The Journal of Lipid Research, Vol. 40, 1495-1500, August 1999
Copyright © 1999 by Lipid Research, Inc.
Interactions between cationic liposomes and bacteria: the physical-chemistry of the bactericidal action
M. T. N. Campanhãa,
E. M. Mamizuka1,a, and
A. M. Carmona-Ribeiroa
a Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, CP 26077, CEP 05599-970 São Paulo SP, Brazil
Correspondence to:
A. M. Carmona-Ribeiro
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ABSTRACT |
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The bactericidal effect of dioctadecyldimethylammonium bromide (DODAB), a liposome forming synthetic amphiphile, is further evaluated for Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa, and Staphylococcus aureus in order to establish susceptibilities of different bacteria species towards DODAB at a fixed viable bacteria concentration (2.5 x 107 viable bacteria/mL). For the four species, susceptibility towards DODAB increases from E. coli to S. aureus in the order above. Typically, cell viability decreases to 5% over 1 h of interaction time at DODAB concentrations equal to 50 and 5 µM for E. coli and S. aureus, respectively. At charge neutralization of the bacterial cell, bacteria flocculation by DODAB vesicles is shown to be a diffusion-controlled process. Bacteria flocculation does not yield underestimated counts of colony forming units possibly because dilution procedures done before plating cause deflocculation. The effect of vesicle size on cell viability demonstrates that large vesicles, due to their higher affinity constant for the bacteria (45.20 M-1) relative to the small vesicles (0.14 M-1), kill E. coli at smaller DODAB concentrations. For E. coli and S. aureus, simultaneous determination of cell viability and electrophoretic mobility as a function of DODAB concentration yields a very good correlation between cell surface charge and cell viability. Negatively charged cells are 100% viable whereas positively charged cells do not survive.
The results show a clear correlation between simple adsorption of entire vesicles generating a positive charge on the cell surfaces and cell death.—Campanhã, M. T. N., E. M. Mamizuka, and A. M. Carmona-Ribeiro. Interactions between cationic liposomes and bacteria: the physical-chemistry of the bactericidal action. J. Lipid Res. 1999. 40: 1495;–1500.
Supplementary key words:
dioctadecyldimethylammonium bromide vesicles, Gram-positive and Gram-negative bacteria, electrophoretic mobility for bacteria, correlation cell charge/cell viability, bacteria aggregation
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INTRODUCTION |
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Liposomes are relatively well established as carriers for antimicrobial and anticancer agents (1) (2). They reduce toxicity of drugs in the target organ by modifying drug distribution and improve the therapeutic index observed with several antimony salts (3) (4), immunomodulators (5) (6), antifungal agents (7) (8), and antibiotics (9) (10). Liposome encapsulation results in sustained local concentrations of antimicrobial agents (11) (12). After in vivo administration via the intravenous route, conventional liposomes are taken up by the reticuloendothelial system (RES) and are potentially useful as antibiotic carriers for treatment of infections involving the RES (13). Alternatively, vesicle size and phospholipid composition may be controlled to change liposome biodistribution and circulation time (14). A general property of conventional liposomes is that by themselves they are generally innocuous.
This work further elucidates the mechanism responsible for the antimicrobial properties of some synthetic cationic liposomes (15) which, by themselves, are not innocuous (15) (16) (17) (18) (19). Dioctadecyldimethylammonium bromide (DODAB) or chloride (DODAC) are quaternary ammonium compounds that form stable and closed bilayers (vesicles) in aqueous solutions (15). The bactericidal, flocculant, and cytotoxic effects of these cationic vesicles on bacteria and cultured mammalian cells were partially described, a differential toxicity being ascribed to DODAB towards bacteria or mammalian cells (16) (17) (18) (19). In this work, we determine: 1) dose and time effects on four different bacteria species of clinical importance at a fixed bacteria concentration; 2) effect of vesicle size on E. coli viability; and 3) simultaneous effect of DODAB vesicles on bacteria electrophoretic mobility and viability. An interesting correlation between sign of the cell surface charge and cell viability is found: negatively charged cells are 100% viable whereas positively charged cells do not survive (ca. 0% viability).
