The study, published in the journal MicrobiologyOpen, focused on Staphylococcus aureus, Pseudomonas aeruginosa, and Enterococcus faecalis, three species that dominate the microbiology of difficult wounds and are notorious for their capacity to develop resistance to conventional antibiotics. The researchers repeatedly grew these bacteria in liquid culture containing concentrations of spruce resin below the level needed to stop growth entirely, a condition that in classical antibiotic therapy is precisely the situation that breeds resistance. Yet when the team measured the minimum inhibitory concentration, or MIC, of the resin at the start, midpoint, and end of the experiment, none of the three organisms came close to the predefined threshold that would signal genuine resistance: a fourfold or greater increase in inhibitory concentration.

The experimental design followed established guidelines from the Clinical and Laboratory Standards Institute for dilution antimicrobial susceptibility testing, adapted for resistance evaluation through serial passage. The resin itself was collected by hand from sustainably managed Finnish forests and purified according to a patented method before being dissolved in ethanol and diluted into Mueller-Hinton broth at concentrations ranging from 10 percent down to just over 0.15 percent. Cultures were transferred daily into fresh medium containing the same sub-inhibitory resin concentration, giving the bacterial populations a sustained opportunity to adapt over ten and then twenty consecutive passages. Every assay was performed in duplicate, and the duplicate trials produced identical MIC endpoints at every time point, lending unusual consistency to the results.

At baseline, the resin inhibited Pseudomonas aeruginosa at a concentration of 5 percent, Staphylococcus aureus at 1.25 percent, and Enterococcus faecalis at 2.5 percent, making the staphylococcus the most susceptible of the three and the pseudomonad the most tolerant, a pattern consistent with the well-known intrinsic hardiness of Gram-negative bacteria. Over the following twenty days, susceptibility remained broadly stable. The inhibitory concentration for Pseudomonas rose from 5 percent to 10 percent by Day 20, a twofold shift, while Staphylococcus aureus doubled from 1.25 percent to 2.5 percent by Day 10 and then held steady. Enterococcus faecalis did not budge at all, remaining at 2.5 percent throughout the entire experiment.

Crucially, the researchers built safeguards into the study to distinguish genuine adaptation from mere laboratory artifact. Because ethanol was used as the solvent, matched vehicle-control cultures were passaged in ethanol without any resin, and validation arms were passaged in plain nutrient broth before being tested against resin or ethanol. These controls remained stable over time and showed no progressive drift, which means the transfer procedure and medium changes did not artificially induce resistance. Tellingly, the Day 20 increase seen in Pseudomonas aeruginosa under resin exposure was not mirrored in the validation arms, supporting the interpretation that the modest shift was minor and not clearly indicative of progressive resistance selection. The vehicle controls also demonstrated that ethanol alone could not explain the inhibitory activity observed at lower resin concentrations, since the ethanol-only endpoints for Staphylococcus and Enterococcus sat at 10 percent, far above the resin-containing test values.

The contrast with conventional antibiotics is striking. In previously published serial-passage experiments, methicillin-resistant Staphylococcus aureus exposed repeatedly to the topical antibiotic mupirocin developed a 64-fold increase in MIC, fusidic acid produced a 256-fold increase, and retapamulin a 16-fold increase. Ciprofloxacin has generated 32-fold MIC increases in both Staphylococcus aureus and Pseudomonas aeruginosa after just 25 passages, and resistance selection has been demonstrated in Enterococcus faecalis with linezolid. Against that backdrop, the twofold fluctuations observed with spruce resin fall well below the fourfold resistance threshold and, according to the authors, do not necessarily indicate stable resistance, especially when they are not supported by consistent drift in the control arms.

Why might a natural resin be so resistant to resistance? The answer likely lies in its chemistry. Spruce resin is not a single active molecule but a complex natural mixture in which resin acids predominate, accompanied by lignans and p-coumaric acid. Earlier mechanistic work has shown that when Staphylococcus aureus is exposed to resin, resin salve, or the resin acid abietic acid, the cells develop thickened walls, aggregate together, alter their fatty-acid composition, and lose their membrane potential, pointing to damage of the bacterial cell envelope and interference with energy metabolism rather than the blockade of any single defined target. Terpenic resin acids are thought to act through nonspecific membrane and cell-wall effects, including disruption of the proton gradient and possible uncoupling of oxidative phosphorylation. A multi-target mode of attack of this kind is classically considered far less amenable to the single-mutation escape routes that bacteria exploit against conventional antibiotics.

