Microbial Population Shifts Research Articles

Ochratoxin A in Ruminants–A Review on Its Degradation by Gut Microbes and Effects on Animals

Ruminants are much less sensitive to ochratoxin A (OTA) than non-ruminants. The ruminal microbes, with protozoa being a central group, degrade the mycotoxin extensively, with disappearance half lives of 0.6–3.8 h. However, in some studies OTA was detected systemically when using sensitive analytical methods, probably due to some rumen bypass at proportions of estimated 2–6.5% of dosage (maximum 10%). High concentrate proportions and high feeding levels are dietary factors promoting the likeliness of systemic occurrence due to factors like shifts in microbial population and higher contamination potential. Among risk scenarios for ruminants, chronic intoxication represents the most relevant.

Microbial Composition in Cadmium Contaminated Soils around Zinc Mining Area, Thailand

According to the problems of cadmium contamination in soil at Mae Sot district, Tak province. This study attempts to examine the shift of active microbial population according to cadmium concentration variation around the zinc mine by using RT-PCR of the bacterial 16S rRNA gene and DGGE profiles analysis. The results showed that the amount of total cadmium and zinc was in the range from 1.1 to 34.95 mg/kg soil and from 27.05 to 1998 mg/kg soil, respectively. At the highest cadmium concentration (34.95 mg/kg soil) the highest diversity index was obtained. Cluster analysis from DGGE pattern indicated two different clusters in the samples. The two soil samples with the cadmium amounts were similar to the EU Maximum Permissible level (3.0 mg/kg soil) belonged to one cluster and the leftovers belonged to another. The Pearson correlation between the diversity index and cadmium values were significantly positive.

Effects of Salt on Microbial Populations and Treatment Performance in Purifying Saline Sewage Using the MUCT Process

AbstractUnderstanding salt effects on activated sludge systems and treatment performance is essential in order to design and operate a full scale treatment plant processing saline sewage. For this purpose, a bench scale modified University of Cape Town (MUCT) process for treating saline sewage was used to provide information with regard to microbial population shift and treatment performance. Various salt levels (5, 8, 10 and 15 g/L) were studied. Experimental results indicated that the salt concentration in sewage significantly affected phosphorous removal, while salt inhibition of ammonium degradation was slight, even in a 15 g/L salt solution. Outstanding nitrite accumulation in aerobic tanks occurred in the MUCT treating 8 g/L saline sewage, which resulted in denitrifying nitrite by phosphorous accumulating organisms (PAOs) in the anoxic tank. Population size of physiological groups (aerobic‐heterotrophic, nitrifying and denitrifying microbial group) was also investigated at different salt levels. The investigation of changes in the dynamics of the activated sludge microbial structure gave an insight into the nature and the magnitude of salt effects on the microbial community.

Aerobic granulation in sequencing batch reactors with different reactor height/diameter ratios

Effects of reactor height/diameter ratios ranged from 24 to 4 corresponding to reactor settling velocities from 12 to 2 m h −1 on aerobic granulation were investigated. It was found that granules appeared after 1-week operation and granule volume percentages exceeded 50% after 2–3 weeks in four reactors. In addition, similar granule fraction of 94–96% was found at steady state in all four reactors. Sludge volume index (SVI), average sludge size, biomass density and granule settling velocity at steady state were around 50 ml g −1, 1800 μm, 53 g l −1 and 40 m h −1, respectively, in four reactors. Extracellular polymeric substances (EPS) and specific oxygen uptake rate (SOUR) were around 38 mg g −1 VSS and 40 mg O 2 g −1 VSS h −1, respectively. Denaturing gradient gel electrophoresis (DGGE) fingerprint of sludge in four reactors showed the same microbial population shift during the start-up period and same microbial community structure during steady-state period. These results recommended strongly that reactor height/diameter ratio or reactor setting velocity in the used range in this study did not affect granule formation, physical characteristics, microbial community structure of granules and stable operation of granular sludge reactor. Reactor height/diameter ratio thus can be very flexible in the practice, which is important for the application of aerobic granule technology.

