Activity of siderophores against drug-resistant Gram-positive and Gram-negative bacteria

Abstract

Infections by drug-resistant bacteria are life-threatening. As iron is a vital element for the growth of bacteria, iron-chelating agents (siderophores) can be used to arrest their multiplication. Exogenous siderophores - exochelin-MS and deferoxamine-B - were evaluated for their inhibitory activity against methicillin-resistant Staphylococcus aureus and metallo-β-lactamase producers - Pseudomonas aeruginosa and Acinetobacter baumannii - by disc diffusion, micro-broth dilution, and turbidimetric growth assays. The drug-resistant isolates were inhibited by the synergistic activity of siderophores and antibiotics. Minimum inhibitory concentration of exochelin-MS+ampicillin for different isolates was between 0.05 and 0.5 mg/mL. Minimum inhibitory concentration of deferoxamine-B+ampicillin was 1.0 mg/mL and greater. Iron-chelation therapy could provide a complementary approach to overcome drug resistance in pathogenic bacteria.

Keywords: deferoxamine B; exochelin MS; iron-chelation; xenosiderophores.

Figure 1

Susceptibility of MRSA to siderophores and antibiotics. Notes: MRSA isolates were swabbed on the surface of MHA plates and antibiotic discs were placed. Then, either Exo-MS or DFO-B (10 μL each) was loaded onto the antibiotic discs. The antibiotics used were Amp and CDR. On each plate, a different isolate was seeded. Zones of inhibition were observed with the siderophore+antibiotic combinations, indicating synergism. For example, in the second row, the left plate showed no zones of inhibition with cefdinir. The cefdinir disc was loaded with 10 μL of distilled water. The siderophore+cefdinir combination showed a zone of inhibition. Exo-MS and DFO-B individually showed no zones of inhibition. The zone of inhibition was negligible when the disc was loaded with ferric ammonium citrate along with DFO-B on the Amp disc. Abbreviations: Amp, ampicillin; CDR, cefdinir; DFO-B, deferoxamine-B; Exo-MS, exochelin-MS; MRSA, methicillin-resistant Staphylococcus aureus; MHA, Mueller–Hinton agar.

Figure 2

Susceptibility of metallo-β-lactamase-producing Pseudomonas aeruginosa to siderophores and antibiotics. Notes: P. aeruginosa isolates were swabbed on the surface of MHA plates and antibiotic discs were placed. Then, either Exo-MS or DFO-B (10 μL each) was loaded onto the antibiotic disc. The antibiotics used were Amp, IMP, and MRP. On each plate, a different isolate was seeded. Larger zones of inhibition were observed with the siderophore+antibiotic combinations, indicating synergism. Such an effect is seen in the first row, middle plate for siderophore+meropenem combinations, where meropenem alone shows a small zone of inhibition; Exo-MS and DFO-B individually showed no zones of inhibition. Abbreviations: Amp, ampicillin; DFO-B, deferoxamine-B; Exo-MS, exochelin-MS; IMP, imipenem; MHA, Mueller–Hinton agar; MRP, meropenem.

Figure 3

Susceptibility of metallo-β-lactamase-producing Acinetobacter baumannii to siderophores and antibiotics. Notes: A. baumannii isolates were swabbed on the surface of MHA plates and antibiotic discs were placed. Then, either Exo-MS or DFO-B (10 μL each) were loaded onto the antibiotic discs. The antibiotics used were Amp, IMP, and MRP. On each plate, a different isolate was seeded. Zones of inhibition were observed with the siderophore+antibiotic combinations, indicating synergism. For example, in the second row, the left plate showed no zones of inhibition with ampicillin and siderophores individually. The siderophore+ampicillin combination showed zones of inhibition. Exo-MS and DFO-B individually showed no zones of inhibition. Abbreviations: Amp, ampicillin; DFO-B, deferoxamine-B; Exo-MS, exochelin-MS; IMP, imipenem; MHA, Mueller–Hinton agar; MRP, meropenem.

Figure 4

Growth curves of methicillin-resistant Staphylococcus aureus in the presence of Exo-MS. Notes: (A) and (B) plot in red: growth curves of the isolate in medium alone and plot in green show curves in the presence of Exo-MS at 0.125 and 0.25 mg/mL, respectively; (C) and (D) plot in red: growth curves of the isolate in the presence of ampicillin 0.01 mg/mL and plot in green show curves in the presence of Exo-MS 0.125 and 0.25 mg/mL, respectively, along with ampicillin 0.01 mg/mL. Abbreviations: Amp, ampicillin; Exo-MS, exochelin-MS; OD, optical density.

Figure 5

Growth curves of an isolate of metallo-β-lactamase-producing Pseudomonas aeruginosa in the presence of Exo-MS. Notes: Plot in red: growth curves of the isolate in medium alone. Plot in green show growth curves: (A) in the presence of Exo-MS at 0.125 mg/mL + ampicillin 0.01 mg/mL, (B) only Exo-MS at 0.125 mg/mL, and (C) only ampicillin 0.01 mg/mL. Abbreviations: Amp, ampicillin; Exo-MS, exochelin-MS; OD, optical density.

Figure 6

Growth curves of metallo-β-lactamases-producing Acinetobacter baumannii in the presence of Exo-MS. Notes: (A) and (B) plot in red: growth curves of the isolate in medium alone and plot in green show curves in the presence of Exo-MS at 0.05 and 0.125 mg/mL, respectively; (C) and (D) plot in red: growth curves of the isolate in the presence of ampicillin 0.01 mg/mL and plot in green show curves in the presence of Exo-MS at 0.05 and 0.125 mg/mL, respectively, in combination with ampicillin 0.01 mg/mL. Abbreviations: Amp, ampicillin; Exo-MS, exochelin-MS; OD, optical density.