Using drug combinations is one of the methods to effectively control the multidrug-resistant (MDR) organisms [3]. Such combinations include antibiotic–antibiotic combinations and the pairing of an antibiotic with a nonantibiotic adjuvant molecule such as an EPI to directly target resistance mechanisms [3]. EPIs such as phenylalanine-arginine β-naphthylamide (PaβN) and carbonyl cyanide 3-chlorophenylhydrazone (CCCP) noticeably decreased the MICs of various antibiotics [4, 5]. Thus, they restored susceptibility to antibiotics that are well-known substrates of efflux pumps [4]. Therefore, EPIs can improve the efficacy of antibiotics and ameliorate the crisis in health care caused by the multidrug resistance of Gram-negative pathogens [6]. In the future, EPIs might be included in therapeutic regimens for
If we want to know whether an efflux pump mechanism contributes to the resistance of the specific antibiotic to the bacteria, monitoring the level of accumulation of the antibiotics after the addition of EPI may provide the clue for that. Previously, the cellular accumulation of antibiotics has been assessed directly using radiochemical and fluorescent techniques that monitor accumulation of specific antibiotics [7]. However, these procedures can be time consuming because separation of bacterial cells and the medium is required [7]. To solve this problem, fluorescent compounds, such as Hoechst 33342 (H33342) and ethidium bromide, were used as surrogate markers to assess efflux activity instead of antibiotics [7, 8]. These fluorescent intercalators change their wavelength of maximal emission in different environments when intercalating with DNA, and this phenomenon has been exploited to enable discrimination between intra- and extracellular localization of the probe [7].
In conjunction with EPIs, the H33342 accumulation assay can be used to assess the contribution of efflux pumps to an MDR phenotype [8] and to the resistance to several antibiotics. H33342, a bis-benzamide fluorescent dye, is readily taken up by living cells and fluoresces upon binding to DNA in the hydrophobic environment of the lipid membrane [8]. If antibiotics and H33342 are the substrates of the same efflux pump system, their pattern of accumulation may be similar. In these conditions, the changing pattern of H33342 accumulation after the addition of EPI may be duplicated by these antibiotics. Therefore, H33342 is adopted as a reporter of accumulation and efflux activity [7].
Because H33342 is a substrate of several efflux pumps, it can be pumped out of bacteria by these pumps. The amount of H33342 accumulation is lower in bacteria having high efflux activity than in bacteria having low efflux activity [8]. The addition of EPI CCCP, which dissipates the proton motive force required by several efflux pumps, may cause a significant increase in H33342 accumulation in bacteria with active efflux [8].
Although the H33342 accumulation assay is useful in assessing efflux activity, it is difficult to find studies that determine which antibiotics are affected by efflux pumps using this method. The aim of the present study is to assess the role of efflux activity on antimicrobials commonly used in hospitals by applying the H33342 accumulation assay to clinical isolates of
After approval by the institutional review board (IRB) of Chosun University Hospital (approval No. NON2016-003), 120 anonymized clinical isolates of
The 120
MDRAB was defined as a strain that was nonsusceptible to at least 1 agent in each of 3 or more antimicrobial categories described previously [11]. Non-MDRAB was defined as a strain that was susceptible to all antibiotics or non-susceptible to antibiotics in fewer than 3 categories. Strain ATCC19606T was used as a reference strain. Bacteria were grown at 37°C in Luria–Bertani (LB) broth and agar (Difco Laboratories). All chemicals used were purchased from Sigma–Aldrich Co.
We performed the H33342 accumulation assay to assess efflux activity of each strain. We compared the H33342 accumulation ratios (HARs) in the following groups: (i) susceptible vs. nonsusceptible for each antibiotic and (ii) MDRAB vs. non-MDRAB.
The H33342 accumulation assay was conducted as described by Richmond et al. with the following modifications [8]. Logarithmic phase cells were adjusted to an OD600 of 0.5 and then transferred to wells of black 96-well plates (Corning). The wells of the black microtiter plates were inoculated with 180 μL of each culture with or without 50 μM CCCP, and then 2.5 μM H33342 was added to each well. Four replicates for each strain were analyzed. Fluorescence was read on a fluorescence microplate reader (SpectraMax Gemini XPS; Molecular Devices) at 37°C using excitation and emission filters of 355 nm and 460 nm, respectively. Each experiment was repeated twice. Heat-killed
The HAR was calculated by dividing the amount of H33342 accumulation in the presence of CCCP (HAC) by the amount of H33342 accumulation in the absence of CCCP (HA).
Efflux activity was considered a contributor to antibiotic resistance if all 3 of the following criteria were met: (1) a significant difference in the mean HAR was noted between the susceptible group and nonsusceptible group for an antibiotic; (2) the mean HA in the nonsusceptible group was lower than that in the susceptible group for an antibiotic; and (3) in the nonsusceptible group, the mean HA was lower than the mean HAC for an antibiotic.
