Biology subjects

Ivask, A.

Publications and source records attributed to Ivask, A..

8 recordsLinked to original sources

bioRxiv2026

Incomplete killing by antimicrobial metals generates transient antibiotic tolerance via delayed recovery in Escherichia coli

Incomplete killing is common after application of disinfectants or use of antimicrobial surfaces, leaving behind bacterial survivors with poorly characterized physiology. Here, we investigated the effects of short biocidal exposure to the common antimicrobial metals copper and silver as metal ions in suspension or under semi-dry conditions on solid metallic surfaces used against Escherichia coli. Survivors exhibited delayed regrowth and increased tolerance to bactericidal antibiotics without changes in MIC, consistent with a transient, non-genetic tolerance phenotype. When the magnitude of killing by silver or copper was matched between exposure formats, antibiotic tolerance patterns converged across metals and exposure conditions, indicating that the survivor state is governed more strongly by the severity of the primary lethal stress than by metal identity or exposure format alone. Incremental copper exposure revealed a dose- dependent increase in delayed regrowth and antibiotic tolerance but reduced tolerance to subsequent copper exposure. Post-exposure antibiotic tolerance as well as copper hypersensitivity phenotypes reverted after regrowth. Together, this supports a model of injury-linked delayed recovery rather than generalized stress priming. These findings demonstrate that incomplete disinfection can reshape survivor physiology, transiently increasing antibiotic tolerance through combined effects of selective survival acting on pre-existing phenotypic heterogeneity and acute injury-linked delayed recovery.

microbiology↗

bioRxiv2026

Copper stress upregulates oxidative stress response, histidine production and iron acquisition genes in E. coli

Copper is widely used as a fast-acting antimicrobial, yet the strategies that allow bacteria to survive copper stress remain incompletely understood. Here, we characterize the transcriptional responses of Escherichia coli MG1655 to excess ionic copper using RNA sequencing and a genome-wide GFP-based promoter library. We applied 2 mM copper, which slows growth, and 8 mM copper, a near-lethal concentration. RNA-seq revealed extensive transcriptome remodeling, with 487 genes upregulated at 2 mM and 364 at 8 mM. Both concentrations strongly induced canonical copper-responsive systems, oxidative stress defenses, histidine biosynthesis, and multiple iron acquisition pathways - including enterobactin biosynthesis and transport - despite external iron failing to reduce copper toxicity. At 2 mM copper, additional pathways were activated, including heat-shock and protein-folding functions as well as lipid A, methionine and arginine biosynthesis. Copper exposure also repressed large gene sets: 486 genes at 2 mM, enriched for biofilm formation and pH elevation, and 217 genes at 8 mM, enriched for anaerobic metabolism. In contrast to the robust RNA seq results, we investigated the Horizon Discovery E. coli genome-wide GFP based promoter library as an alternative screening tool. However, in our experiments it showed low signal to noise ratios, limiting its suitability for large scale gene expression screening.

microbiology↗

bioRxiv2026

Context-dependent effects of antimicrobial coatings on microbial load and bacterial community composition on public high-touch surfaces

BackgroundAntimicrobial surfaces incorporated into high-touch public areas are used as a passive intervention to reduce surface microbial load and reduce the spread of infections. However, usually such surfaces lack proof of their antimicrobial activity in real use conditions and against a wider variety of microbes. This study evaluated the real-world performance of copper, TiO2-, silver- and quaternary ammonium compound (SiQAC)-based surfaces, which are commercially available and have proven antibacterial activity in lab tests. The surfaces were introduced to five study sites in diverse environments to collect data on bacterial load, community structure and taxonomic profile over several months. ResultsCopper surfaces introduced to shopping basket handles consistently exhibited the strongest antimicrobial performance, with significant reductions in aerobic bacterial counts and bacterial DNA, accompanied by clear shifts in microbial community composition. These shifts included reduction of several human-associated and opportunistic taxa and relative enrichment of environmentally resilient, stress-tolerant genera. TiO2-based photocatalytic coating reduced bacterial load in kindergarten tables under field conditions but did not significantly alter overall community structure. Silver-based surfaces on university campus tables showed minimal effects on microbial load and composition despite confirmed antibacterial activity in laboratory testing. Analogously to silver, SiQAC-based coating despite being active in lab conditions showed no decrease in bacterial load in real use conditions. When applied onto cafeteria and animal clinic tables SiQAC coating displayed context-dependent effects, with modest, genus-specific changes and increased richness in a high-contact cafeteria environment, but no significant impact in a low-biomass animal clinic setting. Viability based analysis revealed that on most of the surfaces a notable fraction of detected microbial DNA originated from non-viable cells. ConclusionsThis multisite field study demonstrates that the real-world performance of antimicrobially coated surfaces is strongly context dependent and cannot be reliably predicted from laboratory testing alone. Moreover, to understand the effect of antimicrobial coatings on surface microbial communities, real-use monitoring is needed.

