Standardize Inoculum by OD600

Procedure for standardizing bacterial inocula to a target OD600 using C1V1=C2V2 dilution calculations.

Standardize Inoculum by OD600, v1.0 Kathryn E. Caruso · 0009-0003-2436-1791 Foreman Lab · Center for Biofilm Engineering, Montana State University Updated March 2026

How to cite this protocol Caruso, K.E. (2026). Standardize Inoculum by OD600, v1.0. Foreman Lab, Center for Biofilm Engineering, Montana State University. https://kathryncaruso.github.io/methods/standardize-inoculum/

Standardize Inoculum by OD600

Standard Operating Procedure

Purpose

General procedure for standardizing bacterial inocula to a target starting OD600 using C1V1 = C2V2 dilution calculations. Ensures consistent starting cell density across replicates and experiments. Applicable to any liquid culture inoculation where a target OD is specified.

1 Equipment and materials

  • Spectrophotometer (OD600)
  • Cuvettes (disposable or quartz)
  • Micropipettes (2–1000 µL range)
  • Sterile pipette tips
  • Milli-Q water (for dilution series)
  • Calculator
  • Lab notebook

2 Parameters

Define these before starting — values will vary by experiment.

Parameter Symbol Description
Target starting OD C2 The OD600 you want in each experimental tube after inoculation
Final tube volume V2 Total volume of medium + inoculum in each tube (e.g., 3000 µL)
Rounding precision — Round inoculum volumes for pipetting accuracy (e.g., nearest 5 µL)

Common target ODs used in this lab:

Experiment Target OD600 Tube volume
Jung assay serial transfer 0.025 3000 µL
Carbon source growth assay 0.025 3000 µL

3 Measuring starter culture OD600

Many starter cultures will be too dense to read directly on the spectrophotometer (readings above ~1.0 become nonlinear). Use a cuvette dilution series:

  1. Add 50 µL of starter culture to 1000 µL Milli-Q water in a cuvette (1:20 dilution)
  2. Measure OD600; if reading is >0.3, try a smaller volume or re-dilute
  3. Back-calculate the true OD of the undiluted culture:

True OD = measured OD × (total cuvette volume ÷ sample volume added)

Example: 50 µL culture added to 1000 µL total:

  • True OD = measured OD × (1000 ÷ 50) = measured OD × 20
  • If measured OD = 0.15, then True OD = 0.15 × 20 = 3.0
  1. Repeat with additional volumes if needed to confirm; average confirmatory reads
  2. Record all dilution steps and measured values in lab notebook

Tip

Start with 50 µL into 1000 µL (1:20). This gives readable values on the Genesys 10UV Scanning for cultures in the OD 0.5–6.0 range. If the reading is >0.3, try 20 µL or 10 µL. If too low to read accurately (<0.01), try 100 µL.

The Genesys 10UV Scanning has a reliable lower detection limit of ~0.01–0.02 absorbance units. A 1:100 dilution (10 µL into 1000 µL) often puts readings at the noise floor for typical starter culture densities — start at 1:20 and work from there.

4 Calculating inoculum volume

Use C1V1 = C2V2 with the confirmed true OD:

Variable Meaning Value
C1 True OD of starter culture Measured in Section 3
V1 Inoculum volume to add Solve for this
C2 Target starting OD Defined per experiment (Section 2)
V2 Final tube volume Defined per experiment (Section 2)

V1 = (C2 × V2) ÷ C1

Worked example (target OD 0.025, tube volume 3000 µL):

  • Starter culture True OD = 8.5
  • V1 = (0.025 × 3000) ÷ 8.5 = 75 ÷ 8.5 = 8.8 µL
  • Round to nearest 5 µL → 10 µL inoculum

Important

Round to the nearest 5 µL for pipetting accuracy. For very small volumes (<5 µL), consider diluting the starter culture first to bring the inoculum volume into a more pipettable range (e.g., ≥10 µL).

5 Inoculation

  1. Calculate inoculum volume for each starter culture (Section 4)
  2. Calculate medium volume per tube: V2 − V1
  3. Dispense medium into labeled tubes
  4. Add calculated inoculum volume from starter culture
  5. Cap and gently vortex or invert to mix
  6. Record all values in lab notebook

6 Verification

After inoculation, confirm the starting OD is in the expected range:

  1. Measure OD600 of inoculated tubes (read directly in culture tube if using glass tubes, or transfer to cuvette)
  2. Compare to target OD600
  3. Flag any tubes that fall outside the acceptable range

Note

Small deviations from target are normal due to rounding. The important thing is consistency across replicates.