By Isabelle Hall
The rise of antibiotic resistance and the resultant emergence of superbugs pose an urgent threat to global public health. Whilst novel antibiotics are developed and searched for in nature, other approaches to combating this issue are under exploration. These include the use of: far-UVC – ultraviolet light which is capable of inactivating microbes but cannot penetrate the outer layers of human skin; employment of bacteriophage endolysins to initiate cell lysis; and encouragement of the disease tolerance defence system in patients.
Another method under investigation involves combining antibiotics and other compounds which can enhance their activity, known as antibiotic adjuvants. These compounds generally lack antibiotic activity themselves, but when utilised to create syncretic combinations, can improve efficacy of antibiotics. Adjuvants employ multiple methods – they may target bacterial metabolism, inhibiting enzymes which would otherwise mediate drug resistance. One example is clavulanic acid, a compound that blocks the activity of β-lactamase. This acid has been used in combination with amoxicillin to treat infection, preventing the degradation of the antibiotic. Another adjuvant that obstructs a resistance enzyme is 7-hydroxytropolone, which inhibits aminoglycoside adenyltransferase. Adjuvants may also influence host biology, acting on defence mechanisms (Tyers & Wright, 2019). Research has also revealed numerous interactions among drug pairs which appear to be antagonistic. In this case, combining the two drugs lowers the efficacy of each. Such an effect may be observed if a compound inhibits the mechanism used by the antibiotic (Brochado et al., 2018).
A recent study on antibacterial drug combinations analysed the impact of treating E. coli, S. Typhimurium and P. aeruginosa strains with various groupings of antibiotics, food additives and human-targeted drugs. Here, antagonistic interactions were found to be more prevalent than synergisms. The former occurred most frequently among pairings of drugs affecting different cellular processes, whereas the latter was more common for combinations of drugs with the same target.
In E. coli, combined use of the antibiotic ciprofloxacin with the flavouring compound vanillin resulted in an increased minimal inhibitory concentration for ciprofloxacin, demonstrating an antagonistic interaction between the two. This may be due to vanillin causing overexpression of the transcriptional regulator MarA. Such activity would lead to higher levels of the AcrA protein, thus encouraging expression of AcrAB-TolC, an efflux pump (Brochado et al., 2018). This pump works to remove compounds such as ciprofloxacin, decreasing intracellular concentration of the antibiotic. The pump system consists of the TolC channel in the outer membrane, the inner membrane transporter AcrB, and the AcrA protein which links them (Du et al., 2014).
Antagonistic interactions for ciprofloxacin were also apparent upon treatment with the herbicide paraquat and with caffeine. In both cases, induction of the AcrAB-TolC pump also appeared to be responsible for this effect.
Decreased intracellular levels of another antibiotic, gentamicin, were also observed in E. coli upon treatment with certain drug combinations. This effect was likely the outcome of reduced uptake of the antibiotic, a process mediated by proton motive forces. Drugs targeting the cell membrane may also be used in syncretic combinations with antibiotics to increase uptake or prevent efflux. This may be the mechanism enabling multiple human-targeted drugs to behave as adjuvants, potentiating the effect of antibiotics. Such drugs include the anaesthetic procaine, as well as verapamil – a treatment for high blood pressure and angina (Brochado et al., 2018).
Syncretic interactions have also been shown to occur via mechanisms involving the bacterial cytoskeleton. The compound A22 (S-(3,4-dichlorobenzyl)isothiourea) acts to sensitise E. coli to the antibiotic novobiocin (Tyers & Wright, 2019). A22 binds to the actin homologue MreB, preventing ATP attaching and adversely affecting MreB polymerisation. This hinders formation of long and rigid polymers, disrupting the cytoskeleton and giving rise to changes in cell shape. It is thought that the syncretic interaction between A22 and novobiocin may also be the result of altered influx or efflux systems within bacteria (Taylor et al., 2012).
These findings demonstrate the immense potential that such drug combinations hold for treating infection and addressing the growing issue of antibiotic resistance. Currently this method is used rarely in clinical practice – there is an urgent need for further research into effective combinations and the mechanisms underlying them, particularly those which may be implemented to manage multidrug-resistant superbugs.
Brochado, A.R., Telzerow, A., Bobonis, J., Banzhaf, M., Mateus, A., Selkrig, J., Huth, E., Bassler, S., Beas, J.Z., Zietek, M. & Ng, N. (2018) Species-specific activity of antibacterial drug combinations. Nature. 559 (7713), 259-263. Available from: doi: 10.1038/s41586-018-0278-9
Du, D., Wang, Z., James, N.R., Voss, J.E., Klimont, E., Ohene-Agyei, T., Venter, H., Chiu, W. & Luisi, B.F. (2014) Structure of the AcrAB–TolC multidrug efflux pump. Nature. 509 (7501), 512-515. Available from: doi: 10.1038/nature13205
Taylor, P.L., Rossi, L., De Pascale, G. & Wright, G.D. (2012). A forward chemical screen identifies antibiotic adjuvants in Escherichia coli. ACS chemical biology. 7 (9), 1547-1555. Available from: doi: 10.1021/cb300269g
Tyers, M. & Wright, G.D. (2019) Drug combinations: a strategy to extend the life of antibiotics in the 21st century. Nature Reviews Microbiology. 17 (3), 141-155. Available from: doi: 10.1038/s41579-018-0141-x