Friday, April 11, 2014

Propionibacterium is orthopaedically different from staphylococcus and can be a cause of 'aseptic' failure

Propionibacterium acnes and Staphylococcus   Cause Pyogenic Osteomyelitis in an Intramedullary Nail Model in the Rabbit

Both Propionibacterium acnes and coagulase-negative staphylococci are opportunistic pathogens implicated in prosthetic joint and fracture fixation device-related infection.

These authors isolated Propionibacterium and coagulase-negative staphylococci from ‘aseptically failed’ prosthetic hip joints and attempted to produce osteomyelitis in an established implant-related osteomyelitis model in the rabbit, in the absence of implant material wear debris.

The Propionibacterium LED2 was isolated after ultrasound treatment of a retrieved prosthetic hip joint, due to a supposed aseptic joint failure. Bacterial biofilm was also detected in the sonicate fluid by immunofluorescence microscopy (IFM) after labeling with a P. acnes-specific antibody.

This isolate belonged to the ST5 lineage within the type IB phylogenetic grouping. Isolates from this 
phylogroup are associated with healthy skin, and rarely recovered from acne vulgaris lesions. They
 have, however, been associated with soft tissue and medical device-related infections, although their
exact clinical importance in these cases has remained unclear.

The histological features of Propionibacterium infection in the in vivo rabbit model were consistent with localized pyogenic osteomyelitis, and biofilm was present on all explanted IM nails. The animals displayed no outward signs of infection, such as swelling, lameness, weight loss, or elevated white cell count. 

In contrast, infection with coagulase-negative staphylococci resulted in histological features consistent with both pyogenic osteomyelitis and septic arthritis, and all coagulase-negative staphylococci animals displayed weight loss and an elevated white cell count despite biofilm detection in only two out six rabbits.

The differences in the histological and bacteriological profiles of the two species in this rabbit model of infection are reflective of their different clinical presentations; low-grade infection in the case of Propionibacterium and acute infection for coagulase-negative staphylococci. 

These results are especially important in relation to the growing recognition of chronic Propionibacterium  biofilm infections in prosthetic joint failure and non-union of fracture fixations, which may be currently reported as ‘aseptic’ failure.

The authors conclude that: Propionibacterium has the potential to cause prosthetic joint infection and fracture non-union, in the absence of signs of classical infection and patient morbidity. Propionibacterium should therefore no longer be dismissed as an insignificant pathogen in the setting of failed retrieved implants; clinical diagnostic practice should be tailored to enable the efficient detection of P. acnes. Without this there is the risk of  an incorrect diagnosis of aseptic loosening and subsequent patient treatment may be misinformed. If a failed prosthetic joint with a mis-diagnosis of aseptic loosening is removed, and a new sterile device placed in the underlying infected site it is very possible that higher numbers of bacteria will be present, particularly if adjacent bone has been colonized.

Comment: This animal model of Propionibacterium infection nicely replicates many of the features encountered in revision arthroplasty, where the preoperative impression of 'aseptic' failure is disproven by positive cultures for Propionibacterium.

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