Capsule expression and genotypic differences among Staphylococcus aureus isolates from patients with chronic or acute osteomyelitis

Abstract

There is ample evidence that Staphylococcus aureus capsular polysaccharide (CP) promotes virulence. Loss of capsule expression, however, may lead to S. aureus persistence in a chronically infected host. This study was conducted to determine the relative prevalence of nonencapsulated S. aureus in patients with chronic and acute osteomyelitis. Only 76/118 (64%) S. aureus isolates from patients with osteomyelitis expressed CP, whereas all 50 isolates from blood cultures of patients with infections other than osteoarticular infections expressed CP (P = 0.0001). A significantly higher prevalence of nonencapsulated S. aureus was found in patients with chronic osteomyelitis (53%) than in those with acute osteomyelitis (21%) (P = 0.0046). S. aureus isolates obtained from multiple specimens from five of six patients with chronic osteomyelitis exhibited phenotypic (expression of CP, alpha-hemolysin, beta-hemolysin, slime, and the small-colony variant phenotype) and/or genotypic (pulsed-field gel electrophoresis and spa typing) differences. Nonencapsulated S. aureus was recovered from at least one specimen from each chronic osteomyelitis patient. Fourteen isolates obtained from two patients with acute osteomyelitis were indistinguishable from each other within each group, and all produced CP5. In conclusion, we demonstrated that nonencapsulated S. aureus is more frequently isolated from patients with chronic osteomyelitis than from those with acute osteomyelitis, suggesting that loss of CP expression may be advantageous to S. aureus during chronic infection. Our findings on multiple S. aureus isolates from individual patients allow us to suggest that selection of nonencapsulated S. aureus is likely to have occurred in the patient during long-term bone infection.

FIG. 1.

SmaI PFGE band patterns of S. aureus strains isolated from patients 85 (left panel), 107 (center panel), and 110 (right panel). Lanes λ, lambda ladder. Dendrograms show the similarity among isolates from each patient. Representatives of the prevalent Brazilian (Br), Pediatric (Pe), and Cordobes (Co) MRSA clones in Argentina were included in the analysis. BAL is a representative strain of a new emerging MRSA clone in Buenos Aires, obtained from a respiratory specimen of a child with cystic fibrosis (Liliana Jordá Vargas, personal communication).