ISO 20128:2006

Overview

ISO 20128:2006 (IDF 192:2006) specifies a standardized microbiological method for the enumeration of presumptive Lactobacillus acidophilus in milk products using a colony-count technique at 37 °C on a selective medium. The procedure is designed for fermented and non‑fermented milks, milk powders and infant formulae where presumptive L. acidophilus coexists with other lactic acid bacteria and bifidobacteria. Results are reported as colony‑forming units per gram (CFU/g). The method produces presumptive identifications and cannot distinguish very closely related species (e.g., L. johnsonii, L. gasseri, L. crispatus).

Key topics and requirements

• Selective medium: MRS agar supplemented with clindamycin and ciprofloxacin (MRS/CL/CIP agar). Typical stock additions are 0.1 mg clindamycin and 10 mg ciprofloxacin per litre of medium (per the standard formulation).
• Sample prep and plating: Homogenize a known sample amount, prepare decimal dilutions, spread plate appropriate dilutions onto MRS/CL/CIP plates.
• Incubation: Anaerobic incubation at 37 °C for 72 h ± 3 h, followed by colony counting.
• Colony morphology: Presumptive L. acidophilus colonies are typically flat, mat or rough, grey to whitish, 1–3 mm diameter.
• Limits and limitations:
  • Not applicable when presumptive L. acidophilus < 10^4 CFU/g and competing species (L. rhamnosus, L. reuteri, L. paracasei subsp. paracasei) exceed 10^6 CFU/g.
  • Method yields presumptive counts only; confirmatory identification requires additional tests.
• Quality and storage: Prepared MRS/CL/CIP plates may be stored refrigerated (4–7 °C) for limited time; antibiotic stock solutions are held frozen per the standard.

Applications and users

• Routine quality control and microbiological testing in dairy manufacturing, probiotic product validation, infant formula production and third‑party food testing laboratories.
• Useful for product release testing, shelf‑life studies, and verification of probiotic label claims where L. acidophilus is a target organism.
• Users: food microbiologists, laboratory managers, regulatory compliance teams, and academic researchers working with dairy probiotics.

Related standards

• ISO 7218 - General rules for microbiological examinations.
• ISO 8261 | IDF 122 - Guidance for preparation of test samples, initial suspensions and decimal dilutions for microbiological examination.
• See ISO/TS 11133 (culture media quality/validation) for additional media performance guidance.

Keywords: ISO 20128, Lactobacillus acidophilus enumeration, selective medium, MRS/CL/CIP agar, colony-count technique at 37 °C, milk products, microbiological methods, fermented milk, infant formula.