Bioactivity of Eugenol: A Potential Antibiotic Adjuvant with Minimal Ecotoxicological Impact.

Combining commercial antibiotics with adjuvants to lower their minimum inhibitory concentration (MIC) is vital in combating antimicrobial resistance. Evaluating the ecotoxicity of such compounds is crucial due to environmental and health risks. Here, eugenol was assessed as an adjuvant for 7 commercial antibiotics against 14 pathogenic bacteria in vitro, also examining its acute ecotoxicity on various soil and water organisms (microbiota, Vibrio fischeri, Daphnia magna, Eisenia foetida, and Allium cepa). Using microdilution methods, checkerboard assays, and kinetic studies, the MICs for eugenol were determined together with the nature of its combinations with antibiotics against bacteria, some unexposed to eugenol previously. The lethal dose for the non-target organisms was also determined, as well as the Average Well Color Development and the Community-Level Physiological Profiling for soil and water microbiota. Our findings indicate that eugenol significantly reduces MICs by 75 to 98%, which means that it could be a potent adjuvant. Ecotoxicological assessments showed eugenol to be less harmful to water and soil microbiota compared to studied antibiotics. While Vibrio fischeri and Daphnia magna were susceptible, Allium cepa and Eisenia foetida were minimally affected. Given that only 0.1% of eugenol is excreted by humans without metabolism, its environmental risk when used with antibiotics appears minimal.

Enhancing Commercial Antibiotics with Trans-Cinnamaldehyde in Gram-Positive and Gram-Negative Bacteria: An In Vitro Approach.

One strategy to mitigate the emergence of bacterial resistance involves reducing antibiotic doses by combining them with natural products, such as trans-cinnamaldehyde (CIN). The objective of this research was to identify in vitro combinations (CIN + commercial antibiotic (ABX)) that decrease the minimum inhibitory concentration (MIC) of seven antibiotics against 14 different Gram-positive and Gram-negative pathogenic bacteria, most of them classified as ESKAPE. MIC values were measured for all compounds using the broth microdilution method. The effect of the combinations on these microorganisms was analyzed through the checkboard assay to determine the type of activity (synergy, antagonism, or addition). This analysis was complemented with a kinetic study of the synergistic combinations. Fifteen synergistic combinations were characterized for nine of the tested bacteria. CIN demonstrated effectiveness in reducing the MIC of chloramphenicol, streptomycin, amoxicillin, and erythromycin (94-98%) when tested on Serratia marcescens, Staphylococcus aureus, Pasteurella aerogenes, and Salmonella enterica, respectively. The kinetic study revealed that when the substances were tested alone at the MIC concentration observed in the synergistic combination, bacterial growth was not inhibited. However, when CIN and the ABX, for which synergy was observed, were tested simultaneously in combination at these same concentrations, the bacterial growth inhibition was complete. This demonstrates the highly potent in vitro synergistic activity of CIN when combined with commercial ABXs. This finding could be particularly beneficial in livestock farming, as this sector witnesses the highest quantities of antimicrobial usage, contributing significantly to antimicrobial resistance issues. Further research focused on this natural compound is thus warranted for this reason.

Assessing the Ecotoxicity of Eight Widely Used Antibiotics on River Microbial Communities.

Global prevalence of antibiotic residues (ABX) in rivers requires ecotoxicological impact assessment. River microbial communities serve as effective bioindicators for this purpose. We quantified the effects of eight commonly used ABXs on a freshwater river microbial community using Biolog EcoPlates™, enabling the assessment of growth and physiological profile changes. Microbial community characterization involved 16S rRNA gene sequencing. The river community structure was representative of aquatic ecosystems, with the prevalence of Cyanobacteria, Proteobacteria, Actinobacteria, and Bacteroidetes. Our findings reveal that all ABXs at 100 µg/mL reduced microbial community growth and metabolic capacity, particularly for polymers, carbohydrates, carboxylic, and ketonic acids. Chloramphenicol, erythromycin, and gentamicin exhibited the highest toxicity, with chloramphenicol notably impairing the metabolism of all studied metabolite groups. At lower concentrations (1 µg/mL), some ABXs slightly enhanced growth and the capacity to metabolize substrates, such as carbohydrates, carboxylic, and ketonic acids, and amines, except for amoxicillin, which decreased the metabolic capacity across all metabolites. We explored potential correlations between physicochemical parameters and drug mechanisms to understand drug bioavailability. Acute toxicity effects at the river-detected low concentrations (ng/L) are unlikely. However, they may disrupt microbial communities in aquatic ecosystems. The utilization of a wide array of genetically characterized microbial communities, as opposed to a single species, enables a better understanding of the impact of ABXs on complex river ecosystems.

Antibiotic properties of Satureja montana L. hydrolate in bacteria and fungus of clinical interest and its impact in non-target environmental microorganisms.

The aim of this study was to analyse the microbicidal and microbiostatic activity of S. montana hydrolate L., the water-soluble fraction of the hydro-distillation process used to obtain the essential oil, on 14 Gram-positive and Gram-negative bacteria and a fungus of clinical interest. To consider whether this hydrolate is a more environmentally friendly alternative to traditional antibiotics, its effect on non-target microorganisms in the aquatic and terrestrial environment was analysed using natural soil and river microorganism communities, characterized through 16S rRNA gene sequencing. Results showed that S. montana hydrolate was especially effective (25% v/v concentration) against Pasteurella aerogenes, Streptococcus agalactiae and Acinetobacter baumannii (priority 1, WHO). It was also a microbicide for a further 7 bacterial strains and the fungus Candida albicans (50% v/v concentration). The river and soil communities exposed to the hydrolate showed a decrease in their growth, as well as a decrease in their ability to metabolize polymers and carbohydrates (soil microorganisms) and polymers, carboxylic and ketone acids (river microorganisms). Hydrolates could be an alternative to conventional antibiotics, but their impact on the environment must be taken into account.

Impact of eight widely consumed antibiotics on the growth and physiological profile of natural soil microbial communities.

Antibiotics' (ATBs) occurrence in soil ecosystems has a relevant effect in the structure and functionality of edaphic microbial communities, mainly because of their amendment with manure and biosolids that alter their key ecological functions. In this study, the impact of eight widely consumed ATBs on a natural soil microbial community, characterized through 16 S rRNA gene sequencing, was evaluated. Changes induced by the ATBs in the growth of the soil microbiota and in the community-level physiological profiling (CLPP), using Biolog EcoPlates™, were measured as endpoint. The eight assayed ATBs lead to a significant decrease in the growth of soil microbial communities in a dose-dependent way, ordered by its effect as follows: chloramphenicol > gentamycin > erythromycin > ampicillin > penicillin > amoxicillin > tetracycline > streptomycin. Chloramphenicol, gentamycin, and erythromycin adversely affected the physiological profile of the soil community, especially its ability to metabolize amino acids, carboxylic and ketonic acids and polymers. The analysis of the relationship between the physico-chemical properties of ATBs, as well as their mechanism of action, revealed that, except for the aminoglycosides, each ATB is influenced by a different physico-chemical parameters, even for ATBs of the same family. Significant effects were detected from 100 µg mL to 1, concentrations that can be found in digested sludge, biosolids and even in fertilized soils after repeated application of manure, so cumulative and long-term effects of these antibiotics on soil environment cannot be ruled out.