Filter-Mating Bacterial Conjugation Assay to Measure Plasmid RP4 Transfer Frequency from E. coli Donor to Recipient

Matings are performed in biological triplicate on separate days, each with a fixed donor:recipient ratio (1:1, ~10^8 cells each) on 0.45 µm filters atop non-selective agar. Mating time (37 C, 4 h) is the primary condition; secondary conditions vary time (1, 4, 18 h) and temperature (25 vs 37 C). Donor-only and recipient-only filters serve as spontaneous-resistance controls. CFU are enumerated from serial dilutions plated in technical duplicate on each selective medium.

BSL-1 to BSL-2 depending on strains (laboratory E. coli K-12 derivatives are typically BSL-1; treat clinical/uncharacterized strains as BSL-2). IMPORTANT: this assay propagates antibiotic-resistance plasmids — follow institutional biosafety committee approval for conjugative-plasmid work and prevent environmental release. PPE: lab coat, gloves, eye protection. Autoclave ALL mating mixtures, filters, plates, and dilution tubes before disposal; never drain-dispose live transconjugants. Handle rifampicin (stains, irritant) and kanamycin with gloves.

Donor-only filter plated on transconjugant-selective medium: detects spontaneous recipient-marker mutants (must be << transconjugant count). Recipient-only filter on transconjugant medium: detects spontaneous donor-marker resistance. No-mating control: donor and recipient mixed and immediately diluted/plated (no incubation) to confirm transfer requires mating contact/time. Antibiotic-free CFU counts verify input densities. A reference donor/recipient pair with known transfer frequency validates the run.

Transconjugant colonies on dual-selective plates; transfer frequency ~10^-1 to 10^-3 per recipient for RP4 under optimal conditions. Spontaneous-resistance controls should yield <=10^-7 to 10^-8 (i.e., negligible relative to transfer). CFU counts of 30-300 per countable plate. Shorter times/lower temperature reduce frequency by 1-2 logs. Transconjugant identity can be confirmed by re-streaking on triple-selective medium.

• Donor: E. coli carrying plasmid RP4 (KanR/AmpR/TetR markers; e.g., E. coli HB101(RP4))
• Recipient: rifampicin-resistant or nalidixic-acid-resistant E. coli (e.g., E. coli J53 RifR)
• LB broth and LB agar
• Selective agar plates: donor (kanamycin 50 µg/mL), recipient (rifampicin 100 µg/mL), transconjugant (kanamycin 50 + rifampicin 100 µg/mL)
• Sterile 0.45 µm nitrocellulose membrane filters (25 mm)
• Sterile 0.85% NaCl for dilutions and resuspension
• Sterile forceps, glass spreaders/beads, microcentrifuge tubes
• Phosphate-buffered saline (PBS), pH 7.4

To perform a standardized filter-mating conjugation between an antibiotic-resistance-marked donor and a counter-selectable recipient, and to calculate transfer frequency as transconjugants divided by recipients (and per donor), using selective and counter-selective agar to enumerate each population by CFU. The assay measures the efficiency of plasmid-mediated horizontal gene transfer under defined conditions.

1. Grow donor and recipient separately overnight in LB with their selective antibiotics at 37 C, 200 rpm.
2. Subculture 1:100 into antibiotic-free LB; grow to mid-log (OD600 ~0.5-0.8).
3. Wash cells once in saline to remove residual antibiotic; resuspend to ~10^8-10^9 CFU/mL.
4. Mix equal volumes (e.g., 500 µL each) of donor and recipient; pellet gently at 4000 x g for 5 min.
5. Resuspend the combined pellet in 50-100 µL LB and pipette onto a 0.45 µm filter on a pre-warmed non-selective LB agar plate.
6. Incubate the filter cell-side up at 37 C for 4 h.
7. Transfer the filter to 1 mL saline and vortex/wash to resuspend the mating mixture.
8. Prepare 10-fold serial dilutions (10^0 to 10^-7) in saline.
9. Plate appropriate dilutions on donor-, recipient-, and transconjugant-selective agar in technical duplicate.
10. Incubate plates 18-24 h at 37 C and count colonies (30-300 per plate).

Independent variables: mating time, mating temperature, and donor:recipient ratio. Dependent variable: transfer frequency = transconjugant CFU / recipient CFU (and / donor CFU). Controlled variables: input cell densities (~10^8 each), medium (LB), filter pore size (0.45 µm), antibiotic concentrations in selective plates, dilution scheme, and incubation/enumeration conditions.

RP4-bearing E. coli donors will transfer the plasmid to recipient E. coli at a frequency of approximately 10^-1 to 10^-3 transconjugants per recipient under optimal solid-surface mating conditions at 37 C, and transfer frequency will decline at lower mating temperatures or with shorter mating times, reflecting the energy- and contact-dependence of type IV secretion.

Compute CFU/mL for each population from counts x dilution factor / plated volume, averaging technical-duplicate plates. Calculate transfer frequency as transconjugants/recipient and transconjugants/donor. Log10-transform frequencies for plotting. Subtract or flag spontaneous-mutant background using the control counts. Tabulate mean +/- SD across biological replicates. Manage data and plate maps in a spreadsheet or LIMS; plot with GraphPad Prism or Python (matplotlib).

1. No transconjugants -> donor lost the plasmid or recipient marker overlaps; verify markers by control plating and re-confirm donor RP4 by re-streaking on plasmid-selective medium. 2. High spontaneous background -> increase recipient marker selectivity (use a recipient with two chromosomal markers) and lower antibiotic carryover by washing input cells. 3. Plates overgrown/uncountable -> extend the dilution series and plate 10^-5 to 10^-7. 4. Low frequency vs expected -> ensure mid-log cells, solid-surface (not broth) mating, and 37 C; broth matings give lower RP4 efficiency. 5. Confluent satellite colonies on antibiotic plates -> use fresh plates and correct antibiotic concentrations; beta-lactam satellites indicate degraded drug.

Because transfer frequencies are log-distributed, compare conditions on log10-transformed values using one-way ANOVA with Tukey's HSD or, for two groups, an unpaired t-test (alpha = 0.05). For non-normal data, use Mann-Whitney/Kruskal-Wallis with Dunn's correction. Report geometric means with 95% CIs. n = 3 biological replicates minimum; report whether the transfer frequency exceeds the spontaneous-resistance background by at least 10-fold for validity.