The urgent need for cheap and easy-to-use protection against both unwanted pregnancies and sexually transmitted diseases has stimulated considerable interest in the use of surfactants as microbicides, anti-viral, and contraceptive agents in recent years.
In the present study we report a systematic in vitro evaluation of the microbicidal, anti-viral and contraceptive potential of cationic, anionic, zwitterionic, and non-ionic surfactants. Toxicity was evaluated in mammalian columnar epithelial MDCK cells, human sperm cells, Candida albicans , Escherichia coli , Pseudomonas aeruginosa , Neisseria gonorrhoeae , Streptococcus agalactiae and Enterococcus faecalis.
The inhibition of adenovirus and lentivirus infection of MDCK cells was also tested. A homologous series of cationic surfactants, alkyl-N,N,N-trimethylammonium bromides C n TAB , with varying alkyl chains were shown to be bactericidal and fungicidal at doses that were related to the surfactant critical micelle concentrations CMC , all of them at concentrations significantly below the CMC.
In general, bacteria were more susceptible to this surfactant group than C. This suggests that the C n TAB may be useful as vaginal disinfectants only in so far as bacterial and fungal infections are concerned.
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None of the surfactants examined, including those that have been used in pre-clinical studies, showed inhibition of adenovirus or lentivirus infection of MDCK cells or spermicidal activity at doses that were sub-toxic to MDCK cells.
The treatment and prevention of sexually transmitted diseases STDs is a growing challenge since more and more pathogens are developing multi-drug resistance and effective vaccines do not exist for the majority of them. On the other hand, the world population continues to grow at an alarming rate with a high incidence of unwanted pregnancies. These facts have highlighted the importance of efforts to develop other approaches to prevent STDs and unplanned pregnancies, especially women-targeted methodologies such as vaginal prophylactic products, especially microbicides .
Nonoxynol-9 N-9 , a non-ionic surfactant that is widely used as an over-the counter spermicide was tested as a vaginal prophylactic against HIV infection  ,  and, although this surfactant provides protection against some STDs and, in vitro , destroys HIV  , Phase III studies showed that N-9 tends to irritate vaginal mucosa and facilitate HIV transmission  ,  and that vaginal irritation increases with frequency of use and dose .
The failure of these Phase-III studies have created a sense of urgency to accelerate the pace of research in this area and to increase efforts to study compounds, particularly those that even at high concentrations, appear to have no effect on the structural integrity and function of the cervico-vaginal or rectal epithelium.
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Several candidate microbicides such as vaginal acid-buffering agents that maintain a protective vaginal pH, antiretroviral drugs specific for HIV, inhibitors of viral entry into animal cells and detergents or surfactants have been proposed and some of them are being tested in pre-clinical studies . Because of their amphiphilic nature, surfactants are generally believed to act at the level of the cell membrane.
Differences between the chemical and physical properties of the pathogen and the host membrane as well as the manner in which the membranes are related to the cell physiology could be profitably exploited.
In view of their potential in this regard we attempted to answer the following questions: Are surfactants really useful as vaginal microbicides and contraceptives? Are there good laboratory models to test their utility in this regard?
The answers to these questions require a multi-step, systematic investigation on the effect of different types of surfactants on fungal and bacterial growth, viral infection and the viability of mammalian cells particularly polarized epithelial cells and human sperm cells before any possible application is proposed. The results have also to be related to the physico-chemical properties of the surfactants since these are known to condition the effect of surfactants on lipid bilayer membranes  — .
Therefore, as a first step, we tested anionic sodium dodecylsulfate, SDS , cationic a homologous series of alkyl-N,N,N-trimethyammonium compounds with varying alkyl chain , zwitterionic N-dodecyl-N,N-dimethylammonium-propanesulfonate, DDPS and nonionic Triton X and Nonoxynol-9 detergents.
These surfactants are all commercially available. SDS, known as an antimicrobial agent able to inactivate HIV in vitro  ,  , is widely used in personal hygiene e. C n TAB are also known as microbicides and antifungal agents .