microbiology↗

bioRxiv2025

Sub-MIC kill kinetics of copper and silver in Escherichia coli

Copper and silver are well-known and widely used antimicrobial metals that are often considered to employ diverse overlapping biocidal mechanisms of action and induce corresponding bacterial defense responses. Exposure to antimicrobial metals at concentrations below the minimal inhibitory concentration (sub-MIC) are widespread in natural, clinical and built environments and can shape evolutionary trajectories of the affected microbes that could lead to antimicrobial tolerance and/or resistance. By analyzing growth and kill kinetics of Escherichia coli under copper or silver exposure we observed that sub-MIC copper concentrations resulted in a lasting dose-dependent slowing of exponential growth with reduced yield while silver seemed to cause dose-dependent growth delay without substantially affecting exponential growth or yield. Time-kill experiments revealed minimal loss of viability in early copper exposure while in case of silver a rapid dose-dependent transient killing followed by normal exponential regrowth of the survivors was observed, underlying the seemingly dose-dependently extended lag phase durations. Distinguishing conditions that select for antimicrobial resistance (sustained growth) versus tolerance (survival without growth) is essential for antimicrobial stewardship as acquiring tolerance is considered a steppingstone towards developing resistance. Our results suggest that short-term survival of the initial killing by silver is sufficient for selective advantage while maintaining energy-intensive enhanced growth in the presence of copper is needed to gain competitive benefit over the general population. The findings highlight new and known challenges in antimicrobial characterization and risk assessment of metal-based formulations by using wide-spread non-kinetic endpoint assays such as MIC.

microbiology↗

bioRxiv2024

Experimental evolution of Escherichia coli on solid silver, copper, stainless steel, and glass surfaces

To study bacterial adaptation to antimicrobial metal surfaces in application-relevant conditions, Escherichia coli was exposed to copper and silver surfaces for thirty exposure cycles in low-organic dry or high-organic humid conditions. The evolved populations demonstrated increased metal surface tolerance without concurrent increase in MBC and MIC values of respective metal ions or selected antibiotics. Mutation analysis did not detect increased mutation accumulation nor mutations in cop, cus, cue, sil, pco or general efflux genes known to actively maintain copper/silver homeostasis. Instead, during cyclic exposure mutations in genes related to cellular barrier functions and sulfur metabolism were enriched potentially suggesting that reducing bioavailability and passively restricting uptake of the toxic metals rather than active efflux is selected for on copper and silver surfaces. The changes detected in the evolved populations did not indicate an increased risk of antibiotic cross-resistance as a result of copper or silver surface exposure. However, rapid emergence of mutations in silS activated the cryptic sil efflux locus during silver ion challenge in liquid MBC assay with the evolved populations. The silS mutants showed no benefit on copper and silver surfaces but demonstrated decreased sensitivity to ampicillin and ciprofloxacin as well as copper and silver ions in liquid tests indicating that efflux might be specific to granting heavy metal tolerance in liquid but not surface exposure format. Our findings highlight the critical importance of appropriate exposure conditions not only in efficacy assessment but also risk assessment of antimicrobial surface applications. ImportanceThis study examines the evolutionary adaptations of Escherichia coli after semi-dry exposure to copper and silver surfaces, leading to an increase in surface tolerance but no increase in mutation accumulation or substantially enhanced metal ion tolerance in standard tests. Notably, enriched mutations indicate a shift toward more energy-passive mechanisms of metal tolerance. Additionally, while enhanced silver efflux was rapidly selected for in a single round of silver exposure in liquid tests and substantially increased copper and silver ion tolerance in conventional test formats, the causal mutations did not improve viability on silver and copper surfaces, underscoring the different fitness scenarios of tolerance mechanisms dependent on exposure conditions. These findings emphasize the need for appropriate exposure conditions in evaluating of both efficacy and the potential risks of using antimicrobial surfaces, as the results from conventional liquid-based tests may not apply in solid contexts.

microbiology↗

bioRxiv2023

Antibacterial activity of solid surfaces is critically dependent on relative humidity, inoculum volume and organic soiling

