Acinetobacter baumannii is not associated with osteomyelitis in a rat model: a pilot study.

BACKGROUND

Multidrug resistant Acinetobacter baumannii (MDR AB) with and without Staphylococcus aureus (SA) is a commonly isolated organism in infected segmental bone defects in combat-related trauma in Iraq and Afghanistan. Although MDR AB in visceral infections is a therapeutic challenge, control of infection appears more common for combat-related osteomyelitis.

QUESTIONS/PURPOSES: Using a rat model, we explored the virulence of MDR AB in segmental bone defects alone and in combination with SA.

METHODS

Segmental defects in 60 rat femurs were created, stabilized, and inoculated with MDR AB alone and 60 with MDR AB and SA. We performed qualitative and quantitative bacteriology and radiographic assessments at 2, 4, and 8 weeks for MDR AB and at 1, 2, and 3 weeks for MDR AB and SA.

RESULTS

Quantitative bacteriology revealed a 3- to 5-log decrease in MDR AB from the initial inoculum. After polymicrobial inoculation, only 10 of 60 animals had positive cultures for MDR AB, whereas 59 of 60 animals had positive cultures for SA. Recovered SA were 2 to 5 log greater than the initial inoculum, while there again was a 3- to 5-log decrease in MDR AB. MDR AB alone did not cause bony lysis, but there was radiographic evidence of new bone formation in 67% of the segmental defects. Osteolysis was noted with MDR AB and SA.

CONCLUSIONS

MDR AB did not appear to cause or contribute to clinically apparent osteomyelitis in this pilot study.

CLINICAL RELEVANCE

Resolution of infections in combat-related segmental bone defects inoculated with MDR AB may be attributable to low virulence. Additional studies are needed to confirm low virulence and bone formation with MDR AB.