Nonoxynol-9 is a commonly used spermicide and lubricant in condoms and its structural homolog, Triton X was studied because its action on lipid bilayers and cell membranes is well documented.
Since surfactants are amphiphilic molecules, the first locus of their interaction with cells of any kind is the cell membrane and this interaction is related to their partition coefficient between the aqueous and membrane phases which, in its turn, is related to the Critical Micelle Concentration CMC of the surfactant. Therefore, another novelty of this study is that surfactant effects were compared taking into account their CMC. We show here that only the C n TAB were bactericidal and fungicidal at concentrations that were not toxic to mammalian columnar epithelial cells grown to confluence.
At concentrations that were sub-toxic to the mammalian epithelial cells all the surfactants examined were neither spermicidal nor did they prevent viral infection of the epithelial cells. We chose Escherichia coli , Pseudomonas aeruginosa , Neisseria gonorrhoeae , Streptococcus agalactiae , and Enterococcus faecalis as bacterial models. The CMC of the surfactants was used as a reference concentration for the following reason: The formation of surfactant micelles in aqueous media may be considered, under certain conditions, to be similar to a phase separation between an aqueous solution of surfactant monomers and a micellar phase.
The surfactant concentration at which this phase separation occurs is the CMC. Within a given homologous series of surfactants the CMC is linearly proportional to the free energy of partitioning of the apolar part of the surfactant between an apolar environment such as a micelle or the lipid bilayer of a cell membrane and the aqueous phase .
It follows, then, that the toxic effects of any surfactant homologous series must be compared with the CMC as the reference concentration. Even if the surfactants act at some intracellular level, they must cross the cell membrane in order to do so and, again, their partitioning into the apolar part of the membrane becomes a critical step. We propose that the relationship between toxicity and CMC is simply a result of the thermodynamics of partitioning of amphiphiles between the aqueous phase and an apolar environment, be it a micelle or a membrane.
For convenience the CMC of these detergents is listed in the legend to Figure 1. Bacterial growth was assessed by measuring the absorbance at nm as a function of time, in the presence or absence control of detergents. Surfactants were added at time zero lag-phase growing E. Concentrations of the surfactants are given relative to their respective CMC. The results shown are the mean of three independent experiments. Standard deviations are only shown for the control and for those curves in which there is a noticeable but not complete inhibition of growth, these curves are identified by the number that corresponds to the respective surfactant concentration.
Overlapping curves are not individually identified. The results are also shown in Figure 1.
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Growth of the bacteria was inhibited by all three C n TAB homologues albeit at different concentrations relative to their respective CMC. The bromide anion was not the determinant in the toxicity of these surfactants since no differences were found between the toxicities of the C n TAB and their chloride analogues results not shown. The results are shown in Figure 2 and Table 1. Concentrations are given relative to the CMC of each surfactant. Other details are as in Figure 1.
With regard to E. In order to demonstrate the applicability of the above results to other bacteria involved in STD or urinogenital tract related infections we examined the toxicity of the same surfactants towards Pseudomonas aeruginosa , Neisseria gonorrhoeae , Streptocococus agalactiae , and Enterococcus faecalis.
As seen in Figure 3 , the results with P.
In the case of N. This organism was, therefore, cultured on solid media and surfactant toxicity was evaluated by subjecting the organism for 60 min to the different surfactants at different concentrations before plating the cultures.
The results are shown in Table 2. In the case of S. We chose the yeast Candida albicans as a fungal model to test for toxicity of the surfactants examined in this work. This organism is responsible for vulvovaginal candidiasis, a common mucosal infection in women of childbearing age. Furthermore, fungal infections have recently emerged as a growing threat to human health in seriously debilitated and immunocompromised diseases, such as AIDS and cancer .
Figure 6 shows the effect of all the surfactants examined upon growth curves of C.
Although here, as in the case of E. These differences suggest that surfactant toxicity is not just dependent upon the chemical structure of the surfactants but also upon the nature of the cell membrane, the C.
