Production of microbial oil with high oleic acid content by Trichosporon capitatum

Microbial oils with high unsaturated fatty acids content, especially oleic acid content, are good feedstock for high quality biodiesel production. Trichosporon capitatum was found to accumulate lipid with around 80% oleic acid and 89% total unsaturated fatty acids content on nitrogen-limited medium. In order to improve its lipid yield, effects of medium components and culture conditions on cell growth and lipid accumulation were investigated. Optimization of media resulted in a 61% increase in the lipid yield of T. capitatum after cultivation at 28 °C and 160 rpm for 6 days. In addition, T. capitatum could grow well on cane molasses and afford a lipid yield comparable to that on synthetic nitrogen-limited medium. The biodiesel from the microbial oil produced by T. capitatum on cane molasses displayed a low cold filter plugging point (−15 °C), and so T. capitatum might be a promising strain to provide lipid suitable for high quality biodiesel production.     

Neochloris oleoabundans grown on anaerobically digested dairy manure for concomitant nutrient removal and biodiesel feedstock production

Microalgae have been investigated as a promising biodiesel feedstock; however, large-scale production is not currently cost-competitive with petroleum diesel, and its environmental impacts have received little attention. Using wastewater to supply nutrients for algal growth obviates synthetic fertilizer use, provides on-site nutrient removal, and reduces greenhouse gas emissions. In this work, anaerobically digested dairy manure was used to grow the oleaginous green alga Neochloris oleoabundans. In batch culture experiments with both synthetic media and anaerobic digester effluent, N. oleoabundans assimilated 90-95% of the initial nitrate and ammonium after 6 d and yielded 10-30% fatty acid methyl esters on a dry weight basis. Cellular lipid content and the N concentration in the growth media were inversely correlated. In addition, the proportion of polyunsaturated fatty acids (i.e. C16:3, C18:2, and C18:3) decreased with N concentration over time while the proportion of C18:1 fatty acid increased. Although N deficiency is likely the primary driver behind lipid accumulation, the influence of culture pH confounded results and requires further study. Other living microorganisms in the digester effluent were not observed to affect algal growth and lipid productivity, though the breakdown of organic nitrogen may have hindered lipid accumulation traditionally achieved through the manipulation of synthetic media. This work highlights the potential for waste-grown mono-algal cultures to produce high quality biodiesel while accomplishing simultaneous wastewater treatment.     

Biomass and lipid productivities of marine microalgae isolated from the Persian Gulf and the Qeshm Island

In this work, the screening of 147 microalgal strains from the Persian Gulf and the Qeshm Island (Iran) were done in order to choose the best ones, in terms of growth (biomass) rate and lipid content for biodiesel production. A methodology, combining experiments in lab-scale and pilot plant (open pond) used to produce and evaluate biomass and lipid productivity is presented for the systematic investigation of the potential of different microalgae species. The culture conditions, including photo flux (180 ??E m⁻² s⁻¹), photoperiod (12 h light/dark), temperature (25 °C), pH (≈8), air (carbon dioxide) and growth medium, were kept constant for all experiments. Microalgae were screened in two stages using optical density (for evaluation of biomass concentration) and Nile red and gas chromatography (for determination of lipid content and fatty acid fractions). In general, maximum specific growth rate and the maximum biomass productivity were obtained after 8-12-day culture. Nannochloropsis sp. and Neochloris sp. were selected from the marine microalgal culture collection, due to their high biomass (50 and 21.7 g L⁻¹, respectively) and oil content (52% and 46%, respectively). If the purpose is to produce biodiesel only from one species, Nannochloropsis sp. presented the most adequate fatty acid profile, namely linolenic and other polyunsaturated fatty acids. However, the microalgae Chlorella sp. can also be used if associated with other microalgal oils. In addition, selected strains could be potent candidates for commercial production in the open pond culture.     