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MATERIALS AND METHODS |
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Organisms and culture conditions
Escherichia coli O111H- isolated from diarrheic human feces were obtained from the culture collection at the Universidade Federal de São Paulo (UNIFESP), São Paulo, S.P., Brazil. Salmonella typhimurium ATCC 14028, Staphylococcus aureus ATCC 25923, or Pseudomonas aeruginosa ATCC 27853 were reactivated for 2;–5 h at 37°C in 3 mL of Tryptic Soy Broth, TSB (Oxoid, Unipath Ltd., Basingstoke, Hampshire, UK). Thereafter, bacteria were spread on plates of MacConkey agar (Difco Laboratories, Detroit, MI) for the Gram-negative bacteria or on plates of blood agar for S. aureus and incubated (37°C/24 h). Two colonies of each species were taken from the plates and incubated in 3 mL of liquid TSB (150 rpm, 37°C, 2 h). Thereafter, 3 mL of each inoculum was added to 27 mL of liquid TSB for further incubation (150 rpm, 37°C, 3 h). Finally, 25 mL of each culture was pelleted and separated from its nutritive medium by centrifugation (8000 rpm/15 min). The supernatant was replaced by sterile water and the bacteria pellet was resuspended. The centrifugation/resuspension procedure was repeated twice before using the bacteria for evaluating DODAB bactericidal effects. Bacteria number densities were determined from agar plating and colony forming units (CFU) counting, turbidity against a MacFarland scale, and a correlation established between bacteria number density (obtained by CFU counting) and turbidity at 400 nm. For E. coli, a growth control was done which established the culture obtained as above as a suspension in the stationary phase.
Chemicals
Dioctadecyldimethylammonium bromide 99.9% pure (DODAB) was obtained from Fluka Chemie AG (Switzerland) and used as such without further purification. All other reagents were analytical grade and were used without further purification. Water was Milli-Q quality.
Vesicles preparation
Small unilamellar DODAB vesicles (SV) with 86 ± 1 nm mean diameter (20) were prepared by ultrasonic dispersion with tip in water or in an isotonic 0.264 M D-glucose solution (21). As no effects on viability were obtained for suspensions in water or in 0.264 M D-glucose, most assays for bacteria were performed in water or water solutions. SV were centrifuged at 10,000 rpm for 1 h at 15°C to precipitate multilamellar liposomes and titanium particles ejected from the titanium probe during sonication. The supernatant containing the unilamellar vesicles was used within 1 h of the preparation. Large unilamellar DODAB vesicles (LV) with 250 ± 12 nm mean diameter were prepared by vaporization of a chloroformic DODAB solution as previously described (21). DODAB concentrations were determined by microtitration (22). One should notice that DODAB vesicles contain 100% DODAB.
Viability assays
Colony forming units (CFU) counting was obtained as a function of DODAB concentration at 1 h of interaction time between bacteria and vesicles or as a function of time at 5 µM DODAB. Both time and dose effects were obtained at a fixed bacteria concentration of 2.5 x 107 viable bacteria/mL. After interaction between vesicles and cells, mixtures were diluted (1:20000) and 0.1 mL of the diluted mixtures was spread on agar plates. After spreading, plates were incubated for 24 h at 37°C. CFU counts were made using a colony counter.
Microelectrophoresis of E. coli and S. aureus in the presence of the cationic liposomes
DODAB vesicles over a range of DODAB concentrations (or interaction times) and bacteria were mixed to yield a final bacteria concentration of 2.5 x 107 bacteria/mL, allowed to interact for 1 h at room temperature, and placed in a flat cell to determine bacteria electrophoretic mobility (EM). One should notice that the measurements were done precisely at the same experimental conditions used to obtain the viability curves for E. coli or S. aureus as a function of DODAB concentration in the mixtures. Mobilities were determined using a Rank Brothers microelectrophoresis apparatus (Cambridge, England) with a flat cell at 25°C. The sample to be measured was placed into the electrophoresis cell, electrodes were connected, and a voltage of 60 V was applied across the cell. Velocities of individual bacteria over a given tracking distance were recorded, as was direction of bacteria movement. Average velocities were calculated from data on at least 20 individual bacteria. EM was calculated according to the equation EM = cm(u/V)(1/t), where u is the distance over which the particle is tracked (micrometers), cm is the interelectrodes distance (7.27 cm), V is the voltage applied (±60 V), and t is the average time in seconds required to track one particle a given distance u.
