Non-culture-based identification of mastitis-causing bacteria by MALDI-TOF mass spectrometry

The purpose of this study was to evaluate the detection limit of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) for direct identification, without previous microbiological culture, of bovine mastitis-causing bacteria from milk samples. Milk samples (n = 15) were experimentally contaminated with Staphylococcus aureus, Streptococcus uberis, Streptococcus agalactiae, Streptococcus dysgalactiae, and Escherichia coli to have bacterial counts ranging from 103 to 109 cfu/mL. These contaminated milk samples were subjected to a preparation protocol for bacterial ribosomal protein extraction using the MALDI Sepsityper kit (Bruker Daltonik, Bremen, Germany), which allowed MALDI-TOF MS coupled with Biotyper software (Bruker Daltonik) to identify bacterial fingerprints based on intact ribosomal proteins. The ability of MALDI-TOF MS to correctly identify bacterial strains from experimentally contaminated milk (without previous microbiological culture) depended on the bacterial count of the samples and on the species of the bacteria evaluated. Adequate identification at the bacterial species level (score ≥2.0) directly from milk samples required bacterial counts in the following ranges: ≥106 cfu/mL of Staph. aureus, ≥107 cfu/mL of E. coli, and ≥108 cfu/mL of Strep. agalactiae, Strep. dysgalactiae, and Strep. uberis. We concluded that direct identification of mastitis-causing pathogens is possible for Staph. aureus, E. coli, Strep. agalactiae, Strep. dysgalactiae, and Strep. uberis, but correct identification depended on the bacterial count in the milk samples.