Protein and energy utilization in germ-free and conventional chicks given diets containing different levels of dietary protein

Comparative biology of germ-free and conventional poultry

Interaction between the host and the enteric microbiome is highly complex. Microbial involvement in certain pathologies is moderately well established, but the contribution of the microbiome to animal welfare, behavior, sustainability, immune development, nutritional status, physiology, and maturation is less clear. A valuable experimental model to enable scientists to explore the role of the microbiome in various domains is to compare various phenotypes of a conventionally reared (CV) cohort with those in a germ-free (GF) state. A GF animal is one that is devoid of any detectable microbial life including bacteria, viruses, protozoa and parasites. The GF state is different from gnotobiotic animals where the microbiome is fully described, or ‘specific pathogen free’ (SPF) animals where a moderately normal microbiome is present but devoid of pathogenic microorganisms. Pioneering GF research in poultry in the late 1940s and 1950s has its origin in a need understand the mode of action of antibiotics. Early researchers quickly established that GF chicks responded differently to antibiotics than CV counterparts. The GF experimental model has since been exploited in many divergent fields including pathology, immunology, metabolism, anatomy, physiology, and others. The absence of a microbiome presents the host with a range of advantages and disadvantages. For example, GF chicks often grow more quickly and have lower feed conversion ratio (FCR) than their CV counterparts but may be less resilient to external stress and have a compromised immunological maturation rate. This review will summarize the literature on GF animal research with a special emphasis on poultry. The objective of the review is to establish a frame of reference to understand the extent of the role of the microbiome in animal health, welfare, nutrition, and growth, to provide opportunities for targeted modulation of the microbiome to achieve desired phenotypic responses whilst simultaneously minimizing unintended collateral effects.

Gut Microbiota and Host Juvenile Growth

Good genes, good food, good friends. That is what parents hope will sustain and nurture the harmonious growth of their children. The impact of the genetic background and nutrition on postnatal growth has been in the spot light for long, but the good friends have come to the scene only recently. Among the good friends perhaps the most crucial ones are those that we are carrying within ourselves. They comprise the trillions of microbes that collectively constitute each individual's intestinal microbiota. Indeed, recent epidemiological and field studies in humans, supported by extensive experimental data on animal models, demonstrate a clear role of the intestinal microbiota on their host's juvenile growth, especially under suboptimal nutrient conditions. Genuinely integrative approaches applicable to invertebrate and vertebrate systems combine tools from genetics, developmental biology, microbiology, nutrition, and physiology to reveal how gut microbiota affects growth both positively and negatively, in healthy and pathological conditions. It appears that certain natural or engineered gut microbiota communities can positively impact insulin/IGF-1 and steroid hormone signaling, thus contributing to the host juvenile development and maturation.

An Introduction to the Avian Gut Microbiota and the Effects of Yeast-Based Prebiotic-Type Compounds as Potential Feed Additives

The poultry industry has been searching for a replacement for antibiotic growth promoters in poultry feed as public concerns over the use of antibiotics and the appearance of antibiotic resistance has become more intense. An ideal replacement would be feed amendments that could eliminate pathogens and disease while retaining economic value via improvements on body weight and feed conversion ratios. Establishing a healthy gut microbiota can have a positive impact on growth and development of both body weight and the immune system of poultry while reducing pathogen invasion and disease. The addition of prebiotics to poultry feed represents one such recognized way to establish a healthy gut microbiota. Prebiotics are feed additives, mainly in the form of specific types of carbohydrates that are indigestible to the host while serving as substrates to select beneficial bacteria and altering the gut microbiota. Beneficial bacteria in the ceca easily ferment commonly studied prebiotics, producing short-chain fatty acids, while pathogenic bacteria and the host are unable to digest their molecular bonds. Prebiotic-like substances are less commonly studied, but show promise in their effects on the prevention of pathogen colonization, improvements on the immune system, and host growth. Inclusion of yeast and yeast derivatives as probiotic and prebiotic-like substances, respectively, in animal feed has demonstrated positive associations with growth performance and modification of gut morphology. This review will aim to link together how such prebiotics and prebiotic-like substances function to influence the native and beneficial microorganisms that result in a diverse and well-developed gut microbiota.

