Impact of Coagulase-Negative Staphylococci in Mixed Intramammary Infections with Streptococci on Milk Quality

This study investigated the relationship between milk composition among different mastitis pathogens. The study was conducted from January to February 2025 on three different dairy farms in Cambodia, utilizing 241 crossbred Holstein-Friesian milking cows. Quarter milk samples were aseptically collected from randomly selected milking cows, mostly in mid- and late lactation, including 450 samples from 3, 25, and 213 cows and 11, 88, and 351 quarters from Farms A, B, and C, respectively. The milk samples were analyzed for bacterial identification, somatic cell count, and milk composition. Mastitis pathogens were grouped primarily based on their genus, including streptococci, coagulase-negative staphylococci (CNS), Gram-negative bacteria (Gram-negative), CNS plus streptococci (Mixed), and those with no detected bacteria (None). Pearson's correlation coefficients were used to determine the correlations between milk composition and somatic cell scores (SCSs). The comparison of milk composition and SCS means across bacteria groups, farms, and intramammary infection (IMI) status was performed using repeated linear models. Significance was defined as <i>p</i> < 0.05, and a tendency was defined as <i>p</i> < 0.10. The results show that the collected quarter milk had a low fat concentration. Milk composition and SCS were associated with the farm (<i>p</i> < 0.05). Regarding bacterial groups, the SCS of milk with streptococci was higher than that of milk with CNS and non-IMI milk (<i>p</i> < 0.05). Milk with a mixed IMI of streptococci and CNS had significantly lower lactose and solid-not-fat (SNF) levels than milk with CNS or Gram-negative bacteria. In contrast, the freezing point of the mixed IMI was higher than that of CNS and non-IMI milk (<i>p</i> < 0.05). In conclusion, the compositions of milk from CNS IMI did not deviate from those of milk from healthy quarters; however, the mixed infection of CNS and streptococci caused lower milk quality. These findings underscore the importance of CNS IMI, which can lead to significant economic losses in the case of infection with other pathogens. However, as this study employed a cross-sectional design, which is suitable for identifying associations rather than establishing causation, further studies are needed.