Impacts of subclinical mastitis on milk quality, clotting ability and microbial resistance of the causative Staphylococci
Bovine mastitis is one of the most problematic diseases and continues to be a leading cause of heavy economic losses in the dairy industry and a public health hazard globally. To understand the characteristics of subclinical mastitis (SCM) in lactating cows and their associated effects on milk quality, protein composition, and milk clotting ability, 240 quarter-milk samples were collected and tested by California Mastitis Test (CMT). Milk composition was analyzed using LactoScope FT-A and separation of protein fractions was performed by PAGE-SDS electrophoresis. We also measured the time from rennet addition to milk gelation (RCT) as a traditional milk coagulation trait. Samples with SCM were analyzed bacteriologically, and Staphylococci isolates were tested for antibiotic susceptibility. Higher values of conductivity and pH were recorded from CMT-positive milk samples. Overall, 50/240 (20.83%) quarters suffered from SCM, whose 64% (32/50) infected with Staphylococci. On the 36 tested Staphylococci, resistance to penicillin and erythromycin represented 83.3%, and 61.1% respectively. Resistance to cefoxitin was linked to three isolates while 77.7% were multi-drug resistant, but in proportion that differ between S. aureus (88.8%) and non-aureus Staphylococci (74.1%). Physics-chemical analysis indicated that, quarters with SCM had lower milk-fat content and mineral content compared with quarters without SCM. The profiles of total proteins electrophoresis revealed degradation of casein fractions in milk with SCM. Milk samples subclinically infected with Staphylococci exhibited longer coagulation time (1093.9±781.9 seconds) and weaker clotting activity (2.55±1.49 RU) than milk samples collected from healthy quarters which showed 325.3±177.5 seconds and 7.80 ± 4.46 RU. The increase in conductivity due to intramammary infection, was highly associated with an elongation in RCT. Moreover, clotting activity was inversely proportional to conductivity. Due to its impacts on milk composition, proteins integrity and clotting ability, SCM still a major concern in dairy industry which needs efficient measures to control their occurrence in dairy herds.