The new findings build on a substantial body of earlier evidence. The first modern in vitro work, published in 2007, showed that home-made spruce resin salve inhibited clinically relevant wound bacteria, with the clearest effects against Gram-positive organisms and demonstrable activity against methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. Later European Pharmacopoeia challenge-test studies extended the picture to Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Bacillus subtilis, and Candida albicans, finding that 10 percent resin in the salve medium was sufficient to prevent microbial survival. More recently, a spruce resin-based salve was shown to eradicate mixed Pseudomonas-Staphylococcus biofilms and penetrate their layers, even when the isolates were highly tolerant to bacitracin, neomycin, and polymyxin B. Clinical studies, including a randomized multicentre trial on severe pressure ulcers, have added wound-healing and safety data to the antimicrobial story.

The authors are careful to spell out the limits of what their experiment can show. A twenty-day design cannot exclude slower adaptive responses that might emerge during longer exposure, and phenotypic MIC measurements cannot rule out genetic adaptation that occurs before substantial MIC shifts become detectable. The study used well-characterized reference strains rather than clinical isolates, which may harbor pre-existing resistance determinants that could alter their response to repeated resin exposure. Biofilm growth, which is the norm in chronic wounds, may also change resistance-selection dynamics compared with the planktonic cultures used here. The researchers therefore call for extended experiments reaching fifty or more passages, continuous-flow and biofilm-based models, whole-genome sequencing of baseline and endpoint isolates, fitness and cross-resistance assessments, and testing of multidrug-resistant clinical strains including MRSA, vancomycin-resistant enterococci, and resistant Pseudomonas aeruginosa.

Even with those caveats, the practical implications for wound care are meaningful. Chronic wounds expose microbes to variable and sometimes sub-inhibitory levels of topical antimicrobials, because wound exudate can reduce bactericidal efficacy and biofilms increase tolerance, exactly the conditions under which resistance to classical antibiotics flourishes. The finding that medical-grade spruce resin, used in products such as Abilar, ilon Wundxtra Salbe, and SutriHeal Forte, maintained its inhibitory activity across twenty days of repeated sub-inhibitory exposure strengthens the rationale for its continued topical use. It suggests that any adaptive response to this ancient forest remedy, if present at all, was limited and species-dependent under the conditions tested. Longer and more clinically faithful studies will be needed to confirm the picture, but for now the spruce resin story, running from folk medicine to modern microbiology, has gained one of its most important scientific supports yet.

Subject of Research: Resistance development potential of Norway spruce resin against wound-infecting bacteria assessed by serial passage

Article Title: Serial‐Passage Assessment Shows No Confirmed Resistance Development to Norway Spruce (Picea Abies) Resin in Bacterial Species Relevant to Wound Infection

Article References: Yamileva, K., Parrotta, S., Ghanbarirad, M., & Multia, E. (2026). Serial‐Passage Assessment Shows No Confirmed Resistance Development to Norway Spruce ( Picea Abies ) Resin in Bacterial Species Relevant to Wound Infection. MicrobiologyOpen, 15(5), Article e70436. https://doi.org/10.1002/mbo3.70436

Image Credits: AI Generated

DOI: 10.1002/mbo3.70436

Keywords: Norway spruce resin, antimicrobial resistance, serial passage, wound infection, Staphylococcus aureus, Pseudomonas aeruginosa, Enterococcus faecalis, minimum inhibitory concentration, natural antimicrobials, biofilm, topical treatment, Picea abies

The research, published in the journal MicrobiologyOpen, focused on five strains with established probiotic credentials: Lactiplantibacillus plantarum NBRC 3070, Lactobacillus acidophilus ATCC 4356, Lacticaseibacillus casei ATCC 393, Lacticaseibacillus rhamnosus GG ATCC 53103, and Bifidobacterium animalis subsp. lactis ATCC 27673. These are not obscure laboratory curiosities. They are workhorse organisms used in food fermentation and probiotic formulations worldwide, which means that any antimicrobial compounds they produce could, in principle, be translated into industrial biopreservation strategies with relative speed. The team pitted both the live bacteria and their cell-free metabolites against a panel of seven indicator pathogens, including Escherichia coli, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Shigella sonnei, Pseudomonas aeruginosa, Serratia marcescens, and Bacillus cereus, organisms routinely implicated in contamination of meat, dairy, water, and ready-to-eat foods.

