Bacillithiol: a key protective thiol in Staphylococcus aureus

Abstract

Bacillithiol is a low-molecular-weight thiol analogous to glutathione and is found in several Firmicutes, including Staphylococcus aureus. Since its discovery in 2009, bacillithiol has been a topic of interest because it has been found to contribute to resistance during oxidative stress and detoxification of electrophiles, such as the antibiotic fosfomycin, in S. aureus. The rapid increase in resistance of methicillin-resistant Staphylococcus aureus (MRSA) to available therapeutic agents is a great health concern, and many research efforts are focused on identifying new drugs and targets to combat this organism. This review describes the discovery of bacillithiol, studies that have elucidated the physiological roles of this molecule in S. aureus and other Bacilli, and the contribution of bacillithiol to S. aureus fitness during pathogenesis. Additionally, the bacillithiol biosynthesis pathway is evaluated as a novel drug target that can be utilized in combination with existing therapies to treat S. aureus infections.

Keywords: Staphylococcus aureus; bacillithiol; bacillithiol conjugate amidase; drug resistance; fosfomycin; oxidative stress; pathogenesis.

Figure 1. Structures and pKa values of the major low-molecular-weight thiols found in bacteria

Thiols shown as the major ionized form at pH = 7.7 and represent microscopic thiol pKa values. ^†Rabenstein 1973 [5]; ^‡Sharma et al. 2013 [20]; ^§Benesch and Benesch 1955 [4]; ^¶Keire et al. 1992 [3].

Figure 2. Comparison of the major forms of bacillithiol, cysteine and CoA at pH 7.7

Proportions of each form are based on microscopic pKa values. The four major forms of bacillithiol are depicted. The ionizable phosphate groups of CoA are not listed. Figure from Sharma et al. 2013 [20].

Figure 3. 3-dimensional structure of bacillithiol in the ionized 0 + − − form (Figure 2)

The proximal location of thiol, amine and carboxylate moieties in the energy-minimized structure may support divalent metal ion chelation. Atom labels: C, green; N, blue; O, red; H, white, and S, yellow.

Figure 4. Bacillithiol biosynthesis (top) and bacillithiol-dependent detoxification (bottom) in Staphylococcus aureus

Data from Gaballa et al. 2010 [27], Newton et al. 2011 [56], Newton et al. 2012 [48], Fang et al. 2013 [25], and Perera et al. 2014 [52].

Figure 5. Bacillithiol-dependent detoxification of fosfomycin is catalyzed by S. aureus FosB

Figure from Roberts et al. 2013 [26].

Figure 6. Close relatives of S. aureus Newman BstA in the S-transferase like (STL, formerly DinB/YfiT-like) superfamily

Staphylococcus aureus BstA is distantly related to B. subtilis YfiT (BstA) and a B. anthracis STL superfamily member (BA_2721), and unrelated to the bacillithiol transferase protein FosB. Bootstrap values are shown for the major branches. Figure from Perera et al. 2014 [52].

Figure 7. Genomic context of S. aureus bshA, bshB, bshC and bstA genes in S. aureus Newman

The bacillithiol biosynthesis genes bshA, bshB and bshC and the bstA gene are unlinked (in bold). Overlapping arrows represent overlapping ORFs.

Figure 8. Bacillithiol-dependent methylglyoxal detoxification pathway in Bacillus subtilis

Methylglyoxal can enter the cell via diffusion or can be formed intracellularly (i.e., from dihydroxyacetone phosphate, abbreviated DHAP). Bacillithiol and both glyoxalase A (GlxA) and glyoxylase B (GlxB) are utilized to convert methylglyoxal to D-lactate. Cytoplasmic acidification is mediated by the interplay of the KhtSTU pump with bacillithiol and S-lactoyl-bacillithiol. Model from Chandrangsu et al. 2013 [67].

Figure 9. Natural product and synthetic inhibitors of M. tuberculosis metalloproteins MshB and MCA

Oceanapiside (compound 1) is a non-competitive inhibitor of M. tuberculosis MCA; a brominated derivative of tyrosine (compound 2) and psammplysin A (compound 3) are competitive inhibitors of MCA. Psammaplin A (compound 5) and (compound 2) contain oxime moieties, known to chelate metal ions. Compound 4 is a synthetic substrate analog of GlcNAc-Ins (substrate of MshB), and one of the few compounds that inhibits both M. tuberculosis MshB and MCA.