Parasitism and physiological trade-offs in stressed capybaras

Parasites play a key role in regulating wildlife population dynamics, but their impact on the host appears to be context-dependent. Evidence indicates that a synergistic interaction between stress, host condition and parasites is implicated in this phenomenon, but more studies are needed to better understand this context-dependency. With the goal to assess the net effect of two types of chronic stress on various host-parasite interactions, we conducted an experiment in capybaras to evaluate the impact of food restriction and physical restraint on the infection intensity of specific gastrointestinal nematodes and coccidia, and how these stressors affected the growth, body condition, and some immuno-physiological parameters. Our hypothesis was that both forms of stress would result in an alteration in the host-parasite interactions, with deteriorated condition and reduced immunological investment leading to high parasite burdens and vice versa. Stressed capybaras had significantly higher coccidia infection intensities; but among individuals that were smaller, those stressed consistently showed lower helminth burdens than controls. Both stress treatments had a marked negative impact on growth and body condition, but concomitantly they had a significant positive effect on some components of the immune system. Our results suggest, on the one hand, that during prolonged periods of stress capybaras preventatively invest in some components of their immunity, such as innate humoural defenses and cells that combat helminths, which could be considered a stress-dependent prophylaxis. On the other hand, stress was found to cause greater infection intensities of protozoans but lower burdens of nematodes, indicating that the relationship between stress, physiological trade-offs and infection depends on the type of parasite in question. Moreover, both findings might be related in a causal way, as one of the immunological parameters enhanced in stressed capybaras is associated with the immune response to control helminths.

Dietary plant bioactives for poultry health and productivity

1. Plants and their biologically active chemical constituents, sometimes called secondary metabolites or bioactives, present numerous opportunities for the improvement of livestock production by inclusion in the diet. 2. Many such plant derived materials have well established therapeutic values in man; however, their potential as feed additives in animal production, particularly of poultry, remains largely unexploited. 3. There is increasing evidence indicating that they can be efficient in controlling diseases, and plant bioactives may also influence production parameters such as feed efficiency and product quality. 4. It has been reported that they may even replicate some of the effects of antibiotic growth promoters, which were banned from use in Europe from 2006. 5. This review assesses the status of plant bioactives in poultry production and their mode of action on avian physiology, particularly in the digestive tract.

Absorption of methionine and 2-hydroxy-4-methylthiobutoanic acid in conventional and germ-free chickens

The apparent absorption of 3H-labeled L-Met and L-2-hydroxy-4-methylthiobutoanic acid (MHA-FA) was compared in germ-free and conventional broiler chickens to determine the effect of intestinal bacteria on the absorption of Met and MHA-FA. The two diets contained 0.236% of added Met or MHA-FA. Nineteen germ-free birds were maintained in two isolators and fed diets that had been sterilized by gamma irradiation (50 kilogreys). Nineteen conventional birds were reared in batteries and received nonirradiated feed. Diets were fed ad libitum for 3 wk. On d 21 of the experiment, the birds fasted overnight and were refed the experimental diets to which 1.11 x 10(7) Bq of 1-[methyl3H]MHA-FA or 1-[methyl3H]Met/kg of feed had been added. 51CrCl3 (1.11 x 10(7) Bq/kg of feed) was added as an indigestible marker. After 3 h the birds were euthanized, and their intestinal tracts were removed and partitioned into six sections. Residual Met and MHA-FA in digesta were calculated as the ratio of 3H:51Cr in each sample divided by the ratio of 3H:51Cr in the feed. The residual MHA-FA in the distal ileum of germ-free broilers was lower than in conventional birds (4.7 and 10.2% respectively; P < 0.05). In contrast the residual Met in the distal ileum of germ-free broilers was not different than in conventional birds (3.0 and 3.7% respectively; P > 0.05). This study demonstrates that intestinal bacteria significantly reduce the apparent absorption of MHA-FA from the intestinal tract of broiler chickens.

Modification of energy metabolism by the presence of the gut microflora in the chicken

Whether the association with gut microflora modifies the energy metabolism of chickens was investigated by varying the metabolizable energy consumption level from zero to above the maintenance requirement in the germ-free and conventional states. Single comb White Leghorn chicks were either fasted for 3 d (Expt 1), or fed for 6 d at a fixed daily meal intake of 2, 5 or 8 g/d (Expt 2), or 5, 10 or 15 g/d (Expt 3). Changes in carcass energy deposition and heat production indicated that when no dietary energy was available the presence of the gut microflora could benefit the birds by reducing energy losses, whereas when dietary energy was supplied the efficiency of energy utilization was reduced by the presence of the gut microflora. It was concluded, therefore, that the heavy burden of the gut microflora modifies energy metabolism by exerting a buffering or a counter-productive action on the energy utilization of the chicken.

