Microbial assimilation of hydrocarbons. II. Intracytoplasmic membrane induction in Acinetobacter sp

Transcriptional profiling of genes involved in n-hexadecane compounds assimilation in the hydrocarbon degrading Dietzia cinnamea P4 strain

The petroleum-derived degrading Dietzia cinnamea strain P4 recently had its genome sequenced and annotated. This allowed employing the data on genes that are involved in the degradation of n-alkanes. To examine the physiological behavior of strain P4 in the presence of n-alkanes, the strain was grown under varying conditions of pH and temperature. D. cinnamea P4 was able to grow at pH 7.0–9.0 and at temperatures ranging from 35 ºC to 45 ºC. Experiments of gene expression by real-time quantitative RT-PCR throughout the complete growth cycle clearly indicated the induction of the regulatory gene alkU (TetR family) during early growth. During the logarithmic phase, a large increase in transcriptional levels of a lipid transporter gene was noted. Also, the expression of a gene that encodes the protein fused rubredoxin-alkane monooxygenase was enhanced. Both genes are probably under the influence of the AlkU regulator.

Synthesis of rhamnolipid biosurfactant and mode of hexadecane uptake by Pseudomonas species

Background

Microorganisms have devised ways by which they increase the bioavailability of many water immiscible substrates whose degradation rates are limited by their low water solubility. Hexadecane is one such water immiscible hydrocarbon substrate which forms an important constituent of oil. One major mechanism employed by hydrocarbon degrading organisms to utilize such substrates is the production of biosurfactants. However, much of the overall mechanism by which such organisms utilize hydrocarbon substrate still remains a mystery.

Results

With an aim to gain more insight into hydrocarbon uptake mechanism, an efficient biosurfactant producing and n-hexadecane utilizing Pseudomonas sp was isolated from oil contaminated soil which was found to produce rhamnolipid type of biosurfactant containing a total of 13 congeners. Biosurfactant action brought about the dispersion of hexadecane to droplets smaller than 0.22 μm increasing the availability of the hydrocarbon to the degrading organism. Involvement of biosurfactant was further confirmed by electron microscopic studies. Biosurfactant formed an emulsion with hexadecane thereby facilitating increased contact between hydrocarbon and the degrading bacteria. Interestingly, it was observed that "internalization" of "biosurfactant layered hydrocarbon droplet" was taking place suggesting a mechanism similar in appearance to active pinocytosis, a fact not earlier visually reported in bacterial systems for hydrocarbon uptake.

Conclusion

This study throws more light on the uptake mechanism of hydrocarbon by Pseudomonas aeruginosa. We report here a new and exciting line of research for hydrocarbon uptake involving internalization of biosurfactant covered hydrocarbon inside cell for subsequent breakdown.

The concentrations of hexadecane and inorganic nutrients modulate the production of extracellular membrane-bound vesicles, soluble protein, and bioemulsifier by Acinetobacter venetianus RAG-1 and Acinetobacter sp. strain HO1-N

In the present study, we addressed the possibility that the production of both bioemulsifiers and membrane-bound vesicles may be a common feature of the growth of Acinetobacter spp. on alkanes, and we determined the extent to which the release of extracellular products by these organisms is regulated by the concentrations of the alkane substrate and inorganic nutrients. To accomplish this objective, we grew Acinetobacter venetianus RAG-1 and Acinetobacter sp. strain HO1-N with different concentrations of nutrients and assayed for extracellular products. The results indicated that the release of vesicles, soluble protein, and bioemulsifier was promoted in various degrees by higher concentrations of hexadecane and inorganic nutrients, while the specific activities of the bioemulsifiers were enhanced with lower nutrient concentrations. Based on our findings, we propose that under conditions of nutrient excess, these strains produce membrane-bound vesicles to function in "luxury uptake" of the alkane substrate for delivery and storage in the form of inclusions. Under the same conditions, soluble bioemulsifier and protein may perform auxiliary roles in cell desorption and (or) alkane uptake. With low concentrations of nutrients, the decreased production of vesicles, protein, and bioemulsifier and the increased activity of the emulsifier may represent a mechanism for reducing biosynthetic demands and conserving cellular material.

