Ampicillin killing curve patterns for 20 strains of ampicillin-susceptible nontypeable Haemophilus influenzae were determined by the agar dilution plate count method. The paradoxical effect was detected in the 24-h killing curve patterns for each strain. For the biphasic effect, minimum survivor percentages (maximum killing) occurred over a narrow range of ampicillin concentrations immediately above the MIC, with survivor percentages then rising rapidly to peak at --loglo-unit increment higher. The 24-h minimum survivor percentages for the 20 strains ranged from -0.01% (rapid killing) to >10% (slow killing). In comparison with the previous results for typeable strains, the present findings suggest that nontypeable stains are, on average, killed much more slowly. Based on the initial 24-h killing curve patterns for the 20 strains, 4 strains were selected as putative representatives of the range of bactericidal responses encountered. These strains were then studied to examine the reproducibility of the 24-h patterns and to determine sequential killing curves. These patterns were found to be reproducible and served to characterize the relative killing responses of the strains. In the sequential studies of three of the four strains, tiny colonies having the gross and microscopic characteristics of L-forms were found to be present on the agar dilution plate count plates prior to the application of penicillinase at 48 and 72 h. Such colonies reverted to vegetative forms within 24 to 48 h after application of penicillinase to the panels. Of particular interest was the observation that the paradoxical effect was manifested both by the L-form colonies and by the reverted vegetative colonies. The late development of L-forms was observed for both rapidly and slowly killed strains.In a previous study (35) we described ampicillin killing curve patterns for type b Haemophilus influenzae strains as determined by the agar dilution plate count (ADPC) method. Killing curve patterns were found to be strain dependent and ranged from strains that were killed slowly to others that were killed rapidly. The paradoxical effect (7) was demonstrated at some time during the killing sequence for each strain studied. We present here a similar study of ampicillin killing curve patterns for ampicillin-susceptible nontypeable H. influenzae strains. MATERIALS AND METHODSStudy design. Twenty ampicillin-susceptible nontypeable H. influenzae strains were studied. On retrieval from stock culture, each strain was confirmed to by nontypeable H. influenzae and ampicillin susceptible by standard microbiological methods. Several 24-h killing curve patterns were then determined by the ADPC method for each strain (35). On the basis of the 24-h patterns, several putative rapidly to slowly killed strains were studied further to determine the reproducibility of the 24-h patterns and to determine patterns of sequential killing after various times of ampicillin action.H. influenzae strains. Of the 20 ampicillin-susceptible nontypeable H. influenzae isolates stu...

Quality control for in vitro antimicrobial susceptibility testing of Pseudomonas aeruginosa versus aminoglycosides is improved by P. aeruginosa ATCC 49189, which was developed in the Clinical Microbiology Laboratory at St. Paul-Ramsey Medical Center. This strain, used by us for daily testing for the past 6 years, requires MICs that approximate therapeutic concentrations, are midrange in most dilution schemes, and are stable and reproducible.