The results of the H33342 accumulation assay as an indicator of efflux activity are shown according to susceptibility to each antibiotic (
Results of Hoechst 33342 dye-accumulation assays of 120 Calculated by subtraction of the mean HAC from the mean HA: Calculated by dividing the amount of HAC by the amount of HA: HAR = HAC/HA All 3 criteria (C1–C3) suggest the contribution of efflux activity to antibiotic resistance. Criterion 1 is a significant difference in the mean HAR between the susceptible group and non-susceptible group for an antibiotic. Criterion 2 is a mean HA in the non-susceptible group that is lower than that in the susceptible group for an antibiotic. Criterion 3 is a mean HA that is lower than the mean HAC in the non-susceptible group for an antibiotic. 1 means the criterion is met. 0 means the criterion is not met AC, all 3 criteria (C1–C3); C1, criterion 1; C2, criterion 2; C3, criterion 3; CCCP, carbonyl cyanide 3-chlorophenylhydrazone; Antibiotic(s) Group No. HA HAC HAR Suitability to each criterion Mean SD Mean SD Mean SD C1 C2 C3 AC Cefepime Susceptible 19 742.1 223.1 0.028 685.1 164.9 0.447 57.03 0.9 0.2 0.000 1 1 1 1 Non-susceptible 101 607.5 245.4 655.3 154.4 –47.81 1.2 0.5 Cefotaxime Susceptible 17 740.8 242.0 0.042 696.9 172.5 0.295 43.94 1.0 0.3 0.010 1 1 1 1 Non-susceptible 103 610.4 243.0 654.0 152.8 –43.61 1.2 0.5 Ceftazidime Susceptible 19 742.1 223.1 0.028 685.1 164.9 0.447 57.03 0.9 0.2 0.000 1 1 1 1 Non-susceptible 101 607.5 245.4 655.3 154.4 –47.81 1.2 0.5 Imipenem Susceptible 20 726.4 228.3 0.052 690.4 162.2 0.342 36.00 1.0 0.3 0.004 1 1 1 1 Non-susceptible 100 609.3 246.0 654.0 154.5 –44.65 1.2 0.5 Meropenem Susceptible 20 726.4 228.3 0.052 690.4 162.2 0.342 36.00 1.0 0.3 0.004 1 1 1 1 Non-susceptible 100 609.3 246.0 654.0 154.5 –44.65 1.2 0.5 Piperacillin Susceptible 20 726.4 228.3 0.052 690.4 162.2 0.342 36.00 1.0 0.3 0.004 1 1 1 1 Non-susceptible 100 609.3 246.0 654.0 154.5 –44.65 1.2 0.5 Piperacillin/tazobactam Susceptible 20 726.4 228.3 0.052 690.4 162.2 0.342 36.00 1.0 0.3 0.004 1 1 1 1 Non-susceptible 100 609.3 246.0 654.0 154.5 –44.65 1.2 0.5 Ticarcillin/clavulanic acid Susceptible 20 726.4 228.3 0.052 690.4 162.2 0.342 36.00 1.0 0.3 0.004 1 1 1 1 Non-susceptible 100 609.3 246.0 654.0 154.5 –44.65 1.2 0.5 Ampicillin/sulbactam Susceptible 25 675.4 237.5 0.289 694.4 169.0 0.216 –19.00 1.1 0.4 0.387 0 1 1 0 Non-susceptible 95 616.6 248.1 651.0 151.7 –34.42 1.2 0.5 Aztreonam Susceptible 1 487.5 NA 485.1 NA 2.34 1.0 NA 0 0 1 0 Non-susceptible 119 630.0 246.9 661.5 155.5 –31.49 1.2 0.5 Tigecycline Susceptible 31 732.0 236.9 0.006 666.1 164.7 0.804 65.92 1.0 0.2 0.000 1 1 1 1 Non-susceptible 89 592.9 240.3 657.9 153.4 –65.04 1.3 0.5 Minocycline Susceptible 111 636.7 250.5 0.219 656.3 147.7 0.555 –19.56 1.2 0.5 0.200 0 1 1 0 Non-susceptible 9 531.6 164.6 706.5 241.5 –174.84 1.4 0.4 Ciprofloxacin Susceptible 19 742.1 223.1 0.028 685.1 164.9 0.447 57.03 0.9 0.2 0.000 1 1 1 1 Non-susceptible 101 607.5 245.4 655.3 154.4 –47.81 1.2 0.5 Gentamicin Susceptible 25 700.5 218.8 0.102 682.8 156.7 0.414 17.70 1.0 0.2 0.002 1 1 1 1 Non-susceptible 95 610.0 250.5 654.0 155.7 –44.08 1.2 0.5 Colistin Susceptible 119 626.6 246.0 NA 660.9 156.1 NA –34.25 1.2 0.5 NA 0 0 0 0 Non-susceptible 1 893.5 563.1 330.33 0.6 Trimethoprim/sulfamethoxazole Susceptible 31 628.2 238.9 0.979 684.4 165.3 0.276 –56.23 1.2 0.4 0.767 0 0 1 0 Non-susceptible 88 629.6 251.3 649.0 151.2 –19.39 1.2 0.5 Multidrug resistance Non-MDR 20 726.4 228.3 0.052 690.4 162.2 0.342 36.00 1.0 0.3 0.004 1 1 1 1 MDR 100 609.3 246.0 654.0 154.5 –44.65 1.2 0.5
We compared these results to determine which antibiotics met the criteria, suggesting a role for efflux activity in resistance to the antibiotic. The following 11 of 16 antibiotics met all 3 criteria, suggesting that efflux activity contributed to resistance to these antibiotics: cefepime, cefotaxime, ceftazidime, ciprofloxacin, gentamicin, imipenem, meropenem, piperacillin, piperacillin/tazobactam, ticarcillin/ clavulanic acid, and tigecycline. The numbers of antibiotics that met the first, second, and third criteria were 11 (69%), 13 (81%), and 15 (94%) of the 16 antibiotics tested, respectively. The mean HAC was subtracted from the mean HA in the susceptible and nonsusceptible groups of each antibiotic. The difference between them was a negative quantity in 15 of 16 antibiotics tested in the nonsusceptible group, because the mean HA was lower than the mean HAC in these cases. The difference between these values was positive in 12 of 16 antibiotics tested in the susceptible group, because the mean HA was higher than the mean HAC in these cases (
