Potential therapeutic applications of microbial surface-active compounds

Microorganisms synthesise a wide range of surface-active compounds (SAC), generally called biosurfactants. These compounds are mainly classified according to their molecular weight, physico-chemical properties and mode of action. The low-molecular-weight SACs or biosurfactants reduce the surface tension at the air/water interfaces and the interfacial tension at oil/water interfaces, whereas the high-molecular-weight SACs, also called bioemulsifiers, are more effective in stabilising oil-in-water emulsions. Biosurfactants are attracting much interest due to their potential advantages over their synthetic counterparts in many fields spanning environmental, food, biomedical, and other industrial applications. Their large-scale application and production, however, are currently limited by the high cost of production and by limited understanding of their interactions with cells and with the abiotic environment. In this paper, we review the current knowledge and the latest advances in biosurfactant applications and the biotechnological strategies being developed for improving production processes and future potential.

Surfactants are surface-active compounds capable of reducing surface and interfacial tension at the interfaces between liquids, solids and gases, thereby allowing them to mix or disperse readily as emulsions in water or other liquids. The enormous market demand for surfactants is currently met by numerous synthetic, mainly petroleum-based, chemical surfactants. These compounds are usually toxic to the environment and non-biodegradable. They may bio-accumulate and their production, processes and by-products can be environmentally hazardous. Tightening environmental regulations and increasing awareness for the need to protect the ecosystem have effectively resulted in an increasing interest in biosurfactants as possible alternatives to chemical surfactants. Biosurfactants are amphiphilic compounds of microbial origin with considerable potential in commercial applications within various industries. They have advantages over their chemical counterparts in biodegradability and effectiveness at extreme temperature or pH and in having lower toxicity. Biosurfactants are beginning to acquire a status as potential performance-effective molecules in various fields. At present biosurfactants are mainly used in studies on enhanced oil recovery and hydrocarbon bioremediation. The solubilization and emulsification of toxic chemicals by biosurfactants have also been reported. Biosurfactants also have potential applications in agriculture, cosmetics, pharmaceuticals, detergents, personal care products, food processing, textile manufacturing, laundry supplies, metal treatment and processing, pulp and paper processing and paint industries. Their uses and potential commercial applications in these fields are reviewed.

The global emergence of multidrug-resistant gram-negative bacilli has spurred a renewed interest in polymyxins. Once discarded due to concerns regarding nephrotoxicity and neurotoxicity, polymyxins now hold an important role in the antibiotic armamentarium. However, more reliable information is needed to determine the optimal dosing of these agents. Also, unanswered questions regarding in vitro testing remain, including questions regarding the reliability of automated systems and the establishment of appropriate breakpoints for defining susceptibility. Most contemporary clinical studies examining the use of these agents have involved patients with infections due to multidrug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii strains. It has been reassuring that polymyxin therapy for resistant bacteria has resulted in clinical responses and toxicity rates similar to those for carbapenem therapy for susceptible isolates. While most surveillance studies demonstrated high rates of susceptibility, several reports noted the emergence of polymyxin-resistant nosocomial pathogens. Polymyxins have assumed an important antibiotic niche for therapy for hospital-acquired infections; further studies defining the optimal use of these agents will likely extend the duration of their clinical usefulness.