Enterococci are ubiquitous in the environment; they can grow in the intestinal tracts of humans and animals. They cause numerous infections, including urinary tract infections and endocarditis, as well as various nosocomial infections. In order to curb the spread of such diseases, it is important to identify enterococci at the species level in diagnostic laboratories (Vu; Kirschner). Further, with the recent increase of clinical cases in which less common species, such as E. durans, E. hirae, E. gallinarum and E. casseliflavus, are implicated, rapid and accurate identification at the species or subspecies level is necessary for the control of infections and the study of epidemiology (Willey et al. 1999; Kirschner et al. 2001).
Similar results were observed in previous studies, in which MALDI-TOF MS correctly identified all clinical isolates tested, including E. faecalis, E. faecium, E. casselifalvus, and E. gallinarum. In contrast, VITEK 2 misidentified 10 of 131 isolates (Fang et al.2012). Such misidentification may reflect unreliable identification of uncommon Enterococcus species, or the occurance of atypical phenotypic properties (Singer et al. 1996; Tsakris et al. 1998). This suggests that the VITEK 2 database needs further improvement to increase accuracy of Enterococcus species identification. Quintela-Baluja et al. (2013) revealed that enterococci can have different mass spectral profiles, depending on whether they originate from dairy or meat (Quintela‐Baluja et al. 2013). Therefore, to ensure accurate species identification, enterococci originating from animal-derived sources should be evaluated by MALDI-TOF MS. This study is the first to assess the discriminative power of MALDI-TOF MS in relation to common and uncommon Enterococcus species from dairy samples, including E. durans, E. hirae, and E. gallinarum. Although the source of the isolates in our study was dairy samples, our results indicate that MALDI-TOF MS could be a useful alternative diagnostic tool for screening enterococci species not only from foods, but also in etiological clinical applications which require fast and reliable species- or strain-level identification.
Although each method differentiates based on different characteristics (i.e., protein spectrum versus biochemical patterns) (Sala-Comorera et al. 2016), the phylogenetic trees obtained from the two systems placed the isolates in identical positions (Fig. 1). In accordance with our results, Sala-Comorer et al. (2016) demonstrated that dendrograms obtained from MALDI-TOF MS showed taxonomic resolution similar to that of the PhenePlate™ system, a biochemical phenotyping method (Sala-Comorer et al. 2016). These results suggest that although both methods display similar levels of resolution in discriminating the isolates, MALDI-TOF MS is a much more accurate method of species identification than VITEK 2.
In the present study, we compared two bacterial diagnostic techniques which are currently used in laboratories, VITEK 2 and MALDI-TOF MS. The latter has been used as an alternative tool for identification and phylogenic study of enterococci. Our evaluation of the accuracy of each system in identifying species among Enterococcus isolates showed MALDI-TOF MS to be more accurate than VITEK 2. Whereas MALDI-TOF MS misidentified only a single E. durans isolate, VITEK 2 showed less discriminative capability regarding both common (E. faecium) and uncommon species (E. durans and E. hirae). These results imply that MALDI-TOF MS can be used as fast and reliable tool for bacterial identification, and for analyzing phylogenetic relationships within genera.
In addition to its high level of accuracy, MALDI-TOF MS was the most time- and cost-effective method among various other automated biochemical identification methods, such as API and the VITEK system (Seng et al. 2009). A previous study similarly showed that MALDI-TOF MS is more efficient than VITEK 2 in identifying Enterococcus species other than faecalis and faecium (Fang et al. 2012). Because the results of MALDI-TOF MS are based on a software-installed database of mass spectral profiles, this method requires more extensive spectrum data than other systems. However, its reliability is superior; 38 out of 39 Enterococcus isolates in this study were accurately identified.
In conclusion, the results of this study indicate that MALDI-TOF MS can be used for routine identification of both usual and unusual enterococci from dairy products, as it shows greater species-level discriminatory power than VITEK 2. Further, MALDI-TOF MS is cost-effective and requires less time to identify the bacteria, making it a useful technique for diagnostic laboratories which need rapid identification of bacterial species. However, one limitation to this study is that we only tested for species in the Enterococcus genus, and have not tested the sensitivity of MALDI-TOF MS to species of other genera. Further study is needed which incorporates numerous species from various genera in order to further evaluate the discriminative capacity of MALDI-TOF MS.