Chapter Four · failure evidence
What Microbial Colony Screening & Selection got wrong, from 38 dissertations
Microbial colony screening and selection workflows encounter frequent failures stemming from assay matrix interference, selective circuit escape, and poor correlation between agar-based phenotypes and liquid performance. Researchers repeatedly face false-positive hits from non-specific binding artifacts, insufficient phenotypic discrimination on solid media, and overgrowth by contaminating organisms. These records come from PhD theses at 16 institutions, 2021 to 2026. Each links to its thesis. They were extracted by language models reading the full text, so treat each as a lead to read, not a verdict.
Microbial evolution systems frequently suffer from selection circuit evasion and off-target adaptation
Microbial populations and phage systems under continuous or growth selection circumvent intended evolutionary pressures by acquiring circuit-escaping mutations or non-coding modifications. These selective regimes frequently favor broken regulatory elements, deletion products, or alternative survival adaptations over the intended catalytic improvements.
Tried and failed
directed evolution in bacterial selection system applied to genome editing enzymes for mammalian cells. Outcome: did not generalise. Reason: evolved variants optimized propagation by escaping bacterial circuit inhibition rather than improving target mammalian editing activity
Tried and failed
metabolic detoxifier coexpression in growth selection applied to enzyme directed evolution library screening. Outcome: no signal. Reason: alleviating intermediate toxicity sped growth but failed to alter evolutionary selection pressure away from low-activity variants
Engineering of human enzymes for amino acid depletion therapy in cancer · UT Austin
Tried and failed
continuous directed evolution using bacteriophage selection circuits applied to multicomponent precision genome editing agents. Outcome: did not generalise. Reason: evolved mutations adapted to phage circuit dynamics by reducing repression rather than improving catalytic activity in target hosts
Enhancing precision genome editing agents through directed evolution and protein engineering · Harvard
Tried and failed
targeted genetic knockouts to prevent biofilm formation applied to continuous bacterial culture systems. Reason: bacteria rapidly evolved alternative adaptive mechanisms to re-establish biofilm phenotypes under continuous selection pressure
Understanding the Limits on Exponential Growth of Bacteria Through Long Term Evolution · Harvard
Tried and failed
binding-only selection in continuous directed evolution applied to CRISPR Cas ortholog PAM recognition. Outcome: did not generalise. Reason: enriching for target binding failed to couple with or retain downstream functional editing activity
Engineering and evolution of precision genome editing agents · Harvard
Tried and failed
serial passaging under antibiotic selection pressure applied to heterologous plasmid reporter expression stability. Outcome: unstable. Reason: loss of reporter expression occurred through mechanisms outside coding sequence mutations depending on selection marker
Antibiotic Markers: An Overlooked Design Choice in Synthetic Biology? · UT Austin
Tried and failed
pre-screening library variants in an intermediate host applied to synthetic promoter library construction. Reason: selection pressure caused severe diversity bottleneck, yielding broken promoters and sequence deletions
Tried and failed
non-PCR synthetic gene library assembly applied to protein display library generation. Reason: synthesis errors generated frameshifted deletion products that bypassed initial selection screening
Engineering Nanobody Intracellular Behavior Through Framework Mutagenesis · Harvard
Colony and clone screening assays are hindered by poor signal discrimination and throughput limits
Assays evaluating microbial clones or metagenomic libraries often fail to generate detectable fluorescence or sufficient phenotypic contrast across test conditions. In addition, low-throughput sample preparation and poor barcode recovery restrict the scalability of colony-picking and replica-plating workflows.
Tried and failed
peroxidase coupled fluorescent functional metagenomic screening applied to bacterial genomic DNA library. Outcome: no signal. Reason: Screening of selected library clones yielded no verified substrate-responsive oxidase activity
Finding novel oxidases specific to hydrocortisone in bacterial genomes · OpenBU
Considered and rejected
Considered and rejected: Rejected MALDI-based mass spectrometry imaging (MSI) as a primary screening method for large candidate probiotic libraries due to multi-hour sample prep per slide, matrix optimization challenges, and low throughput.
Metabolomic Analysis of Chemical Interactions between Burkholderia thailandensis and Host to Inform Alternative Treatments · Georgia Tech
Tried and failed
solid medium fluorescence biosensor screening applied to bacterial biosensor strains. Outcome: no signal. Reason: the biosensor strain failed to generate sufficient fluorescence signal on agar plates
High-throughput isolation of anaerobic arsenic-transforming microorganisms · EPFL
Tried and failed
patch and replica plating for selective screening applied to measuring antibiotic selection sensitivity across media. Outcome: no signal. Reason: insufficient phenotypic discrimination between media water sources and antibiotic concentrations
Impacts of Water Quality of G418 Effectiveness · Iowa State
Considered and rejected
Considered and rejected: Rejected individually picked colony replica-plating for library curing despite preserving uniform barcode abundance, because total recovered barcode and assigned sensor counts were substantially lower than pooled liquid outgrowth.
