Staphylococcus aureus has been an important human pathogen throughout history and is currently a leading cause of bacterial infections worldwide. S. aureus has the unique ability to cause a continuum of diseases, ranging from minor skin infections to fatal necrotizing pneumonia. Moreover, the emergence of highly virulent, drug-resistant strains such as methicillin-resistant S. aureus in both healthcare and community settings is a major therapeutic concern. Neutrophils are the most prominent cellular component of the innate immune system and provide an essential primary defense against bacterial pathogens such as S. aureus. Neutrophils are rapidly recruited to sites of infection where they bind and ingest invading S. aureus, and this process triggers potent oxidative and non-oxidative antimicrobial killing mechanisms that serve to limit pathogen survival and dissemination. S. aureus has evolved numerous mechanisms to evade host defense strategies employed by neutrophils, including the ability to modulate normal neutrophil turnover, a process critical to the resolution of acute inflammation. Here we provide an overview of the role of neutrophils in host defense against bacterial pathogens and discuss strategies employed by S. aureus to circumvent neutrophil function.

Staphylococcus epidermidis is a leading nosocomial pathogen. In contrast to its more aggressive relative S. aureus, it causes chronic rather than acute infections. In highly virulent S. aureus, phenol-soluble modulins (PSMs) contribute significantly to immune evasion and aggressive virulence by their strong ability to lyse human neutrophils. Members of the PSM family are also produced by S. epidermidis, but their role in immune evasion is not known. Notably, strong cytolytic capacity of S. epidermidis PSMs would be at odds with the notion that S. epidermidis is a less aggressive pathogen than S. aureus, prompting us to examine the biological activities of S. epidermidis PSMs. Surprisingly, we found that S. epidermidis has the capacity to produce PSMδ, a potent leukocyte toxin, representing the first potent cytolysin to be identified in that pathogen. However, production of strongly cytolytic PSMs was low in S. epidermidis, explaining its low cytolytic potency. Interestingly, the different approaches of S. epidermidis and S. aureus to causing human disease are thus reflected by the adaptation of biological activities within one family of virulence determinants, the PSMs. Nevertheless, S. epidermidis has the capacity to evade neutrophil killing, a phenomenon we found is partly mediated by resistance mechanisms to antimicrobial peptides (AMPs), including the protease SepA, which degrades AMPs, and the AMP sensor/resistance regulator, Aps (GraRS). These findings establish a significant function of SepA and Aps in S. epidermidis immune evasion and explain in part why S. epidermidis may evade elimination by innate host defense despite the lack of cytolytic toxin expression. Our study shows that the strategy of S. epidermidis to evade elimination by human neutrophils is characterized by a passive defense approach and provides molecular evidence to support the notion that S. epidermidis is a less aggressive pathogen than S. aureus.

The yeast mitochondrion is shown to contain a pool of copper that is distinct from that associated with the two known mitochondrial cuproenzymes, superoxide dismutase (Sod1) and cytochrome c oxidase (CcO) and the copper-binding CcO assembly proteins Cox11, Cox17, and Sco1. Only a small fraction of mitochondrial copper is associated with these cuproproteins. The bulk of the remainder is localized within the matrix as a soluble, anionic, low molecular weight complex. The identity of the matrix copper ligand is unknown, but the bulk of the matrix copper fraction is not protein-bound. The mitochondrial copper pool is dynamic, responding to changes in the cytosolic copper level. The addition of copper salts to the growth medium leads to an increase in mitochondrial copper, yet the expansion of this matrix pool does not induce any respiration defects. The matrix copper pool is accessible to a heterologous cuproenzyme. Co-localization of human Sod1 and the metallochaperone CCS within the mitochondrial matrix results in suppression of growth defects of sod2⌬ cells. However, in the absence of CCS within the matrix, the activation of human Sod1 can be achieved by the addition of copper salts to the growth medium.Copper is an essential cell nutrient acting as a cofactor in nearly 20 enzymes (1). However, excess accumulation of copper ions results in toxicity. Evidence for the effectiveness of copper ions as a toxin comes from its historic use as a fungicide, molluscide, and algicide. Homeostatic mechanisms exist in cells to regulate the cellular concentration of copper ions, thus maintaining copper balance and minimizing deleterious effects. Cells appear to maintain a quota for essential metal ions; this quota is primarily the quantity necessary to metallate the various copper proteins (2, 3). Copper ions are required for at least three key enzymes in Saccharomyces cerevisiae. The cuproenzymes include the cytosolic superoxide dismutase Sod1, 1 the plasma membrane ferroxidase Fet3, and the mitochondrial inner membrane enzyme cytochrome c oxidase (CcO). The copper quota of the yeast S. cerevisiae is about 5 ϫ 10 5 atoms per cell (3, 4).It is unclear what fraction of the 5 ϫ 10 5 copper atoms per cell is from copper in Sod1, Fet3, and CcO. Expression of these copper-binding proteins varies with growth conditions, suggesting that the copper may be distributed differently depending on growth conditions. Clearly, a significant fraction of the cellular copper is associated with Sod1; however, not all Sod1 molecules are metallated (4, 5). Fet3 requires four copper ions for activity, but levels of this protein are dependent on iron status of the medium (6). CcO levels vary depending on whether the cells are grown by fermentation or respiration. In addition, a varying quantity of cellular copper exists bound to two metallothioneins, Cup1 and Crs5 (7,8). Expression of CUP1 and CRS5 is regulated by copper levels through the copper-responsive transcription factor Ace1 (9, 10). An increase in the free Cu(I) ion pool activates Ace1 throu...