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The phosphorus (P) requirements of meat-type poultry cannot be met by plant-based diets because approximately two thirds of total phosphorus in cereals and leguminous feed materials is in the unavailable form of phytate (inositol hexaphosphate or phytic acid). Phosphorus supplements are expensive and over supplementation can increase excretion of P, with negative implications on the environment. For some time, research has focussed on the development of exogenous phytase enzymes capable of releasing phytate phosphorus for absorption, thus reducing the environmental impact of poultry production and requirements for costly dietary inorganic P supplementation. The absolute adoption of this strategy has, however, been stalled by apparent variability in rate of release of phytate by phytase enzyme. The aim of this project was to examine the availability and reactivity of phytate in diets and raw materials fed to broilers in a range of physiological environments, and to develop strategies for combating their effects. A series of in vitro studies and bird trials were conducted to investigate phytate reactivity by observing the relationship between pH, protein phytate interactions and phytate susceptibility to the effects of phytase. A bird trial was undertaken comparing the current commercial level of 500 FTU/kg phytase and a superdose of 5000 FTU/kg phytase in soyabean meal and rapeseed meal based broiler diets. Feeding phytase doses above the commercial recommendation resulted in increased bird performance through phytate destruction, leading to heightened alleviation of the anti-nutritional effects of phytate. This suggests that superdosing broiler diets can improve profitability by both increasing availability of phosphorus and removing restrictions of using cheaper feed ingredients which are viewed as undesirable due to their high phytate content. There is a common misconception that poultry lack any endogenous phytase, but a study sequentially sampling broilers from age d4 to d14 illustrated that phytase activity from intestinal mucosa and bacteria and diet is quantifiable in regulating phytate-P digestion. Endogenous phytase contributes significantly towards degradation of phytate at bird age d4. By d14, ileal phytase activity levels were approximately 45 U/kg and the amount of total dietary phytate hydrolysed ranged from 21% to 36%. Measurements of total phytate-P content of diets may be deceptive as they do not indicate substrate availability for phytase; measurements of phytate susceptible to the effects of phytase may be a more accurate measure of phosphorus availability to the bird. To verify this hypothesis in practice, a bird study was designed to compare diets formulated to contain high or low susceptible phytate. This study showed that at age d28, birds fed diets with high susceptible phytate content had significantly better cumulative BWG (p=0.015) and FCR (p=0.003) than birds fed diets with low susceptible phytate content. Furthermore, in vitro screening of raw materials revealed that phytate susceptibility varies considerably between ingredients and batches of ingredients, and total phytate content bears no relation to susceptible phytate content of an ingredient. Therefore, for optimum phytase efficacy, it may be advantageous to formulate diets and develop phytase matrix values based on the susceptible phytate content of the individual batch of ingredients being fed. Both total and susceptible phytate content can be measured using simple colorimetric assays. Examination of in vitro digestion models and subsequent correlation to in vivo studies indicate that exposing samples to conditions that mimic the gastrointestinal environment is the most accurate way to predict phytate degradation and mineral availability in diets prior to feeding. The relationship between phytate reactivity and pH was investigated throughout this project. It was found that gastric pH is closer to the optimum for pepsin activity and phytate-complex degradation when in the presence of phytase. This is because phytate reduces protein digestibility, resulting in heightened presence of intact proteins in the tract, which instigates an increase in secretion of HCl and pepsin. More bicarbonate ions are released in response to this increased acidity, causing an increase in intestinal pH and resulting reduction in solubility of phytate-protein complexes. To conclude, dietary phytate that is susceptible to phytase effects as opposed to total phytate content should potentially be considered when determining phytase matrix values. For optimum response to phytase, it may be beneficial to formulate diets based on the susceptible phytate content of the individual batch of ingredients being fed. Some raw materials have high total phytate content but also high susceptible phytate content, meaning they have the potential to be used as replacements for more expensive feed ingredients if in the presence of phytase. This is particularly the case in diets supplemented with high doses of phytase (>500 FTU/kg), because high phytase doses alleviate the extra-phosphoric anti-nutritional effects of phytate as well as increase mineral availability.