The results were striking. When whole bacterial cultures of the lactic acid bacteria were tested using agar well diffusion, inhibition zones ranged from roughly 15 to 23 millimeters, placing most of the target pathogens in the very sensitive to extremely sensitive categories. Lactiplantibacillus plantarum NBRC 3070 emerged as a standout performer, producing the largest average inhibition zones, while E. coli ATCC 25922 proved the most vulnerable target overall, with a mean inhibition zone exceeding 21 millimeters across all five producer strains. Shigella sonnei, by contrast, showed the greatest resilience, a reminder that even nature’s antimicrobials face pathogen-specific limits.

Crucially, the antimicrobial activity survived the removal of the living cells. Cell-free supernatants, the liquid fraction left after the bacteria are spun away, still inhibited pathogen growth by 84 to 89 percent in broth microdilution assays against E. coli, Salmonella, and S. aureus, particularly when derived from L. plantarum and L. casei. This matters because it demonstrates that the inhibitory power resides in secreted metabolites, a class of compounds often called postbiotics, rather than in any competitive behavior of the living microbes. For food manufacturers, that opens the door to using purified or concentrated extracts as shelf-life extenders without introducing live cultures into products where they might alter taste or texture.

The researchers then dissected what exactly was doing the killing. Lactic acid bacteria deploy a layered chemical arsenal: organic acids such as lactic and acetic acid, hydrogen peroxide, and proteinaceous toxins known as bacteriocins and bacteriocin-like inhibitory substances, or BLIS. By neutralizing the supernatants to remove the effects of acids and peroxide, then lyophilizing them into a stable powder, the team isolated the contribution of the BLIS fraction. Four of the five strains retained measurable activity after this treatment, with inhibition zones between 10 and 15.8 millimeters, confirming genuine BLIS production. Notably, Bifidobacterium animalis subsp. lactis lost its activity entirely once acids were neutralized, suggesting its antimicrobial punch comes primarily from pH effects rather than stable protein toxins.

Enzyme digestion experiments sealed the case for the proteinaceous nature of the compounds. Treating the lyophilized supernatants with trypsin, pepsin, and papain significantly reduced antimicrobial activity, with reductions ranging from about 1.3 to 11 percent. Pepsin inflicted the heaviest damage, hinting that the active peptides contain cleavage sites vulnerable to that particular enzyme. This enzyme sensitivity is a hallmark of bacteriocin-like substances and distinguishes them from non-protein antimicrobials. It also carries a safety implication: proteinaceous compounds are typically digested in the human gut, reducing the risk that they would disrupt beneficial microbiota or drive resistance the way broad-spectrum synthetic antibiotics can.

Quantification revealed just how potent these substances can be. Using arbitrary units and minimum inhibitory concentrations, the team found activity ranging from 133 to 1280 AU/mL across the target pathogens. L. rhamnosus GG delivered the single strongest result, achieving 1280 AU/mL against Salmonella Typhimurium with a remarkably low MIC of 1.54 mg/mL. Salmonella proved broadly susceptible across all five strains, an encouraging sign given the persistent burden of salmonellosis worldwide. Against E. coli, S. aureus, and P. aeruginosa, MIC values were higher, between 6.26 and 12.5 mg/mL for most strains, indicating moderate but still meaningful activity. The study also established a direct correlation between arbitrary units and MIC values, a methodological bridge that is rarely quantified and should make future comparisons between laboratories far more rigorous.