Practical Use of New Microbiology Tools in Oil Production

Summary Culture-based methods of traditional microbiology applied to the microbiological processes involved in souring of oil fields and microbiologically influenced corrosion (MIC) pose a risk of yielding inadequate and contradictory results. Any cultivation step will almost certainly alter the population characteristics and, thus, alter the results on which any evaluation will be based. The need for cultivation-independent methods has, over the past 10 years, facilitated the development of several analytical methods for the determination of microbial identity, quantity, and, to some extent, function, applied directly to samples of the native population. This development so far has been fairly limited regarding practical application, and it has only recently been transferred to the offshore industry. In this paper, we demonstrate the features of these novel techniques and the benefits of applying them to two situations often encountered in offshore oil production in the North Sea—nitrate injection and MIC. The microbiological tools are based on the detection of the genetic material in microorganisms. The methods include direct counting of specific groups of microorganisms with microscopy by use of fluorescent in-situ hybridization (FISH) and other methods that are based on direct extraction of cell genetic material (i.e., DNA/RNA), such as quantitative polymerase chain reaction (qPCR) and denaturing-gradient gel electrophoresis (DGGE). The paper will describe these relatively novel molecular techniques briefly. The paper documents the microbial-population shifts related to water breakthrough in a nitrate-treated reservoir and shows that key microbial populations can be identified and, thereby, this can lead to the creation of new and strengthened surveillance strategies on microorganisms that cause souring in these systems. Additionally, we have shown that when applying these novel techniques to aggressive corrosion attacks, especially under deposit corrosion, molecular techniques are powerful tools in identifying the most probable corrosion process in which microorganisms are implicated. These examples are described and related to offshore operations. Special focus is given to the use of the new and improved microbiological data in relation to designing and testing remedial actions toward oilfield souring and MIC.

Dietary protein concentration affects intestinal microbiota of adult cats: a study using DGGE and qPCR to evaluate differences in microbial populations in the feline gastrointestinal tract

The objective of this study was to identify qualitative and quantitative differences in microbial populations of adult cats fed diets containing different protein concentrations. Following a 4 week baseline period, eight healthy adult domestic short-hair queens (>1-year-old) were randomly allotted to a moderate-protein (MP; n = 4) or high-protein (HP; n = 4) diet for 8 weeks. Fresh faecal samples were collected after baseline and 8 weeks on treatment and stored at -80 degrees C. Following DNA extraction, samples were analyzed using denaturing gradient gel electrophoresis to distinguish qualitative changes between diets. Quantitative polymerase chain reaction was used to measure E. coli, Bifidobacterium, Clostridium perfringens, and Lactobacillus populations. Compared to baseline, cats fed MP had a bacterial similarity index of 66.7% as opposed to 40.6% similarity for those fed HP, exhibiting marked changes in intestinal bacteria of cats fed HP. Bifidobacterium populations were greater (p < 0.05) in cats fed MP versus HP (9.44 vs. 5.63 CFU/g). Clostridium perfringens populations were greater (p < 0.05) in cats fed HP than MP (12.39 vs. 10.83 CFU/g). In this experiment, a high-protein diet resulted in a dramatic shift in microbial populations. Decreased Bifidobacterium population in cats fed HP may justify prebiotic supplementation for such diets.

