These authors investigated the susceptibility of 55 clinical isolates of P. acnes, obtained from orthopaedic implant-associated infections of the knee joint (n = 5), hip joint (n = 17), and shoulder joint (n = 33), to eight antimicrobial agents: benzylpenicillin, clindamycin, metronidazole, fusidic acid, doxycycline, moxifloxacin, linezolid and rifampicin. Synergy testing was also conducted, in which rifampicin was combined with each of the remaining seven antibiotics.
Phylogenetic typing based on tly sequence analysis showed that 31 isolates were type IA, 13 isolates type IB, and 11 isolates type II.
All isolates (n = 55) were susceptible to most of the antibiotics tested, with the exception of 100% resistance to metronidazole, five (9.1%) isolates displaying decreased susceptibility to clindamycin, and one (1.8%) to moxifloxacin.
None of the antimicrobial agents investigated were synergistic with each other when combined and nine isolates were antagonistic for various antimicrobial combinations. The majority of the antimicrobial combinations had an indifferent effect on the isolates of P. acnes. However, the combination of rifampicin and benzylpenicillin showed an additive effect on nearly half of the isolates.
Comment: This article is very interesting from a number of standpoints:
(1) 22 of the 55 isolates came from hip and knee infections - Propionibacterium is not just a shoulder issue
(2) Clindamycin is not universally effective against Propionibacterium. This is important in that many surgeons use this antibiotic for prophylaxis.
(3) Rifampin appears to be a useful adjunct against Propionibacterium.
Here is another important presentation regarding antibiotics and Propionibacterium. It makes the important observation that a significant percentage of the bacteria isolated from acne patients are now resistant to the most common antibiotics used in acne treatment: Clindamycin, Erythromycin, Tetracycline, Doxycycline and Minocycline. We can only assume that the same is true for the Propionibacterium found in failed arthroplasty.
As our infectious disease experts point out, we seeing two worrisome trends: (1) there are fewer antibiotics available to us for the prevention and management of infections and (2) the bacteria are becoming increasingly resistant to the antibiotics that we do have.
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