Antimicrobial surface materials potentially prevent pathogen transfer from contaminated surfaces. Efficacy of such surfaces is assessed by standard methods using wet exposure conditions known to overestimate antimicrobial activity compared to dry exposure. Some dry test formats have been proposed but semi-dry exposure scenarios e.g., oral spray or water droplets exposed to ambient environment, are less studied. We aimed to determine the impact of environmental test conditions on antibacterial activity against the model species Escherichia coli and Staphylococcus aureus. Surfaces based on copper, silver, and quaternary ammonium with known or claimed antimicrobial properties were tested in conditions mimicking microdroplet spray or larger water droplets exposed to variable relative air humidity in the presence or absence of organic soiling. All the environmental parameters critically affected antibacterial activity of the tested surfaces from no effect in high-organic dry conditions to higher effect in low-organic humid conditions but not reaching the effect size demonstrated in the ISO 22169 wet format. Copper was the most efficient antibacterial surface followed by silver and quaternary ammonium based coating. Antimicrobial testing of surfaces using small droplet contamination in application-relevant conditions could therefore be considered as one of the worst-case exposure scenarios relevant to dry use surfaces. Featured image + One Sentence summaryAntibacterial activity of copper and silver surfaces is highly dependent on environmental testing conditions with maximum efficiency in low-organic wet conditions to no antibacterial activity in high-organic dry conditions indicating the need to test antimicrobial surface materials in application-relevant test formats as opposed to current standards. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=127 SRC="FIGDIR/small/534510v3_ufig1.gif" ALT="Figure 1"> View larger version (36K): org.highwire.dtl.DTLVardef@17eb30eorg.highwire.dtl.DTLVardef@11d9a72org.highwire.dtl.DTLVardef@86ea55org.highwire.dtl.DTLVardef@1828c5d_HPS_FORMAT_FIGEXP M_FIG C_FIG

microbiology↗

bioRxiv2020

Selective antibiofilm properties and biocompatibility of nano-ZnO and nano-ZnO/Ag coated surfaces

Spread of pathogenic microbes and antibiotic-resistant bacteria in health-care settings and public spaces is a serious public health challenge. Materials that prevent solid surface colonization or impede touch-transfer of viable microbes could provide means to decrease pathogen transfer from high-touch surfaces in critical applications. ZnO and Ag nanoparticles have shown great potential in antimicrobial applications. Less is known about nano-enabled surfaces. Here we demonstrate that surfaces coated with nano-ZnO or nano-ZnO/Ag composites are not cytotoxic to human keratinocytes and possess species-selective medium-dependent antibiofilm activity against Escherichia coli, Staphylococcus aureus and Candida albicans. Colonization of nano-ZnO and nano-ZnO/Ag surfaces by E. coli and S. aureus was decreased in static oligotrophic conditions (no planktonic growth). Moderate to no effect was observed for bacterial biofilms in growth medium (supporting exponential growth). Inversely, nano-ZnO surfaces enhanced biofilm formation by C. albicans in oligotrophic conditions. However, enhanced C. albicans biofilm formation on nano-ZnO surfaces was effectively counteracted by the addition of Ag. Possible selective enhancement of biofilm formation by the yeast C. albicans on Zn-enabled surfaces should be taken into account in antimicrobial surface development. Our results also indicated the importance of the use of application-appropriate test conditions and exposure medium in antimicrobial surface testing.

microbiology↗

bioRxiv2018

Propidium iodide staining underestimates viability of adherent bacterial cells

Combining membrane impermeable DNA-binding stain propidium iodide (PI) with membrane-permeable DNA-binding counterstains is a widely used approach for bacterial viability staining. In this paper we show that PI staining of adherent cells in biofilms may significantly underestimate bacterial viability due to the presence of extracellular nucleic acids. We demonstrate that gram-positive Staphylococcus epidermidis and gram-negative Escherichia coli 24-hour initial biofilms on glass consist of 76 and 96% PI-positive red cells in situ, respectively, even though 68% the cells of either species in these aggregates are metabolically active. Furthermore, 82% of E. coli and 89% S. epidermidis are cultivable after harvesting. Confocal laser scanning microscopy (CLSM) revealed that this false dead layer of red cells is due to a subpopulation of double-stained cells that have green interiors under red coating layer which hints at extracellular DNA (eDNA) being stained outside intact membranes. Therefore, viability staining results of adherent cells should always be validated by an alternative method for estimating viability, preferably by cultivation.

microbiology↗