Thesis (MNutr (Human Nutrition))--University of Stellenbosch, 2006. / INTRODUCTION: The primary objective was to determine whether acidified formulae (pH 3.5 and 4.5) decreased gastric and tracheal colonisation, as well as microbial contamination of the enteral feeding delivery system, compared with a non-acidified control formula (pH 6.8) in critically ill patients. Secondary objectives included tolerance of the trial formulae and mortality in relation to the administration of acidified formulas. DESIGN: The trial was a controlled, double-blinded, randomised clinical trial of three parallel groups at a single centre. METHOD: Sixty-seven mechanically ventilated, medical and surgical critically ill patients were randomised according to their APACHE II scores and included in the trial. Patients received either an acidified (pH 3.5 or 4.5) or control polymeric enteral formula via an 8-Fr nasogastric tube at a continuous rate. Daily samples were taken for microbiologic analyses of the enteral formulae at various stages of reconstitution and at 6-hour and 24-hour intervals during administration thereof (feeding bottle and delivery set). Daily patient samples included nasogastric and tracheal aspirates, haematological evaluation and gastro-intestinal tolerance. The trial period terminated when patients were extubated, transferred from the ICU, enteral nutrition became contraindicated, a patient died, or for a maximum of 21 days. RESULTS: Gastric pH showed no significant difference (p = 0.86) between the 3 feeding groups [pH 3.5 (n = 23), pH 4.5 (n = 23) and pH 6.8 (n = 21)] at baseline prior to the administration of enteral formulae. After initiation of feeds, the gastric pH decreased significantly (p< 0.0001) in the acidified formulae as compared to the control formula during the trial period. Patients who received acidified enteral formulae (pH 3.5 and 4.5) had significantly less (p < 0.0001) contamination from the feeding bottles and delivery systems in respect of Enterobacteriacea, and Enterococcus., The more acidified group (pH 3.5) showed significantly less gastric contamination (p = 0.029) with Enterobacteriacea, , but not for fungi. The 3.5 acidified group also had the lowest gastric growth in terms of colony counts (≤104) of these organisms, but not for fungi, when compared to the control group (≤105). Vomiting episodes were 22% and abdominal distension 12%, with a higher incidence in the control group. Adverse events occurred equally between the groups with a higher, but not significantly different incidence of 37% in the control group and 32% for the acidified groups. There was no evidence of gastro-intestinal bleeding in any patient. Overall, the mortality rate in this trial was 6%, with 6.5% for the acidified groups (n=46) and 4.8% for the control group (n=21), a statistically insignificant difference. CONCLUSION: Acidified enteral formulae significantly decrease gastric colonisation by preserving gastric acidity that decreases the growth of Enterobacteriaceaes organisms. Acidified formulae significantly decrease bacterial contamination of the enteral feeding system (bottle and delivery set) of Enterobacteriaceae and Enterococcus organisms. Acidified formulae are tolerated well in critically ill patients.