## 4 Discussion
(from [[https://www.barthhaas.com/](https://www.barthhaas.com/)]([https://www.barthhaas.com/](https://www.barthhaas.com/)) (accessed on 30 May 2025). This variability in methodologies adds complexity and discrepancy in research and industrial tests and underscores the need for careful selection of extraction methods, solvents, and compound standardization--a critical issue raised in other meta-analyses on botanical antimicrobials [69]. An unexpected finding from our study was the variability in antimicrobial activity between the incubation periods of 24 and 48 h. Contrary to general expectations, many MIC values at 48 h were higher than at 24 h. This inconsistency primarily reflects the substantial variation in bacterial species and strains tested across these time points. Similar observations have been reported previously, suggesting differential microbial adaptive responses over extended exposure times [70]. This further emphasizes the urgent need for the scientific community to adopt a standardized panel of microbial strains and time points for evaluating MIC, thus enhancing comparability and reproducibility of antimicrobial studies. One of the critical contributions of this work is the stratified meta-analysis that distinguishes between effects on food spoilage microorganisms, and probiotics. This functional stratification is of growing importance, especially in the context of microbiome-conscious antimicrobial development. For instance, xanthohumol showed promising selective action, exerting stronger effects on non-probiotic and food spoilage bacteria in several contexts, but also inhibiting probiotics at certain time points, underscoring the complexity of predicting microbial selectivity. Such diversifying behavior reinforces the need for integrated evaluations combining in vitro data with microbiome-sensitive in vivo testing, as proposed in diagnostics-focused meta-evaluations [68; 70]. Our stratified analyses paint a straightforward picture of where hop compounds work best. They were consistently more effective against Gram-positive bacteria than Gram-negative ones, a pattern that fits with cell-wall architecture: the open, thick peptidoglycan of Gram-positives lets hydrophobic hop acids slip through, whereas Gram-negatives possess an outer lipopolysaccharide membrane that blocks many large, oily molecules [71]. We also saw lower MIC values in anaerobic and facultative anaerobic bacteria compared with strict aerobes. Because these low-oxygen microbes rely on redox-sensitive enzymes and a fragile proton-motive force to make energy, hop preynylated flavonoids and bitter acids--known to disrupt both--exert a markedly stronger inhibitory effect on them [65]. Taken together, the data show that hop compounds favor organisms with permeable walls and oxygen-sensitive metabolism, offering a predictable spectrum of activity. From an application standpoint, the potency patterns uncovered in our meta-analysis align with several formulation routes that are already moving from bench to pilot scale: (a) food protection [72; 73], (b) cosmetic antimicrobials [15], (c) therapeutic adjuvants [38; 74]. Together, these proof-of-concept studies outline a concrete industrial and clinical roadmap: hop \(\beta\)-acids and prenyl-chalcones can be selected from a larger pool of candidates for (i) controlled-release films and coatings for \"clean-label\" foods, (ii) cosmetic formulations targeting acne and body odor, and (iii) oral or topical antibiotic-potentiating adjuncts. Embedding such compound-specific formulation scenarios into future trials will accelerate the translation of our evidence-based efficacy estimates into real-world antimicrobial products. However, our study is not without limitations. Primarily, the absence of complete methodological details in several original studies introduced uncertainties in data interpretation.