Commercial identification systems often fail to identify Providencia stuartii.

We tested 145 clinical isolates in an attempt to evaluate some of the most widely used commercial identification systems in Europe in terms of their ability to identify Providencia strains. Two manual miniaturized systems (API 20E and Enterotube II) and three mechanized-automated systems (Cobas-Bact, Sceptor System, and Titertek-Enterobac-Rapid Automated System) were evaluated. Providencia alcalifaciens and Providencia rettgeri strains were correctly identified by all systems in all cases, and in most cases identification was achieved without the aid of supplementary tube tests. By contrast, Providencia stuartii was identified without the aid of supplementary tube tests for only 42.5% (API 20E), 37.5% (Enterotube), 68.7% (Sceptor), and 71.2% (Cobas-Bact) of the isolates. The overall misidentification rates were 16.3, 11.3, 11.3, and 10%, respectively. The Titertek-Enterobac-Rapid Automated System failed to identify only 1 of 80 strains (1.3%) and required supplementary tests in 2 other cases (2.5%). Since four of the multitest systems examined often failed to correctly identify P. stuartii, we conclude that supplementary conventional tube tests should always be used to distinguish this species from the other taxa of the Proteeae tribe.