<p class="p p-first" id="idm140479872088432"><strong>Introduction:</strong> Histopathological definition of bone and joint infection (BJI) is based on Mirra's criterion (≥ 5 polymorphonuclears (PMNs) per field in 5 high power fields (HPFs)). However, this definition does not seem appropriate for chronic BJIs caused by slow-growing germs such as <em>Cutibacterium acnes</em> (<em>C. acnes</em>). The aim of this study was to confirm that Mirra's criterion is not adequate for diagnosis of BJIs due to <em>C. acnes</em>. The second objective was to determine if plasma cell infiltration could be useful for the diagnosis of chronic BJIs due to <em>C. acnes</em>.</p><p id="idm140479872084848"><strong>Methods:</strong> We retrospectively selected 25 consecutive patients from 2009 to 2013 with chronic BJIs due to <em>C. acnes</em>. Histological analysis was performed on the 21 cases with at least two <em>C. acnes</em> positive cultures. In addition of Mirra's criterion, the number of plasma cells (≥5 plasma cells/5 HPFs, defined as “CRIOAc Lyon's criterion”) was implemented in the histopathological analysis. Patients were defined as infected, if at least one of the two criteria were present.</p><p id="idm140479872082656"><strong>Results:</strong> According to Mirra's and CRIOAc Lyon's criteria, positive histopathology was observed in 12 (57.1%) and 15 (71.4%) cases respectively. Considering the 9 cases with negative Mirra's criterion, high plasma cell infiltration (≥5 plasma cells per field/5 HPFs) was observed in 5 cases (55.6%), and low plasma cells infiltration (2-5 plasma cells per field/5 HPFs) was observed in 4 other cases (44.4%).</p><p class="p p-last" id="idm140479872081728"><strong>Conclusions:</strong> Adding CRIOAc Lyon's criterion to Mirra's criterion might restore some histopathological diagnosis of chronic BJIs due to <em>C. acnes</em> when a chronic BJI is clinically suspected.</p>