*2.1.2 Coagulase-negative Staphylococcus species (CNS)*

More recently, coagulase-negative *Staphylococcus* species (CNS) such as *S. chromogenes, S. simulans, S. xylosus, S. haemolyticus, S. hyicus*, and *S. epidermidis* are increasingly isolated from bovine milk [7, 25–27] with *S. chromogenes* being the most increasingly diagnosed species as a cause of subclinical mastitis. *Staphylococcus chromogenes* [28] and other CNS [4, 8] have been shown to cause subclinical infections in dairy cows that reduce the prevalence of contagious mastitis pathogens.

*Staphylococcus chromogenes* is most commonly isolated from mammary secretions rather than from the environment itself [8, 29]. *S. chromogenes* consistently isolated from the cow's udder and teat skin [30], and some studies showed that it causes long-lasting, persistent subclinical infections [26]. The CNS causes high somatic cell counts in milk on some dairy farms [29, 31]. Woodward et al. [32] evaluated the normal teat skin flora and found that 25% of the isolates exhibited the ability to prevent the growth of some mastitis pathogens. An *in vitro* study conducted on *S. chromogenes* showed that this organism could inhibit the growth of major mastitiscausing pathogens such as *Staph. aureus*, *Strep. dysgalactiae*, and *Strep. uberis* [28]. In a study conducted on conventional and organic Canadian dairy farms, CNS were found in 20% of the clinical samples [33]. Recently, mastitis caused by CNS increasingly became more problematic in dairy herds [30, 34–36]. However, mastitis caused by CNS is less severe compared to mastitis caused by *Staphylococcus aureus* [26].
