Antibacterial drugs (oxytetracycline, streptomycin and sulphonamides) were included in the drinking water of healthy broiler chicks from the sixth to the twentieth day of life to select a resistant gut flora. On the twenty-first day the birds were divided into three groups and reared in separate rooms until 100 days of age. One group was housed in cages with wire floors while the others were reared on litter. Faeces from adult hens were added regularly to the litter of one of these groups to determine its effect on the gut flora of the chicks.

The ecology of Escherichia coli was studied using O-serotyping, biotyping and antibacterial drug resistogram typing. The proportion of E. coli in the dominant faecal flora resistant to two to four antibacterial drugs increased with time to reach a peak several days after the drugs were withdrawn. Thereafter, the level of drug resistance in the E. coli declined equally in all three groups. The majority of organisms with multiple resistance were derived from biotypes of O-serotypes initially resistant to only one drug and were identified before the drugs were administered. The decline in the level of resistance in the dominant faecal flora after the fourth week was due to the appearance of either new O-serotypes or new biotypes of O-serotypes previously shown to be multiply resistant, and which were either sensitive or resistant to only one drug. It is probable that these new strains were derived from the food since several O-serotypes appeared simultaneously in all three groups of birds.