Culture Media Preparation: Sources and QC

Short answer

Culture media preparation is the work of turning a defined medium formulation into a sterile, performance-tested medium that supports the organisms a method is meant to detect. ISO 11133 covers how culture media are prepared, produced, stored and performance-tested; a published formula such as the FDA’s Plate Count Agar specifies ingredients, sterilisation and final pH, but quality control comes from the laboratory’s own standard or from the supplier. This page is a reference on sources, sterilisation and quality control, not a benchtop protocol.

General information only, not laboratory-accreditation advice. Follow your method, your quality manual and the manufacturer’s instructions.

Definition and scope

A culture medium is the nutrient formulation on or in which microorganisms are grown. Preparing it covers everything between the formulation and a usable plate, tube or flask: obtaining the ingredients, dissolving, sterilising, dispensing, storing, and showing that the finished medium performs as intended.

The ISO technical committee for food microbiology (ISO/TC 34/SC 9) describes ISO 11133 as the standard on how culture media are prepared, produced, stored and performance-tested, together with performance testing of reagents, for food and water microbiology; some media and reagents from the milk and milk products committee are also included. Its stated aim is to make media “fit for purpose” by defining the minimum acceptance criteria needed to achieve consistently reliable microbiological results. The control strains for performance testing are specified in Annexes E, F and K. The current edition is ISO 11133:2014, with Amendment 1 (2018) correcting the text and Amendment 2 (2020) addressing confirmation media and reagents. Where a newer standard covers performance testing, it takes precedence. (Source: ISO/TC 34/SC 9, checked 1 November 2026.)

How a published specification reads: the FDA Plate Count Agar example

The FDA Bacteriological Analytical Manual (BAM) medium M124, Plate Count Agar (Standard Methods), shows what a specified preparation looks like. Per litre it lists tryptone 5 g, yeast extract 2.5 g, dextrose 1 g, agar 15 g and distilled water 1 L. The page states that the ingredients are heated to dissolve, dispensed into suitable tubes or flasks and autoclaved for 15 minutes at 121 °C, with a final pH of 7.0 ± 0.2. (Source: FDA BAM M124, content current as of 13 November 2017, checked 1 November 2026.)

Three points follow from that one specification. First, the formula, the sterilisation condition and the pH target are separate, checkable items. Second, the page states a final pH tolerance, so pH is part of the preparation record. Third, the page contains no quality-control statement: it does not say how to show that the finished plates support growth or stay sterile. That is the gap the standards below fill.

For the sterilisation step itself, see what an autoclave is and how autoclave cycles are checked with biological and chemical indicators. For worked formulas of common media, see tryptic soy broth and LB agar composition; for solutions that are not growth media, see buffer and solution preparation.

Who is responsible for quality control: what each source covers

Three sources a laboratory is likely to meet answer different questions, and together they show where preparation instructions end and quality control begins.

Source	What it covers	Limits to note
FDA BAM M124 Plate Count Agar	Formula per litre, heating to dissolve, autoclave 15 min at 121 °C, final pH 7.0 ± 0.2	No quality-control statement on the page
ISO 11133	Preparation, production, storage and performance testing of media and reagents; minimum acceptance criteria; control strains in Annexes E, F and K	Scoped to food and water microbiology, with some dairy media included
CLSI M22-A3 (third edition, 20 June 2004, archived)	Quality control of commercially prepared media, regardless of packaging; guidance for users and manufacturers	No longer applies to US laboratories under CLIA, though it remains applicable to international laboratories

CLSI M22-A3 incorporated data from three College of American Pathologists surveys, and the third survey (2001) led to the addition of 27 exempt media. Because the standard no longer applies to US laboratories under CLIA, and CMS has required an individualized quality control plan (IQCP) for applicable media or compliance with CLIA quality assurance guidelines since January 2016, a US laboratory should treat M22-A3 as historical context and rely on its current accreditation and method requirements. (Source: CLSI M22, checked 1 November 2026.)

Where it matters for labs, clinics and supply decisions

Whether a laboratory prepares media itself or receives them prepared changes who carries the quality-control record. CLSI M22 is written for both users and manufacturers of commercially prepared media, which is why the incoming-media record is a supply-chain question as well as a bench question. ISO 11133 sets minimum acceptance criteria for media that are prepared, produced, stored and tested, so a laboratory that makes its own medium works to the same performance expectations as a producer.

A practical way to use this: for each medium in use, write down which of the ISO 11133 activities (preparation, production, storage, performance testing) the laboratory performs and which the supplier performs, and keep the corresponding record. Which records an accreditation body or method requires is set by that body and method, so check them directly.

Agar plate pouring in context

Agar plate pouring is the dispensing step in which sterilised agar medium goes into sterile dishes and sets. This page does not give a volume, temperature or technique for it; those belong to the method in use. For plate types, media classes and storage, see what an agar plate is and what a Petri dish is. For how plates are then used, see culturing microorganisms and streak plate versus spread plate.

Common misunderstandings

A published recipe is a quality-controlled medium

The FDA M124 entry specifies the medium’s formula, sterilisation condition and pH, but contains no quality-control statement. Showing that a batch is fit for purpose is a separate step governed by the laboratory’s standard, such as ISO 11133.

Autoclaving at 121 °C for 15 minutes is the same for every medium

That condition is what BAM M124 states for Plate Count Agar. Other media have their own specifications, so use the one in the method or the manufacturer’s instructions for the medium in hand.

CLSI M22 is the current requirement everywhere

The third edition is archived. It remains applicable to international laboratories, but it no longer applies to US laboratories under CLIA.

ISO 11133 applies to every kind of laboratory

ISO describes it for food and water microbiology, with some dairy media included. A clinical or research laboratory should confirm which standard its method or accreditation body names.

FAQ

What is culture media preparation?

It is making a defined culture medium ready for use: obtaining and dissolving the ingredients, sterilising, dispensing, storing, and checking that the medium performs. ISO 11133 describes the preparation, production, storage and performance testing of culture media.

What does ISO 11133 cover?

ISO 11133:2014 covers the preparation, production, storage and performance testing of culture media, plus reagent performance testing, for food and water microbiology. Amendment 1 (2018) corrected the text and Amendment 2 (2020) added confirmation media and reagents.

What sterilisation conditions does FDA give for Plate Count Agar?

BAM M124 says to autoclave for 15 minutes at 121 °C, with a final pH of 7.0 ± 0.2. That applies to that medium only.

Does CLSI M22 still apply to US laboratories?

No. M22-A3 (2004) is archived and no longer applies to US laboratories under CLIA, though it remains applicable to international laboratories. Follow the requirements of your accreditation body and method.