Disparity between timed-kill and checkerboard methods for determination of in vitro bactericidal interactions of vancomycin plus rifampin versus methicillin-susceptible and -resistant Staphylococcus aureus

Abstract

p.g/ml (8). The 22 MSSA isolates were all recent blood isolates from the Clinical Microbiology Laboratory of the Harbor-UCLA Medical Center; the 26 MRSA isolates (kindly provided by Francisco Sapico, Downey, Calif.) were also recent clinical isolates from a variety of body sites. There were no duplicate isolates from individual patients. The isolates were maintained on Mueller-Hinton (MH) agar slants until the susceptibility studies were performed.Antibiotics. Vancomycin was supplied by Eli Lilly & Co. (Indianapolis, Ind.), and rifampin was provided by MerrillDow Research (Indianapolis, Ind.). Stock solutions of each agent at a concentration of 10,000 ,ug/ml were prepared and kept at -70°C until thawed on the day of the in vitro studies.Antibiotic susceptibility testing. The MICs and MBCs of vancomycin and rifampin against these 48 isolates were determined by the microtiter broth dilution technique (14). Pilot MIC and MBC determinations in our laboratory, utilizing several study staphylococcal isolates, disclosed close agreement between results in the microtiter and macrobroth dilution methods (15). Fifty-microliter volumes of each antibiotic in twofold serial dilutions were added to a separate microtiter row of wells. A 50-,ul sample of each MRSA or MSSA isolate grown in MH broth (MHB) to mid-logarithmic phase (-2 h) was added to each antibiotic-containing well and mixed without splashing by a microdiluter to achieve a final concentration of -2 x 105 CFU/ml. The final drug concentrations in the wells were 0.007 to 16 ,ug/ml for both drugs. These concentrations were chosen to encompass readily attainable levels in serum for each agent at standard clinical dosages (1, 13). For each isolate tested, one microtiter well contained only the organism in antibiotic-free MHB as a growth control. After inoculation and gentle mixing, all plates were incubated for 24 h at either 30°C for

Determination of Fungicidal Activities against Yeasts and Molds: Lessons Learned from Bactericidal Testing and the Need for Standardization

“…The time-kill method has been used widely for the evaluation of new antimicrobial agents and allows the determination of whether an agent produces concentration-dependent killing (where the extent of killing increases with increased drug concentrations) or time-dependent killing (where the killing continues only as long as the concentrations are in excess of the MIC) (53). Time-kill studies are used infrequently to guide therapy in an individual patient; however, interest in such studies is sustained, given the fact that (i) killing curves show good correlation with cure in experimental models of endocarditis (22,61), (ii) time-kill studies are useful in determining tolerance to the lethal activity of antibacterial agents (12,28,68,82) (see below), and (iii) the time-kill method is useful for determining synergy or antagonism between two (or more) antimicrobial agents (4,5,53). This VOL.…”

Rifampin Combination Therapy for Nonmycobacterial Infections

“…For example, vancomycin penetration into lung tissue is not sustained, and the same was seen for the meninges and infected bone, whereas rifampin easily penetrates these tissues (61,109,120). The rifampin-vancomycin in vitro interaction appears to range from antagonistic to indifferent, with very few synergistic studies (25,26,112,119,158,218,246,251,258,261). Tuazon et al showed that the rifampin-vancomycin combination had "partial synergism" with only 25% of the MSSA isolates, with the remainder being indifferent, while the nafcillin-rifampin combination had "partial synergism" for 60% of the isolates (246).…”

“…Rifampin in combination with vancomycin, a macrolide, and/or a quinolone is used for the treatment of this infection (60, 164, 173, 244). Clinical 26 FORREST AND TAMURA CLIN. MICROBIOL.…”

Testing the optimality properties of a dual antibiotic treatment in a two-locus, two-allele model

“…It has also been shown that synergistic combinations can accelerate the evolution of antibiotic resistance [25] and, as a result, it was suggested that 'antagonistic' drug interactions, seemingly neglected in clinical practice, might slow resistance evolution [26]. In practise, drugs are used even when their synergy is not proved, for example the anti-MRSA combination vancomycin and rifampicin [27,28].…”