Energy utilisation in germ-free and conventional chicks fed diets containing sorbose

1. In experiment 1, growing conventional (CV) chicks were fed on diets containing graded amounts (0, 100, 200 and 300 g/kg diet) of sorbose from 4 to 14 d. Protein, fat and energy deposition were determined after carcase analysis. The values for growth, food efficiency, metabolisable energy (ME) and fat and energy depositions declined as the dietary sorbose content increased. 2. In experiment 2, the performances of germ-free (GF) and CV chicks fed on diets with (100 g sorbose/kg diet) or without sorbose were investigated. On both diets, body weight gain, food consumption and protein accumulation in GF chicks were significantly higher than those in CV birds. No significant differences were observed between the dietary treatment except for ME values, which were significantly lower for the sorbose diet. 3. It is suggested that dietary sorbose decreased energy utilisation, and that the microbial contribution to the utilisation of dietary sorbose was negligible in the chicken.

Nitrogen and energy utilization in germ-free and conventional chicks at early stages of growth

Influence of aging on N and ME utilization in germ-free and conventional (CV) chicks was investigated at Stage 1 (2 to 6 days old) and Stage 2 (10 to 14 days old). Chicks were fed diets for ad libitum consumption with protein concentrations of 116, 220, or 445 g/kg diet. The N utilization was not affected by the gut microflora at either stages, but it significantly decreased for chicks on the high-protein diet compared with those on the low-protein and adequate-protein diets. The ME values of diets were not influenced by treatments, except for the high-protein diet at Stage 1, in which ME was significantly reduced.

Utilization of dietary diammonium citrate by germ-free chicks

This study was conducted to clarify the effect of the gut microflora on the utilization of dietary diammonium citrate in the chick. Germ-free and conventional Single Comb White Leghorn chicks were given a basal diet, which contained adequate amounts of all essential amino acids but none of the nonessential amino acids as nitrogen sources, or a diammonium citrate-supplemented diet. Irrespective of presence or absence of the gut microflora, values for body weight gain, feed efficiency, and protein and energy utilization were improved by supplementing the basal diet with diammonium citrate. This suggests that the gut microflora are not required for the chick to utilize diammonium citrate.

Lower fat deposition and energy utilization of growing rats fed diets containing sorbose

1. Growing rats were fed diets containing graded levels (0, 100, 200 and 300 g/kg diet) of sorbose for 6 weeks. Protein, fat and energy deposition were determined by carcass analysis. 2. The values for growth, serum insulin level, digestible energy (DE), metabolizable energy (ME) and fat and energy deposition declined with the increment of dietary sorbose. 3. The efficiency of protein utilization (protein retained/protein consumed) was hardly affected by dietary sorbose. The DE and ME of sorbose per se was calculated as 14.09 and 12.35 kJ/g respectively. The efficiency of energy utilization (energy retained/ME intake) decreased with the increase of dietary sorbose, although sorbose had an ME. 4. The relative weights of gastro-intestinal tract and liver were positively associated with dietary sorbose level, although the reverse was true for the amount of stomach content, being heavier with higher dietary sorbose. 5. It is suggested that dietary sorbose, as a sweetener as well as a bulky agent, seems to be a suitable sugar for the obese and diabetic with special reference to lower body fat and energy deposition without reducing protein utilization.

Influence of the gut microflora on fasting heat production in chicks

1. The influence of the gut microflora on fasting heat production in chicks was investigated. 2. Single Comb White Leghorn chicks were fed on an adequate diet from 2 to 12 d of age and then fasted for 3 d. Fasting heat production was estimated from changes in body composition and energy content of droppings from days 1 to 3 of fasting. Measurements of body temperature and plasma thyroxine concentration were also made. 3. Body fat content was consistently higher in germ-free (GF) chicks than in conventional (CV) counterparts during the starvation period, whereas no difference was found in body protein content. The GF birds had a significantly higher body temperature than the CV controls with no change in plasma thyroxine concentration. 4. It was concluded that estimated fasting heat production of chicks was increased by the absence of the gut microflora.

