MICROBIOLOGICAL SPECTRUM EVOLUTION IN PROSTHETIC JOINT INFECTIONS BETWEEN 2003 AND 2008, AND 2009 AND 2013: RESULTS OF A SINGLE INSTITUTION IN PORTUGAL

We aim to describe the microbiological spectrum and relevant antibiotic susceptibility profile of PJI in our institution over a five-year period(2009–2013) and determine its evolution considering the preceding six years(2003–2008) thus evaluating the adequacy of our empirical antibiotic regimen. We retrospectively reviewed the records of 96 consecutive PJI (51 hips:45 knees) treated from May 2009-December 2013. Demographics, microbial species and antibiotic susceptibility were recorded. These results were then compared to those previously obtained by studying the 2003–2008 time period. Infections were polymicrobial in 27 cases(28.1%) and only two cases(2.1%) were culture-negative accounting for a total of 132 different culture results. S.aureus grew in 37 samples(28.0%) being the most frequently isolated microorganism. Coagulase-negative staphylococci grew in 32 samples(24.2%) and gram negative bacteria in 35 samples(26.5%). Other Gram positive species (most commonly enterococci and streptococci) were isolated in 26 samples(19.7%). Comparing 2009–2013 to 2003–2008, there was a significant increase of polymicrobial infections – 28% vs. 8%(OR=4.6, 95%CI [1.9–11.3]) and a significant decrease of culture-negative cases – 2% vs. 18%(OR=0.1, 95%CI [0.02–0.4]). It is also noteworthy that the prevalence of gram negative isolates was significantly increased – 26.5% vs. 13.3%(OR=1.3, 95%CI [1.1–1.6]). Antibiotic susceptibilities study showed a 41.4% methicillin resistance among S.aureus and even higher among coagulase-negative staphylococci isolates(57.7%). This is a not quite significant decrease compared to the earlier period(p=0.10). We also found a high rate antibiotic resistance among gram negative: ampiciline(81.8%), amoxicilin/clavulanate(59.1%), ciprofloxacin(19.2%), aminoglycosides(17%), third generation cephalosporins(14.6%) and even carbapenems(13.6%). These results show that our sampling protocol has improved considerably as the proportion of culture-negative cases has dramatically decreased. On the other hand this may also help explain the increase in polymicrobial infections. We have no clear explanation for the increase in gram negative bacteria. Despite the downward trend we still face a very significant proportion of methicillin-resistant staphylococci infections. The antibiotic resistance profile among gram negative bacteria is also worrying. As such we believe a regimen consisting of vancomycin and gram-negative coverage such as aminoglycosides or a third generation cephalosporin is still warranted in our institution.