Improved identification including MALDI-TOF mass spectrometry analysis of group D streptococci from bovine mastitis and subsequent molecular characterization of corresponding Enterococcus faecalis and Enterococcus faecium isolates

Abstract

We examined 199 group D streptococci isolated from clinically defined and epidemiologically unrelated cases of bovine mastitis. Samples were collected during a 5-month period from 2010 to 2011 from diseased animals in 199 herds (1 isolate per herd) raised in different counties and federal states in Germany. A classical enterococcal species identification procedure started with PYRase and catalase assays, growth on Enterococcoselagar(®) and GCG(®) agar plates and in 6.5% NaCl followed by a biochemical reaction panel. All 199 isolates were also subjected to MALDI-TOF MS diagnostics in which a simple and an extended direct transfer protocol were compared. The latter revealed a much better performance (higher log (score) values) although the same result was obtained in all but three cases. Classical and MALDI TOF MS analyses identified 64 Enterococcus faecalis and 37 Enterococcus faecium isolates which were confirmed by species-specific PCRs. These 101 enterococcal isolates did not display a specific multi-resistance phenotype and resistances to glycopeptides and antibiotics of last resort (linezolid, daptomycin, tigecycline) were absent, resistance to tetracycline was the most frequent resistance feature. Molecular typing of the 64 E. faecalis isolates revealed 3 main PFGE clusters of related strains represented by three MLST types (ST40, ST211, ST268). PFGE and MLST analysis of E. faecium isolates revealed several smaller clusters of only a few related strains and identified a number of previously unknown allele and MLST types (n=6; ST624-ST629) besides known variants (ST22, ST32). One of the 37 E. faecium strains showed properties of hospital-associated E. faecium strains (ampicillin resistance, IS16-positive; MLST CC17).