This rod- shaped, Gram-negative species of bacteria has numerous tail-like flagella which help it move. E. coli is a normal inhabitant of the human intestine and is usually harmless. Under certain conditions its numbers may increase and cause infection. Some strains are responsible for gastroenteritis in children, particularly in tropical countries. It can cause 'traveller's diarrhoea' and 80% of all urinary tract infections. It is also the organism most used in genetic studies. Escherichia coli can carry the gene for NDM-1, an enzyme which makes bacteria resistant to a broad range of beta-lactam antibiotics. Bacteria which carry such genes are often referred to as 'superbugs', since infections with these bacteria are resistant to all standard antibiotics.