Glycerol monolaurate (GML), a fatty acid monoester, is generally recognized as safe by the Food and Drug Administration for topical skin and mucous membrane uses at doses up to 100 mg/ml; this is based on many years of experience with GML as an additive to cosmetics and foods. Previously, the compound was shown in vitro to inhibit exotoxin production by gram-positive bacteria, including production of staphylococcal toxic shock syndrome (TSS) toxin-1 and alpha-toxin, at concentrations of ≥20 μg/ml, acting at the level of transcription (24
). Streptococci and other gram-positive cocci, which do not produce glycerol ester hydrolases (lipases), are killed by GML at concentrations of ≥10 μg/ml, but production of their exotoxins is inhibited at even lower GML doses (28
). Staphylococcus aureus
and coagulase-negative staphylococci secrete lipases, and thus, the organisms are resistant to killing by GML except at concentrations approaching 500 μg/ml (28
). Studies to assess the effect of GML on normal vaginal microflora lactobacilli have not been performed.
Unlike gram-positive cocci, gram-negative Enterobacteriaceae
are resistant to GML at even very high concentrations (>2,000 μg/ml); however, rough mutants, lacking intact lipopolysaccharide (LPS), are highly susceptible to the bactericidal effects of GML (at concentrations of ≥20 μg/ml), indicating that the external components of LPS facilitate GML resistance (28
). Interestingly, other gram-negative bacteria, such as Helicobacter
, and Gardnerella
spp., which have lipooligosaccharides instead of LPS like Enterobacteriaceae
, are susceptible to the killing effects of GML (≥20 μg/ml) (unpublished observations).
Some studies have evaluated the effect of monoglycerides, including GML, on enveloped viruses. Thormar et al. demonstrated in vitro that GML at concentrations as low as 250 μg/ml causes significant reductions in vesicular stomatitis virus, herpes simplex virus, and visna virus infectious titers (33
). Other investigators also showed monoglycerides in vitro reduce herpes simplex virus 1 and 2 titers, though in these studies, GML was not tested (26
). We have shown in vitro that GML at 100 μg/ml inhibits human immunodeficiency virus type 1 (HIV-1) infection of human T cells (unpublished data).
GML is considered a surfactant, and thus, the compound might be expected to solubilize lipid bilayers (13
). However, unlike many surfactants, GML is relatively insoluble at body temperature, with a solubility limit in aqueous solutions of 50 to 100 μg/ml. We previously showed that the major effect of GML is plasma membrane stabilization and interference of signal transduction rather than solubilization (21
). For example, GML stabilizes red blood cells, preventing lysis by either hypotonic solutions or bacterial cytolysins (21
). GML prevents superantigen and LPS activations of T and B lymphocytes, respectively (36
). The compound inhibits superantigen-induced production of proinflammatory cytokines by human vaginal epithelial cells (HVECs) (21
) and recently was shown to reduce human vaginal interleukin 8 (IL-8) production when added to tampons (20a
). GML does not appear cytotoxic for mammalian cells, but rather their metabolic functions in the presence of GML (100 μg/ml) are slowed (21
). These slowed cellular functions result from transitory GML effects on plasma membranes, returning to normal when GML is eliminated by cells, presumably through the action of mammalian lipases.
We hypothesized that GML may be useful in preventing both vaginal pathogen transmission and pathogenesis because of its in vitro ability to (i) inhibit gram-positive bacterial pathogen growth and exotoxin production, (ii) reduce enveloped virus infection of mammalian cells, and (iii) stabilize mammalian cell plasma membranes. The present studies assessed the safety of GML as a potential topical microbicide to prevent HIV-1 transmission to women, with use of a 5% GML gel administered in the simian immunodeficiency virus-rhesus macaque animal model. Studies to assess its effects on lactobacilli in vitro were also undertaken.
Our studies demonstrate that GML neither has adverse effects on normal flora lactobacilli vaginally nor alters mucosal surface integrity due to its surfactant properties. Even though lactobacilli are gram positive, they are not inhibited from growing in vitro by GML. Additionally, daily vaginal application of GML does not alter vaginal microflora lactobacilli of rhesus monkeys over a period of 6 months. This study provides the first quantitative determination of rhesus monkey aerobic vaginal microflora and indicates that GML potentially may be useful as a safe topical microbicide to prevent transmission and pathogenesis due to vaginal pathogens while at the same time maintaining normal flora lactobacilli and mucosal integrity.