The following 5 of 16 antibiotics did not meet criteria 1, 2, or 3: ampicillin/sulbactam, aztreonam, colistin, minocycline, and trimethoprim/sulfamethoxazole (
The present study on the role of efflux activity in resistance to each antibiotic assessed by the H33342 accumulation assay suggests that efflux activity in clinical isolates of
By contrast, ampicillin/sulbactam, minocycline, and trimethoprim/sulfamethoxazole did not meet criterion 1 or 2 of the 3 criteria. Therefore, efflux activity did not appear to exert a major effect on resistance to these antibiotics. However, there is the possibility that efflux pump activity contributed to resistance to a minor degree in strains having other stronger resistance mechanisms. Although minocycline is a substrate of adeIJK [1] and trimethoprim and sulfonamide are substrates of adeABC, adeIJK, and adeFGH [1, 12, 13], they did not show significant differences in mean HARs of the susceptible and nonsusceptible groups. The reason for this is not clear. We suspect that other resistance determinants in some test strains might have distorted or masked the usual pattern attributed to efflux pumps with ampicillin/sulbactam, minocycline, or trimethoprim/sulfamethoxazole.
In addition, previous reports have shown various discrepancies in the activity of resistance-modifying EPIs. Thus, the level of antibiotic activity restored by the effect of EPIs will depend on the antibiotic class and type of EPI used [4]. The activity of a specific EPI is reported to differ by antibiotic.
Thus, PaβN, which drastically decreases the MIC of levofloxacin in MexAB-OprM-overproducing
Not all the efflux pump activity in bacterial strains could be assessed by H33342 accumulation assay because H33342 is not the substrate of the entire pump. We cannot preclude the possibility that H33342 may not be appropriate for screening efflux activity of antibiotics such as minocycline and trimethoprim/sulfamethoxazole. Even if such a situation arises, it may not affect our research goal. Our research goal was to screen candidate antibiotics useful for combination therapy using EPI. It may be a better strategy to select drugs with a strong efflux activity to ensure a higher effect with combination therapy with EPI. Therefore, selecting the candidate antibiotics among drugs showing high efflux activity may be a good starting point to investigate better combination model.
Although an assessment of the effect of efflux activity on resistance to colistin was not possible in this study because of the disproportionate number of strains allocated to the susceptible and nonsusceptible groups, efflux activity may not contribute to colistin resistance. Colistin, which acts on the surface of cells, has not been identified as a substrate for any efflux pumps [1]. Kuo et al. showed that the MIC value for ampicillin/sulbactam was not changed by the addition of EPIs, which suggested a negligible effect of efflux activity on resistance to this antibiotic [15]. This finding is compatible with our results, suggesting that efflux pumps may not exert a major effect on resistance to ampicillin/sulbactam.
The present study shows significant differences in efflux activity between bacteria belonging to the MDRAB and non-MDRAB groups, which is consistent with previous findings that increased expression of chromosomal genes for efflux systems plays a major role in MDR [1, 12].
The results of the H33342 accumulation assay as a tool to assess efflux activity were presented as the HAR, HA, and HAC in this study (
In the future investigation, effects of EPIs other than CCCP, preferably specific EPIs against tested pumps, on the efflux pump activity need to be studied to get more detailed information about them. In addition, further in-depth study is needed to elucidate the interrelationships between the following related parameters: (1) structural and physiological features of efflux pumps and interacting ligands such as antibiotics and EPIs to develop better ligands and (2) interactions between efflux pumps and other determinants of resistance to a given substrate (an antibiotic) in MDRAB clinical isolates.
Efflux activity may contribute to resistance to numerous antibiotics used in hospitals and multidrug resistance. Antibiotics against which resistance is mediated by efflux activity would be good targets for combination therapies to combat drug efflux.