Oleksii Shemchuka,
Dario Braga
*a,
Fabrizia Grepioni
a and
Raymond J. Turner
*b
aMolecular Crystal Engineering Laboratory, Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via F. Selmi 2, 40126 Bologna, Italy. E-mail: dario.braga@unibo.it
bDepartment of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. E-mail: turnerr@ucalgary.ca
First published on 10th January 2020
Co-crystallization of the antibacterial agents proflavine and methyl viologen with the inorganic salts CuCl, CuCl2 and AgNO3 results in enhanced antimicrobial activity with respect to the separate components.
Furthermore, it has been shown that ionic co-crystals,19 whereby the active organic molecule (urea, thiourea) is co-crystallized with inorganic salts such as KCl, ZnCl2, can be successfully utilized to inhibit the activity of metalloenzymes involved in the degradation of urea.15,20
Neutral proflavine (acridine-3,6-diamine) and the dichloride salt of methyl viologen (1,1′-dimethyl-4,4′-bipyridinium dichloride) (see Scheme 1) have been chosen for this preliminary investigation, since promising results have been obtained by using quaternary-cation compounds (QCC) and metals mixed with extracted biosurfactants,21,22 where the mixtures had greater efficacy than each compound alone. Synthetic QCCs are widely used as antiseptics, cationic surfactants, disinfectants, herbicides and dyes in domestic, clinical and industrial setting.23
The structure of PF·CuCl (1a) was determined from X-ray powder diffraction (XRPD) data (see ESI†). The structure can be described as layers of PF molecules, arranged in a herring-bone fashion; the acridine nitrogens coordinate the copper(I) cations of the CuCl pairs, which in turn form a (Cu–Cl⋯Cu–Cl⋯)n 1D chain (Fig. 1).
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Fig. 1 Relevant packing features in crystalline PF·CuCl (1a), showing the herring-bone arrangement of the proflavine molecules and the 1D (CuCl⋯CuCl⋯)n chains. |
The reaction of PF and AgNO3, both by slurry and grinding, yielded the anhydrous, novel solid PF·AgNO3 (1b), with a consistent and reproducible XRPD pattern different from the starting material (see ESI†). Unfortunately, in spite of multiple crystallization attempts, the low quality of the diffraction patterns has thus far prevented a full structural characterization from powder data.
Co-crystallization of the MVCl2 salt with copper(I) and copper(II) chlorides was also performed via slurry at ambient conditions (see ESI†). The reaction resulted in the formation of two solid compounds, 2a and 2b, whose XRPD patterns were found to match those of catena-(MV2+)tetrakis(μ2-chloro)-di-copper(I), (CSD refcode DMDPCU,24 obtained in different conditions in the original paper) and N,N′-dimethyl-4,4′-bipyridinium tetrachloro-copper(II), (CSD refcode MBPYCU25 – no synthesis reported in the original paper), respectively (see ESI†).
The organics PF and MV as well as salts of Cu and Ag have been shown to have antimicrobial activity. Therefore, we performed standard agar media plate antimicrobial assays to determine if the co-crystal of the compounds maintained or changed their antimicrobial efficacies. For the antimicrobial assays working stocks of 1a,b and 2a,b as well as of PF and MV and of the inorganic salts CuCl, CuCl2 and AgNO3 were made fresh at a concentration of 25 mg mL−1 in double distilled water producing solutions or slurries. Antimicrobial testing was performed using the pathogen indicator strains: Pseudomonas aeruginosa ATCC27853, Staphylococcus aureus ATCC25923, and Escherichia coli ATCC25922. Efficacy of antimicrobials used the established method of measuring a zone of growth inhibition which was performed either by adding the compounds to a well in the agar media or through soaking the compounds into an antimicrobial susceptibility filter disk. An example of zones of inhibition is shown in Fig. 2. In order to deal with subtle differences in biological growth between trials, zones in a given technical trial plate were normalized to allow more effective comparisons between trials and antimicrobial formulations.
