Growth, inhibition and pathogenicity of microorganisms in enteral nutrient solutions

Enteral nutrient solutions (ENS) which are contaminated with microorganisms from exogenous sources or from the microbial flora of the patient's own gastrointestinal tract are associated with bacteraemia, diarrhoea, respiratory infections and septicaemia. These infections seriously undermine some of the advantages of enteral feeding by increasing patient morbidity, mortality and lengthening hospital stay. The strategies which have been employed to prevent contamination of enteral feeding systems and the subsequent incidence of disease have been shown to be ineffective or impractical in a clinical environment. Therefore, this thesis investigated the possibilities of using novel antimicrobial agents (oil of fennel and parabens) to inhibit the growth of microorganisms that commonly contaminate ENS and/or to alter the expression of products which mediate in their pathogenicity. In order to achieve this, the study of growth and the virulence determinants of microorganisms in enteral feeding solutions was necessary. Gram-negative bacteria are the most important group of microorganisms that contaminate ENS and therefore this group of bacteria and lipopolysaccharide, an important Gram-negative bacterial virulence determinant, were particularly singled out for study. The results showed that the growth of all test strains of microorganisms was similar in milk-based enteral nutrient solutions and laboratory media, while only Candida albicans and Klebsiella aerogenes grew in the "fruit-based" ENS. None of microbial strains were inhibited in milk-based ENS by the concentrations of oil of fennel or parabens used singly. However, oil of fennel and parabens used in combination had synergistic antimicrobial activity and inhibited the growth of all microbial strains. Growth in milk-based enteral nutrient solutions was found to alter the phenotypic expression and biological activity of lipopolysaccharide. Results showed differences in the O-polysaccharide portion of the lipopolysaccharide of bacteria cultured in milk-based enteral nutrient solutions compared to those cultured in laboratory media. Bacteria cultured in milk-based enteral feeds had increased serum resistance and also induced a significantly greater release of nitric oxide from a human monocyte cell line than bacteria cultured in laboratory media. The concentrations of oil of fennel and methyl parabens used in experiments did not alter the expression of O-polysacchande. Enteral nutrient solutions can therefore, not only support the growth of microorganisms but alter virulence determinants. This could have important consequences for immunocompromised patients who receive ENS