The current epidemiological situation is especially worrying with MDR Gram-negative bacteria spreading worldwide and with a paucity of novel marketed antibiotics, thus colistin is gaining increasing interest from many clinicians worldwide [16]. However, the increasing numbers of discoveries of colistin-resistant pathogens broke the last barrier between superbacteria and humans and new therapy strategies are urgently required [17]. A large quantity of previous studies have reported the synergistic activity of colistin combined with other antibiotics against colistin-resistant or carbapenem-resistant strains [18, 19, 20]. Besides, a few studies based on colistin in combination with the nonantibiotics were also performed previously [21, 22, 23]. In this study, we evaluated the synergistic activity of colistin combined with PFK-158 against colistin-resistant and colistin-susceptible Gram-negative bacteria.
Firstly, as shown in Table 2, we noticed that for the 18 colistin-resistant Gram-negative bacteria, the addition of PFK-158 demonstrated the enhanced colistin susceptibility by reducing colistin MICs and fractional inhibitory concentration indices (FICIs) of the combination of colistin and PFK-158 were all below 0.5 in all 18 colistin-resistant strains, indicating that there was a synergistic activity of colistin combined with PFK-158. It should be emphasized that a synergistic activity was observed against all tested colistin-resistant P. aeruginosa, A. baumannii, E.coli and K. pneumoniae, suggesting that the combination may be a potential strategy to treat the infections caused by these four kinds of colistin-resistant Gram-negative bacteria. Recently, Xuefu You et al. have reported on the synergistic activity of colistin combined with PFK-158 against colistin-resistant Enterobacteriaceae [13], including E.coli, K. pneumoniae and E. cloacae, the result of which is consistent with the result in our study. Compared to the study of Xuefu You et al., the advantage of our study is that we have explored not only the synergistic activity of colistin in combination with PFK-158 against Enterobacteriaceae but also non-fermenting bacteria (P. aeruginosa, A. baumannii) and colistin-susceptible Gram-negative bacteria. We found that PFK-158 could potentiate colistin activity through a specific mechanism against all the colistin-resistant Gram-negative bacteria no matter what kinds of bacteria among the tested colistin-resistant P. aeruginosa, A. baumannii, E. coli and K. pneumoniae. In other words, the combination of colistin and PFK-158 showed a wide range of synergistic activity on colistin-resistant Gram-negative bacteria. This suggests that the mechanism of the synergistic activity of the two drugs combination is probably not related to the difference of the strains property.
However, data of the previous studies are available against only colistin- resistant strains, and little is known regarding the potential for the synergistic activity of colistin in combination with PFK-158 against colistin-susceptible Gram-negative bacteria. Therefore, to explore the reason why the combination of the two drugs could show synergistic activity and whether it is because PFK-158 changes the colistin resistance mechanism of the colistin-resistant Gram-negative bacteria, we further examined the synergistic activity of colistin combined with PFK-158 against colistin-susceptible Gram-negative bacteria as a comparison. The synergistic activity was also noted against most tested Gram-negative bacteria, and for the remaining susceptible Gram-negative bacteria, the combination displayed additive activity as shown in Table 3. When comparing the results between colistin-resistant and colistin-susceptible strains, no statistical significance was found (p < 0.5). This tells us that the mechanism of the synergy of the two drugs combination may be not related to the resistance mechanism of the strains.
So, what are the specific mechanism and reasons why the combination of colistin and PFK-158 can displayed the synergistic activity against colistin-resistant and colistin-susceptible Gram-negative bacteria? According to the research having been published, colistin is known to interact with lipopolysaccharide (LPS) and phospholipids present at the surface of the outer membrane, to disturb membrane permeability, and finally to bind to phospholipids present at the surface of the cytoplasmic membrane [24, 25]. The last interaction is thought to result in disruption of the osmotic equilibrium and leakage of the cell contents [26]. Besides, PFK-158 shows extensive anti-tumor activity by reducing the uptake of glucose in cancer cells, the production of ATP, the release of lactic acid and inducing apoptosis and autophagy [11, 12]. On the basis of the results as described above, the antibacterial mechanism of colistin and the anti-tumor mechanism of PFK-158, it reminded us that PFK-158 might confer colistin stronger ability to enter bacteria by influencing the metabolism of the strains so as to change the structure of lipid A moiety of LPS on the strains membrane, which is the main site of action of colistin. However, the mechanism of synergism observed between colistin and PFK-158 against colistin-resistant and colistin-susceptible Gram-negative bacteria remains unknown and will be the subjects of further investigations.
Finally, it is crucial to evaluate the clinical significance of these observations and to better understand the dose-response relationships of combination of colistin combined with PFK-158. As we all know, colistin has the nephrotoxicity and neurotoxicity [27], thus, if the combination therapy can lower the concentration and dosage of colistin, it will be extremely meaningful for the clinical treatment of colistin-resistant and colistin-susceptible Gram-negative bacteria infections, especially for the patients with cancer, who are a special group with the low immunity and are more prone to be infected.