Understanding bovine mammary gland niches for staphylococci using different milk sampling techniques

Abstract

Mastitis is usually caused by a bacterial intramammary infection (IMI) with Staphylococcus being the most commonly isolated genus. The standard method for diagnosing an IMI is to aseptically prepare the teat end and collect milk via the teat orifice into a sterile vial for bacterial culture. While this method has been advocated for decades, there has been debate about the importance of some bacteria isolated from such samples when the milk somatic cell count (SCC) in the mammary quarter is < 200,000 cells/ml (threshold for a healthy mammary gland). Recently, Hiitio et al. (2016) showed that milk samples collected directly from the gland cistern were less likely to contain staphylococcal DNA when using a commercial PCR diagnostic test kit than milk samples that were collected via the teat orifice. Unfortunately, the PCR only differentiated staphylococci into Staphylococcus spp. and Staphylococcus aureus, and so it is unknown which of the nonaureus staphylococcal (NAS) species may be more likely to inhabit the teat canal than those that cause a true IMI. These data provide preliminary evidence that some staphylococci in milk may be teat canal inhabitants and not intramammary pathogens that cause mastitis. The objective of this study was to determine if some species of staphylococci isolated from milk are only isolated when the sample is collected through the teat orifice and absent in cisternal milk collected using the method described by Hiitio et al. (2016).