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Interspecific allometry of locomotor morphology, gait kinematics and energy metabolism is well described, but the underlying reasons for these patterns are not yet fully understood. Between disparate species, a plethora of confounding morphological and physiological variables precludes identifying the independent effects of body size, particular morphological characteristics or gait kinematics on metabolic costs. The main objective of my thesis is to elucidate the links between these integrated components of locomotion by exploiting a range in morphological and physiological similarities and differences in a single species to have resulted from selective breeding. My model species, the leghorn chicken (Gallus gallus domesticus), is selectively bred for egg laying productivity and varieties of different body size which are physiologically and geometrically similar: standard (large) and bantam (small). At sexual maturity, leghorns also exhibit male-biased sexual size dimorphism and males and females differentially bias tissue allocation to pelvic limb muscle and viscera/reproductive organs, respectively. Using respirometry measurements, I first demonstrate that between males of standard and bantam leghorn varieties, the cost of transport is independent of body mass. Using limb posture, muscle architecture and kinematic measurements, I provide the first empirical evidence in support of a potential mechanism behind a lack of scaling in the cost if transport at the intraspecific level. I also provide the first evidence of a greater metabolic cost of transport in a male compared to a female in any species. I demonstrate that the sexual dimorphism in energy metabolism cannot be accounted for by comparison at dynamically similar speeds, limb posture, muscle architectural properties or the rate of muscle force generation. Furthermore, by comparing with a younger cohort of standards, I demonstrate that sex differences in locomotor energy metabolism manifest before the onset of sexual maturity and must, therefore, result from alternative sub-organismal sexual dimorphisms. I suggest that these differences may relate to physiological specialisation for to economical egg-load carriage and performance in inter-male aggressive combat. I also provide the first detailed comparison of the gait dynamics of the sexes in any species. I demonstrate that female leghorns walk with proportionally more foot-ground contact than their conspecific males. Furthermore, these kinematic differences are concomitant with the increase in anatomical mass at sexual maturity loading the swing and stance muscles of males and females, respectively. Perhaps the most striking finding in this thesis is that, while small cursosrial birds generally walk with crouched limbs, female standard leghorns walked with upright limbs. Just as larger species tend to have upright limbs to support a disproportionate amount of weight, compared to smaller more crouched species, these females may do the same due to artificial selection for increased egg production. Together, the locomotor measurements of the leghorns presented this thesis appear to support a recent hypothesis, that animals select postures and gaits to optimize muscle mechanical work and power demands and minimize active muscle volume.

Extended spectrum β-lactamases (ESBLs) are enzymes produced by bacteria that confer resistance to cephalosporins, antibiotics that are widely used to treat infections in animals and humans. The emergence of plasmid-borne ESBL resistance in humans and animals, especially poultry, is of particular concern. Avian pathogenic E. coli (APEC) cause colibacillosis, an economically important disease of poultry, but have also been linked to human disease. Of growing concern is the potential transfer of ESBL-producing plasmids between E. coli isolates and their impact on the host bacterium. Thus, the aim of this study was to characterise ESBL plasmids in APEC isolated from UK poultry and examine their impact on host bacterial fitness and virulence. Plasmid profiling and genome sequencing revealed that all three APEC isolates harboured at least two plasmids, belonging to the IncF and IncI replicon types. The ESBL plasmid was identical in each isolate, being of 105,610 Kb in size, belonging to the IncI1-Iy family and carrying the blaCTX-M-1 allele. Upstream of blaCTX-M-1 gene was an ISEcp1 insertion element, which likely contributes to the transmissibility of the element. The plasmid also carried genes coding for tetracycline resistance that were co-transferred with the β-lactamase genes. The APEC isolates also harboured a large virulence plasmid of similar size, but belonging to the IncF replicon type. Competitive growth studies in rich or minimal media found no significant differences in the growth of CTX-M-1 plasmid-harbouring and plasmid-cured derivatives. However, the presence of the CTX-M-1 plasmid in APEC O78 was beneficial to their growth in iron-limited media, but caused a greater fitness burden at low pH. When present as the only plasmid in APEC, the ESBL-containing derivatives produced more biofilm at human and chicken body temperatures and were better able to invade and survive in human (THP-1) and chicken (HD11) macrophage cell lines than the cured isolates. Finally, the substrate utilisation profiles of the plasmid-containing derivatives differed to that of the parent strains, indicating that the plasmid may influence the metabolic capability of the organism. The studies presented here indicate that ESBL plasmids of the IncI1-1Y type spread readily to unrelated APEC isolates and can influence the fitness of APEC under certain conditions.

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