Enzyme Activities and Shifts in Microbial Populations Associated with Activated Sludge Treatment of Textile Effluents

ABSTRACTThe enzyme studies on an activated sludge (AS) were aimed at providing information about adaptive response of microbial community during four significant alterations in textile wastewater treatment plant behavior associated with: shock loading with azo compounds; recovery after the xenobiotic shock; building up additional aerobic reactor; breakdown of facility exploitation for 5 months. The activity of several enzymes associated with target biodegradation processes was assessed. Induction of catechol 1,2—dioxygenase (C12DO), catechol 2,3—dioxygenase (C23DO), protocatechate 3,4—dioxygenase (P34DO) occurred under aerobic and notably under anoxic/microaerobic conditions, depending on the control point. Key microbial groups in the AS consortium were determined by cultivation-based-methods. The densities of bacteria from g. Pseudomonas (Ps) and Acinetobacter (Acin) were increased during the toxic shock. Aerobic heterotrophs (AH) and g. Acinetobacter decreased after pause of facility exploitation. The numbers of denitrifying bacteria (Dn) in denitrifying reactor were significantly decreased after each of the critical events.

Memory and learning behavior in mice is temporally associated with diet-induced alterations in gut bacteria

The ability of dietary manipulation to influence learning and behavior is well recognized and almost exclusively interpreted as direct effects of dietary constituents on the central nervous system. The role of dietary modification on gut bacterial populations and the possibility of such microbial population shifts related to learning and behavior is poorly understood. The purpose of this study was to examine whether shifts in bacterial diversity due to dietary manipulation could be correlated with changes in memory and learning. Five week old male CF1 mice were randomly assigned to receive standard rodent chow (PP diet) or chow containing 50% lean ground beef (BD diet) for 3 months. As a measure of memory and learning, both groups were trained and tested on a hole-board open field apparatus. Following behavioral testing, all mice were sacrificed and colonic stool samples collected and analyzed by automated rRNA intergenic spacer analysis (ARISA) and bacterial tag-encoded FLX amplicon pyrosequencing (bTEFAP) approach for microbial diversity. Results demonstrated significantly higher bacterial diversity in the beef supplemented diet group according to ARISA and bTEFAP. Compared to the PP diet, the BD diet fed mice displayed improved working ( P = 0.0008) and reference memory ( P < 0.0001). The BD diet fed animals also displayed slower speed ( P < 0.0001) in seeking food as well as reduced anxiety level in the first day of testing ( P = 0.0004). In conclusion, we observed a correlation between dietary induced shifts in bacteria diversity and animal behavior that may indicate a role for gut bacterial diversity in memory and learning.

The use of oxygen uptake rate to monitor discovery of microbial and enzymatic biocatalysts

Arising from the requirement for discovery of novel biocatalysts with unusual properties, a process was developed which uniquely combines aspects of continuous culture with the measurement of oxygen uptake. This adaptation of the chemostat can be used to facilitate the isolation of a number of microorganisms with desirable properties, particularly those with useful metabolic capabilities and/or enzymes. The technique was also used to provide feedback on the metabolic status of a microbial population and increase the feed flow rate (i.e., dilution rate) thereby enabling the isolation of microorganisms with enhanced 1,3-propanediol dehydrogenase activity. The use of oxygen uptake as an indicator of cellular activity enables indirect measurement of substrate utilization and provides a real-time online assessment of the status of microbial enrichment or evolutionary processes and provides an opportunity, through the use of feedback systems, to control these processes. To demonstrate the utility of the technique, oxygen uptake rate (OUR) was compared with a range of conventional analytical techniques that are typically used to monitor enrichment/evolutionary processes and showed good correlation. Further validation was demonstrated by monitoring a characterizable microbial population shift using OUR. The population change was confirmed using off-line analytical techniques that are traditionally used to determine microbial activity. OUR was then used to monitor the enrichment of microorganisms capable of using a solvent (1-methyl-2-pyrrolidinone) as the sole source of carbon for energy and biomass formation from a heterogeneous microbial population. After purification the microorganisms taken from the enrichment process were able to completely utilize 1 g L(-1) 1-methyl-2-pyrrolidinone within 24 h demonstrating that the technique had correctly indicated the enriched population was capable of growth on 1-methyl-2-pyrrolidinone. The technique improves on conventional microbial enrichment that utilizes continuous culture by providing a real-time assessment of the enrichment process and the opportunity to use the OUR output for automated control and variation of one or more growth parameters.