Biotechnological conversions of biodiesel derived waste glycerol by yeast and fungal species

Fifteen eukaryotic microorganisms were tested for their ability to assimilate biodiesel derived waste glycerol and convert it into value-added metabolic products. For this purpose yeast and Zygomycetes strains were cultivated in nitrogen-limited raw glycerol-based media (initial glycerol concentration 30 g/L). Yeasts tested accumulated restricted lipid quantities (up to ∼22%, wt/wt, in the case of Rhodotorula sp), while differentiations in their fatty acid composition were recorded in relation to the yeast strains employed and the fermentation time. On the contrary, fungi accumulated higher quantities of lipid inside their mycelia (ranging between 18.1 and 42.6%, wt/wt, of dry biomass) that contained in variable amounts the medically important GLA (γ-linolenic acid). Moreover, Yarrowia lipolytica, Pichia membranifaciens and Thamnidium elegans were further studied in media having increased initial glycerol concentrations. In these conditions Y. lipolytica secreted significant amounts of acetic acid (29.2 g/L), as well as mannitol (19.4 g/L) while P. membranifaciens reached 28.4 g/L of biomass at glycerol concentration 90 g/L. T. elegans produced 11.6 g/L of oil, with 71.1%, wt/wt, of fat in biomass, while the maximum concentration of GLA was 371 mg/L. Detailed analysis of T. elegans lipids indicated that the phospholipids fraction was particularly rich in polyunsaturated fatty acids.     

Effects of furfural and acetic acid on growth and lipid production from glucose and xylose by Rhodotorula glutinis

Microbial conversion of lignocellulosic sugars to triacylglycerols (a biodiesel or renewable diesel feedstock) was investigated using the oleaginous yeast Rhodotorula glutinis (ATCC 15125). In the shake flask experiments, R. glutinis was first grown in a nitrogen-rich medium utilizing an artificial acid hydrolysate of lignocellulosic biomass switchgrass as the sole carbon and energy source. Once the culture had reached the stationary phase, the cells were harvested and transferred to a fresh nitrogen-free media containing artificial acid hydrolysate sugars for lipid accumulation. Analysis of the data collected showed that the yeast were able to grow in the medium containing artificial acid hydrolysate sugars as the carbon and energy source. The net specific Growth rate(s) indicated that the presence of acetic acid and furfural in the artificial acid hydrolysate inhibited the growth of R. glutinis on glucose, but not the growth on xylose. The lipid accumulated in the cells, determined by gravimetrical method, increased from initial 4.3%-39.0% of dry cell mass weight. The major fatty acids of the accumulated lipids were palmitic acid, stearic acid, oleic acid, linoleic acid and ??-linoleic acid. These results indicate that it is feasible to convert the sugars in acid hydrolysate of lignocellulosic biomass to triacylglycerols using R. glutinis.     

Microbial lipid production by the oleaginous yeast Cryptococcus curvatus O3 grown in fed-batch culture

This study investigates the capability of the oleaginous yeast Cryptococcus curvatus O3 to synthesize microbial lipids using glucose as its sole carbon source. Both glucose concentration and varying nitrogen sources have a significant effect on cell growth and microbial lipid accumulation in batch and fed-batch cultures. When cultivated in a shaking flask at 30 °C with glucose as sole carbon source, the cellular biomass and lipid content reached 51.8 kg m⁻³ and 651 g kg⁻¹, respectively. The fed-batch culture in a 30 × 10⁻³ m³ stirred-tank fermentor run for 185 h produced a cellular biomass, lipid content, and lipid productivity rate of up to 104.1 kg m⁻³, 827 g kg⁻¹, and 0.47 kg m⁻³ h⁻¹, respectively. These data indicate that C. curvatus O3 can be used as an ideal oleaginous yeast for microbial lipid production. Gas chromatography analysis of the synthesized microbial lipids revealed that the major constituents are long-chain fatty acids, such as palmitic acid, stearic acid, oleic acid, and linoleic acid. The results suggest that the microbial lipids produced by C. curvatus O3 can be used to produce biodiesel.     

Analytical evaluation of different carbon sources and growth stimulators on the biomass and lipid production of Chlorella vulgaris – Implications for biofuels

The key challenges in lipid production from marine microalgae include the selection of appropriate strain, optimization of the culture conditions and enhancement of biolipid yield. This study is aimed at evaluating the optimal harvest time and effect of chlorella growth factor (CGF) extract, carbon sources and phytohormones on the biomass and lipid production in Chlorella vulgaris. CGF, extracted using hot water from Chlorella has been reported to possess various medicinal properties. However, in the present study, for the first time in C. vulgaris, CGF was found as a best growth stimulator by enhancing the biomass level (1.208 kg m⁻³) significantly on day 5. Gibberellin and citrate augmented the biomass by 0.935 kg m⁻³ and 1.025 kg m⁻³. Combination of CGF and phytohormones were more effective than CGF and carbon sources. Analysis of fatty acid methyl esters indicated that the ratio of saturated to unsaturated fatty acids is higher in cytokinin, abscisic acid and CGF, and are also rich in short chain carbon atoms, ideal criteria for biodiesel. Nitrogen starvation favoured synthesis of more unsaturated fatty acids than saturated. This study shows that CGF enhances the biomass and lipid significantly and thus can be used for large scale biomass production.     

