| Summary: | Abnormal gastrointestinal colonisation has been implicated in the development of necrotising enterocolitis (NEC). Organisms capable of rapid fermentation of excess carbohydrate in the small bowel were postulated to produce adverse intraluminal conditions leading to intestinal injury. Quantitative culture of duodenal aspirates from 122 very low birth weight newborns revealed a high prevalence of Gram-negative colonisation, with counts up to 10<sup>8</sup> cfu/g. Gram-negative colonisation occurred in infants who had been fed, increased with postnatal age and was associated with a longer stay on the neonatal unit. Molecular typing of <i>E. coli, Klebsiella </i>spp. and <i>Enterobacter</i> spp. demonstrated marked temporal clustering of indistinguishable strains. Colonisation with particular strains of <i>Enterobacteriaceae</i> was not associated with subsequent development of NEC. Faecal flora, from NEC cases and control infants, comprised predominantly aerobic Gram-negative organisms and enterococci, with anaerobes isolated infrequently. Prior to clinical onset of confirmed NEC, <i>Enterobacteriaceae</i> isolates remained at high levels or increased, while there was a significant fall in enterococci. This fall in enterococci was not apparent in infants with suspected NEC. Asymptomatic infants were colonised with <i>Enterobacteriaceae</i> isolates indistinguishable from NEC cases, but enterococci were also present in high numbers, often being the predominant strain. One <i>E. coli</i> from a post-mortem peritoneal sample, was fully induced for <span style='font-family: Symbol'>b-galactosidase expression. Isolates identical by PFGE, from the same infant and controls were not induced. Other strains, from NEC cases and control infants, showed no difference in <span style='font-family:Symbol'>b-galactosidase activity. NEC appears not to be associated with colonisation with particular strains of <i>Enterobacteriaceae</i>, but the decline in enterococci preceding NEC should be investigated.
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