Beyond their chemical weapons, the strains passed several key probiotic fitness tests. All five displayed strong cell surface hydrophobicity above 50 percent, with three strains reaching approximately 65 percent, a trait associated with the ability to adhere to intestinal surfaces. Autoaggregation climbed steadily over time, with L. rhamnosus GG, L. casei, and B. animalis subsp. lactis all exceeding 85 percent after 24 hours. The bacteria also coaggregated with pathogens, clumping with S. aureus and E. coli at rates up to roughly 38 percent, a mechanism that can physically block pathogens from colonizing surfaces. Statistical analysis confirmed significant positive correlations between hydrophobicity, autoaggregation, and coaggregation, suggesting these adhesion traits work in concert. On the safety side, most strains were sensitive to clinically important antibiotics such as azithromycin, tetracycline, and penicillin, though all showed intrinsic resistance to aminoglycosides and norfloxacin, a pattern considered typical and non-transferable in lactic acid bacteria. One caveat stands out: L. plantarum NBRC 3070 was resistant to every antibiotic tested, a finding the authors flag as warranting deeper genetic investigation before any food application.

Perhaps the most practically valuable insight concerns timing and temperature. The team tracked antimicrobial production over 48 hours of anaerobic growth at both 30 and 37 degrees Celsius. Activity appeared as early as four hours into growth, but peak production, exceeding 90 percent inhibition of the indicator organism, arrived during the stationary phase: at 36 hours when cultures were held at 30 degrees, and at 24 hours at 37 degrees. Warmer temperatures accelerated growth and acidification but did not proportionally boost antimicrobial yield, and temperature alone had no significant effect on production levels. Instead, the interplay of time, temperature, and strain identity governed the output, with falling pH and accumulating biomass acting as triggers for biosynthesis. For industrial fermenters, this defines a precise harvest window, after which activity gradually declines, likely due to proteolytic degradation of the active compounds.

The implications extend well beyond the laboratory bench. With the World Health Organization identifying antimicrobial resistance as one of the top global public health threats, and with chemical preservatives facing growing consumer skepticism, bacteriocin-based biopreservation occupies a rare sweet spot: effective, natural, and generally regarded as safe. Nisin, a bacteriocin from a related lactic acid bacterium, has already been licensed as a food additive for decades, proving the commercial pathway exists. What this study adds is a rigorous, comparative evaluation of five industry-relevant strains, complete with optimized production conditions, quantified potency, and enzyme-confirmed mechanisms. The authors are candid about the limitations: the work was conducted entirely in vitro, the molecular structures of the BLIS remain uncharacterized, and the resistance profile of L. plantarum needs clarification. Still, the convergence of strong antimicrobial activity, favorable surface properties, and acceptable antibiotic susceptibility profiles positions these five strains, and especially L. plantarum and L. rhamnosus GG, as leading candidates for the next generation of natural food preservatives. In the escalating contest between superbugs and the food supply, the humble yogurt bacterium may prove to be an unlikely but formidable ally.

Subject of Research: Probiotic lactic acid bacteria and their antimicrobial substances against foodborne pathogens

Article Title: Evaluation of Probiotic Properties and Antimicrobial Substances Produced by Five Lactic Acid Bacteria Against Foodborne and Spoilage Pathogens

Article References: Rahman, M. M., Sazili, A. Q., Ahmad, S. A., Khalil, K. A., Ismail‐Fitry, M. R., & Sarker, M. S. K. (2026). Evaluation of Probiotic Properties and Antimicrobial Substances Produced by Five Lactic Acid Bacteria Against Foodborne and Spoilage Pathogens. MicrobiologyOpen, 15(5), Article e70052. https://doi.org/10.1002/mbo3.70052

Image Credits: AI Generated

DOI: 10.1002/mbo3.70052

Keywords: lactic acid bacteria, probiotics, bacteriocin-like inhibitory substances, foodborne pathogens, antimicrobial resistance, biopreservation, Lactiplantibacillus plantarum, Lacticaseibacillus rhamnosus GG, postbiotics, food safety, minimum inhibitory concentration, cell surface hydrophobicity

Fulvic acid belongs to a family of organic materials known as humic substances, the dark, chemically complex residues left behind when microorganisms decompose plant and animal matter. Humic substances are conventionally divided into three fractions according to their solubility: humic acid, which dissolves in alkaline solutions but precipitates when the pH drops below 2; humin, which is insoluble under all conditions; and fulvic acid, the fraction that remains soluble across the entire pH range. This solubility gives fulvic acid unusual versatility, and its molecular architecture is studded with reactive functional groups, including carboxyl, phenolic, quinone, carbonyl, hydroxyl, and amine moieties, many of which have documented biological activity. The exact structure of fulvic acid remains incompletely understood, but capillary electrophoresis and mass spectrometry studies suggest it should be viewed not as a single high-molecular-weight compound but as an assembly of smaller molecules that associate in solution.