Characterization of fatty acids and hydrocarbons of chlorophycean microalgae towards their use as biofuel source

Microalgae accumulate important biofuel precursors such as fatty acids and hydrocarbons. Identification of microalgal strains with ideal fuel quality precursor profile is important during bioprospection studies. In this direction, thirty two freshwater green microalgae were characterized for their biomass productivity, fatty acid and hydrocarbon composition under autotrophic growth conditions. Scenedesmus dimorphus CFR 1-05/FW, Oocystis pusilla CFR 6-01/FW and Quadrigula lacustris CFR 7-01/FW were high biomass producing strains with shorter doubling time. Lipid accumulation was monitored by nile red staining with mild acetic acid pretreatment (at 7 m mol L⁻¹) to microalgal cells. Six strains viz., S. dimorphus CFR 1-05/FW, Scenedesmus obtusus CFR 1-09/FW, Chlorococum sp. CFR 2-01/FW, C. humicola CFR 2-03/FW, Chlorella sorokiniana CFR 3-01/FW, Dictyosphaerium CFR 5-01/FW showed lipid accumulation of >20% mass fraction at stationary phase. Palmitic, oleic and alpha linolenic acids were major fatty acids in all the chlorophycean species. Fuel grade hydrocarbons which can be directly blended with petroleum fuels were identified. Fourteen strains showed hydrocarbon content of >10% mass fraction of dry biomass. n-Paraffins of chain length between C15 to C20 were predominant hydrocarbons in all the strains. Branched isoprenoid hydrocarbons were detected in Scenedesmus sp. CFR 1- 13/FW constituting 49% mass fraction of total hydrocarbons. High quantities of n-tetradecane (40%) was detected in Kirchneriella cornuta CFR 8-01/FW. The similarity of microalgal hydrocarbon profiles with paraffinic and isoparaffinic fraction of petroleum diesel and compliance of FAME based biodiesel to international standards indicate the suitability of algae derived biofuels for blending with conventional petroleum fuels.     

Isochrysis sp. IOAC724S,a newly isolated,lipid-enriched,marine microalga for lipid production,and optimized cultivation conditions

An oleaginous, unicellular, marine microalga termed IOAC724S was isolated from the South China Sea. Morphology and genetic analyses indicated it belongs to the genus Isochrysis. Gas chromatography (GC) results showed that more than 10 types of fatty acids existed in Isochrysis sp. IOAC724S and that 90% of them were suitable for lipid production. The culture conditions suitable for cell growth were progressively optimized through photosynthetic and respiratory analyses. The optimal culture conditions were: photon flux 200–500 μmol m⁻² s⁻¹, temperature 35 °C during daytime and 24 °C at night, pH value between 7 and 8, NaNO₃ 160 g m⁻³ and NaH₂PO₄·2H₂O 80 g m⁻³ for starting culture. When microalgal cultures were exposed to these optimal conditions, the specific growth rate reached to 0.26 d⁻¹ on average and 1.0 d⁻¹ in MAX. Lipid production was optimized through nutrient starvation processes, including nitrate or phosphate deprivation and simultaneous nitrate and phosphate deprivation. The highest lipid mass fraction of dry cell weight (about 55.6%) was obtained after the stationary phase algal culture was transferred into phosphate-free medium for 3 days. GC data demonstrated that the enhancement of lipid accumulation in algal cells maintained under nutrient starvation came mainly from an increase of C16:0 and C18:1 fatty acids; however, the lipids with a chain length appropriate for fuel use (C14 to C18) were unchanged at 90% mass fraction of the dry cell weight. Based on these good characteristics, Isochrysis sp. IOAC724S appeared to be a strong candidate for lipid production.     

