Effect of Amoxycillin on Colonisation Resistance in Human Volunteers

The concept of colonisation resistance is based on animal experiments. This study was designed to quantify the defence against colonisation by ingested exogenous microorganisms in human volunteers, and to measure the effect of oral amoxycillin on colonisation resistance. Ten healthy volunteers were challenged with increasing concentrations of a test strain of Klebsiella oxytoca , 10 3 , 10 5 , 10 7 and 10 9 colony forming units (cfu). Oral washings and faecal specimens were collected three times a week and were processed qualitatively and quantitatively. After a challenge dose of 10 7 cfu transient intestinal colonisation occurred in one volunteer, and after 10 9 cfu in two volunteers. The experiment was repeated after three days of amoxycillin intake with a challenge dose of 10 5 cfu. In five of nine volunteers the exogenous microorganisms were able to colonise the intestine, with severe clinical symptoms of profuse diarrhoea in one volunteer. This study confirms the existence of colonisation resistance in humans and the adverse effect of amoxycillin.