The Metabolic Evolution of Staphylococcus aureus

Abstract. Methicillin Resistant Staphylococcus aureus (MRSA) is an important human pathogen that exerts a tremendous negative impact on human health. S. aureus stands out among other species of Coagulase- Negative Staphylococci (CoNS) in that S. aureus specifically has evolved high pathogenic potential. Some of this is explained by the arsenal of virulence traits present in the genomes of MRSA isolates. However, the metabolic evolution of S. aureus contributes equally to the success of this pathogen. That is, S. aureus has evolved to thrive at sites if inflammation, which are very often hypoxic, replete with immune radicals (e.g. nitric oxide, NO·) and devoid of free iron. All these attributes limit bacterial respiration and S. aureus has evolved to better deal with such respiratory stress than most CoNS. In response to respiratory stress, S. aureus elicits a metabolic state that maximizes glycolytic flux coupled to lactate excretion. This Warburg-like metabolism is reminiscent of that employed by host immune cells infiltrating to sites if infection. Thus, S. aureus must compete with the host for available glucose. As such, S. aureus encodes three glucose-specific transporters not present in most CoNS. Moreover, S. aureus encodes a unique lactate dehydrogenase (Ldh1) that supports redox balance in the absence of respiration. Our LC-MS metabolomics survey of S. aureus under respiratory stress has uncovered many new pathways beyond lactate production that contribute to redox balance. Aim 1 of this proposal seeks to identify the genes that encode the enzymes in these pathways as none have been annotated. We hypothesize that many of these genes share a common regulatory mechanism and are not encoded in the genomes of most CoNS. Thus, these newly acquired metabolic genes represent additional examples of the metabolic evolution that allowed for the emergence of a pathogen from a genus of skin commensals. We will also explore whether the certain genes exhibit signatures of forward selection specifically among NO·-resistant staphylococci possibly highlighting new mechanisms of metabolic evolution. The second Aim focuses on uncovering the mechanism by which all of these newly acquired metabolic genes, and major virulence regulons (Agr) are controlled by the presence of glucose. We contend that S. aureus uses glucose as a signal that it has penetrated into deeper tissue given that carbohydrates are scarce on the skin surface. Coordinating the expression of virulence genes as well as metabolic genes that contribute to growth in inflamed tissue with the availability of glucose underscores the importance of this carbon source to the metabolic evolution of S. aureus. As such, the final aim investigates whether excessive blood glucose in diabetic patients drives MRSA virulence factor production thereby worsening infection. In total, this proposal seeks to finalize our understanding of the metabolic evolution of an important human pathogen and how it is intricately tied to blood glucose. Perhaps the rise in metabolic disorder in the US accounts for more of the rise in MRSA incidence over the last several decades that previously appreciated.