The activity of Sch 29482 against type b Haemophilus influenzae lacking or possessing detectable -lactamase activity

To gain insight into the potential role of Sch 29482 (SCH) in the treatment of Haemophillus influenzae infections, we examined the activity of this compound against 50 ampicillin-sensitive and 50 ampicillin-resistant strains, comparing its activity to cefaclor, cefamandole and moxalactam. All susceptibility testing was performed on supplemented BHI agar with 18 h incubation in 5% CO 2 at 37°C. With an inoculum of 10 5 cfu moxalactam was the most potent, while cefaclor was the least: cefaclor MIC 90% 8 0, cefamandole 2.0, SCH 0.5 and moxalactam 0.5 mg/l, respectively. If the inoculum is increased from 10 3 to 10 7 with both types of strains, there is a marked increase in apparent MIC to values not readily achievable in serum with standard doses. The apparent MIC increased 150-fold with cefaclor, 600-fold with cefamandole, 90-fold with moxalactam and 20-fold with SCH. We conclude that SCH is less potent than moxalactam, but its apparent MIC is considerably less affected by the inoculum used.