Engineering bacterial biosensors to explore the mammalian gut · Imperial
Considered and rejected
Considered and rejected: Rejected Sp-biosensor in favor of Nd-biosensor for isolate screening due to lower sensitivity to MMAs(III), inability to sense DMAs(III), and weak agar signal.
High-throughput isolation of anaerobic arsenic-transforming microorganisms · EPFL
Considered and rejected
Considered and rejected: Rejected Sanger sequencing for screening large clone libraries in favor of a barcoding and next-generation sequencing (NGS) workflow.
Systematic analysis of biomolecular binding reactions using microfluidic systems · EPFL
Solid agar matrices and colony biomass introduce physical interference and counting variability
Agar matrix components and dense cellular colonies cause enzymatic inhibition or spectral shifts during colony PCR and qPCR assays. Furthermore, high counting variability at low colony densities and visual occlusion from opaque formulations compromise quantitative colony evaluations.
Considered and rejected
Considered and rejected: Rejected using pour-plating for colony PCR screening due to agar contamination inhibiting PCR and causing false positives.
Establishment of tools for genetic modification of the thermophilic methanogenic archaeon Methanothermobacter thermautotrophicus deltaH · Publikationssystem UB Tuebingen
Considered and rejected
Considered and rejected: Rejected using melting temperatures (Tm) from purified plasmid test qPCRs as a control filter for colony qPCR screening because E. coli colonies alter the Tm.
Towards mapping large-scale deletions in synthetic yeast chromosomes in high-throughput Auf dem Weg zur Kartierung großflächiger Deletionen in synthetischen Hefechromosomen mit hohem Durchsatz · open_UMR Marburg DSpace 10.0
Considered and rejected
Considered and rejected: Rejected colony counting as the quantification method for stent biofilm assays because of lower repeatability compared to fluorescence microscopy imaging.
Effect of Aligned Nanoscale Surface Structures on Microbial Adhesion · Virginia Tech
Considered and rejected
Considered and rejected: Rejected colony count plates yielding fewer than 30 colonies due to high variability.
Transduction in Synchronously Dividing Cultures of Salmonella typhimurium · TXST Digital Repository
Considered and rejected
Considered and rejected: Directly testing Pluronic-formulated nanosuspensions on agar plates was rejected because the milky opaque liquid obscured yeast colony counting, requiring testing in DMSO instead
Antifungal Drug Formulation for Vaginal Yeast Infection · JScholarship
Phenotypic bioactivity observed on solid agar plates fails to translate to liquid cultures
Antimicrobial and enzymatic activities observed on solid agar or surrogate substrates frequently fail to replicate in liquid culture media. Microbial isolates often secrete insufficient concentrations of active metabolites into broth or show degradation rates that do not correlate with agar clearance zones.
Tried and failed
bioassay screening of microbial isolates applied to antimicrobial and toxicity hit identification. Outcome: did not generalise. Reason: initial phenotypic bioactivity hits failed to reproduce during subsequent follow-up testing
An exploration of chemical cues as mediators of marine predator-prey interactions · Georgia Tech
Tried and failed
screening liquid culture broth for secreted antimicrobial activity applied to bacterial isolates from natural microbiome samples. Outcome: no signal. Reason: most solid-media active isolates failed to produce or excrete active compounds in liquid culture broth
HONEY MICROBIOME AND METABOLOME: A VAST RESERVOIR OF NATURAL ANTIMICROBIALS · Cornell
Tried and failed
agar well diffusion of culture supernatants applied to screening bacterial metabolites for fungal inhibition. Outcome: no signal. Reason: supernatants lacked sufficient concentration or active antimicrobial metabolites to produce measurable inhibition zones
Tried and failed
surrogate substrate agar clearance assay for screening applied to bacterial lipid degradation screening. Outcome: did not generalise. Reason: clearing zones on short-chain triglyceride agar did not correlate with actual long-chain oil degradation rates in liquid
Biological FOG degradation: development of a standardised bioadditive protocol. · Cranfield
Selective plating and isolation strategies fail due to microbial overgrowth and harsh stress
Direct plating and selective media fail to isolate target organisms when fast-growing bacterial contaminants overgrow the desired colonies. Extreme thermal conditions or direct plating without liquid pre-culture also cause severe mortality and poor recovery of viable target cells.
Lost to a baseline
Antibiotic selection (e.g., rifampicin) and semi-solid agar migration methods failed as effective strategies for the isolation/enrichment of termite gut treponemes, leading to the use of membrane filtration combined with deep-agar dilution series.
