Oxacillin killing curve patterns of Staphylococcus aureus isolates by agar dilution plate count method

Abstract

The bactericidal dynamics of oxacillin against four Staphylococcus aureus isolates with known 24-h "persister" percentages were studied by using the agar dilution plate count method. Isolates were selected to provide a representative spectrum whose individual 24-h trough intrinsic persister percentages ranged from greater than 1 to less than 0.01%. Resultant agar dilution plate count method killing curve patterns were found to be reproducible and served to characterize each isolate. The paradoxical effect was observed for each isolate, with paradoxical peaks tending to develop and diminish sequentially during the course of oxacillin action. The observed strain-dependent dynamics of oxacillin killing underscore the artifactual nature of the so-called tolerance phenomenon and negate the usefulness of MBCs and MBC/MIC ratios for the characterization of S. aureus isolates.

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

“…Studying S. aureus and oxacillin by ADPC, we found responses to be strain dependent, with strains constituting a spectrum ranging from some which are slowly killed to others which are rapidly killed (29). The paradoxical effect (5), although irregularly observed by BDPC methods, was observed by us for all S. aureus isolates tested, with the phenomenon becoming manifest over the course of antimicrobial action.…”

“…We recently introduced the ADPC method (27)(28)(29) as an alternative to the broth dilution plate count (BDPC) approach for the quantitative measurement of bacterial inhibition and killing by P-lactam agents. The procedure was designed to avoid the severe dependency on technical factor variations which have been shown to confound BDPC results.…”

“…We recently described a novel agar dilution plate count (ADPC) method for the quantitative measurement of Staphylococcus aureus inhibition and killing by oxacillin (27)(28)(29). Since Each culture was then further diluted as needed with MHB-C to produce standardized inoculum preparations for the ADPC tests.…”

Tests for bactericidal effects of antimicrobial agents: technical performance and clinical relevance

“…Bactericidal analysis of antimicrobial activity in solid rather than liquid medium has also been proposed (16,50,68,69,82,122). Most techniques require inactivation of the antimicrobial agent under evaluation after the initial 18-to 24-h incubation period, and the majority of these studies have involved inactivation of ,-lactam agents with a 3-lactamase (50,68,69,82 (45).…”

“…The appearance of bacteriostatic or bactericidal activity is typically determined for each tube in the set, with the result indicating the optimal concentrations of each drug in combination for inhibitory and bactericidal effects. IN (59,67,78,85,99,104,114,120), the use of cation supplements (41,92,107,120), whether the culture is mixed or otherwise agitated during incubation (48,104,105,108,120), the growth phase of the inoculum (7,39,48,57,70,104,105,108,114,120), the pH of the medium (34,105,114,120), the use of serum (binding) proteins (92,120), the presence of complement (21,34,120), the elimination of antibiotic carryover during subculture (8,25,35,47,93,103,104,119,122), the final density of the bacterial inoculum …”