Synergism between penicillin G and the antimicrobial ether lipid, rac-1-dodecylglycerol, acting below its critical micelle concentration

Natural and Enantiopure Alkylglycerols as Antibiofilms Against Clinical Bacterial Isolates and Quorum Sensing Inhibitors of Chromobacterium violaceum ATCC 12472

Resistance mechanisms occur in almost all clinical bacterial isolates and represent one of the most worrisome health problems worldwide. Bacteria can form biofilms and communicate through quorum sensing (QS), which allow them to develop resistance against conventional antibiotics. Thus, new therapeutic candidates are sought. We focus on alkylglycerols (AKGs) because of their recently discovered quorum sensing inhibition (QSI) ability and antibiofilm potential. Fifteen natural enantiopure AKGs were tested to determine their effect on the biofilm formation of other clinical bacterial isolates, two reference strains and their QSI was determined using Chromobacterium violaceum ATCC 12472. The highest biofilm inhibition rates (%) and minimum QS inhibitory concentration were determined by a microtiter plate assay and ciprofloxacin was used as the standard antibiotic. At subinhibitory concentrations, each AKG reduced biofilm formation in a concentration-dependent manner against seven bacterial isolates, with values up to 97.2%. Each AKG displayed QSI at different levels of ability without affecting the growth of C. violaceum. AKG (2S)-3-O-(cis-13’-docosenyl)-1,2-propanediol was the best QS inhibitor (20 μM), while (2S)-3-O-(cis-9’-hexadecenyl)-1,2-propanediol was the least effective (795 μM). The results showed for the first time the QSI activity of this natural AKG series and suggest that AKGs could be promising candidates for further studies on preventing antimicrobial resistance.

Characterizing the Membrane-Disruptive Behavior of Dodecylglycerol Using Supported Lipid Bilayers

Monoglycerides are esterified adducts of fatty acid and glycerol molecules that disrupt phospholipid membranes, leading to a wide range of biological functions such as antimicrobial activity. Among monoglycerides, glycerol monolaurate (GML) exhibits particularly high antimicrobial activity, although enzymatic hydrolysis of its ester group can diminish potency. Consequently, there have been efforts to identify more chemically stable versions of GML, most notably its alkylglycerol ether equivalent called dodecylglycerol (DDG). However, despite high structural similarity, biological studies indicate that DDG and GML are not functionally equivalent and it has been speculated that the two compounds might have different interaction profiles with phospholipid membranes. To address this outstanding question, herein, we employed supported lipid bilayer (SLB) platforms to experimentally characterize the interactions of DDG with phospholipid membranes. Quartz crystal microbalance-dissipation experiments identified that DDG causes concentration-dependent membrane morphological changes in SLBs and the overall extent of membrane remodeling events was greater than that caused by GML. In addition, time-lapsed fluorescence microscopy imaging experiments revealed that DDG causes extensive membrane tubulation that is distinct from how GML induces membrane budding. We discuss how differences in the head group properties of DDG and GML contribute to distinct membrane interaction profiles, offering insight into how the molecular design of DDG not only improves chemical stability but also enhances membrane-disruptive activity.

In vitro surfactant structure-toxicity relationships: implications for surfactant use in sexually transmitted infection prophylaxis and contraception

BACKGROUND

The need for woman-controlled, cheap, safe, effective, easy-to-use and easy-to-store topical applications for prophylaxis against sexually transmitted infections (STIs) makes surfactant-containing formulations an interesting option that requires a more fundamental knowledge concerning surfactant toxicology and structure-activity relationships.

METHODOLOGY/PRINCIPAL FINDINGS

We report in vitro effects of surfactant concentration, exposure time and structure on the viability of mammalian cell types typically encountered in the vagina, namely, fully polarized and confluent epithelial cells, confluent but non-polarized epithelial-like cells, dendritic cells, and human sperm. Representatives of the different families of commercially available surfactants--nonionic (Triton X-100 and monolaurin), zwitterionic (DDPS), anionic (SDS), and cationic (C(n)TAB (n = 10 to 16), C(12)PB, and C(12)BZK)--were examined. Triton X-100, monolaurin, DDPS and SDS were toxic to all cell types at concentrations around their critical micelle concentration (CMC) suggesting a non-selective mode of action involving cell membrane destabilization and/or destruction. All cationic surfactants were toxic at concentrations far below their CMC and showed significant differences in their toxicity toward polarized as compared with non-polarized cells. Their toxicity was also dependent on the chemical nature of the polar head group. Our results suggest an intracellular locus of action for cationic surfactants and show that their structure-activity relationships could be profitably exploited for STI prophylaxis in vaginal gel formulations. The therapeutic indices comparing polarized epithelial cell toxicity to sperm toxicity for all surfactants examined, except C(12)PB and C(12)BZK, does not justify their use as contraceptive agents. C(12)PB and C(12)BZK are shown to have a narrow therapeutic index recommending caution in their use in contraceptive formulations.

CONCLUSIONS/SIGNIFICANCE

Our results contribute to understanding the mechanisms involved in surfactant toxicity, have a predictive value with regard to their safety, and may be used to design more effective and less harmful surfactants for use in topical applications for STI prophylaxis.

In vitro antimicrobial activity and mechanism of action of novel carbohydrate fatty acid derivatives against Staphylococcus aureus and MRSA

AIMS

This study investigates the antimicrobial activity and mode of action of novel carbohydrate fatty acid (CFA) derivatives against Staphylococcus aureus and methicillin-resistant Staph. aureus (MRSA).

