Ampicillin killing curve patterns of Haemophilus influenzae type b isolates by agar dilution plate count method

Abstract

The responses of 20 ampicillin-susceptible Haemophilus influenzae type b clinical isolates to the bactericidal action of ampicillin were studied by using a modified agar dilution plate count method. A well-defined paradoxical effect was observed in each of the 24-h

Ampicillin killing curve patterns for ampicillin-susceptible nontypeable Haemophilus influenzae strains by the agar dilution plate count method

“…influenzae strains. Of the 20 ampicillin-susceptible nontypeable H. influenzae isolates studied, 15 were obtained through the courtesy of Timothy F. Murphy (17) (34,35) we encountered some loss of ampicillin activity in ADPC plates made with MHA and 1% supplement C (Difco Laboratories, Detroit, Mich.) as a consequence of various lots of supplement C containing small amounts of P-lactamase. To avoid this problem in the present study, ADPC panels were prepared with the recently introduced HTM (12).…”

“…Material from a number of colonies on the second stock retrieval plate was inoculated into 5 ml of HTM broth (12) contained in a borosilicate glass tube (16 by 125 mm), which was then incubated at 35°C in air until the broth approached the density of a 0.5 McFarland standard. This procedure was previously shown (35,39) to produce a log-phase culture of -3 x 108 CFU/ml. The culture was then diluted 1:10, 1:100, and so on with Mueller-Hinton broth to produce the appropriate inoculum densities as needed for the ADPC inoculation.…”

“…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.…”

“…ADPC panels were prepared (35) with HTM agar containing ampicillin in twofold dilution concentrations ranging from 0.25 through 128 ,ug/mil and were used within 1 week of preparation, most often on the day they were made. Immediately after preparing the standardized inoculum, 0.050 ml was pipetted onto the surface of each plate and streaked with a bacteriological loop to disperse thoroughly the inoculum evenly over the entire surface.…”

“…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.…”