We evaluated first the MV compounds. MV salts with CuCl or CuCl2 showed maximum zones of inhibition only on the order of 10–12 mm. However, one can see in the normalized results (see Fig. 3) that the antimicrobial activity was greater than the metal salts or MV alone. This establishes that generating a crystal combining these two antimicrobial agents provides enhanced antimicrobial activity compared to Cu chlorides or MV alone towards all three bacteria.
With the success of the MV based compounds we explored the antimicrobial efficacy of our new proflavine-based materials. These compounds showed more impressive antimicrobial activities. For the agar-well approach, zones of inhibition were as much as 150 mm in some experiments and with less variability between biological trials. Disk diffusion assays were more reproducible with zones of inhibition maxing out at 60 to 90 mm. The normalized data is compared in Fig. 4.
As expected, differences in susceptibility were observed between bacterial species, as seen with previous studies.26,27 Yet overall the co-crystals showed better efficacy than the metal salts on their own. Experiments using method 1 (see ESI†), where a physical mixture of PF and metal salt gave rise to a zone of inhibition on the same order as the most toxic compound of the two, suggest a degree of synergy of the components as a co-crystal. Although proflavine has poor efficacy on its own towards P. aeruginosa, it shows quite a good efficacy against S. aureus and E. coli on its own. But in the agar-well assay (Fig. 4 top), PF compounds 1a and 1b show a significant improved efficacy (p < 0.0001) over PF alone. For the complementary assay this improvement is lost. Regardless there is still improved efficacy against P. aeruginosa. These results imply delivery approach of the engineered crystals is an important variable when considering the target organism.
An alternative antimicrobial assay is to evaluate contact killing against an existing growth of bacteria. We tested to see if contact killing and cell lysis will occur, using a compound impregnated dried susceptibility disk applied to a robustly grown lawn of bacteria. We found that, under the media and growth conditions used here, all compounds tested demonstrated the ability to kill and lyse the existing lawn of bacteria, generating a zone of clearing beneath the disk (see Fig. 5). No zones of clearing beyond the disk were observed for E. coli lawns. However, for 1a, an additional 0.5–1.0 mm zone was observed in the S. aureus lawn. Several compounds showed additional zone of clearing beyond the disk against P. aeruginosa with the comparator compounds of AgNO3 and Ag2O nanoparticles showing the largest with a 3 mm zone. Compounds 1a, 1b, 2a and 2b all showed a 1 to 1.5 mm zone, however MV and PF also showed the same sized zone. This experiment demonstrated that our new formulations have contacting killing activity.
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Fig. 5 Killing and lysing of established cultures (method 3, see ESI†). (1) MV; (2) 2a; (3) 2b; (4) CuSO4; (5) CuCl2; (6) AgNO3; (7) silver oxide; (8) PF; (9) 1a; (10) 1b. The clearing demonstrates the ability to kill and lyse a pre-grown culture of bacteria compared to the other methods that measure the ability of compounds to prevent growth. |
Using such compounds led to experimental challenges between experiments. The issue was primarily producing reproducible slurries. Since it was noted that the silver based compounds were more photosensitive, all data shown is with fresh stocks prepared within 24 hours stored in the dark. Additionally, zones of inhibition were not always distinct, with two zones often observed, one of full clearing of growth and a second larger ring where growth was inhibited but not prevented.
We envisage the use of our compounds as coating materials in the prevention of infectious disease transfer. The challenges that exist is producing a metal formulation that releases the metal at a level that sustains antimicrobial properties and that is stable to the physical manipulation of the material that has been coated. This opens avenues for development of unique metal chelates and organo-metal compounds.
Footnote |
† Electronic supplementary information (ESI) available: Synthesis, DSC and TGA, XRPD. CCDC 1964562. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9ra10353h |
This journal is © The Royal Society of Chemistry 2020 |