Gnotobiotes in nutritional studies

Studies with gnotobiotic animals have demonstrated extensive synthesis of vitamins, involvement in the metabolism of nitrogenous compounds and modification of lipids among the activities of the conventional microflora. The extent to which they affect the host depends on factors such as the structure of the gut, the position of the sites of bacterial proliferation and the host's nutritional status. Thus the findings in experimental animals may not always be applicable to man. Results so far indicate that in circumstances of dietary inadequacy bacterial activity may be detrimental or beneficial. However, these effects are small, and it seems reasonable to conclude that the influence of the indigenous microflora on the host's nutrition is of relatively minor importance compared with its role as a barrier against invasion by undesirable organisms.

Gordon memorial lecture. The biologists' debt to the domestic fowl

Although it is highly desirable to reduce the need for experiments with animals, in vitro methods cannot entirely supplant them. Observations made in simple systems must be checked in a live subject if they are to be relevant to man or other higher animals. Young growing chicks are very susceptible to vitamin deficiencies. Biological assays in chicks have been used to check the validity of chemical and microbiological methods of measuring vitamins in foods. Experiments with chicks and chick embryos deprived of vitamin B12 have served to predict the likely clinical effects of analogues of the vitamin. The discovery of the growth-promoting properties of dietary antibiotics stimulated research into the influence of the gut microflora on its host. Studies in germ-free and gnotobiotic chicks have implicated Streptococcus faecium as one of the organisms responsible for the growth depression reversed by antibiotics. In general the growth of conventional chicks given adequate diets is slightly less good than that of their germ-free counterparts, although small beneficial effects of the microflora have been observed in special circumstances. The most important function of the indigenous microflora appears to be as a barrier against invasion by pathogens. To sustain this protective barrier may incur a small cost to the host in terms of dietary energy and other nutrients.

Influence of energy intake on growth and utilisation of dietary protein and energy in germ-free and conventional chicks

The effect of metabolisable energy (ME) intake on the growth and utilisation of dietary protein and energy in germ-free (GF) and conventional (CV) chicks was investigated in two experiments. In experiment 1 a high energy diet (HED, 14.8 kJ ME/g) and a marginally-adequate energy diet (AED, 11.7 kJ ME/g) were fed to the GF and CV chicks at 240 g/2 birds/10 d. In experiment 2 a diet with 13.7 kJ ME/g was fed at 118 g (low level, LL) or 128 g (high level, HL)/bird/10 d. Body weight gain, protein retention and protein retention rate were similar in GF and CV chicks on both AED and HED in the first experiment, but in the second were higher in GF than in CV chicks. The increased ME intake of the CV chicks in experiment 2 may be too small to compensate for the increased requirement. ME intake was significantly higher in the CV chicks than in the GF chicks, whereas energy retention was similar in both groups.

Effect of the gut microflora on chick growth and utilisation of protein and energy at different concentrations of dietary protein

The effect of the gut flora on chick growth and protein and energy utilisation at a marginal dietary energy level (calculated metabolisable energy value 11.7 MJ/kg) was determined with diets containing 227 or 293 g protein/kg. Germ-free (GF) and conventional (CV) chicks were reared for 10 d on the diets, and excreta were collected during the last 4 d. The chicks were killed and carcases and droppings were analysed for N and fat. The GF chicks grew significantly faster than the CV controls on both diets. The growth of CV chicks given the high protein diet was similar to that of GF chicks given the adequate protein diet. Protein and energy utilisation were significantly less for CV than for GF chicks on both diets. It was concluded that the energy requirement of CV chicks was greater than that of their GF counterparts, and that their poorer growth was due to utilisation of some dietary protein as an energy source.

Effect of the gut microflora on the size and weight of organs of chicks fed diets of different protein content

Germ-free (GF) and conventional (CV) chicks were reared for 14 d on diets containing 50 (LD), 200 (AD) and 400 (HD) g protein/kg. Food and water were provided ad libitum. The size and weight of the proventriculus, gizzard, duodenum, jejunum, ileum, caeca, colon, liver, pancreas, heart, spleen, adrenal glands and kidneys were measured, and liver composition was examined. The absolute and relative (weight/kg body-weight) weights of the intestine of CV chicks fed AD and HD diets were greater than those of GF chicks. The LD diet did not affect the absolute and relative weights of the duodenum and jejunum. The absolute weight, relative weight and fat content of the liver of GF chicks given LD diet were greater than those of their CV counterparts. The size and weight of some organs are affected by the diet (dietary protein content)-microflora-host interaction.