Microbial oil production from thermophile cyanobacteria for biodiesel production

Compared to lipid extraction from algae, little work has been performed for cyanobacteria. In this article it is aimed to show high lipid accumulation potential of Synechococcus sp., Cyanobacterium aponinum and Phormidium sp. cells in BG-11 medium. Four different pH values (6–9) and NaNO₃ (0.25, 0.5, 1.0, 1.5 g/L) concentrations were examined at different incubation days to discover the highest lipid accumulation. The maximum lipid content could be achieved in the medium containing 0.25 g/L NaNO₃ at pH 7 for Synechococcus sp., pH 8 for C. aponinum and pH 9 for Phormidium sp. after 15 days. The maximum lipid contents and C16 and C18 methyl ester yields were measured as 42.8% and 46.9% for Synechococcus sp., 45.0% and 67.7% for C. aponinum, 38.2% and 90.6% for Phormidium sp. The saturated compounds were 74.5%, 77.9%, 84.7% for Synechococcus sp., C. aponinum and Phormidium sp., respectively. These crude lipids could be promising feedstock for biodiesel production.     

Bio-hydrogen production by mixed culture of photo- and dark-fermentation bacteria

Clostridium butyricum and Rhodopseudomonas faecalis RLD-53 were employed to produce hydrogen in mixed culture with glucose as sole substrate. Due to the great difference on growth rate and acid-resistant capacity between photo-fermentative bacteria and dark-fermentative bacteria, directly mixed culture of the two kinds of bacteria in different ratio was studied in this work. Hydrogen yield, volatile acids, pH and biomass in different periods were evaluated. Acetic acid and butyric acid produced by C. butyricum were dominant terminal fermentation products, and they were effective substrates for photo-fermentative bacteria. The cooperation was formed in a way like food chain. But compared to the production rate of volatile acids produced by C. butyricum, the utilization rate by photo-fermentative bacteria was far slower. The results demonstrated that the growth of photo-fermentative bacteria was limited when pH decreased sharply. The best ratio of C. butyricum to R. faecalis RLD-53 was 1:600. The maximum yield of hydrogen reached 122.4 ml-H₂/vessel and hydrogen production rate was 0.5 ml-H₂/ml-culture/day.     

Morphological,physico-chemical and micropropagation studies in Jatropha curcas L. and RAPD analysis of the regenerants

Four accessions of Jatropha curcas L. were characterized with respect to their yield and physico-chemical properties of seed-oil. Further, an efficient micropropagation protocol was developed followed by RAPD analysis of the regenerants. Although the highest 1000-seed-weight and seed-oil content were recorded in OJC1 and OJC5 accessions respectively, the seed-oil of OJC9 accession was more suited for biodiesel production due to the desirable characteristics like low values of free fatty acids, acid value, moisture content and total unsaturated fatty acids. An accession – independent micropropagation protocol of Jatropha was optimized by using axillary nodal explants with MS medium supplemented with a combination of growth regulators, glutamine and citric acid. All the accessions were amenable to in vitro culture, and the highest number of plantlets could be regenerated from the OJC5 accession. Although no somaclonal variation was recorded in the regenerants of all the accessions, RAPD analysis confirmed their genetic similarity and also the similarity of the clones with their respective mother plants.     

Lipid accumulation in the new oleaginous yeast Debaryomyces etchellsii correlates with ascosporogenesis

The growth characteristics, lipid accumulation and composition during the life cycle of a newly isolated strain of Debaryomyces etchellsii were studied under nitrogen limiting conditions. This yeast, grown in batch flask or bioreactor cultures, reproduced asexually by buds when nitrogen was available in the growth medium, or sexually by ascospores after nitrogen exhaustion, producing more than 7 g L⁻¹ biomass. During ascosporogenesis, an important increase in the cellular lipid content in dry cell mass occurred, i.e. from a mass fraction of 11.9% in the vegetative phase to 22.4%, in the ascosporogenic phase. During transition of D. etchellsii from batch to continuous cultures using dilution rates 0.026 and 0.019 h⁻¹, a shift from sexual to asexual reproduction was observed. At 0.019 h⁻¹, few pseudomycelia were also formed. The yeast synthesized lipids containing long chain fatty acids (mainly C16 and C18). Budded cells at steady-states contained only 8.6–9.3 % of lipids mass fraction per dry cell mass that were composed of oleic and linoleic acids and, to a lesser extent, of palmitic and palmitoleic acids. Neutral lipids were the major fraction represented 61.8–66.1%, of total lipids followed by phospholipids, which was the only fraction in which linoleic acid predominated over oleic acid.     