METHODS AND RESULTS

Minimum inhibitory concentrations (MICs) and the effect of CFA derivatives on lag phase were determined using a broth microdilution method. Lauric acid carbohydrate esters and corresponding ether analogues showed the greatest antimicrobial activity with MIC values between 0.04 and 0.16 mmol l(-1). Leakage studies at 260 nm following exposure to CFA derivatives at 4x MIC showed a significant increase in membrane permeability for all compounds, after c. 15 min exposure except for the lauric beta ether CFA derivative. Further assessment using both BacLight and luminescence ATP assays confirmed that an increase in membrane permeability and reduced metabolic activity was associated with CFA treatment.

CONCLUSIONS

All strains were significantly inhibited by the novel compounds studied, and efficacy was related to specific structural features. Cell-membrane permeabilization was associated with CFA treatment and may account for at least a component of the mode of action of these compounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study reports the antimicrobial action of CFA compounds against a range of Staph. aureus and MRSA strains, and provides insights into their mode of action.

Glycerol monolaurate and dodecylglycerol effects on Staphylococcus aureus and toxic shock syndrome toxin-1 in vitro and in vivo

Background

Glycerol monolaurate (GML), a 12 carbon fatty acid monoester, inhibits Staphylococcus aureus growth and exotoxin production, but is degraded by S. aureus lipase. Therefore, dodecylglycerol (DDG), a 12 carbon fatty acid monoether, was compared in vitro and in vivo to GML for its effects on S. aureus growth, exotoxin production, and stability.

Methodology/Principal Findings

Antimicrobial effects of GML and DDG (0 to 500 µg/ml) on 54 clinical isolates of S. aureus, including pulsed-field gel electrophoresis (PFGE) types USA200, USA300, and USA400, were determined in vitro. A rabbit Wiffle ball infection model assessed GML and DDG (1 mg/ml instilled into the Wiffle ball every other day) effects on S. aureus (MN8) growth (inoculum 3×10⁸ CFU/ml), toxic shock syndrome toxin-1 (TSST-1) production, tumor necrosis factor-α (TNF-α) concentrations and mortality over 7 days. DDG (50 and 100 µg/ml) inhibited S. aureus growth in vitro more effectively than GML (p<0.01) and was stable to lipase degradation. Unlike GML, DDG inhibition of TSST-1 was dependent on S. aureus growth. GML-treated (4 of 5; 80%) and DDG-treated rabbits (2 of 5; 40%) survived after 7 days. Control rabbits (5 of 5; 100%) succumbed by day 4. GML suppressed TNF-α at the infection site on day 7; however, DDG did not (<10 ng/ml versus 80 ng/ml, respectively).

Conclusions/Significance

These data suggest that DDG was stable to S. aureus lipase and inhibited S. aureus growth at lower concentrations than GML in vitro. However, in vivo GML was more effective than DDG by reducing mortality, and suppressing TNF-α, S. aureus growth and exotoxin production, which may reduce toxic shock syndrome. GML is proposed as a more effective anti-staphylococcal topical anti-infective candidate than DDG, despite its potential degradation by S. aureus lipase.

Synergism between the antifungal agents amphotericin B and alkyl glycerol ethers

The alkyl glycerol ether rac-1-O-dodecylglycerol inhibited the growth of members of two genera of yeasts, Candida and Cryptococcus, and was strongly synergistic with amphotericin B. At one-half its MIC, dodecylglycerol decreased the MIC of amphotericin B by as much as 80-fold. This high degree of synergism between dodecylglycerol and amphotericin B was demonstrated against a number of species of yeasts including Candida albicans, Candida tropicalis, Candida parapsilosis, Cryptococcus neoformans, Cryptococcus albidus, and Cryptococcus laurentii. All fractional inhibitory concentrations (for all strains and species) were calculated to be less than 1, and most were less than 0.6, again demonstrating strong synergism. Other alkyl glycerol ethers with alkyl chain lengths ranging from 8 to 18 carbon atoms were also found to be synergistic with amphotericin B against C. neoformans and C. albicans. Electron microscopy experiments showed that C. neoformans grown in the presence of dodecylglycerol had severely abnormal, deformed capsules. Although the mechanism of action of dodecylglycerol is not known, dodecylglycerol was not simply acting as a detergent. The natural detergent sodium deoxycholate could not substitute for dodecylglycerol. At comparable and higher concentrations, sodium deoxycholate had no fungicidal effect on its own, nor did it potentiate the activity of amphotericin B. Dodecylglycerol did not interact synergistically with the water-soluble antifungal agent fluconazole. The lipid-soluble hydrophobic properties of amphotericin B appear to be important for this synergistic effect, in that alkyl glycerol ethers could promote synergism with amphotericin B by potentially increasing the interaction between membrane-bound ergosterol and amphotericin B.

Regulation of neuronal differentiation in neuron-enriched primary cultures from embryonic rat cerebra by platelet activating factor and the structurally related glycerol ether lipid, dodecylglycerol

Primary cultures enriched in neurons dissociated from embryonic rat cerebra were used to demonstrate that platelet activating factor and the structurally related ether glycerolipid, dodecylglycerol, are readily taken up in small amounts by neurons and that they stimulate the differentiation of neurons. The stimulation of neuronal differentiation was observed as a precocious development of axon-like extensions which correlated with a concentration-dependent increase in neuronal-specific enzyme activities. This stimulation of morphological and neurochemical factors by either platelet activating factor or dodecylglycerol was almost completely abolished by triazolam, a known inhibitor of platelet activating factor function. Neither platelet activating factor nor dodecylglycerol at the concentrations used to achieve stimulation of neuronal differentiation compromised the plasma membrane, as indicated by the lack of leakage of cytoplasmic lactic acid dehydrogenase.