Spot Test Protocol 5.6 | Phage Isolation Lab Demo

Bacteriophage isolation involves collecting environmental samples (such as soil from Isabela, Puerto Rico), mixing with liquid solution, incubating in a shaker, and centrifuging. The supernatant is filtered through a 0.22-micrometer filter to isolate bacteriophages from other particles. Two methods exist: direct isolation (immediate plaque assay) and enriched isolation (incubation with host bacteria to increase concentration). Multiple rounds of purification yield clonal populations (genetically identical phages) confirmed by consistent plaque morphology. The purified phage is then named (e.g., 'Cyro') and prepared for genomic analysis.

To pick a bacteriophage plaque, gently touch a micropipette tip to the clear zone of the plaque without breaking through the agar, then transfer the sample to phage buffer and perform serial dilutions to isolate and purify a single phage type for further study.

The three-phase streak protocol is a laboratory technique used to purify a single species of bacteriophage from a mixed sample by repeatedly diluting the phage across three sections of an agar plate over three weeks, ensuring that only one species remains for characterization. The process involves selecting a well-isolated plaque (at least 0.5 cm from others), using a fresh sterile wooden stick for each streak, and progressively widening the streaks from section 1 to section 3 to achieve dilution. Top agar containing host bacteria is then poured into the most dilute section to grow plaques, allowing researchers to isolate a clonal phage population for further study.

This video explains two methods for purifying bacteriophages: the streak plate method, which uses serial dilution and dragging a pipette tip across an agar plate to progressively isolate single viruses, and the plaque assay method, which uses multiple plates with serial dilutions (typically 10^-4 to 10^-7) to identify plates with well-isolated plaques. The streak plate method is simpler and faster but provides less confidence in obtaining a completely clonal population, while the plaque assay method, though more labor-intensive, offers higher confidence that each plaque originated from a single bacteriophage particle, making it preferable for downstream genetic analysis.

Bacteriophage purification involves isolating individual plaques from infected bacterial cultures through repeated rounds of plaque picking, filtration, and serial dilution to obtain a pure phage lysate suitable for titer determination; the process includes dissolving isolated plaques in LB broth, filtering the suspension through a 22-micron syringe filter, preparing serial dilutions (5-7 dilutions), mixing with host bacterial suspension, and pouring onto LB agar plates for overnight incubation at 37°C to visualize clear plaques, with the procedure repeated 4-5 times to achieve high-purity phage preparation.

Bacteriophage purification involves isolating individual plaques from infected bacterial cultures through repeated rounds of plaque picking, filtration, and serial dilution to obtain a pure phage lysate suitable for titer determination; the process includes dissolving isolated plaques in LB broth, filtering the suspension through a 22-micron syringe filter, preparing serial dilutions (5-7 dilutions), mixing with host bacterial suspension, and pouring onto LB agar plates for overnight incubation at 37°C to visualize clear plaques, with the procedure repeated 4-5 times to achieve high-purity phage preparation.

This video demonstrates a DNA isolation procedure for phage hunters that involves treating phage lysate with nuclease to degrade bacterial nucleic acids, followed by DNA cleanup resin treatment to break open phage capsids and remove contaminating enzymes, then performing multiple isopropanol washes to purify the DNA, and finally eluting the purified phage DNA using a column-based method with heat-treated water.

This video demonstrates a column-based DNA extraction method for isolating phage genomic DNA using the Promega Wizard DNA Clean-up kit, which employs ion exchange chromatography instead of traditional phenol-chloroform extraction. The protocol involves four main steps: (1) concentration of phage particles using polyethylene glycol (PEG) to reduce sample volume from 10-20 mL to ~500 μL, (2) purification where guanidine thiocyanate denatures proteins and releases DNA, followed by binding to a resin column, (3) washing with isopropyl alcohol to remove proteins while retaining DNA, and (4) elution using hot water (80°C) to release purified DNA. The final DNA yield is quantified using spectrophotometry to assess purity and concentration.

This comprehensive phase covers the complete preparation of bacterial lysate for plasmid DNA extraction. It begins with resuspending a bacterial pellet in Solution One (175 μL), mixing until homogeneous, then adding Solution Two (175 μL) without pipetting and inverting 10 times. The mixture sits for 3 minutes before adding N3 buffer (85 μL) with 10 inversions, followed by GBT buffer (100 μL) with 10 more inversions. White flocculent particulates form during this process. The lysate is then loaded onto a nested column system consisting of a 2ml collection tube containing a High Bind mini column with a Fast Filter mini column inside. The uncleared lysate (cloudy and chunky) is directly pipetted into the Fast Filter tube and centrifuged at 13,000 g for 1 minute, separating clear liquid from captured particulate material.

Transmission electron microscopy (TEM) is the primary method for visualizing bacteriophages. The process involves preparing samples on metal grids, washing with water and stain, and imaging using electron beams. TEM produces high-resolution images revealing phage morphology including capsid (head) and tail structures.