Comparative study of biohydrogen production by four dark fermentative bacteria

Dark fermentation is a promising biological method for hydrogen production because of its high production rate in the absence of light source and variety of the substrates. In this study, hydrogen production potential of four dark fermentative bacteria (Clostridium butyricum, Clostridium pasteurianum, Clostridium beijerinckii, and Enterobacter aerogenes) using glucose as substrate was investigated under anaerobic conditions. Batch experiments were conducted to study the effects of initial glucose concentration on hydrogen yield, hydrogen production rate and concentration of volatile fatty acids (VFA) in the effluents. Among the four different fermentative bacteria, C. butyricum showed great performance at 10 g/L of glucose with hydrogen production rate of 18.29 mL-H₂/L-medium/hand specific hydrogen production rate of 3.90 mL-H₂/g-biomass/h. In addition, it was found that the distribution of volatile fatty acids was different among the fermentative bacteria. C. butyricum and C. pasteurianum had higher ratio of acetate to butyrate compared to the other two species, which favored hydrogen generation.     

Application of sweet sorghum for biodiesel production by heterotrophic microalga Chlorella protothecoides

Microalga Chlorella protothecoides can grow heterotrophically with glucose as the carbon source and accumulate high proportion of lipids. The microalgal lipids are suitable for biodiesel production. To further increase lipid yield and reduce biodiesel cost, sweet sorghum juice was investigated as an alternative carbon source to glucose in the present study. When the initial reducing sugar concentration was 10 g L⁻¹ in the culture medium, the dry cell yield and lipid content were 5.1 g L⁻¹ and 52.5% using enzymatic hydrolyzates of sweet sorghum juice as the carbon source after 120 h-culture in flasks. The lipid yield was 35.7% higher than that using glucose. When 3.0 g L⁻¹ yeast extract was added to the medium, the dry cell yield and lipid productivity was increased to 1.2 g L⁻¹ day⁻¹ and 586.8 mg L⁻¹ day⁻¹. Biodiesel produced from the lipid of C. protothecoides through acid catalyzed transesterification was analyzed by GC–MS, and the three most abundant components were oleic acid methyl ester, cetane acid methyl ester and linoleic acid methyl ester. The results indicate that sweet sorghum juice could effectively enhance algal lipid production, and its application may reduce the cost of algae-based biodiesel.     

Impact of light intensity,CO2 concentration and bubble size on growth and fatty acid composition of Arthrospira (Spirulina) platensis KMMCC CY-007

Studies on lipid accumulation and fatty acid composition of microalgae as a feedstock for biodiesel have been reported in numerous papers in recent years. However, in contrast to microalgae, documentation on cyanobacteria such as Arthrospira platensis with regard to lipid production has been sparse thus far. In this paper, we show the relatively high lipid accumulation potential of A. platensis KMMCC CY-007 in modified Schlösser medium. Four different growth conditions [7.5 candela (cd) and 1% CO₂, 15 cd and 1% CO₂, 7.5 cd and air, and 15 cd and air] were examined over 7 days in order to compare the impact of light intensity. Using two different growth conditions (7.5 cd and 1% CO₂ bulk bubble and 7.5 cd and 1% CO₂ fine bubble), changes in growth, lipid accumulation and fatty acid composition were compared. The highest lipid content of 37.96% was obtained at 7.5 cd and 1% CO₂ fine bubble after 7 days, and the maximum net production (72.56%) of long chain fatty acid methyl esters (C16:0 and C18:0) detected at 7.5 cd and 1% CO₂ bulk bubble. These data suggest the potential of A. platensis for production of crude lipid as a future feedstock for biodiesel production.     

A potent lipid producing isolate of Epicoccum purpurascens AUMC5615 and its promising use for biodiesel production

Oils of oleaginous microorganisms are a powerful alternative to vegetable oils for biodiesel production. In this study, the fungus Epicoccum purpurascens AUMC5615 isolated from Egypt showed a potent high lipid content (80% lipid) when grown on 4% sucrose in submerged culture under continuous illumination. Under dark submerged conditions the lipid content has drastically decreased to 12%. In light static conditions, the lipid content was 70%; however, the net lipid yield was significantly lower than that of light submerged cultures because of the decrease in growth under light static conditions in comparison to light submerged cultures. Under dark static conditions the lipid content of the fungus has declined to 30%. These results indicate that light plays a crucial role in the lipid accumulation whereas submersion enhances the growth of the fungus. Concomitantly, the highest yield of carotenoids was obtained under light submerged conditions followed respectively by light static, dark submerged and dark static. This synchronized increase in carotenoids content might be implicated in protecting the high lipid pools in the fungus from peroxidation. Growing the fungus on 4% of crude molasses resulted in a net lipid production of 26.8 g per liter under light submerged conditions. The determination of fatty acids by GC/MS revealed that the major constituents are four saturated fatty acids, hexadecanoic, n-decanoic, dodecanoic and octadecanoic acids. These saturated fatty acids would give valuable stability properties of such fungal biodiesel. The current investigation opens the scope for the possible use of this promising fungal isolate in biodiesel production.     

