Discipline: mastitis; Keywords: coagulase negative staphylococci, Maldi Tof, CNS strains, routine diagnostic identification, somatic cell count.
The 2018 Progress Report of a study by Dr Inge-Marie Petzer of the Faculty of Veterinary Science of the University of Pretoria.
Three hundred coagulase negative staphylococci (Non aureus staphylococci) (CNS / NAS) were identified using the Maldi Tof (Bruker) method. These isolates were used to upgrade the Maldi Tof library. In addition, 200 of these isolates that showed good identification (>1.90) with the Maldi Tof (HPLC based) were further identified with a biochemical method, the Staph API (Biomerieux), which was thought to be a more accessible method for routine identification in laboratories. The results of these two methods did not compare well. An overall correlation of just 0.29 was found. In some CNS strains (S. epidermidis) the correlation was higher (0.64) but this differed between strains. A third method, 16s identification sequencing was explored to identify the isolates. One hundred and seventy CNS/NAS isolates were sequenced and the results were compared to those from the Maldi Tof.
The most common strains of CNS/NAS isolated from 20 SA dairy herds were Staphylococcus chromogenes, Staphylococcus epidermidis and Staphylococcus haemolyticus. The correlation between the results obtained using the Maldi Tof and 16s sequencing was satisfactory for CNS/NAS. It was thus decided to use the Maldi Tof method in future for routine diagnostic identification of CNS/NAS when necessary, as it is much more cost effective.
We also investigated the probable effect of parity and days in milk (DIM) on the infection rate of the different CNS/NAS isolates. The isolation of the numbers of all CNS/NAS strains increased as lactation progressed.
The results were evaluated to identify a probable correlation between the CNS/NAS strains and somatic cell count (SCC) levels. Out of the three most prominent isolates, S. epidermidis had the highest percentage of isolates with SCC in excess of 1 million cells per ml milk followed by S. haemolyticus and the most commonly isolated CNS/NAS overall was S. chromogenes.