Role of the accessory gene regulator (agr) in pathogenesis of staphylococcal osteomyelitis

Abstract

To examine the role of the accessory gene regulator (agr) in staphylococcal osteomyelitis, we compared a Staphylococcus aureus osteomyelitis isolate (UAMS-1) with a derivative of the same strain (UAMS-4) carrying an inactivated agr locus. Virulence was assessed with a rabbit model of acute, exogenous osteomyelitis. Bacteria were delivered by microinjection into the midradial region of the forelimb. After 4 weeks, UAMS-1 was identified in the bone of 12 of 13 rabbits infected with ≥2 x 106 CFU and 5 of 6 infected with ≤2 x 105 CFU. In contrast, UAMS-4 was found in 6 of 13 infected with the higher dose and 1 of 6 infected with the lower dose. Additionally, on the basis of a five-point scale assessing radiographic evidence of disease, rabbits infected with UAMS-1 had average scores of 2.64 ± 0.30 (high dose) and 1.43 ± 0.39 (low dose) while rabbits infected with UAMS-4 had average scores of 0.95 ± 0.23 (high dose) and 0.63 ± 0.20 (low dose). Uninfected controls had an average score of 0.53 ± 0.08. The results obtained with UAMS-1 were significantly different from those obtained with UAMS-4 at both doses (P ≤ 0.047). The results obtained with UAMS-4 were not significantly different from those obtained with the controls at either dose of UAMS-4 (P ≥ 0.150). On the basis of a similar five-point scale assessing histopathological evidence of disease, rabbits infected with UAMS-1 had average scores of 2.31 ± 0.22 (high dose) and 1.96 ± 0.36 (low dose) while rabbits infected with UAMS-4 had average scores of 1.58 ± 0.29 (high dose) and 0.83 ± 0.32 (low dose). Controls had an average score of 0.33 ± 0.05. The results obtained with UAMS-1 were significantly different from those obtained with UAMS-4 at both doses (P ≤ 0.040). However, the results obtained with UAMS-4 were significantly different from the controls only at the high dose of UAMS-4 (P = 0.025). We conclude that mutation of agr reduces the incidence and severity of disease but does not eliminate the ability to colonize bone and cause histopathological evidence of osteomyelitis.