Molecular detection of Streptococcus agalactiae in bovine raw milk samples obtained directly from bulk tanks

Mastitis is the most common infectious disease affecting dairy cattle; in addition, it remains the most economically important disease of dairy industries around the world. Streptococcus agalactiae, a contagious pathogen associated with subclinical mastitis, is highly infectious. This bacterium can cause an increase in bulk tank bacterial counts (BTBC) and bulk tank somatic cell counts (BTSCC). The microbiological identification of S. agalactiae in samples from bulk tanks is an auxiliary method to control contagious mastitis. Thus, there are some limitations for time-consuming cultures or identification methods and additional concerns about the conservation and transport of samples. Bulk tank samples from 247 dairy farms were cultured and compared through polymerase chain reaction (PCR), directed to 16S rRNA genes of S. agalactiae, followed by BTBC and S. agalactiae isolation. The mean value of BTBC was 1.08 × 106 CFU mL−1 and the bacterium was identified through the microbiological method in 98 (39.7%; CI95% = 33.8–45.9%) and through PCR in 110 (44.5%; CI95% = 38.5–50.8%) samples. Results indicated sensitivity of 0.8571 ± 0.0353 (CI95% = 0.7719–0.9196) and specificity of 0.8255 ± 0.0311 (CI95% = 0.7549–0.8827). The lack of significant difference between microbiological and molecular results (κ = 0.6686 ± 0.0477 and CI95% = 0.5752–0.7620) indicated substantial agreement between the methods. This suggests that PCR can be used for bulk tank samples to detect contagious mastitis caused by S. agalactiae.