Microbial population dynamics in nitrifying reactors: Experimental evidence explained by a simple model including interspecies competition

Key biological processes such as nitrification do not result from the work of a single bacterial species but are performed by a wide variety of bacteria. However, this microbial diversity is usually not tracked during reactor operation and mostly neglected in present mathematical models, which distinguish at the most between ammonium oxidizers and nitrite oxidizers, assuming the same properties for all bacteria of each group. Nevertheless, experimental evidence is available that different process conditions may favour the selection of different types of bacteria. This contribution deals with observed differences between two nitrifying inverse turbulent bed reactors (ITBRs) with different solid hold-ups. It also contains additional experimental data, revealing a microbial population shift upon changing the operating conditions in one of these reactors. The reactor behaviour is described with a basic two-step nitrification model, which includes the competition between two different types of ammonium oxidizers, besides nitrite oxidizers. Considering the oxygen concentration as the key variable governing the population shift, the influence of microbial growth parameters on the competition dynamics is assessed.

Effect of continuous Cd feeding on the performance of a nitrification reactor

The inhibitory effect of Cd on nitrification was investigated in a continuous-flow system with enriched nitrifying bacteria. The maximum specific ammonium utilization rate and the half-saturation constant were found as 671 mg NH(4)-N/g VSS day and 0.48 mg/l, respectively. In the case of continuous Cd input at 1 and 2.5 mg/l, nitrification was inhibited by 30% and 47%, respectively. Inhibition ranged from 20% to 40% and no further increase in inhibition was exhibited in new runs except at 10 mg/l influent Cd. At 10 mg/l influent Cd, specific ammonium utilization and nitrate production rates were inhibited by 90%. On the contrary, a serious nitrite accumulation was not observed during this period. When Cd feeding was stopped, recovery from inhibition was observed after 37 day which was seen by the improvement in ammonium utilization and nitrate production rates. A shift in microbial population from the initial Nitrosomonas sp. to the Cd-tolerant Nitrosospira sp. was observed in the recovery period from severe Cd inhibition. After the domination of Nitrosospira species, redosing at 10 mg/l and then at 15 mg/l did not affect the performance as before.

Influence of plant developmental stage on microbial community structure and activity in the rhizosphere of three field crops.

Seasonal shifts in rhizosphere microbial populations were investigated to follow the influence of plant developmental stage. A field study of indigenous microbial rhizosphere communities was undertaken on pea (Pisum satvium var. quincy), wheat (Triticum aestivum var. pena wawa) and sugar beet (Beta vulgaris var. amythyst). Rhizosphere community diversity and substrate utilization patterns were followed throughout a growing season, by culturing, rRNA gene density gradient gel electrophoresis and BIOLOG. Culturable bacterial and fungal rhizosphere community densities were stable in pea and wheat rhizospheres, with dynamic shifts observed in the sugar beet rhizosphere. Successional shifts in bacterial and fungal diversity as plants mature demonstrated that different plants select and define their own functional rhizosphere communities. Assessment of metabolic activity and resource utilization by bacterial community-level physiological profiling demonstrated greater similarities between different plant species rhizosphere communities at the same than at different developmental stages. Marked temporal shifts in diversity and relative activity were observed in rhizosphere bacterial communities with developmental stage for all plant species studied. Shifts in the diversity of fungal and bacterial communities were more pronounced in maturing pea and sugar beet plants. This detailed study demonstrates that plant species select for specialized microbial communities that change in response to plant growth and plant inputs.

