Antibacterial properties of some cyclic organobismuth(III) compounds

Abstract

Bismuth compounds are known for their low levels of toxicity in mammals, and various types of bismuth salts have been used to treat medical disorders. As part of our program to probe this aspect of bismuth chemistry, cyclic organobismuth compounds 1 to 8 bearing a nitrogen or sulfur atom as an additional ring member have been synthesized, and their antimicrobial activities against five standard strains of gram-negative and gram-positive bacteria were assessed. The eight-membered-ring compounds, compounds 1 to 3, exhibited MICs of less than 0.5 microg/ml against Staphylococcus aureus and were more active than the six-membered ones, compounds 5 to 8 (MICs, 4.0 to 16 microg/ml). The gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis, and Enterococcus faecalis) were more susceptible to both types of ring compounds than the gram-negative ones (Escherichia coli and Pseudomonas aeruginosa). Treatment with polymyxin B nonapeptide increased the susceptibility of E. coli to cyclic organobismuth compounds, indicating the low permeability of the outer membrane of gram-negative bacteria to the compounds. Compound 1 also had activity against methicillin-resistant S. aureus, which had an MIC for 90% of the hospital stock strains of 1.25 microg/ml. The killing curves for S. aureus treated with compound 1 or 3 revealed a static effect at a low dose (2x the MIC). However, when S. aureus was treated with 10x the MIC of compound 1 or 3, there was an approximately 3-log reduction in the viable cell number after 48 h of treatment. Electron microscopic inspection demonstrated a considerable increase in the size of S. aureus and the proportion of cells undergoing cell division after treatment with compound 1 at 0.5x the MIC.

FIG. 1.

Routes of synthesis of compounds 1 and 2 (A), compounds 3 and 4 (B), and compounds 5 to 7 (C). BuLi, butyllithium.

FIG. 2.

Structures of the synthetic cyclic organobismuth(III) compounds used in this study: compound 1, N-tert-butyl-bi-chlorodibenzo[c,f][1,5]azabismocine; compound 2, N-tert-butyl-bi-(4-methylphenyl)dibenzo[c,f][1,5]azabismocine; compound 3, bi-chlorodibenzo[c,f][1,5]thiabismocine; compound 4, bi-(4-methylphenyl)dibenzo[c,f][1,5]thiabismocine; compound 5, bi-chlorophenothiabismin-S,S-dioxide; compound 6, bi-(4-methylphenyl)phenothiabismin-S,S-dioxide; compound 7, bi-vinylphenothiabismin-S,S-dioxide; and compound 8, bi-(4-methylphenyl)phenothiabismin.

FIG. 3.

Effect of compound 1 on the growth of S. aureus. Cells were grown in MHB medium, and compound 1 was added to the culture at the following concentrations (at time zero): control (no drug added; ♦), 0.125 μg/ml (0.5× the MIC; ▪), 0.5 μg/ml (2× the MIC; ×); 1.0 μg/ml (4× the MIC; ○). The growth rate gradually decreased with an increase in the drug concentration.

FIG. 4.

Killing curves for compounds 1 (A) and 3 (B) tested against S. aureus. Compound 1 or 3 was added to the logarithmic phase of S. aureus broth cultures at 30°C. At the indicated times, 10 ml of culture was withdrawn, placed on ice, and subsequently washed by three cycles of centrifugation with MHB medium. After the final wash, the bacteria were resuspended in MHB medium and viable counts were made in triplicate on MHB agar plates. Control (no drug addition; ◊); 2× the MIC (○); 5× the MIC (▵); 10× the MIC (×).

FIG. 5.

Transmission electron microscopy. (A) Thin section of untreated S. aureus culture for 18 h. Staining was with uranyl acetate-lead citrate. (B) Thin section of S. aureus culture after exposure to compound 1 at 0.125 μg/ml for 18 h. The morphology of the treated bacteria is very similar to that of the bacteria in the control culture, except that many cells are larger and the proportion of dividing cells (ca. 40%) is significantly higher than that in the control culture (ca. 5%). Bar, 1 μm.

FIG. 6.

Reversible inversion of stereochemistry at the bismuth center of compound 3 demonstrated on variable temperature dynamic ¹H nuclear magnetic resonance spectra. ΔG, free energy of activation.