Drop-Plate CFU Enumeration of Bacterial Viability for Time-Kill Kinetics of Gentamicin Against Klebsiella pneumoniae

Time-kill is performed at 0.25x, 1x, 4x, and 8x MIC plus a no-drug growth control, with sampling at 0, 2, 4, 6, 8, and 24 h. Each condition is run in biological triplicate on separate days from independent cultures. At each time point, three technical-replicate dilution series are drop-plated (3 x 10 µL drops per dilution). Starting inoculum is standardized to ~5-6 x 10^5 CFU/mL in CAMHB. Plating and counting are performed by a reader blinded to the concentration label where feasible.

BSL-2 for K. pneumoniae (opportunistic, often multidrug-resistant); work in a Class II biosafety cabinet, avoid aerosols, and never mouth-pipette. PPE: lab coat, nitrile gloves, eye protection. Gentamicin is nephrotoxic/ototoxic — avoid inhalation/skin contact when weighing (use a hood and mask). Autoclave or 10% bleach all cultures, dilution plates/tubes, and MHA plates before disposal as biohazard. Handle carbapenem/aminoglycoside-resistant isolates with heightened containment per institutional policy.

No-drug growth control: confirms strain viability and provides the baseline growth trajectory for log-reduction calculations. Sterility control: uninoculated CAMHB + drug, plated to confirm no contamination. Antibiotic-carryover control: at the highest gentamicin multiple, compare drop-plate counts with and without a dilution/wash step to verify carryover is not falsely suppressing colonies. Method QC: enumerate E. coli ATCC 25922 against a known count to validate drop-plate accuracy (within ~0.3 log10).

No-drug control rises ~2-3 log10 over 24 h to stationary (~10^8-10^9 CFU/mL). At >=4x MIC, a steep decline reaching >=3 log10 reduction (bactericidal) by 4-6 h. At 1x MIC, partial inhibition with possible regrowth by 24 h; at 0.25x, growth approaching control. Countable drops contain 3-30 colonies; the technique is reproducible within ~0.2-0.3 log10. Bactericidal defined as >=99.9% kill vs initial inoculum.

• Klebsiella pneumoniae test strain (and E. coli ATCC 25922 for method QC)
• Cation-adjusted Mueller-Hinton Broth (CAMHB)
• Gentamicin sulfate reference standard, stock 10 mg/mL in sterile water
• Mueller-Hinton agar (MHA) plates, well-dried, gridded into 6-8 sectors on the base
• Sterile 0.85% NaCl or PBS for serial dilutions
• 96-well plates or microtubes for 10-fold dilution series
• Calibrated single/multichannel pipette (10 µL), sterile tips
• 0.5 McFarland standard / densitometer, vortex mixer

To quantify viable bacterial counts (CFU/mL) at defined time points during exposure to gentamicin at multiples of the MIC, using the drop-plate technique (10 µL drops of serial 10-fold dilutions) for efficient, reproducible enumeration, and to plot log10 CFU/mL versus time to characterize the rate and extent of killing. The method supports rigorous bactericidal/bacteriostatic classification.

1. Determine the gentamicin MIC for the strain by broth microdilution first; set 1x = that MIC.
2. Prepare a mid-log culture in CAMHB; standardize and dilute to ~5-6 x 10^5 CFU/mL in tubes containing gentamicin at 0.25x, 1x, 4x, 8x MIC, plus a drug-free control (total ~10 mL each).
3. Incubate all tubes at 35 +/- 2 C with shaking (~150 rpm).
4. At t = 0, immediately sample 100 µL and perform 10-fold serial dilutions (10^0 to 10^-7) in saline.
5. Drop 3 x 10 µL of each appropriate dilution onto a labeled sector of a dried MHA plate; let drops absorb fully without running.
6. Repeat sampling/dilution/drop-plating at 2, 4, 6, 8, and 24 h.
7. For high-concentration samples, wash or dilute to minimize antibiotic carryover before plating.
8. Incubate plates upright 18-24 h at 35 C.
9. Count drops with 3-30 colonies; average the three drops per dilution.
10. Calculate CFU/mL = mean colonies per drop x dilution factor x 100 (since 10 µL = 0.01 mL).

Independent variables: gentamicin concentration (0.25-8x MIC) and exposure time (0-24 h). Dependent variable: viable count (log10 CFU/mL) and derived log-reduction from the t=0 inoculum. Controlled variables: starting inoculum (~5 x 10^5 CFU/mL), medium (CAMHB), temperature (35 C), shaking, dilution diluent, drop volume (10 µL), MHA dryness, and incubation time.

Gentamicin, a concentration-dependent aminoglycoside, will produce rapid bactericidal killing of K. pneumoniae at >=4x MIC, achieving a >=3 log10 CFU/mL reduction within 4-6 h, with greater killing at higher concentrations, while the no-drug control grows to stationary phase. Sub-MIC exposure will show regrowth, and high-inoculum effects may blunt killing.

Convert mean colonies per drop to CFU/mL (multiply by dilution factor and by 100 for the 10 µL drop). Average technical replicates, then plot log10 CFU/mL versus time per concentration (GraphPad Prism or Python). Compute log10 reduction at each time point relative to t=0 and to the growth control. Determine time to 3-log kill and the killing rate (slope of the log-linear decline). Flag the lower limit of detection (~2 x 10^2 CFU/mL for 10 µL drops, lower with concentrated plating).

1. Drops run together / smear -> plates not dry enough; pre-dry MHA at 37 C for 20-30 min with lids ajar and dispense smaller drops. 2. Counts higher than expected at high drug -> antibiotic carryover insufficient or regrowth on plate; add a wash/dilution step and confirm with the carryover control. 3. No countable dilution -> widen the dilution range (plate 10^-1 to 10^-7) and include low dilutions for late time points near detection limit. 4. Confluent or no growth -> wrong dilution chosen; always plate at least 3 adjacent dilutions. 5. Poor reproducibility between drops -> vortex dilutions thoroughly and change tips between dilutions to avoid carryover.

Compare log10 CFU/mL across concentrations and time using two-way repeated-measures ANOVA (factors: concentration x time) with Bonferroni or Sidak correction, alpha = 0.05, on biological-replicate means (n = 3). Compare killing slopes by linear regression and an extra-sum-of-squares F-test. Report mean +/- SD and define bactericidal endpoints (>=3 log10 reduction). Power analysis targets 80% power to detect a 1-log10 difference between concentrations at alpha 0.05.