Modeling benzene plume elongation mechanisms exerted by ethanol using RT3D with a general substrate interaction module

A mathematical model was developed to evaluate the effect of the common fuel additive ethanol on benzene fate and transport in fuel‐contaminated groundwater and to discern the most influential benzene plume elongation mechanisms. The model, developed as a module for the Reactive Transport in 3 Dimensions (RT3D) model, includes commonly considered fate and transport processes (advection, dispersion, adsorption, biodegradation, and depletion of molecular oxygen during biodegradation) and substrate interactions previously not considered (e.g., a decrease in the specific benzene utilization rate due to metabolic flux dilution and/or catabolite repression) as well as microbial population shifts. Benzene plume elongation predictions, based on literature model parameters, were on the order of 40% for a constant source of E10 gasoline (10% vol/vol ethanol), which compares favorably to field observations. For low benzene concentrations (&lt;1 mg/L), oxygen depletion during ethanol degradation was the principal mechanism hindering benzene natural attenuation. For higher benzene concentrations (exerting an oxygen demand higher than the available dissolved oxygen), metabolic flux dilution was the dominant plume elongation process. If oxygen were not limiting, as might be the case in zones undergoing aerobic biostimulation, model simulations showed that microbial growth on ethanol could offset negative substrate interactions and enhance benzene degradation, resulting in shorter plumes than baseline conditions without ethanol.

Fermentative hydrogen production from starch using natural mixed cultures

Batch and continuous tests were conducted to evaluate fermentative hydrogen production from starch (at a concentration of chemical oxygen demand (COD) 20 g/L) at 35 °C by a natural mixed culture of paper mill wastewater treatment sludge. The optimal initial cultivation pH (tested range 5–7) and substrate concentration (tested range 5–60-gCOD/L) were evaluated by batch reactors while the effects of hydraulic retention time (HRT) on hydrogen production, as expressed by hydrogen yield (HY) and hydrogen production rate (HPR), were evaluated by continuous tests. The experimental results indicate that the initial cultivation pH markedly affected HY, maximum HPR, liquid fermentation product concentration and distribution, butyrate/acetate concentration ratio and metabolic pathway. The optimal initial cultivation pH was 5.5 with peak values of HY 1.1 mol-H 2/mol-hexose maximum HPR 10.4 mmol-H 2/L/h and butyrate concentration 7700 mg-COD/L. In continuous hydrogen fermentation, the optimal HRT was 4 h with peak HY of 1.5 mol-H 2/mol-hexose, peak HPR of 450 mmol-H 2/L/d and lowest butyrate concentration of 3000 mg-COD/L. The HPR obtained was 280% higher than reported values. A shift in dominant hydrogen-producing microbial population along with HRT variation was observed with Clostridium butyricum, C. pasteurianum, Klebshilla pneumoniae, Streptococcus sp., and Pseudomonas sp. being present at efficient hydrogen production at the HRTs of 4–6 h. Strategies based on the experimental results for optimal hydrogen production from starch are proposed.

Lairage during transport of eighteen-kilogram pigs has an impact on innate immunity and commensal bacteria diversity in the intestines1,2

Long distance transportation may affect the health of pigs; thus, adding a rest stop (lairage) during long journeys may improve their well-being. The objective of this study was to determine whether a mid-journey lairage influenced swine innate immunity and intestinal microbial populations after a 16-h transport. Four replications were conducted, 1 in each of 4 seasons. Eighteen-kilogram pigs were housed in 16 pens (13 to 16 pigs/pen) with 8 pens/treatment. Lairage pigs were transported for 8 h, given a rest with food and water for 8 h, then transported for an additional 8 h. Continuous pigs were continuously transported for 16 h. Jugular blood samples and intestinal tissue and contents were collected from 16 pigs (8/treatment) on d 1, 3, 7, and 14 posttransport. Hematocrit and white blood cell counts were determined and neutrophil cell functions, including phagocytosis/oxidative burst and phagocytosis of latex beads and leukocyte phenotypic cell markers (CD14 and CD18), were analyzed using flow cytometry. Expression of toll-like receptors 2, 4, and 5; IL-8 (a cytokine that is a chemoattractant for neutrophils); CCL20 (a chemokine that is a chemoattractant for dendritic cells); and the antimicrobial peptide PR39 were determined from ileal and jejunal total RNA. Denaturing gradient gel electrophoresis was used to determine shifts in intestinal microbial populations. Total white blood cell and granulocyte counts in continuous pigs were greater (P < 0.01) on d 1 than in lairage pigs. Phagocytosis of microbeads was greater in continuous (P < 0.05) than in lairage pigs on d 7. Expression of IL-8 in jejunum was greater (P < 0.05) for continuous than for lairage pigs on d 1. Expression of CCL20 in the ileum was greater (P < 0.05) on d 14 for the continuous pigs. Expression of PR39 was greatest (P < 0.05) in the jejunum of lairage pigs on d 3. Lairage pigs had a greater (P < 0.05) variation in microbial populations (lower similarity coefficient) in the jejunum contents on d 1 and 7, in the cecum contents and tissue on d 3, and in the jejunum contents and tissue on d 14. However, continuous pigs had greater (P < 0.05) variation in the ileal tissues on d 14. This study indicates that adding a lairage to an extended transport alters immune functions, receptor, cytokine and chemokine expression, and gut microbiota compared with pigs transported for 16 h without lairage.

