2.1. MaterialsChemicals and enzymes, unless specified otherwise, were purchasedfrom Sigma-Aldrich Canada Ltd, Oakville, ON.2.2. Construction and synthesis of the oligonucleotide libraryTo test the efficacy of this novel high throughput screeningmethod, a random oligonucleotide library containing Sph I and HindIII restriction sites was designed and synthesized as follows: 5′cagaattcggatccgcatgcat-(NNN)25-taagcttctcgagagatctga3′ (116 nts)(Alpha DNA Inc., Montreal, QC). Double-stranded DNA was synthesizedfrom single-stranded templates using Klenow fragment followingthe supplier's protocol (GE Healthcare, Baie d'Urfé, QC). The doublestranded DNA was purified using QIAEX II gene cleaning kit anddouble digested by Sph I and Hind III (Qiagen, Mississauga, ON).Purified DNA fragments were cloned into pET-CoCo vector (Novagen,Madison, WI) downstream of the T7 promoter and transformed intoE. coli BL21-AI host cells.2.3. Library screeningMembrane filters (d=9.0 cm; Chemical School Factory, Beijing)were placed onto Luria-Bertani (LB) agar plates (100×100 mm)supplemented with 100 mg/L ampicillin (Fig. 1A). Host cell suspensions(50 μL) containing the oligonucleotide library were spread onmembranes with a conventional plastic bacteriological glass spreader,air-dried for 10 min in a sterile laminar flow hood and incubatedovernight at 37 °C. Those membranes with evenly distributed bacterialcolonies (600–800 colonies per plate) were lifted from the agarsurface by forceps and transferred (colonies facing up) to a new LBagar plate containing 1 mM isopropyl-beta-D-thiogalactopyranoside(IPTG) as an inducer, 100mg/L ampicillin, 0.00125% trypan blue (M.W.=960.81) and 0.0025% bromophenol blue (M.W.=670). Colonies werefurther incubated at room temperature for 1–3 h to allow for inductionof peptide expression. Those few colonies which appeared to havebeen stained blue, in relation to the whitemajority, were picked usingsterile toothpicks and stored as glycerol stocks. Clone rescuing ispossible, since not all cells within any given colony have yet expressedthe antagonistic insert.2.4. Bacterial inhibition assay for verification of antagonistic nature ofDNA insertA bacterial growth inhibition assay was performed to rule out falsepositives. To ensure each clone contained a DNA insert, cells wereinduced with a second round of IPTG and the growth rate measuredcompared to a no insert control. Host cells harboring an emptyexpression vector were detected to be trypan blue permeable at0.0001% of cases and were detected using this high throughputscreening method. Blue staining colonies (positive hits) from thepotentially lethal gene induction assays and the unstained controlcells (BL21-AI harboring empty pET-CoCo vector) were rescued bybeing picked with sterile toothpicks and grown in LB brothsupplemented with ampicillin at 37 °C overnight. The overnightcultureswere diluted to OD600=0.004 in M9 broth supplemented withampicillin. IPTG was added to the diluted bacterial cultures to a finalconcentration of 0.5 mM to induce gene expression of the clonedinsert (random oligonucleotide). The diluted cultures were incubatedat 25 °C for 6 h and their absorbancies at OD600 (A) were examined(Table 1). An arbitrary 75% inhibitory cut-off OD600 was defined asfollows: Acut-off=(Acontrol−0.004)×25%. The positive hits with absorbanciesless than Acut-off were selected for the next round ofverification.