Antimicrobial resistance remains one of the most urgent global health challenges of our time. Despite warnings from researchers and clinicians about pathogens such as methicillin-resistant Staphylococcus aureus (MRSA), carbapenem-resistant Enterobacteriaceae (CRE), and the broader ESKAPE group, the scale of the threat is still often underestimated outside scientific and medical communities.
At the same time, the pipeline of new drugs addressing resistant infections is diminishing, and the traditional paradigm of “kill the bacterium” is not enough. We cannot win a long-term battle against microbial adaptation if our only tool is a hammer aimed at bacterial viability. Instead we should rethink how we discover, test, and characterize antimicrobial agents, and we must open our minds to mechanisms beyond straightforward bactericidal activity.
In a recent study published in Food Science & Nutrition, Silvia Bittner Fialová’s research group and collaborators from Comenius University in Bratislava explore an alternative perspective.
Rather than evaluating plant-derived compounds solely based on their ability to inhibit growth, the study investigates how oregano extracts influence microbial metabolic activity using both conventional methods and isothermal microcalorimetry.
This more nuanced approach highlights an important shift in antimicrobial research, moving beyond binary outcomes toward a deeper understanding of microbial response. Such perspectives may play a key role in identifying new strategies to combat resistant infections.
For decades, antimicrobial discovery relied on fairly narrow readouts: does a compound prevent visible growth on a plate or in broth? Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) have been standard metrics. While these tests are valuable, they tell us only part of the story—and often a misleading one.
A simple binary readout of “growth or no growth” misses critical questions, such as:
Is the organism truly dead or merely growth-arrested?
Is metabolism suppressed without loss of “culturability”?
Are subpopulations of viable but non-culturable cells persisting?
Does the compound affect virulence factors or metabolic pathways?
These questions matter because pathogens in clinical contexts rarely exist as isolated, rapidly dividing populations in a flask. In human tissue, biofilms, and chronic infections, bacteria often exhibit altered metabolic states and tolerance to stress, meaning conventional screens may overlook promising agents that disrupt key aspects of pathogenic behavior rather than outright killing.
Plants and other natural sources remain a rich reservoir of bioactive molecules, but their exploration has historically been constrained by classical assays. Natural product extracts often contain diverse compounds with multiple activities—some antimicrobial, some regulatory, and others with entirely different mechanisms.
The Origanum vulgare (oregano) study by Kurin et al. exemplifies this complexity. Researchers identified 14 phenolic compounds in water extracts of oregano leaves and rhizomes and used both traditional broth microdilution and isothermal microcalorimetry to evaluate activity against Staphylococcus aureus (including MRSA) and Enterococcus faecalis.
Critically, microcalorimetry revealed how living cells respond to compounds in real time, not just whether they grow or not. This enables differentiation between bacteriostatic, bactericidal, and metabolic effects—information that traditional MIC values alone cannot provide.
Indeed, oregano rhizome extracts showed strong inhibition of staphylococcal metabolic activity at lower concentrations than leaf extracts, suggesting mechanisms beyond simple antimicrobial activity.
Using advanced kinetic approaches like microcalorimetry reflects a broader shift needed in drug discovery:
1. Mechanistic Insight: Instead of only cataloguing whether a compound inhibits growth, kinetic data reveal how it affects bacterial physiology. Is energy production impaired? Are stress responses triggered? Is replication slowed?
2. Resistance Avoidance: Agents that reduce metabolic activity without strong bactericidal pressure may exert less evolutionary pressure toward resistance. This is especially important for chronic and biofilm-associated infections, where tolerance rather than outright resistance is often clinically relevant.
3. Synergistic Potential: Some natural molecules do not kill on their own but can enhance antibiotic efficacy, inhibit efflux pumps, disrupt signaling pathways, or modulate host responses. These modes of action are often overlooked by binary screening approaches.
4. Viability vs Culturability: There is growing recognition that viable but non-culturable (VBNC) cells can persist after treatment and contribute to relapse. Kinetic metabolic profiling helps detect such populations where standard plate assays fail.
Integrating broader assays, such as time-resolved metabolic measurements, omics profiling, and advanced imaging, will be essential to accelerate discovery. Traditional high-throughput screening of natural products, for example screening hundreds of thousands of fractions against standard strains, has shown that many active compounds remain undiscovered simply because our tools are limited.
However, even classical HTS can miss molecules with non-lethal modes of action or effects that only manifest under specific physiological conditions. Screening conditions should better reflect in vivo environments, including biofilms, host immune factors, and nutrient limitations. This approach increases the likelihood of identifying agents that disrupt infection processes rather than simply inhibiting growth in broth.
The urgency of antimicrobial resistance demands a corresponding urgency in how we approach discovery. We should move beyond simplistic kill-or-don’t metrics, broaden our understanding of what constitutes a meaningful antimicrobial effect, and open the door to new mechanisms rooted in metabolism, synergy, environmental modulation, and host–pathogen interactions.
The study by Kurin et al. highlights a promising example of this shift. By combining detailed phytochemical analysis with kinetic metabolic profiling, it demonstrates that natural products still have much to teach us. This includes not only what inhibits microbial growth, but also what modulates microbial activity in ways that can be harnessed therapeutically.
To truly accelerate antimicrobial discovery, we need not only new molecules, but also new ways of thinking, screening, and interpreting microbial responses. Only then can we outpace microbial adaptation and develop treatments that reflect the complexity of real infections.
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Read more about biofilm testing with calScreener biocalorimeter and clinical research using biocalorimetry.