Reliability of thermonuclease production for the identification of human and animal Staphylococcus aureus

A total of 314 clinical and non-clinical isolates of the genus Staphylococcus was tested for coagulase production and glucose and mannitol fermentation. The isolates were tested for thermonuclease production and agglutination by sera 17H and 61218, which were specific for human and canine S. aureus biotypes, respectively. All produced coagulase and fermented glucose. A majority fermented mannitol anaerobically except for the canine isolates. A majority of human isolates produced thermonuclease (64.3%) and most were agglutinated by serum 17H. There was good correlation between thermonuclease production and agglutination by serum 17H of human and bovine clinical isolates (86.6 and 80%, respectively). This was also true of clinical canine isolates agglutinated by serum 61218, of which 75% were thermonuclease-positive. Over half of canine isolates (52.8%) were thermonuclease-positive and most were agglutinated by serum 61218. Bovine and caprine isolates were 34.1 and 25% thermonuclease-positive, respectively, while ovine isolates were only 14.2% thermonuclease-positive. Isolates from these ruminant sources were also poorly agglutinated by either serum. It was concluded that a greater number of clinical human and canine biotypes of S. aureus produced thermonuclease than their non-clinical isolates, and that a majority of other animal isolates were negative for thermonuclease. Therefore, the thermonuclease test may not be very useful for confirming the animal origin of S. aureus isolates.