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Aerobic plate counting

Distinguish dilution fraction, colony count and inoculum volume; calculate with eligible replicate plates.

    Food Lab · aerobic plate counting

    An educational model. It does not replace actual test conditions or safety procedures.

    Dilution fraction and CFU

    From plate colonies to original sample

    CFU estimates units capable of forming colonies under the specified culture conditions.

    Dilution fraction d is the original-sample proportion in the final mixture, obtained by multiplying actual transfer volume ratios.

    A colony may arise from one cell or a cell cluster; CFU does not equal total cell count.

    Use ΣC/(n×v×d) for one step and ΣC/[v×(n1+0.1n2)×d] for two consecutive steps.

    Applies to decimal serial dilutions and eligible plates. The reference is an entry point, not verification of all current official conditions.

    Zero colonies describe the observed volume and conditions; they do not guarantee sterility or food safety.

    An independent educational simulation. Not affiliated with or endorsed by HRD Korea, Q-net or MFDS. Virtual results do not establish food safety or guarantee exam success.

    Food Lab · Microbiological analysis / principles and inquiry

    Aerobic plate count guide

    Estimate CFU/mL from eight plates using actual dilution lineage and inoculum volume.

    How does it work?

    Use dilution fraction d and volume v in the denominator, not dilution factor. Weight two consecutive steps by replicate counts.

    Key relationship

    CFU/mL = ΣC / [v × (n1 + 0.1n2) × d]

    Try this investigation

    1. Repeat 1 mL transfer into 9 mL diluent and mixing four times; prepare two plates per step.
    2. Read all eight plates; record counts, selection and exclusion reasons. The educational range is 30–300.
    3. Keep normal records and compare a reused-tip case with 12 extra contaminating colonies on one plate.

    Interpreting your results

    Is d=1000 for a 10⁻³ dilution?

    d=0.001. Dilution fraction and dilution factor are reciprocals.

    Assumptions and limitations

    The food-edu-v1 range 30–300 is educational. Zero colonies do not guarantee sterility. Distinguish contamination, spreading colonies, low counts and excessive counts. Do not culture unknown microbes at home or open plates.