Phylogeny and metabolism of novel spirochetes from cockroaches and evolutionary origin of reductive acetogenesis in termite gut treponemes · open_UMR Marburg DSpace 10.0
Considered and rejected
Considered and rejected: Rejected direct colony streaking onto sucrose plates without liquid pre-culture for SacB curing due to poor survival and low efficiency.
Toward optimized genetic transformation techniques for soybean [Glycine max (L.) Merr.] · Iowa State
Tried and failed
thermal selective screening assay applied to detecting low-abundance cold-tolerant spoilage microbes. Outcome: no signal. Reason: the specific target bacterial subtypes were not recovered under standard laboratory thermal stress conditions
Tried and failed
direct agar plating of raw environmental samples applied to fungal isolation from urban water. Reason: bacterial contamination outgrew and physically obstructed fungal colony expansion
An Investigation of Fungal Species in a Heavy Metal-Contaminated Urban Waterway · UT Austin
Non-specific binding artifacts and contaminants generate false-positive clone hits
Screening bacterial clones and expression supernatants with ELISA formats frequently enriches for non-specific plastic-binding molecules rather than target-specific binders. Copurifying contaminants in crude expression mixtures similarly distort normalization controls to produce false-positive screening hits.
Tried and failed
high-throughput directed evolution screening applied to engineered enzyme variants. Outcome: did not generalise. Reason: apparent screening hits were false positives caused by copurifying contaminants affecting initial normalization and readout
Tried and failed
ELISA screening of bacterial expression clones applied to single-domain antibody selection. Reason: Screening selected non-specific plastic-binding clones rather than target-specific binders, generating false positives.
Considered and rejected
Considered and rejected: Rejected ELISA-based screening of bacterial supernatants for phage display candidates due to high rates of false positives from plastic-binding nanobodies, switching to flow cytometry.
Left open by the authors
Problems the authors named and did not get to.
Left open
Perform directed evolution with negative screening on SalR variants to eliminate unintended cross-reactivity with cellobiose. Blocker: Requires a molecular biology wet lab for directed evolution and screening assays
Engineering Synthetic Allosteric Transcription Factors · Georgia Tech
Left open
Perform directed evolution and selection on genetically encoded non-canonical macrocycle libraries to discover macrocycles with novel functions. Blocker: Requires a wet lab with engineered E. coli strains and biological screening apparatus.
Genetically programmed cell-based synthesis of non-natural peptide and depsipeptide macrocycles. · Cambridge
Left open
Optimize the 96-well conjugation screening protocol to minimize false positives and negatives caused by motile colony lifting in soft agar overlays. Blocker: Requires a wet-lab setup and bacterial culture experiments
Synthetic biology approaches for engineering bacteria as living therapeutics · MIT
Left open
Implement negative selection schemes in the continuous adenovirus directed evolution platform to evolve biomolecules with enhanced specificity. Blocker: Requires a molecular biology wet lab and mammalian cell culture facilities.
Left open
Establish a directed evolution and selection platform for non-canonical macrocycles with new functions. Blocker: Requires a wet lab, engineered E. coli strains (Syn61Δ3(ev5)), and experimental screening assays
Genetically programmed cell-based synthesis of non-natural peptide and depsipeptide macrocycles. · Cambridge
Left open
Incorporate kinetic selection into directed evolution of hemi-methylated DNA binding domain to enhance epigenotyping assay sensitivity. Blocker: Requires wet lab facilities for yeast surface display, FACS, and directed evolution experiments
Applications of Engineered Proteins in Redox Biology and Biomarker Detection Assay Development · MIT
Left open
Perform multiple rounds of directed evolution library screening on MIOX from glucose substrate using the developed biosensor. Blocker: Requires a wet lab, engineered bacterial strains, and experimental screening apparatus
Towards biosensor-assisted directed evolution of myo-inositol oxygenase · MIT
Left open
Clone two gIII-neg construct copies with unique off-target PAMs into the PACE selection system to test multiplexed negative selection stringency. Blocker: Requires wet lab molecular biology, phage culturing, and continuous directed evolution experimental setup
Engineering and evolution of precision genome editing agents · Harvard
Left open
Conduct a suppressor genetic screen to validate the relationship between Hfq foci dispersal dynamics and bacterial recovery/survival. Blocker: Requires a wet biology laboratory, bacterial culturing, genetic screening, and live-cell microscopy apparatus.
Glutathione contributes to the adaptive response to nitrogen starvation in Escherichia coli · Imperial
Left open
Apply purifying selection iteratively in continuous evolution experiments until desired enzymatic activity or physical constraints are achieved. Blocker: Requires wet lab facilities, microfluidic droplet sorting apparatus, and continuous directed evolution reagents
Techniques for High-Throughput Directed Enzyme Evolution · Harvard
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