Selective transport and accumulation of alkanes by Rhodococcus erythropolis S+14He

Selective transport and accumulation of n-alkanes by Rhodococcus erythropolis S+14He was studied by growing cells on n-hexadecane, n-octadecane or the branched alkane pristane, and on mixtures of hydrocarbons. Ultrastructural analysis by transmission electron microscopy (TEM) revealed hydrocarbon inclusion bodies present in cells grown on the three alkanes, but not in cells grown on soluble media or exposed to nonmetabolized 2,2,4,4,6,8,8-heptamethylnonane (HMN). n-Hexadecane had the highest rates of accumulation within the cells and higher overall consumption rates relative to the other alkanes. These rates decreased when the molar concentration of n-hexadecane was decreased in hydrocarbon mixtures, but at the same time the accumulation of n-hexadecane in intracellular inclusions became increasingly selective. Sodium azide significantly inhibited the accumulation of n-hexadecane, consistent with an active transport mechanism for accumulation. These results indicate that R. erythropolis S+14He is able to selectively discriminate and preferentially transport n-hexadecane from mixtures of structurally similar alkanes into intracellular inclusions by an energy-driven transport system. This selective membrane transport of hydrocarbon isomers has potential application for separations, bioprocessing, and the development of novel biosensors.

Degradation of diclofop-methyl by pure cultures of bacteria isolated from Manitoban soils

Pure cultures of Chryseomonas luteola and Sphingomonas paucimobilis isolated from Manitoban soils were able to utilize diclofop-methyl (methyl-2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate) as the sole source of carbon and energy. An actively growing culture of C. luteola completely degraded 1.5 micrograms diclofop-methyl.mL-1 to diclofop acid and 4-(2,4-dichlorophenoxy)phenol within 71 h, as determined by gas chromatographic analysis. The accumulation of these metabolites in the growth medium resulted in the cessation of growth, indicating the organism's inability to degrade phenoxyphenol in the presence of diclofop acid. Sphingomonas paucimobilis mineralized 1.5 micrograms diclofop-methyl.mL-1 to diclofop acid within 54 h. A biphasic growth pattern indicated that this organism was capable of degrading diclofop acid to 4-(2,4-dichlorophenoxy)phenol and 2,4-dichlorophenol and (or) phenol. Neither of the organisms was able to utilize 2,4-dichlorophenol as the sole source of carbon and energy.

Metabolism of n-alkanes by Aspergillus japonicus

Cells of Aspergillus japonicus could degrade n-alkanes as a sole source of carbon. One of the pathways operative during the degradative process was the terminal pathway. Electron micrographs showed that the hydrocarbons were present in the cells as microdroplets.

Ultracytochemical localization of aldehyde dehydrogenase in Acinetobacter calcoaceticus

A membrane-bound aldehyde dehydrogenase is induced in Acinetobacter calcoaceticus grown on aliphatic hydrocarbons as sole carbon source. This enzyme is NADP-dependent and is able to oxidize medium- and long-chain aliphatic aldehydes to their corresponding fatty acids. Electron micrographs of sectioned alkane-adapted bacteria showed hydrocarbon inclusions in the cytoplasmic matrix. The cytochemical phenazine methosulphate-tetranitro tetrazolium blue capture reaction allowed to localize the activity of the aldehyde dehydrogenase near the surface of these inclusions. At the same location we also found a NADPH tetrazolium-reducing activity.

Isolation and Characterization of a Cyclohexane-Metabolizing Xanthobacter sp

An unusual Xanthobacter sp., capable of independent growth on cyclohexane as the sole source of carbon and energy, has been isolated from soil by using classical enrichment techniques. The mean generation time for growth on cyclohexane was 6 h. The microorganism showed a limited ability to utilize hydrocarbons, with only alicyclic hydrocarbons closely related to cyclohexane supporting growth. Ultrastructural studies indicated the presence of electron-transparent vesicles in the cyclohexane-grown Xanthobacter sp., but the presence of complex intracytoplasmic membranes could not be identified. A soluble inducible enzyme capable of oxidizing cyclohexane was identified in cell extracts. This enzyme had a pH optimum of 6.5, an absolute specificity for NADPH, and a stoichiometric requirement for molecular O 2 which was consistent with the formation of cyclohexanol. The enzyme showed no activity towards straight chain alkanes and only a limited activity towards unsaturated ring compounds. Enzymatic studies with cell extracts have indicated the main route of metabolism of cyclohexane by this Xanthobacter sp. to proceed via cyclohexane → cyclohexanol → cyclohexanone → 1-oxa-2-oxocycloheptane (ε-caprolactone) → 6-hydroxyhexanoate (6-hydroxycaproate) → → adipic acid. Alternative routes involving initial double hydroxylation of the cyclohexane ring may operate fortuituously but are unlikely to represent major pathways for the dissimilation of cyclohexane by this microorganism.

Ultrastructural alterations at the cell wall and plasma membrane of Candida spec. H induced by n-alkane assimilation

The ultrastructure of n-alkane-grown cells of Candida spec. H was investigated by thin sectioning and freeze-etching techniques. On the plasma membrane shallow depressions could be observed. n-Alkane deposits were localized within the cell wall and at the plasma membrane. Peri-plasmic multilamellar structure appear in connection with n-alkane accumulation. Their participation in hydrocarbon assimilation and the nature of the lamellar structures are discussed.