Determination of viability for flocculated bacteria
DODAB—induced bacteria flocculation is described elsewhere (18). Assuming neutralization of the cell surface charge by DODAB and an interaction time between bacteria and vesicles larger than the half-time for flocculation, bacteria flocculation should be diffusion-controlled (18) (19). From the DODAB concentration necessary to neutralize E. coli cells at EM = 0 (18) and flocculation kinetics followed over times larger than the half-time for flocculation, the flocculation effect on bacteria viability determined by plating was evaluated. Bacteria flocculation was determined by measuring turbidity at 400 nm as a function of time after vesicle addition using a Hitachi U-2000 spectrophotometer in the double beam mode. In the cuvette used as a reference, a bacteria suspension at the same bacteria concentration as that in the sample cuvette was added of pure water instead of adding the DODAB vesicles dispersion. Thus, turbidity measured after vesicle addition is basically due to bacteria aggregation induced by the vesicles. The time lag between mixing and recording was usually smaller than 10 s. Before dilution and plating of the flocculated samples, the occurrence of flocculation was double checked by direct microscopic visualization using an optical microscope.
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RESULTS AND DISCUSSION |
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Susceptibilities of different bacteria species towards DODAB cationic vesicles
The effect of DODAB concentration (C) on cell viability at a fixed bacteria concentration (ca. 2.5 x 107 bacteria/mL) and interaction time (1 h) for 4 different model microorganisms is shown in Figure 1. As cell concentration is the same for the 4 species, all other experimental conditions also being the same, susceptibilities of the 4 bacteria towards DODAB can be compared for 50% and 5% viability ( Table 1). The more resistant bacteria is E. coli, which requires ca. 50 µM DODAB to remain 5% viable (Table 1). E. coli is followed by S. typhimurium, P. aeruginosa, and the Gram-positive bacterium S. aureus (Table 1). Table 1 also illustrates the consistency between the present data and data previously published by our group. For an interaction time of 2 h, at 8 µM DODAB, 0% viability for S. aureus was previously obtained (17) consistent with the 5% viability at 12 µM DODAB for a smaller interaction time, 1 h (Table 1). One should notice the very large cell number density in the interaction mixtures with DODAB (Figure 1). Because DODAB-induced bacteria flocculation was previously described at these high number densities, plating of aggregated viable cells could yield underestimated CFU counts, as an aggregate of several viable cells would result in a single count on the plate. In order to investigate this possibility, viability for flocculated against non-flocculated bacteria samples was determined. Two different experimental conditions at charge neutralization on the cell surface were tested: one yielding extensive flocculation at 4 x 108 bacteria/mL and the other yielding basically no flocculation at all, at 4 x 107 bacteria/mL ( Table 2). Because the expected viability of 50% was indeed obtained for both samples, viability underestimation due to DODAB-induced clustering of several viable cells yielding a single count is not a possibility. Possibly, the dilution procedure done before plating caused bacteria defflocculation.
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Figure 1.
Cell viability (%) as a function of DODAB concentration at 1 h of interaction time between DODAB SV and bacteria. About 100% of viability was obtained for each control sample containing bacteria only (in absence of DODAB). Before plating 0.1 mL in agar, interaction mixtures were diluted 1:20000. Viable bacteria number densities in each interacting mixture were fixed at ca. 2.5 x 107 CFU/mL for the four bacteria species. For E. coli 2.2 x 107 CFU/mL (a), S. typhimurium 2.0 x 107 CFU/mL (b), S. aureus 2.5 x 107 CFU/mL (c), and P. aeruginosa 3.0 x 107 CFU/mL (d).
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Table 1.
DODAB concentration required for 5% and 50% cell viability for four different bacteria species of clinical importance
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Susceptibilities of the 4 different bacteria species towards DODAB are possibly higher than shown in Table 1. Because our cultures are in the stationary phase and DODAB interacts equally well with viable or nonviable cells in the final bacteria suspension (19), the DODAB dose required for killing a certain number of bacteria in exponential growth, a growth phase for which all cells are viable, is certainly smaller than the dose required to kill all alive cells in the same total number of dead and alive cells at the stationary phase. This reasoning is further confirmed from the comparison between viabilities for E. coli in exponential growth (6.4 x 107 bacteria/mL) (17) and the present results for E. coli in the stationary phase (2.5 x 107 CFU/mL) (Figure 1), both suspensions in the presence of 5 µM DODAB. For the former, a viability of 0% (17) contrasts with 100% of viability obtained for the latter suspension (Figure 1).
