In vitro activity of daptomycin against Enterococcus faecalis under various conditions of growth-phases, inoculum and pH

Bacterial interspecies interactions modulate pH-mediated antibiotic tolerance

Predicting antibiotic efficacy within microbial communities remains highly challenging. Interspecies interactions can impact antibiotic activity through many mechanisms, including alterations to bacterial physiology. Here, we studied synthetic communities constructed from the core members of the fruit fly gut microbiota. Co-culturing of Lactobacillus plantarum with Acetobacter species altered its tolerance to the transcriptional inhibitor rifampin. By measuring key metabolites and environmental pH, we determined that Acetobacter species counter the acidification driven by L. plantarum production of lactate. Shifts in pH were sufficient to modulate L. plantarum tolerance to rifampin and the translational inhibitor erythromycin. A reduction in lag time exiting stationary phase was linked to L. plantarum tolerance to rifampicin, opposite to a previously identified mode of tolerance to ampicillin in E. coli. This mechanistic understanding of the coupling among interspecies interactions, environmental pH, and antibiotic tolerance enables future predictions of growth and the effects of antibiotics in more complex communities.

Population Density Modulates Drug Inhibition and Gives Rise to Potential Bistability of Treatment Outcomes for Bacterial Infections

The inoculum effect (IE) is an increase in the minimum inhibitory concentration (MIC) of an antibiotic as a function of the initial size of a microbial population. The IE has been observed in a wide range of bacteria, implying that antibiotic efficacy may depend on population density. Such density dependence could have dramatic effects on bacterial population dynamics and potential treatment strategies, but explicit measures of per capita growth as a function of density are generally not available. Instead, the IE measures MIC as a function of initial population size, and population density changes by many orders of magnitude on the timescale of the experiment. Therefore, the functional relationship between population density and antibiotic inhibition is generally not known, leaving many questions about the impact of the IE on different treatment strategies unanswered. To address these questions, here we directly measured real-time per capita growth of Enterococcus faecalis populations exposed to antibiotic at fixed population densities using multiplexed computer-automated culture devices. We show that density-dependent growth inhibition is pervasive for commonly used antibiotics, with some drugs showing increased inhibition and others decreased inhibition at high densities. For several drugs, the density dependence is mediated by changes in extracellular pH, a community-level phenomenon not previously linked with the IE. Using a simple mathematical model, we demonstrate how this density dependence can modulate population dynamics in constant drug environments. Then, we illustrate how time-dependent dosing strategies can mitigate the negative effects of density-dependence. Finally, we show that these density effects lead to bistable treatment outcomes for a wide range of antibiotic concentrations in a pharmacological model of antibiotic treatment. As a result, infections exceeding a critical density often survive otherwise effective treatments.

Towards a definition of daptomycin optimal dose: Lessons learned from experimental and clinical data

Daptomycin exhibits excellent antibacterial activity against a wide range of Gram-positive bacteria. The on-label standard daily doses for daptomycin are 4 mg/kg for skin infections and 6 mg/kg for bacteraemia or right-sided endocarditis. Daptomycin bactericidal activity is predominantly concentration-dependent and by considering the values of pharmacokinetic targets established by several authors as well as the peak and trough concentrations of daptomycin obtained at various daily dosages, it appears that these targets can easily be reached with a dose of 6 mg/kg but only for a minimum inhibitory concentration (MIC) at 0.1 mg/L, and that for increasing MICs (e.g. 0.5 mg/L or 1 mg/L) these targets may only be attained with higher dosages (i.e. ≥10 mg/kg). High-dose (HD) daptomycin therapy has also been proven to be effective for reducing the risk of selection of daptomycin-resistant strains. Given the concentration-dependent bactericidal activity of daptomycin, the absence of a dose-toxicity relationship and the need to prevent the selection of resistant strains, we propose to consider for staphylococcal (i) skin and soft-tissue infections, daily doses of daptomycin of 6 mg/kg (new standard dose) and (ii) endocarditis or bacteraemia including those associated with intravascular catheter and implant-related infections, ≥10 mg/kg (HD) when the MIC is unknown or >0.25 mg/L, and 6-10 mg/kg (intermediate doses) when the MIC is ≤0.25 mg/L. For severe and deep-seated enterococcal infections, we propose high (≥10 mg/kg) daily doses of daptomycin in combination with another active agent, especially a β-lactam.

Evaluation of the toxicity and antimicrobial activity of hydroethanolic extract of Arrabidaea chica (Humb. & Bonpl.) B. Verl

ETHNOPHARMACOLOGICAL RELEVANCE

Arrabidaea chica (Bignoniaceae) is a vine native to the Amazon Rainforest, popularly known as "crajiru" and whose infusion and decoction of the leaves are used to treat diseases such as gastric ulcers, inflammations, infections, anemia, herpes, jaundice among others. It is also used as a natural dye. This work aimed to evaluate the in vitro and in vivo toxicity, antimicrobial activity including analysis of chemical constitution of the hydroethanolic extract of the leaves of Arrabidaea chica (HEAc).

MATERIALS AND METHODS

Acute and subchronic toxicity of HEAc was evaluated in mice and rats, respectively, and by Alamar blue (cytotoxicity assay) using CHO-K1 cells. Antimicrobial activity of HEAc was tested by broth microdilution method using a panel of bacteria and yeast of clinical interest. The preliminary phytochemical analysis of HEAc was performed by electrospray ionization mass spectrometry [ESI(+)-MS]. Secondary metabolites were quantified by colorimetric methods.

RESULTS

When administered in vivo at doses up to 3000 mg/kg v.o., HEAc did not cause any signs and symptoms of acute toxicity in mice and no cytotoxicity in CHO-K1 cells. Administration for 30 days caused leukocytosis (200 mg/kg) and reversible reductions in non-dose dependent of body weight, total weight gain and feed intake in rats given 200mg/kg and 500 mg/kg of HEAc, but were not accompanied by behavioral and clinical changes (laboratory and histopathological) that may have demonstrated evidences of subchronic toxicity HEAc demonstrated a pronounced activity against Helicobacter pylori (MIC=12.5 μg/mL) and moderate activity against Enterococcus faecalis (MIC=100 μg/mL) in broth microdilution. Preliminary phytochemical analysis of HEAc by colorimetric methods revealed that mainly the presence of phenolic compounds (16.6%), especially flavones and flavonols (4.02%). [ESI(+)-MS] fingerprint analyses of HEAc revealed the presence of 3-deoxyanthocyanidins and kaempferol.

CONCLUSION

Our data provide evidence that HEAc is safe and can be useful in infections related to Helicobacter pylori and Enterococcus faecalis. Phytochemical analysis revealed the predominant presence of flavones and flavonols, possibly involved in the antimicrobial action of HEAc.