Prevalence and Identification of Lactobacillus Species Isolated from Infected Root Canals by MALDI-TOF Mass Spectrometry, 16S rRNA Gene Sequencing and API 50 CHL

Lactobacillus, a part of the commensal oral microflora, is frequently found in infected root canals but is not considered to be an endodontic pathogen. Lactobacilli have proven to be difficult to identify on species level with biochemical and gene sequencing methods. MALDI-TOF is a new identification method and to our knowledge it has not been used on lactobacilli from infected root canals. The aims of this study were to evaluate the prevalence of lactobacilli in infected root canals and to examine if MALDI-TOF is a suitable method for identifying lactobacilli species. In the retrospective study, we evaluated 449 microbial samples obtained from 361 patients. In the prospective study, 100 consecutive microbial samples were collected from 93 patients with infected root canals. Twelve clinical isolates from eight patients were obtained and six selected reference strains were included in the study. MALDI-TOF, 16S rRNA gene sequencing and API 50 CHL identification methods were used to identify lactobacilli isolates and reference strains on species level. In conclusion, the prevalence of lactobacilli in infected root canals was 22% in our material. Molars were the most frequent tooth group infected with lactobacilli. For identification of the reference strains, MALDI-TOF performed slightly better than the other methods. The identification of clinical isolates was inconclusive. MALDI-TOF is an inexpensive, simple and rapid method for identification of lactobacilli and performs well in comparison with conventional methods. However, all of the three identification methods used in this study have limitations when differentiating between closely related lactobacilli species.