The effect of time on cell viability was obtained at a fixed DODAB (5 µM) and bacteria concentration ( Figure 2). One should notice that a comparison between different species cannot be performed if cell number densities are not the same or, at least, very similar. For this reason we fixed the bacteria concentration in order to establish an order for bacteria susceptibilities towards the bactericidal vesicles. The most resistant among the 4 species tested is E. coli (Figure 2, Table 1).
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Figure 2.
Cell viability (%) as a function of interaction time between bacteria and DODAB SV at 5 µM. Viable bacteria number densities in each interacting mixture were fixed at ca. 2.1 x 107 CFU/mL for E. coli (a), 2.8 x 107 CFU/mL for S. typhimurium (b), 2.7 x 107 CFU/mL for S. aureus (c), and 2.8 x 107 CFU/mL for P. aeruginosa (d). Plating on agar (0.1 mL) was done after a 1:20000 dilution of the mixture. Viability determined for control mixtures (in absence of DODAB) was close to 100%.
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Effect of bacteria surface charge and vesicle size on viability
In Figure 3, the simultaneous effect of DODAB concentration on cell viability and electrophoretic mobility for E. coli and S. aureus suspensions at ca. 2.5 x 107 cells/mL is presented. Negatively charged cells have survival percentages close to 100% whereas positively charged cells do not survive. This is better seen by plotting electrophoretic mobility against cell viability for both bacteria species in Figure 4.
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Figure 3.
Electrophoretic mobility (EM) and cell viability for E. coli (2.2 x 107 CFU/mL) and S. aureus (2.5 x 107 CFU/mL) as a function of DODAB concentration. Bacteria and vesicles in the mixtures interacted for 1 h before dilution and plating or EM measurements.
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Figure 4.
Correlation between sign of the cell surface charge and life or death. For both E. coli and S. aureus, positively charged cells do not survive. The correlation was found from data in Figure 3.
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DODAB concentration required for neutralization of the cell surface charge at an E. coli concentration of ca. 2 x 107 bacteria/mL is ca. 7 µM DODAB (18). Because positively charged cells die (Figure 3 and Figure 4), at charge neutralization (EM = 0), when 50% of the cells are negatively charged whereas the other half is positively charged, viability has to be close to 50%. In fact, results in Table 3 confirm this prediction. At 5 µM DODAB, one is slightly below the DODAB amount required to cause charge neutralization, which is 7 µM, and viability is ca. 35 ± 5%, i.e., slightly below the 50% predicted (Table 3).
From adsorption isotherms for small DODAB vesicles or large DODAC vesicles onto E. coli (19) and assuming the Langmuir adsorption model, it is possible to calculate affinity constants for both vesicle types. Affinity of small DODAB vesicles is much lower than affinity exhibited by the large ones. Therefore, the bactericidal effect of large vesicles should be more pronounced than the bactericidal effect exhibited by the small vesicles. The experiment shown on Figure 5 confirms this reasoning. Large vesicles are indeed more effective for inducing the bactericidal effect than small vesicles, as expected from the higher affinity constant of the large vesicles towards the cell surface.
In summary, DODAB seems to act differently when compared with other cationics that disrupt the cell membrane. The mechanism of DODAB bactericidal action possibly involves damage to protein function at the bacterial external wall level, where vesicles do adhere without vesicle rupture or cell lysis (17).
The aim of this study was threefold: 1) to reconfirm the bactericidal effect of cationic vesicles previously described for E. coli (18) (19) establishing an order of bacterial susceptibilities towards DODAB dispersions; 2) to observe whether vesicle size would be important in determining the bactericidal action; and 3) to investigate the mechanism of death as related to the cell surface charge. We have shown that bacterial death caused by DODAB vesicles is associated with a positive charge on the cell surface (Figure 3 and Figure 4).