Molting in Salmonella Enteritidis-Challenged Laying Hens Fed Alfalfa Crumbles. II. Fermentation and Microbial Ecology Response

The objective of this study was to examine microbial population shifts and short-chain fatty acid (SCFA) responses in the gastrointestinal tract of Salmonella Enteritidis-challenged molted and nonmolted hens fed different dietary regimens. Fifteen Salmonella-free Single Comb Leghorn hens (>50 wk old) were assigned to 3 treatment groups of 5 birds each based on diet in 2 trials: 100% alfalfa crumbles (ALC), full-fed (FF, nonmolted) 100% commercial layer ration, and feed withdrawal (FW). A forced molt was induced by either a 12-d alfalfa diet or FW. In all treatment groups, each hen was challenged by crop gavage orally 4 d after molt induction with a 1-mL inoculum containing 106 cfu of Salmonella Enteritidis. Fecal and cecal samples (d 4, 6, 8, 11, and necropsy on d 12) were collected postchallenge. Microbial population shifts were evaluated by PCR-based 16S ribosomal RNA gene amplification and denaturing gradient gel electrophoresis, and SCFA concentrations were measured. Total SCFA in fecal and cecal contents for FW molted hens were generally lower (P ≤ 0.05) in the later stages of the molt period when compared to ALC and FF treatment groups. The overall trend of SCFA in cecal and fecal samples exhibited similar patterns. In trials 1 and 2, hens molted with ALC diet generally yielded more similar amplicon band patterns with the FF hens in both fecal and cecal samples by the end of the molting period than with FW hens. The results of these studies suggest that ALC molted hens supported microflora and fermentation activities, which were more comparable to FF hens than FW hens by the end of the molting period.

Anaerobic degradation of dimethyl phthalate in wastewater in a UASB reactor

Over 99% of dimethyl phthalate (DMP) and 93% of chemical oxygen demand (COD) were effectively removed in a continuous upflow anaerobic sludge blanket (UASB) reactor from a wastewater containing 600 mg/L DMP at 8 h of hydraulic retention time (HRT), corresponding to a loading rate of 3 g-COD/(L d). Each gram of sludge, expressed as volatile suspended solids (VSS), had a maximum specific methanogenic activity (SMA) of 24 mg-CH 4/(g-VSS d) using DMP as the sole carbon source. The sludge yield was estimated as 0.08 g-VSS/g-COD. During anaerobic degradation, DMP was de-esterified, first to mono-methyl phthalate (MMP) and then to phthalate, before being de-aromatized and subsequently converted to CH 4 and CO 2. The maximum specific degradation rates of DMP, MMP and phthalate were 415, 88 and 36 mg/(g-VSS d), respectively. Analysis based on polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) showed a gradual shift of microbial population with the increase of DMP loading.

