Effect of actual cefquinome concentration on loss of susceptibility and mutant enrichment. Tissue-cage implantation and Escherichia coli ATCC 25922 infection were done as described in Meterials and Methods. Piglets having above 1 × 108 CFU/mL E.coli in tissue-cage fluid 2 days after infection were treated with various intramuscular doses of cefquinome once daily and twice daily for 5 times beginning 3 days after infection. The dosage, administration at hour or day 0 of the x-axis, protocol for each panel was as follows: A1, 4 received 0.1 mg/kg; A2, 3 received 1.0 mg/kg; A3, 4 received 0.2 mg/kg and 2 received 0.4 mg/kg; A4, 4 received 0.8 mg/kg and 1 received 1.0 mg/kg; A5, 3 received 0.4 mg/kg; A6, 2 received 0.2 mg/kg and 3 received 0.4 mg/kg; A7, 1 received 0.2 mg/kg and 4 received 0.4 mg/kg; A8, 3 received 0.2 mg/kg. Cefquinome concentration in tissue-cage fluid was monitored at the indicated times after the administration of each dose (panels A1-A8). The boundaries of the mutant selection window (the MIC99 and MPC) were determined with the E.coli inoculum by agar plate assays. Tissue-cage fluid was sampled for bacteria at 24 h and 12 h intervals for 7 times starting immediately before the administration of the first dose of cefquinome. Loss of susceptibility (panels B1-B8) was monitored as an increase in MIC averaged for all piglets in the group. The fraction of resistant mutants (panels C1-C8) in each group of piglets was determined daily as the number of colonies grown on cefquinome-containing agar (MIC) relative to the number that grew on drug-free agar.