Immune evasion by Staphylococcus haemolyticus: Attenuated NET-associated responses and resistance to NET-mediated killing.

Neutrophil extracellular traps (NETs) contribute to innate immune defense by trapping and exposing microorganisms to DNA-associated antimicrobial proteins. However, some pathogens can tolerate NET-associated antimicrobial activity and persist within the host. Staphylococcus haemolyticus is an emerging multidrug-resistant coagulase-negative staphylococcal pathogen associated with healthcare-related infections, but its interaction with neutrophil-mediated defenses remains poorly understood. This study investigated whether a multidrug-resistant clinical S. haemolyticus isolate modulates neutrophil antimicrobial activity and NET-associated responses, using a clinical Staphylococcus aureus isolate as a comparator. Extracellular DNA release and NET-associated structures were assessed by fluorescence microscopy, scanning electron microscopy, fluorometric DNA quantification, microscopic enumeration, and MPO-DNA complex ELISA. The tested S. haemolyticus isolate induced extracellular DNA release and MPO-DNA complex formation; however, these responses were lower than those induced by PMA and were attenuated compared with the S. aureus comparator. S. haemolyticus also showed reduced susceptibility to neutrophil-mediated killing and greater resistance to NET-mediated killing than S. aureus. NE and MPO mRNA expression was not significantly altered following bacterial exposure. Collectively, these findings provide initial evidence that a clinical S. haemolyticus isolate can induce attenuated NET-associated responses while resisting NET-mediated antimicrobial activity, highlighting a potential immune-evasion mechanism in this emerging opportunistic pathogen.