Pulse Field Gel Electrophoresis (PFGE) extends DNA sizing beyond standard electrophoresis (0.2-50 kb to 200 bp-12 Mb). Procedure: mix phage suspension with low melting agarose, create plugs, release DNA by overnight shaking at 55°C, wash plugs, perform restriction digestion if needed, load plugs in agarose gel, run at appropriate voltage, stain with ethidium bromide. Transmission Electron Microscopy uses negative staining with uranyl acetate on carbon-coated grids. ~96% of examined phages are tailed phages with characteristic components: head, tail tube, sheath, base plate, spikes, and tail fibers. Different families have distinct morphologies: T4 phages have contractile tails, lambda phages have different tail structures, filamentous phages have long flexible tails. Modern sequencing technologies include Sanger (first generation, ~1000 bp reads), Illumina (next generation, ~300-400 bp fragments requiring assembly), and Nanopore (single-molecule real-time sequencing). Workflow: template preparation, fragmentation, barcoding, PCR amplification, sequencing, and bioinformatics assembly. Coverage of at least 9x ensures reliable reconstruction. Bioinformatics identifies open reading frames (ORFs) coding for structural proteins (capsid/head/tail assembly), enzymes (DNA synthesis), and other functions. Comparative genomics enables species identification and evolutionary analysis.

TEM provides high-magnification images showing topographical and morphological information (shape, size, defects, orientation). Components: electron gun, condenser lenses, condenser aperture, objective lens, intermediate lenses, projector lens, and screen. Resolution is very high. Specimens must be electron-transparent. Images appear as bright/dark spots indicating varying thickness. Advantages: powerful microscope with high magnification, provides topographical/morphological information, easy to operate with proper training. Disadvantages: only produces black-and-white 2D images, specimens must be electron-transparent.

Under electron microscopy, most bacteriophages resemble tadpoles with a well-defined head and tail portion. The head contains the nucleic acid core (single or double-stranded DNA or RNA, but not both) surrounded by a protein coat. There are three basic morphological forms: icosahedral head with tail, filamentous bacteriophages, and complex structures. The tail may be flexible or rigid, very short or up to four times the length of the head. T2 phage of E. coli has a sheath composed of protein subunits and a complex base plate with one to six tail fibers.

Transmission Electron Microscopy (TEM) is a nanoscale characterization tool that provides detailed information about material morphology, including particle size, grain size, crystallite size, dislocation defects, and crystal structure; TEM images reveal whether materials are polycrystalline (showing concentric diffraction circles) or single-crystalline (showing discrete diffraction spots), while high-resolution TEM (HRTEM) can visualize atomic planes and lattice fringes to determine crystallographic orientations and d-spacing values.

Bacteriophages (viruses that infect bacteria) were discovered around World War I and used therapeutically for approximately 25 years before being largely abandoned in the 1940s when antibiotics became commercially viable; however, phage therapy and biocontrol have experienced a resurgence in recent decades, particularly for controlling foodborne pathogens and spoilage bacteria in the agri-food supply chain, with the FDA approving commercial phage preparations in the mid-2000s, offering promising solutions to antibiotic resistance challenges.

Bacteriophages serve diverse practical applications beyond basic biology. As natural bacterial predators, they form the basis of phage therapy for treating bacterial infections, particularly valuable when antibiotic resistance develops. Combined with antibiotics, phage therapy shows promising future potential. In biotechnology, phages enable bacterial transduction for genetic engineering and serve as cloning vectors for transferring recombinant genes between bacteria. In agriculture and food safety, phages function as biocontrol agents, mediating targeted killing of pathogenic bacterial populations while sparing beneficial microbes.

Bacteriophages have diverse practical applications: controlling insect pests by disrupting their bacterial symbionts, treating food to eliminate pathogenic bacteria, treating dysbiosis by removing harmful bacteria, and reducing methane emissions from cattle by targeting methane-producing bacteria. Within mammalian hosts, phages serve dual protective functions: population control by destroying pathogenic bacteria, and host protection by shielding beneficial bacteria from immune destruction. Esther Lederberg discovered that bacteriophages create visible plaques on bacterial lawns, demonstrating their ability to systematically destroy bacterial populations. Phages employ two infection cycles: the lytic cycle where phages inject DNA, replicate, and lyse the host cell within minutes; and the lysogenic cycle where phage DNA integrates into bacterial chromosomes and remains dormant for extended periods.

Bacteriophages (phages) are viruses that specifically infect bacteria, with double-stranded DNA genomes and diverse structural classifications including Podoviridae, Myoviridae, and Siphoviridae families. They outnumber bacterial hosts by a factor of 10, representing Earth's most abundant self-replicating entities. Phage therapy emerged shortly after discovery but declined with antibiotic development in the 1950s. Modern revival stems from antibiotic-resistant bacterial threats. Phages received GRAS status due to safety profiles. Commercial applications include ListShield (reducing L. monocytogenes 10-1000x), Salma Fresh (targeting salmonella), and products from Georgia's Bacteriophage Institute. Phages can be sprayed, dipped, or immobilized onto foods without altering sensory properties.

Phage therapy uses lytic bacteriophages to kill pathogenic bacteria. Discovered in 1915 by d'Herelle, it was pursued at the Eliava Institute in Georgia. During WWII, phages were used in soldier kits. Widespread antibiotic discovery in the 1930s reduced enthusiasm, though research continued at Eliava. Renewed interest emerged due to antimicrobial resistance. Approved applications include Agrophage (EPA-approved for tomato canker) and ListShield (FDA-approved for food safety). Clinical successes include treating aortic graft infections, drug-resistant Mycobacterium abscesses, and disseminated Acinetobacter infections, demonstrating potential for personalized antibacterial therapy.