Assessment of partial nitrification reactor performance through microbial population shift using quinone profile, FISH and SEM

In engineered systems, biological nitrogen removal through partial nitrification to nitrite is of great interest. Accordingly, effect of operating parameters such as pH, DO and temperature on the accumulation of ammonia-oxidizers was investigated. pH of 8, DO of 0.3–0.5 mg/l and temperature of 35 °C yielded a ratio of 0.9–1.5 of NO 2N:NH 4N in the effluent suitable as a feed for Anammox reactor. Microbial population shift during start-up was assessed using quinone profile, SEM and FISH. UQ-8 in the biomass, which is the predominant quinone in ammonia-oxidizers, increased from 24.8% on Day 1 to 61.2% on Day 136. Fluorescence in situ hybridization analysis in the reactor showed that ammonia-oxidizing bacteria gradually outcompeted other bacteria and was the dominant population. The morphology and inner structure of the granular sludge was observed using SEM and the photographs indicated that the aerobic granular sludge showed a shift towards spherical and small rod-shaped clusters.

Microbial population structures in activated sludge before and after the application of synthetic polymer

Since Juang (2005) has found that the sludge settleability became much worse and the flocs displayed as pin flocs in only a few days after the addition of synthetic polymer was halted, it is hypothesized that the shift of microbial population in activated sludge before and after the addition of synthetic polymer might have occurred. Therefore, the identification of microbial population in the activated sludge of a lab-scaled continuous-flow type of treatment reactor was conducted at different phases of this study. The results of this study showed that the presence of synthetic polymer inhibited the growth of dominant bacteria and floc-formers in activated sludge, but gave a competitive advantage of growth to some other bacteria. The addition of synthetic polymer has caused the shift of microbial population and affected the growth of floc-formers in activated sludge. It took more than one month for the population structure of predominant microorganisms in activated sludge to return more closely to the initial population structure.

Effect of grazing pastures with different botanical composition by lambs on rumen fatty acid metabolism and fatty acid pattern of longissimus muscle and subcutaneous fat

In order to study the effect of grazing pastures with a different botanical composition on rumen and intramuscular fatty acid metabolism, 21 male lambs were assigned to three botanically different pastures: botanically diverse (BD) (consisting for 65% of a variety of grass species); Leguminosa rich (L) (consisting for 61% of Leguminosae) and intensive English ryegrass (IR) (with 69% Lolium perenne). Pastures were sampled weekly for 12 weeks for analysis of their fatty acid content and composition and on nine occasions to determine the botanical composition. Ruminal and abomasal contents were sampled at slaughter and muscle and subcutaneous fat 24 h after slaughter. All samples were prepared and analysed for fatty acid composition. The L pasture showed a higher fatty acid content (29.8 mg/g dry matter (DM) v. 18.5 and 25.5 mg/g DM, for BD and IR pastures, respectively), but the sum of the proportions of the major polyunsaturated fatty acids, C18:2 n-6 and C18:3 n-3, were similar for the three pastures (69.9, 69.4 and 71.1% of fatty acids methyl esters (FAME) for BD, L and IR pastures, respectively). The BD pasture was richer in C18:2 n-6 (18.2% of FAME), while IR pasture had a higher C18:3 n-3 content (57.2% of FAME). Rumen data showed that animals grazing the BD pasture presented higher proportions of biohydrogenation intermediates, mainly C18:1 t11, C18:2 t11c15 and CLA c9t11, suggesting an inhibition of biohydrogenation. These changes were associated with shifts in the rumen microbial population as indicated by differences in the rumen pattern of volatile fatty acids, microbial odd- and branched-chain fatty acids. In L pasture animals, the content of C18:2 n-6 and C18:3 n-3 in the abomasum and subcutaneous fat was higher. Finally, higher proportions of C20:4 n-6, C20:5 n-3 and C22:5 n-3 and higher indices for elongation and desaturation activity in the intramuscular fat of BD grazing animals suggest some stimulation of elongation and desaturation of long-chain fatty acids, although this also might have been provoked partially by reduced fat deposition (due to a lower growth rate of the animals).