Staphylococcus aureus causes a variety of infections, from skin and soft tissue infections to more serious infections including endocarditis, osteomyelitis, and bacteremia. While we have antibiotics to treat S. aureus infections, treatment failure is incredibly common. In 2017, S. aureus infections killed 20,000 people in the United States alone. So if we have antibiotics, why aren’t they working?

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Antibiotic resistance might be the first explanation that comes to mind when we think about treatment failure. Although antibiotic resistance definitely contributes to the rising rates of antibiotic treatment failure, it cannot explain the high rates of failure that occur with particular infections, like S. aureus infections. In addition to resistant populations, subpopulations of antibiotic tolerant cells, sometimes called persisters, also contribute to antibiotic treatment failure. However, despite the apparent clinical importance of antibiotic tolerant cells, we know very little about how they arise during infection. 

S. aureus has been shown in vitro to survive intracellularly in various cell types, such as endothelial cells, epithelial cells, neutrophils, and macrophages [1-3]. Work from Paul Kubes’ group shows that during systemic infection in mice, S. aureus is readily phagocytosed by macrophages and trafficked to the liver[4]. However, despite the importance of macrophages in initial control of pathogen burden, macrophages fail to eradicate the intracellular S. aureus population. Additionally, the intracellular reservoir seemingly seeds secondary infections in other organs, like the kidneys. Because macrophages represent such an important niche during infection and have been shown to induce tolerance in other clinically important pathogens [5,6], we sought to determine how macrophages might be contributing to antibiotic treatment failure during S. aureus infection.

We hypothesized that stressors encountered in the phagosome of macrophages were inducing antibiotic tolerance in S. aureus. Using both macrophage tissue culture and in vitro bacterial culture models, we observed that exposure to oxidative stress completely protected S. aureus from antibiotic killing by rifampicin. But how? 

Antibiotic tolerance has been associated with decreased metabolic activity. Following exposure of S. aureus to oxidative stress, we observed a collapse in ATP levels and TCA cycle activity, suggesting that oxidative stress forces S. aureus into a metabolic state incompatible with antibiotic killing.

Our next question was whether oxidative stress-induced tolerance was relevant during host infection. By comparing wildtype mice to mice deficient in oxidative burst, we found that oxidative stress indeed contributes to antibiotic tolerance in mice. Excitingly, in both our macrophage and animal model, we were able to resensitize S. aureus to antibiotics by treating either the macrophages or the mice with antioxidants. 

Despite the importance of oxidative stress in control of burden in early infection, we found that oxidative burst, a major component of the innate immune system, directly antagonizes antibiotic function, contributing to treatment failure. By modulating oxidative stress during murine S. aureus infection, we were able to significantly improve antibiotic efficacy. We are excited by the potential implications of this work and the prospect of developing host-directed therapeutics to maximize antibiotic efficacy and clear bacterial infections. 

Check out the full paper here: https://www.nature.com/articles/s41564-019-0627-y

References

1. Gresham HD, Lowrance JH, Caver TE, Wilson BS, Cheung AL, Lindberg FP. Survival of Staphylococcus aureus inside neutrophils contributes to infection. J Immunol. 2000;164(7):3713-22.

2. Koziel J, Maciag-Gudowska A, Mikolajczyk T, Bzowska M, Sturdevant DE, Whitney AR, Shaw LN, DeLeo FR, Potempa J. Phagocytosis of Staphylococcus aureus by macrophages exerts cytoprotective effects manifested by the upregulation of antiapoptotic factors. PLoS One. 2009;4(4):e5210.

3. Tuchscherr L, Medina E, Hussain M, Völker W, Heitmann V, Niemann S, Holzinger D, Roth J, Proctor RA, Becker K, Peters G, Löffler B. Staphylococcus aureus phenotype switching: an effective bacterial strategy to escape host immune response and establish a chronic infection. EMBO Mol Med. 2011;3(3):129-141.

4. Surewaard BGJ, Deniset JF, Zemp FJ, Amrein M, Otto M, Conly J, Omri A, Yates RM, Kubes P. Identification and treatment of the Staphylococcus aureus reservoir in vivo. J Exp Med. 2016;213(7):1141-1151.

5. Helaine S, Cheverton AM, Watson KG, Faure LM, Matthews SA, Holden DW. Internalization of Salmonella by macrophages induces formation of nonreplicating persisters. Science. 2014;343(6167):204-208.

6. Liu Y, Tan S, Huang L, Abramovitch RB, Rohde KH, Zimmerman MD, Chen C, Dartois V, VanderVen BC, Russel DG. Immune activation of the host cell induces drug tolerance in Mycobacterium tuberculosis both in vitro and in vivo. J Exp Med. 2016;213(5):809-825.