which clinical isolates are utilized, and the conflict in proposed roles for capsular polysaccharides on S. aureus is explained.

Proinflammatory response of human osteoblastic cell lines and osteoblast-monocyte interaction upon infection with Brucella spp

The ability of Brucella spp. to infect human osteoblasts and the cytokine response of these cells to infection were investigated in vitro. Brucella abortus, B. suis, B. melitensis, and B. canis were able to infect the SaOS-2 and MG-63 osteoblastic cell lines, and the first three species exhibited intracellular replication. B. abortus internalization was not significantly affected by pretreatment of cells with cytochalasin D but was inhibited up to 92% by colchicine. A virB10 mutant of B. abortus could infect but not replicate within osteoblasts, suggesting a role for the type IV secretion system in intracellular survival. Infected osteoblasts produced low levels of chemokines (interleukin-8 [IL-8] and macrophage chemoattractant protein 1 [MCP-1]) and did not produce proinflammatory cytokines (IL-1beta, IL-6, and tumor necrosis factor alpha [TNF-alpha]). However, osteoblasts stimulated with culture supernatants from Brucella-infected human monocytes (THP-1 cell line) produced chemokines at levels 12-fold (MCP-1) to 17-fold (IL-8) higher than those of infected osteoblasts and also produced IL-6. In the inverse experiment, culture supernatants from Brucella-infected osteoblasts induced the production of IL-8, IL-1beta, IL-6, and TNF-alpha by THP-1 cells. The induction of TNF-alpha and IL-1beta was largely due to granulocyte-macrophage colony-stimulating factor produced by infected osteoblasts, as demonstrated by inhibition with a specific neutralizing antibody. This study shows that Brucella can invade and replicate within human osteoblastic cell lines, which can directly and indirectly mount a proinflammatory response. Both phenomena may have a role in the chronic inflammation and bone and joint destruction observed in osteoarticular brucellosis.

Staphylococcus aureus: new evidence for intracellular persistence

Many reports have documented that Staphylococcus aureus can invade host cells and persist intracellularly for various periods of time in cell culture models. However, it is not clear whether intracellular persistence of S. aureus also occurs in the course of infections in whole organisms. This is a subject of intense debate and is difficult to assess experimentally. Intracellular persistence would provide S. aureus with an ideal strategy to escape from professional phagocytes and extracellular antibiotics and would promote recrudescent infection. Here, we present a brief overview of the mounting evidence that S. aureus has the potential to internalize and survive within host cells.

Low relapse with oral antibiotics and two-stage exchange for late arthroplasty infections in 40 patients after 2-9 years

BACKGROUND AND PURPOSE

Exchange surgery in late arthroplasty infection is directed against bacteria adhering to implants. Therapies based on antibiotics that are effective intracellularly have been proposed recently. We have combined both strategies to improve the cure rate.

METHODS

40 consecutive patients (16 hips, 24 knees) were diagnosed with late arthroplasty infection. The organisms isolated were 35 Staphylococcus, 19 of which were methicillin-resistant, 4 Enterococcus, 6 Gram-negative bacilli, and 4 Corynebacterium. The infections were managed by a combined therapy consisting of two-stage exchange surgery and two oral intracellularly-effective antibiotics. The antibiotics were selected according to bacterial sensitivity and intracellular and biofilm effectiveness. Second re-implantation surgery was delayed until clinical and analytical normalization. Patients were in hospital for only 1 week after each surgery, and were followed up prospectively on an outpatient basis (2-9 years). Cure of the infection was defined as absence of clinical, serological, and radiographic signs of infection during the whole follow-up.

RESULTS

The infection was resolved in 38/40 patients (15/16 hips and 23/24 knees).

INTERPRETATION

Oral antibiotics that are effective intracellularly in combination with two-stage exchange surgery is a promising alternative for treating late arthroplasty infections. Oral antibiotics shorten hospitalization and reduce patient discomfort.