Metabolic reprogramming promotes Staphylococcus aureus serum resistance during bacteraemia.

Staphylococcus aureus is a leading cause of bloodstream infections causing an estimated 300,000 deaths worldwide. Using a functional genomics approach, our group previously identified that adaptation of S. aureus to human serum is polygenic, with a clinically occurring non-synonymous mutation (V76I) in dihydrolipoamide dehydrogenase (lpdA1/pdhD) improving bacterial survival. In this work we establish that improved serum survival of strains expressing PdhD V76I is underpinned by enhanced resistance to host-derived antimicrobials including antimicrobial peptides and host-defence fatty acids which are prevalent in the bloodstream. Here we demonstrate that the PdhD V76I variant has enhanced diaphorase activity, with both clinical and laboratory strains expressing this variant recycling NADH to NAD+ without using respiration, leading to a reduction in membrane potential. This conferred persister / small colony variant (SCV) like phenotypes on strains expressing PdhD V76I including increased resistance to gentamicin, hydrogen peroxide, LL37, HNP-1 and arachidonic acid. However, strains which utilise PdhD V76I do not display growth defects typical of persisters and SCVs, with reduced NADH accumulation in serum leading to enhanced glycolysis/TCA cycle activity, enhancing bacterial replication in human serum. Whilst establishment of SCV and persister populations is a key survival strategy in the bloodstream, this work demonstrates how intermediate phenotypes can also be effective at promoting survival in this hostile environment. This highlights the heterogenous nature of S. aureus adaptation to the host-environment, with an improved fundamental understanding of these processes required to allow for the development of novel therapeutics which target this process of host-adaptation. IMPORTANCEThe establishment of a bloodstream infection requires the bacteria to be able to withstand the antibacterial factors found there. As one of the major global causes of these types of infections, S. aureus, utilises many diverse strategies to survive and replicate in the bloodstream, which can include modifications to either the cell envelope, or to core metabolic processes such that the hosts immune features are withstood. While extreme adaptations to metabolism can lead to the major growth defects associated with persisters and small colony variants, here we describe a clinically relevant adaption where metabolism was altered such that the bacteria could survive and replicate in serum, but where the growth characteristics of the bacteria were not affected. Mutation of the gene encoding the dihydrolipoamide dehydrogenase enzyme (lpdA1/pdhD) enhanced it diaphorase activity such that it could recycle NADH to NAD+ without using respiration, which led a reduction in membrane potential that conferred resistance to gentamicin, hydrogen peroxide, LL37, HNP-1 and arachidonic acid and human serum, but did not affect growth. This work uncovers a novel mean of adaptation to the bloodstream by S. aureus and highlights how it plasticity underlies its success as a major human pathogen.