Influence of adhesion on aerobic biodegradation and bioremediation of liquid hydrocarbons.

Biodegradation of poorly water-soluble liquid hydrocarbons is often limited by low availability of the substrate to microbes. Adhesion of microorganisms to an oil-water interface can enhance this availability, whereas detaching cells from the interface can reduce the rate of biodegradation. The capability of microbes to adhere to the interface is not limited to hydrocarbon degraders, nor is it the only mechanism to enable rapid uptake of hydrocarbons, but it represents a common strategy. This review of the literature indicates that microbial adhesion can benefit growth on and biodegradation of very poorly water-soluble hydrocarbons such as n-alkanes and large polycyclic aromatic hydrocarbons dissolved in a non-aqueous phase. Adhesion is particularly important when the hydrocarbons are not emulsified, giving limited interfacial area between the two liquid phases. When mixed communities are involved in biodegradation, the ability of cells to adhere to the interface can enable selective growth and enhance bioremediation with time. The critical challenge in understanding the relationship between growth rate and biodegradation rate for adherent bacteria is to accurately measure and observe the population that resides at the interface of the hydrocarbon phase.

Adhesion to the hydrocarbon phase increases phenanthrene degradation by Pseudomonas fluorescens LP6a.

Microbial adhesion is an important factor that can influence biodegradation of poorly water soluble hydrocarbons such as phenanthrene. This study examined how adhesion to an oil-water interface, as mediated by 1-dodecanol, enhanced phenanthrene biodegradation by Pseudomonas fluorescens LP6a. Phenanthrene was dissolved in heptamethylnonane and added to the aerobic aqueous growth medium to form a two phase mixture. 1-Dodecanol was non-toxic and furthermore could be biodegraded slowly by this strain. The alcohol promoted adhesion of the bacterial cells to the oil-water interface without significantly changing the interfacial or surface tension. Introducing 1-dodecanol at concentrations from 217 to 4,100 mg l(-1) increased phenanthrene biodegradation by about 30% after 120 h incubation. After 100 h incubation, cultures initially containing 120 or 160 mg l(-1) 1-dodecanol had mineralized >10% of the phenanthrene whereas those incubated without 1-dodecanol had mineralized only 4.5%. The production and accumulation of putative phenanthrene metabolites in the aqueous phase of cultures likewise increased in response to the addition of 1-dodecanol. The results suggest that enhanced adhesion of bacterial cells to the oil-water interface was the main factor responsible for enhanced biodegradation of phenanthrene to presumed polar metabolites and to CO(2).

Two different mechanisms for adhesion of Gram-negative bacterium, Pseudomonas fluorescens LP6a, to an oil-water interface.

Microbial adhesion to the oil-water interface is an important parameter in biodegradation of hydrocarbons to enhance uptake and metabolism of compounds with very low aqueous solubility, but the mechanisms of adhesion are not well understood. Our approach was to study a range of compounds and mechanisms to promote the adhesion of a hydrophilic bacterium, Pseudomonas fluorescens strain LP6a, to an oil-water interface. The cationic surfactants cetylpyridinium chloride (CPC), poly-l-lysine and chlorhexidine gluconate (CHX) and the long chain alcohols 1-dodecanol and farnesol increased the adhesion of P. fluorescens LP6a to n-hexadecane from ca. 30 to 90% of suspended cells adhering. In contrast, adjusting the ionic strength of the suspending medium only increased the adhesion from about 8 to 30%. The alcohols, 1-dodecanol and farnesol, also caused a dramatic change in the oil-water contact angle of the cell surface, increasing it from 24 degrees to 104 degrees , whereas the cationic compounds had little effect. In contrast, cationic compounds changed the electrophoretic mobility of the bacteria, reducing the mean zeta potential from -23 to -7 mV in 0.01 M potassium phosphate buffer, but the alcohols, 1-dodecanol and farnesol, had no effect on zeta potential. Even though both types of compounds promoted cell adhesion to the n-hexadecane interface, the mechanisms were different. Alcohols acted through altering the cell surface hydrophobicity, whereas cationic surfactants changed the surface charge density.