Enterococcus faecalis can be distinguished from Enterococcus faecium via differential susceptibility to antibiotics and growth and fermentation characteristics on mannitol salt agar

Enterococcus faecalis and Enterococcus faecium are both human intestinal colonizers frequently used in medical bacteriology teaching laboratories in order to train students in bacterial identification. In addition, hospitals within the United States and around the world commonly isolate these bacteria because they are a cause of bacteremia, urinary tract infections, endocarditis, wound infections, and nosocomial infections. Given that enterococci are becoming more of a world health hazard, it is important for laboratories to be able to distinguish these bacteria within hospitalized patients from other bacterial genera. In addition, laboratories must differentiate different species within the Enterococcus genus as well as different strains within each species. Though enterococci are differentiated from other bacterial genera via classical culture and biochemical methods, nucleic acid sequencing is required to differentiate species within the genus--a costly, time consuming, and technically challenging procedure for laboratory technicians that, in itself, does not necessarily lead to speedy identification of bactericidal antibiotics. In this study, we perform antibiogram analysis to show （1） that penicillin can be rapidly employed to distinguish strains and clinical isolates of E. faecalis from E. faecium, （2） E. faecalis is susceptible to ampicillin, and （3） that vancomycin resistance in enterococci shows no sign of abating. Additionally, we show that E. faecalis can grow on mannitol salt agar and ferment mannitol, while E. faecium lacks these phenotypes. These data reveal that we now have rapid, cost effective ways to identify enterococci to the species, and not just genus, level and have significance for patient treatment in hospitals.