Comparison of in vitro static and dynamic assays to evaluate the efficacy of an antimicrobial drug combination against Staphylococcus aureus

Abstract

An easily implementable strategy to reduce treatment failures in severe bacterial infections is to combine already available antibiotics. However, most in vitro combination assays are performed by exposing standard bacterial inocula to constant concentrations of antibiotics over less than 24h, which can be poorly representative of clinical situations. The aim of this study was to assess the ability of static and dynamic in vitro Time-Kill Studies (TKS) to identify the potential benefits of an antibiotic combination (here, amikacin and vancomycin) on two different inoculum sizes of two S. aureus strains. In the static TKS (sTKS), performed by exposing both strains over 24h to constant antibiotic concentrations, the activity of the two drugs combined was not significantly different the better drug used alone. However, the dynamic TKS (dTKS) performed over 5 days by exposing one strain to fluctuating concentrations representative of those observed in patients showed that, with the large inoculum, the activities of the drugs, used alone or in combination, significantly differed over time. Vancomycin did not kill bacteria, amikacin led to bacterial regrowth whereas the combination progressively decreased the bacterial load. Thus, dTKS revealed an enhanced effect of the combination on a large inoculum not observed in sTKS. The discrepancy between the sTKS and dTKS results highlights that the assessment of the efficacy of a combination for severe infections associated with a high bacterial load could be demanding. These situations probably require the implementation of dynamic assays over the entire expected treatment duration rather than the sole static assays performed with steady drug concentrations over 24h.

Effects of Incubation Time and Inoculation Level on the Stabilities of Bacteriostatic and Bactericidal Antibiotics against Salmonella Typhimurium

“…The results suggest that ASST and ARST developed the ability to survive prolonged periods of exposure to antibiotics. This observation is in good agreement with the previous report that the development of resistance was noticeable when bacteria were exposed to sublethal concentrations of antibiotics [ 23 ]. The MIC values of tetracycline and tobramycin were increased against ASST after an extended incubation time of 40 h, while those of cephalothin and tobramycin were increased against ARST ( Figure 3 and Figure 4 ).…”

Classification of In Vitro Phage–Host Population Growth Dynamics

“…The dynamics of in vitro phage-host interactions may differ from those observed in vivo. This is similar to what is seen also with antibiotics [19,20]. Still, in vitro observations constitute a necessary initial step in understanding and predicting phage-host population dynamics in in vivo settings [7].…”

Effect of Ciprofloxacin, Levofloxacin, and Ofloxacin on Pseudomonas aeruginosa : A case control study with time kill curve analysis

“…It was also explained earlier that as a weakness of the time-kill test, several factors influence the measurement of the lethal effect of antibiotics in vitro studies, including persistence, paradoxical and resistance. Factors from the technique, such as bacterial growth phase and inoculum size, must be calculated [ 22 ].…”