The reason for this choice was that: 1 This cell line is derived from a columnar epithelium and the vaginal columnar epithelium was shown to be the primary site of damage in studies of surfactant use in animal models  ; 2 MDCK cells can be grown to a completely polarized state and there is a vast literature on the nature and properties of this polarized epithelial cell line in culture as we have used it; 3 In culture MDCK cells can be grown to confluence with relatively non-leaky tight junctions.
We studied the toxicity of the surfactants used in the cases of the microorganisms as described above towards the MDCK cells and, in addition, studied the toxicity of N-9 as well since this surfactant is widely used as a spermicide in condoms and was the object of unsuccessful Phase III studies  , . The results showed that 16—18 h post-addition, the surfactants exhibited different degrees of toxicity. However, all surfactants tested exhibited CMC-dependent differences in cytotoxicity i.
It is interesting to note that for the homologous series of cationic surfactants examined the toxicity to MDCK cells was not dependent upon the surfactant chain length in contrast to what was observed in the case of bacteria.
Also, surfactant toxicity towards the mammalian cells is different, both qualitatively and quantitatively, from the toxicity that these same surfactants exhibited towards the bacterial and the fungal models. We suggest that this may have to do with the differences in physico-chemical properties of the cell membranes, the putative loci of interaction of surfactants with cells. Alternatively, surfactant toxicity towards cells may involve interactions that are not limited to the membranes alone.
Microbicides designed to protect against STD pathogens must do so without causing unacceptable toxic effects towards the vaginal epithelial cells.
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Indeed, the protective agents used would ideally be non-toxic towards the epithelial cells but highly toxic to the agents of infection.
In practice, this ideal goal is difficult to attain and one must settle for a preferred toxicity towards the agent of infection, i. Viral infection of the vaginal epithelium is one of the most significant problems in STDs.
We, therefore, tested the effect of surfactants on the ability of viruses to infect polarized MDCK cells.
Adenovirus a non-enveloped virus and lentivirus an enveloped retrovirus were chosen as viral models since they are easily and relatively safely handled in the laboratory both viruses are not able to replicate in humans.
The lentivirus used here is derived from HIV-1 and pseudotyped with the envelope G-protein from the Vesicular Stomatitis virus. This confers higher physical stability  and broader tropism to the virus, which is able to infect a broader range of cell types.
Both viral models were engineered to express a Green Fluorescent Protein GFP which facilitates microscopic inspection and analyses of infection. The surfactant concentrations used were, in all cases, inferior to those that had been previously judged to be toxic to the MDCK cells.
The results Figure 7 , showed that in all cases there were no sub-toxic concentrations of the surfactants with respect to MDCK cells that could inhibit viral infection. It must be noted, however, that no surfactant promoted viral infection, comparable levels of infection being obtained in all cases. Polarized MDCK cells grown on cover slips or polycarbonate filters were exposed to the adenovirus or lentivirus after the addition of the surfactants for about 1 h after which the cells were washed and placed in a fresh detergent- and virus-free culture medium.
Infectivity was assessed 16—18 h or 48 h after virus and surfactant removal. The efficiency of infection was assessed by counting the number of infected cells green cells, insert pictures relatively to the total number of cells given by staining the cells with rhodamine-phalloidin that labels the actin-cytoskeleton red cells, insert pictures.
Triton X, black columns. DDPS, empty columns.
C 10 TAB, light grey columns. C 12 TAB, dark grey columns. C 14 TAB, columns filled with dots. SDS, columns filled with diamonds. Concentrations of the surfactants relative to their respective CMC are indicated at the graph abcissa of B.
Infectivity is expressed as a percentage relative to cells infected in the absence of surfactant.
N-9, a nonionic surfactant, widely used as spermicide with demonstrable in vitro toxicity to a number of species of bacteria and viruses, including HIV,  was used as control. The effect of the surfactants on human sperm viability was measured by fluorescence after addition of SYBR and propidium iodide.
The sperm cells were incubated for 1 or 24 h with different surfactants and the results are summarized in Table 5.
Except for Triton X, sperm viability decreased significantly when incubated with the surfactants, as compared with control. The viable sperm count declined with time and was CMC-dependent. The results indicated again that the toxic effects of surfactants depend on the target cell.