To evaluate the impact of ceftiofur treatment in calves on fecal shedding of ceftriaxone-resistant bacteria, 3 female Holstein dairy calves were treated by intramuscular injection with EXCENEL RTU (ceftiofur hydrochloride, Pharmacia and Upjohn) at a therapeutic dosage of 2.2 mg/kg/day for 5 consecutive days following label directions. Three untreated calves were housed separately and served as controls. One to 3 days following the initial administration of ceftiofur, there was a 14% and 2% increase of fecal bacteria resistant to 16 and 64 microg ceftriaxone/mL, respectively. This response remained unchanged from days 6 to 13, and increased resistance was seen at day 17. Randomly selected isolates of gram-positive and gram-negative bacteria with elevated resistance to ceftriaxone (minimal inhibitory concentration (MIC) >or=64 microg ceftriaxone/mL) were isolated from calf feces and identified. In vitro conjugation experiments revealed that both the ceftriaxone-resistance gene bla (CMY-2) and class 1 integron were transferred from two bacterial species to Salmonella spp. at a frequency of 10(7) to 10(5). MIC data revealed that Salmonella transconjugants acquired either reduced susceptibility or resistance to ceftriaxone as well as to multiple antibiotics. This genetic transfer occurred both within and between genera. Treatment of calves with therapeutic dosages of ceftiofur can significantly increase for at least 17 days following the initial treatment the fecal excretion of ceftriaxone-resistant bacteria, including Salmonella species.