From the point of view of antibiotics incorporation in DODAB or DODAC liposomes, conventional cationic liposomes usually encapsulate antibiotics such as amikacin, netilmicin and tobramycin (23) more efficiently, a fact that reinforces the importance of prospective applications of our bactericidal liposomes as antibiotics encapsulators. A possible synergistic action between antimicrobial liposomes such as DODAB SV or LV and antibiotics may become useful in clinics. Also, bacterial cell lysis was demonstrated to be absent both for a Gram-negative (E. coli) and a Gram-positive (S. aureus) bacterium, in clear contrast to observations by other authors that detected lytic effects induced by micelle-forming quaternary ammonium surfactants (24) (25). Thus, bilayer-forming surfactants such as DODAB and DODAC clearly behave differently and must cause death via a mechanism that does not include cell lysis. Consistently, interaction between bacteria and large DODAC vesicles did not induce leakage of intravesicle contents (17). Vesicle fusion with the cell membrane would necessarily lead to leakage. Therefore, lipids from the cell membrane and DODAB molecules from the vesicle do not seem to mix, as expected for lipids that are in different physical states (lipids from the cell membrane are in the liquid;–crystalline, more fluid state whereas DODAB or DODAC are in the gel, rigid state). In fact, DODAC or DODAB are not micellizing agents capable of disrupting cell membranes. Furthermore, we have previously quantified the adsorbed DODAB amount on the surface of the bacterial cells (19). At limiting adsorption, a palisade of vesicles, i.e., a single layer of adjacent vesicles, was demonstrated to surround the cell surface (19). The palisade model adequately explained the amphiphile amount adsorbed on the bacterial surface. Thus, mere adhesion of entire vesicles to the cell seems to have devastating effects that lead to cell death. A strong possibility is functional damage of membrane proteins responsible for transport through the bacterial cell wall in the case of the Gram-negative bacteria. Would cell death involve obstruction of porins by entire, non-disrupted vesicles? This is a possibility that requires further investigation.
The results presented in this work establish DODAB or DODAC liposomes by themselves as effective bactericides, pointing out the differential killing effect of a positive charge on the cell surface, in contrast to the usually innocuous effect of conventional liposomes used as drug carriers. Regarding the potential clinical utility of DODAB as a bactericide, the present work conducted in water at practically zero ionic strength may be at odds with any clinical use as under physiological conditions ionic strength is much higher. The low colloidal stability of DODAB vesicles in the presence of salt could be circumvented by using DODAB in a mixture of lipids capable of yielding vesicles of high colloidal stability under physiological conditions. This possibility is being investigated in our laboratory. Furthermore, a bactericidal polyelectrolyte, which as a cationic vesicle is active at low concentrations and ionic strength, may have potential use in preventing enteropathogenic bacterial contamination of water. More importantly, DODAB concentrations required for the bactericidal effect are of the order of 0.01;–0.1 mM (Figure 1) whereas 40;–50% of death for cultured mammalian cells was measured by flow cytometry in the presence of 1 mM DODAB (16). Hence, the bactericidal cationic liposomes do present a potential therapeutic use hitherto unexplored.
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FOOTNOTES |
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1 Present address: Departamento de Análises Clínicas e Toxicológicas, Faculdade de Ciências Farmacêuticas, USP, CP 66083, São Paulo SP, Brazil. 
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ACKNOWLEDGMENTS |
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M. T. N. Campanhã thanks FAPESP for a graduate fellowship (96/07848-8). FAPESP and CNPq are gratefully acknowledged for research grants 96/0704-0 and 520186/96.6, respectively.
Manuscript received December 8, 1998; and in revised form March 9, 1999.
Abbreviations:
DODAB, dioctadecyldimethylammonium bromide; RES, reticuloendothelial system; CFU, colony forming units; SV, small unilamellar vesicles; LV, large unilamellar vesicles; EM, electrophoretic mobility; DODAC, dioctadecyldimethylammonium chloride
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TOP
ABSTRACT
INTRODUCTION
) or large (
) DODAB vesicles prepared using sonication with tip or vaporization of a chloroformic solution, respectively. In comparison to the small vesicles, large vesicles have a larger affinity constant (19) for the bacterium surface consistent with their higher efficiency as bactericides.
