Acinetobacter baumannii is not Associated with Osteomyelitis in a Rat Model: A Pilot Study

Abstract

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 combatrelated segmental bone defects inoculated with MDR AB may be attributable to low virulence.

Bioburden Increases Heterotopic Ossification Formation in an Established Rat Model

“…Conversely, A baumannii was not present in any 12-week cultures, but rather other antibiotic-resistant ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, K pneumoniae, A baumanii, P aeruginosa, and Enterobacter species) such as enteric organisms Enterococcus faecium and Enterobacter cloacae as well as various Gram-positive staphylococci, but not S aureus were isolated at time of culture (Table 1). Negative culture results for A baumannii infection 12 weeks postinoculation are consistent with previous rat studies and may be the result of decreased virulence in bone as compared with other infection sites [9]. Secondary infection with other nosocomial pathogens, particularly Gram-positive organisms, is consistent with clinical findings and suggests that initial infection with A baumannii may produce an environment conducive to secondary infection or overgrowth of other nosocomial organisms [17].…”

Hematogenous Osteomyelitis by Acinetobacter Baumannii: Case Report and Literature Review

“…A recent pilot study using a rat model suggested that MDR A. baumannii did not appear to cause or contribute to osteomyelitis [15]. However, in our patient, isolation of A. baumannii from several bone specimens, including the ones collected intraoperatively, implies a significant role.…”

“…In the experimental rat model, inoculation of S. aureus and A. baumannii was associated with osteolysis, whereas A. baumannii alone lead to osteoblastic activity [15]. The initial bone culture in our patient failed to show S. aureus; however the intraoperative culture grew both S. aureus and A. baumannii.…”

Insights into Acinetobacter baumannii pathogenicity

“…The mammalian models and end-points have varied in complexity and strength, and have included a mouse or rat pneumonia model, as well as a rat soft-tissue model where bacteria are injected into a soft-tissue pouch and then the pouch is sampled over time for bacterial density. A pilot study assessing A. baumannii infection of bone in a rat model has also been performed (45). Virulence factors that have been identified using this approach include penicillinbinding protein 7/8 (20), a glycosyltransferase (LpsB) involved in LPS biosynthesis (23), and genes (ptk and epsA) important for capsule formation (22).…”