Saturday, September 17, 2016

Infections with Propionibacterium - the necessity of a surgical implant

Delayed Propionibacterium acnes surgical site infections occur only in the presence of an implant.

Whether Propionibacterium acnes (P. acnes) causes surgical-site infections (SSI) after orthopedic surgery is controversial. The authors previously reported* that they frequently find P. acnes in intraoperative specimens, yet none of the patients have clinically apparent infections. In this study they tracked P. acnes for 6 months in a mouse osteomyelitis model. They inoculated P. acnes with a 0.5 Å~ 8-mm titanium alloy bar was inserted into the mouse femur along with an inoculation of P. acnes (1 Å~ 106 CFU in 1 μ l medium) in the implant group; the control group was treated with the bacteria but no implant. They then observed over a 6-month period using optical imaging system. During the first 2 weeks, bacterial signals were detected in the femur in the both groups. The bacterial signal completely disappeared in the control group within 28 days. Interestingly, in the implant group, bacterial signals were still present 6 months after inoculation. Histological and scanning electron-microscope analyses confirmed that P. acnes was absent from the control group 6 months after inoculation, but in the implant group, the bacteria had survived in a biofilm around the implant. PCR analysis also identified P. acnes in the purulent effusion from the infected femurs in the implant group.

Interestingly, Propionibacterium showed significantly higher bacterial adhesion to titanium alloy than to other metals used in surgical implants as shown below.

The authors were able to use a bacterial probe to detect the presence of Propionibacterium over time in the implant and non implant groups:

Infected implants were surrounded by areas of bone resorption as is frequently seen in clinical periprosthetic infections:

Their images show Propionibacterium in the biofilm on the surface of the implant

Analysis of the time course in vivo showed that the inoculated P. acnes could not survive in the
femur for > 28 days without an implant. Interestingly, they found that P. acnes was able to survive for > 6 months when associated with a titanium alloy implant, suggesting that the presence of the implant was essential for the survival of the bacteria and the development of the infection. 

Comment: This is a remarkable study that helps clarify many of the observations that have been made on periprosthetic infections of the shoulder with Propionibacterium. 

*Sterility of posterior elements of the spine in posterior correction surgery.
A total of 80 consecutive patients (mean age = 19.5 years) who underwent posterior correction surgery for spinal deformities were included in the study. During surgery, specimens for bacterial culture were obtained by swabs from (1) the skin after cleansing using povidone-iodine scrub solution but before draping, (2) laminae immediately after exposure, (3) laminae immediately after screw placement, (4) laminae immediately before wound closure, (5) a small piece of bone obtained from the spinous process immediately after exposure and placed on a sterile dish during surgery as a control, kept covered, and (6) a similar sample as (5), kept uncovered. The culture was conducted on both blood agar and Gifu anaerobic medium agar plates.

No patient developed surgical-site infection. The culture was positive in specimen (S) 1 in 25 patients (31.3%), S2 in 20 (25%), S3 in 25 (31.3%), S4 in 26 (32.5%), S5 in 6 (7.5%), and S6 in 7 (8.8%). Bacterial species included Propionibacterium acnes in 15 specimens, Propionibacterium species in 9, and others in 10. Of the 25 patients with a positive culture in S1 and 55 patients without, 22 (88%) and 26 (47.3%), respectively, demonstrated a positive culture in specimens obtained during surgery.

The culture was more frequently positive in specimens obtained immediately before wound closure than in those harvested immediately after exposure and isolated from the surgical field. The results suggest that bacterial contamination of the operating field may originate from the skin of the patient during surgery.