The research team collected cow dung from cattle farms in village Chanesar Chachar, Taluka Pano Aqil, District Sukkur, dried it for five to six days at room temperature, and ground it into a fine powder. Extraction followed the protocol of the International Humic Substances Society: the powdered manure was mixed with 80 milliliters of 5 percent sodium hydroxide solution, shaken for four hours, and left to stand for twelve hours. After dilution with deionized water and centrifugation at 3000 revolutions per minute, hydrochloric acid was added to bring the pH down to between 1 and 2, precipitating the humic acid fraction and leaving fulvic acid in the supernatant, which was then collected and dried for analysis. The method is deliberately simple, relying on cheap reagents and standard laboratory equipment, which is precisely what makes the approach attractive for resource-limited settings.

One of the study’s most technically interesting observations concerns extraction efficiency. When 25 grams of dung powder were processed per liter of 0.5 N sodium hydroxide, the yield was 2.5 grams, or 10 percent of the starting mass. Doubling the raw material to 50 grams produced 4 grams, an 8 percent yield, and 100 grams yielded just 6 grams, or 6 percent. The percentage yield therefore falls as the solid loading rises, an inverse relationship the authors attribute to reduced solvent accessibility and less efficient mass transfer at higher concentrations, possibly compounded by saturation effects in the alkaline solubilization process. For anyone hoping to scale up production, the lesson is counterintuitive but clear: less material per batch extracts more efficiently, a trade-off between throughput and yield that will need optimization before any industrial process could be contemplated.

Characterization of the extracted material relied on three complementary analytical techniques. Ultraviolet-visible spectroscopy showed a maximum absorption at 316 nanometers, closely matching a commercial fulvic acid standard from Aldrich, indicating similar chromophoric structures arising from pi-to-pi-star transitions in aromatic and conjugated systems. Fourier transform infrared spectroscopy then mapped the functional group chemistry: a broad band between 3400 and 2900 reciprocal centimeters corresponding to O-H and N-H stretching, a sharp peak near 2929 from aliphatic C-H stretching of methyl and methylene groups, bands between 1598 and 1507 assigned to aromatic carbon-carbon stretching and quinone and conjugated carbonyl contributions, a peak at 1420 reflecting carboxylate asymmetric stretching and phenolic C-O stretching, and a strong absorption at 1034 from C-O stretching of alcohols and polysaccharide structures. The spectrum of the extracted material closely paralleled that of the standard, confirming successful isolation of humic-type functional groups.

X-ray diffraction completed the structural picture. The diffraction pattern displayed a broad background hump between 15 and 35 degrees two-theta, the signature of a largely amorphous material, superimposed with several sharp crystalline peaks. The most intense peak appeared at approximately 44.71 degrees with a d-spacing of 2.0276 angstroms, and a second prominent peak at 31.74 degrees with a d-spacing of 2.8195 angstroms. The combination of diffuse scattering and discrete reflections indicates a semi-crystalline structure in which small crystalline domains are embedded in a dominant amorphous matrix, with minimal impurities, consistent with the successful isolation of a heterogeneous natural organic mixture rather than a single crystalline compound.

