Molecular Mechanisms and Clinical Impact of Acquired and Intrinsic Fosfomycin Resistance

Abstract

Bacterial infections caused by antibiotic-resistant isolates have become a major health problem in recent years, since they are very difficult to treat, leading to an increase in morbidity and mortality. Fosfomycin is a broad-spectrum bactericidal antibiotic that inhibits cell wall biosynthesis in both Gram-negative and Gram-positive bacteria. This antibiotic has a unique mechanism of action and inhibits the initial step in peptidoglycan biosynthesis by blocking the enzyme, MurA. Fosfomycin has been used successfully for the treatment of urinary tract infections for a long time, but the increased emergence of antibiotic resistance has made fosfomycin a suitable candidate for the treatment of infections caused by multidrug-resistant pathogens, especially in combination with other therapeutic partners. The acquisition of fosfomycin resistance could threaten the reintroduction of this antibiotic for the treatment of bacterial infection. Here, we analyse the mechanism of action and molecular mechanisms for the development of fosfomycin resistance, including the modification of the antibiotic target, reduced antibiotic uptake and antibiotic inactivation. In addition, we describe the role of each pathway in clinical isolates.

Keywords: fosfomycin resistance; molecular mechanisms.

Figure 1

Chemical structure of fosfomycin [(2R,3S-3-methyloxiran-2-yl) phosphonic acid].

Figure 2

Although transporters are usually very selective, the chemical structure of fosfomycin mimics both glycerol-3-P (G3P) and glucose-6-P (G6P), which are transported under normal conditions. MurA catalyses the formation of UDP-GlcNac-3-O-enolpyruvate, a peptidoglycan precursor, from UDP-GlcNAc and PEP during the first step of peptidoglycan biosynthesis, allowing cell growth (A). In contrast, when fosfomycin (F) is present, it is transported inside the cell by GlpT and UhpT, blocking the UDP-GlcNac-3-O-enolpyruvate synthesis by mimicking the original substrate of MurA, PEP, avoiding cell wall synthesis and leading to cell death (B). For simplicity, only peptidoglycan and the inner membrane are shown.

Figure 3

Regulation of GlpT and UhpT. In E. coli and several Enterobacteria, the expression of glpT and uhpT requires the presence of the cAMP, which together with the receptor protein complex (CRP) forms the cAMP receptor protein complex (cAMP-CRP). This complex binds to the specific promoter sites of both genes, glpT and uhpT, and promotes their expression. Both transporters experience additional regulation. On the one hand, glpT gene expression is also controlled by the repressor, GlpR, which becomes inactive when it is bound to glycerol-3-P (G3P), and on the other hand, of uhpT; high-level expression also requires the regulatory genes, uhpA, uhpB and uhpC, which sense and transduce signals by phosphorylation when hexose phosphates are detected, thereby positively regulating the transcription of the gene.

Figure 4

Amino acid sequence alignment generated by ClustalX (under Bioedict) of three representative sequences of fosfomycin resistance proteins (Fos) present in different bacterial species. Represented sequences are FosA [P. aeruginosa 18A], FosA3 [E. coli], FosB [S. aureus subsp. aureus USA300_TCH1516], FosC2 [E. coli] and FosX [Clostridium botulinum Ba4 str. 657]. Fos enzymes belong to the same divalent metal-ion dependent metalloenzymes, the vicinal oxygen chelate superfamily (VOC), sharing a high number of core-conserved or identical residues in their sequences.

Figure 5

Reactions catalysed by Fos metalloenzymes (FosA, FosB and FosX) and fosfomycin kinases (FosC, FomA and FomB). Fosfomycin-inactivating enzymes modify the antibiotic, rendering it inactive by opening the oxirane ring (metalloenzymes) or by phosphorylation (fosfomycin kinases). Substrates and the metal requirement for each enzyme are also shown.