1,2,4-Triazolidine-3-thiones Have Specific Activity against Acinetobacter baumannii among Common Nosocomial Pathogens

Abstract

Acinetobacter baumannii are Gram-negative bacilli that pose a constant threat to susceptible patients because of increased resistance to multiple antibiotics and persistence in the hospital environment. After genome analysis, we discovered that A. baumannii harbors genes that share homology to an enzymatic pathway that elongates long-chain fatty acids (LCFA) in fungi. Previously, 1,2,4-triazolidine-3-thiones (T-3-Ts) were shown to inhibit hyphae production in fungi, and this same LCFA elongation pathway was implicated as the possible target. Therefore, we investigated if T-3-Ts also have activity against multidrug-resistant A. baumannii. Surprisingly, all of the clinical isolates of A. baumannii that were tested have susceptibility to ECC145 and ECC188 with MIC₉₀ values of 8.0 μg/mL. In contrast, reference strains and clinical isolates of other common nosocomial bacteria that lack the LCFA pathway also lacked susceptibility. Time-kill experiments revealed that both ECC145 and ECC188 have a bacteriostatic effect against A. baumannii. Mass spectrometry analysis suggested that exposure to T-3-Ts resulted in less LCFA production. Supplementation of media with either 0.02% w/v oleic or linoleic acid abrogated the bacteriostatic effect of the compounds, which again implicated LCFA elongation as the target. Our results suggest these molecules could be a promising start to further exploit what appears to be an important aspect of A. baumannii membrane function and integrity.

Keywords: ESKAPE pathogens; Gram-negative membrane; antibacterial; antibiotic; fatty acids; fungi.

Figure 1

Chemical structures of the 1,2,4-triazolidine-3-thiones, ECC145 and ECC188, used in this study.

Figure 2

Time–kill assays displaying mean and standard error of the mean of CFU/mL at time of initial treatment and at 4 and 24 h post-treatment for ECC145 (A) and ECC188 (B) at 1 and 4 times the MIC determined via microbroth dilution.

Figure 3

Thin-section transmission electron microscopy images (20,000×) of AB5075 untreated (A), exposed to ECC145 at 8.0 µg/mL (B), or exposed to ECC188 at 8.0 µg/mL (C) for 1 h. Aggregates are visible in the cytoplasm (black arrows), and membrane disruption (white arrows) is visible on the bacterial surface, distended from the cytoplasm, of the treated bacteria.

Figure 4

Mass spectrometry analysis of A. baumannii that was untreated (A) or treated with ECC145 for 6 h (B) or with ECC188 for 6 h (C).

Figure 5

Time–kill assays displaying the mean and standard error of the mean of CFU/mL at time of initial treatment and at 4 and 24 h post-treatment for AB5075 treated with either ECC145 at 4 times the MIC, linoleic acid at 0.02% w/v, or a combination of ECC145 and linoleic acid.