The antimicrobial testing followed classical microbiological methodology. Reference strains of E. coli (ATCC 25922), P. aeruginosa (ATCC 27853), and Salmonella Typhi (ATCC 14028) were cultured on Mueller-Hinton agar, and inocula were standardized to the 0.5 McFarland turbidity standard. Fulvic acid was dissolved in dimethyl sulfoxide, with the solvent held at or below 1 percent by volume and a solvent-only control included to rule out false positives. In the agar well diffusion assay, the dose-response relationship was unambiguous. At 0.1 grams per 5 milliliters, no inhibition zones appeared against any of the three organisms. At 0.2 grams per 5 milliliters, zones of 11, 10, and 9 millimeters emerged for E. coli, P. aeruginosa, and S. Typhi respectively. Raising the concentration to 0.4 grams per 5 milliliters expanded the zones to 12, 11, and 11 millimeters, and at 0.8 grams per 5 milliliters the zones reached 14, 15, and 12 millimeters. The maximum concentration tested, 1.6 grams per 5 milliliters, produced the largest zones: 16 millimeters against E. coli, 15 against P. aeruginosa, and 14 against S. Typhi.

Quantitative susceptibility testing sharpened the hierarchy. Using a two-fold dilution series from 1000 down to 31.25 micrograms per milliliter, the team determined minimum inhibitory concentrations ranging from 250 to 1000 micrograms per milliliter and minimum bactericidal concentrations from 500 to 2000 micrograms per milliliter. E. coli proved the most susceptible, inhibited at 250 micrograms per milliliter, followed by P. aeruginosa at 500, while Salmonella Typhi required a full 1000 micrograms per milliliter. The authors connect this graded susceptibility to differences in cell envelope architecture: P. aeruginosa deploys a famously impermeable outer membrane and active efflux pumps, while E. coli and S. Typhi present different membrane compositions and permeability characteristics. Mechanistically, the antimicrobial action is attributed to fulvic acid’s phenolic, carboxylic, and quinone functional groups, which may disrupt microbial membranes, chelate essential metal ions, and generate redox-mediated oxidative stress, mechanisms well documented for structurally related phytophenolic compounds.

The authors are careful to frame these results as preliminary, and the caveats deserve emphasis. The activity was demonstrated only against planktonic, free-floating bacteria in vitro, using agar diffusion and broth dilution assays. No mechanistic studies of bacterial interaction pathways, no cytotoxicity profiling on mammalian cell lines, no stability or bioavailability assessments, and no tests against clinically resistant strains or biofilm models were performed. The dataset, as the researchers themselves state, is insufficient to support direct application in biomedical, agricultural, or environmental systems, and any translational implications must be considered tentative. What the study does establish is a proof of concept: cow dung is a viable, low-cost raw material for fulvic acid extraction, the isolated product carries the structural fingerprints of authentic fulvic acid, and it exerts measurable, concentration-dependent antibacterial activity against three major Gram-negative pathogens.

Even within those limits, the work lands at a compelling moment. Humic substances are already used in agriculture as soil conditioners and fertilizers, added to animal feed to improve growth and immunity, and investigated as anti-inflammatory, antitumor, and wound-healing agents. If subsequent research confirms that fulvic acid’s antimicrobial effects survive purification, formulation, and contact with living systems, a substance that farmers currently discard, or that poses a disposal and sanitation burden in much of the developing world, could be converted into a feedstock for natural antimicrobials. The road from a petri dish in Khairpur to a clinic or a feed additive is long, and the required in vivo studies, toxicity evaluations, and resistance testing will take years. But the underlying idea, that the answer to drug-resistant bacteria might be composting at the edge of the village, is exactly the kind of cross-disciplinary leap that the antimicrobial resistance crisis demands, and this study offers the first careful, spectroscopically verified evidence that the leap is worth taking.

Subject of Research: Antimicrobial activity of fulvic acid extracted from cow dung against Gram-negative bacteria

Article Title: Antimicrobial activity of fulvic acid extracted from bovine dung against selected gram-negative pathogens: a preliminary in vitro assessment

Article References: Chachar, Z. A., Dharejo, S. A., & Pirzada, T. (2026). Antimicrobial activity of fulvic acid extracted from bovine dung against selected gram-negative pathogens: a preliminary in vitro assessment. Discover Chemistry, 3(1), Article 548. https://doi.org/10.1007/s44371-026-00935-8

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00935-8

Keywords: fulvic acid, cow dung, antimicrobial resistance, Gram-negative bacteria, Escherichia coli, Pseudomonas aeruginosa, Salmonella Typhi, humic substances, FTIR spectroscopy, X-ray diffraction, minimum inhibitory concentration, natural antimicrobials