Effects of flocculants on lipid extraction and fatty acid composition of the microalgae Nannochloropsis oculata and Thalassiosira weissflogii

The aim of this study was to investigate the possible interference of anionic and cationic flocculants in the lipid extraction and fatty acid profiles of two species of marine microalgae: Nannochloropsis oculata and Thalassiosira weissflogii. Cells were grown in batch cultures (f/2 medium, salinity of 28, temperature of 20 °C, light intensity of 40 ??mol photons  m^-2 s^-1 and 12/12 h L/D photoperiod) and concentrated using sodium hydroxide (control), sodium hydroxide and the anionic polyacrylamide flocculant Magnafloc^® LT-25 (APF treatment) and sodium hydroxide plus the cationic polyacrylamide flocculant Flopam^® (CPF treatment). There were no statistically significant differences among treatments with respect to lipid extraction for both species. However, N. oculata which presented higher percentages of C16:0, C16:1 and C20:5 fatty acids showed an increase of C14:0 and a decrease of C20:5 with the use of anionic flocculant. Additionally, T. weissflogii which had high percentages of C16:0, C16:1, C16:3 and C20:5, showed a decrease of C18:0 and C18:1n9c when both flocculants were used and a small decrease of C16:0 in the APF treatment. The results indicate that the choice of flocculant should be based on the level of saturation desirable, i.e., if the goal is to produce more stable biodiesel, with low percentage unsaturated fatty acids, then anionic flocculants should be used. On the other hand, if the aim is to produce unsaturated fatty acids for commercial uses in the pharmacy or food industries, then anionic polymers should be avoided.     

Screening of marine microalgae for biodiesel feedstock

Biodiesel production from microalgae lipids is increasingly regarded as a more sustainable and feasible alternative to conventional biodiesel feedstocks derived from terrestrial bioenergy crops. A total of ninety-six strains of marine microalgae, with an elevated biomass productivity and intracellular lipid content, were isolated from the coastal waters of Singapore using an automated flow cytometric cell-sorting technique. Cell sorting was based on the two-dimensional distribution of algal cells for red fluorescence (representing chlorophyll auto-fluorescence) against forward-light scatter (representing cell size) and red vs. green fluorescence. Twenty-one of the strains were further characterized with respect to cell growth rate, biomass concentration, lipid content (total and neutral lipid) and fatty acid profile. The growth rates of Skeletonema costatum, Chaetoceros and Thalassiosira species were greatest among the entire strains, but in terms of absolute lipid yield Nannochloropsis strains predominated. Nannochloropsis strains had a lipid content ranging from 39.4% to 44.9% of dry weight biomass. Transesterification of the lipids yielded 25-51% of fatty acid methyl ester (FAME) i.e. biodiesel, where total FAME content ranged between 11 and 21% of dry weight biomass. This study describes the microalgae screening process and demonstrates that Nannochloropsis is a promising species for biodiesel feedstock.     

A viable antibiotic strategy against microbial contamination in biotechnological production of polyhydroxyalkanoates from surplus whey

The study demonstrates the successful application of vancomycin for inhibiting growth of the contaminant Bacillus cereus in a polyhydroxyalkanoate (PHA) production process with Hydrogenophaga pseudoflava on whey. The minimum inhibiting concentration (MIC) of vancomycin for both strains was calculated in minimal medium H3 as well as in H3 medium supplemented with yeast extract. MIC for B. cereus was 0.57 ??g mL⁻¹ and 1.63 ??g mL⁻¹ in H3 and H3 plus yeast extract respectively. For H. pseudoflava, MIC amounted to 2.8 ??g mL⁻¹ in H3 medium and 6.88 ??g mL⁻¹ in H3 plus yeast extract. The effect of vancomycin on PHA production was minimal or negligible up to a vancomycin concentration of 1 ??g mL⁻¹ in both media. The specific PHA production rates of H. pseudoflava decreased with increasing antibiotics in a minimal medium but in media supplemented with yeast extract the specific PHB production rates increased as the antibiotics concentrations increased.