Good laboratory practice is essential to ensure that:
1 The organisms isolated from a food sample originated from that sample.
2 The organisms in the food do not contaminate the environment or other samples.
3 The organism counts obtained truly reflect the organism levels in the food.
4 The techniques used in the laboratory are reproducible between operators in the same laboratory and repeatable in other laboratories.
In order to achieve this:
(a) Environmental contamination should be minimized. This may require a filter ventilation system for incoming air or the use of a clean-air laminar flow cabinet.
(b) A strict regime for cleaning and disinfection of surfaces and equipment should be in operation.
(c) Staff should be well trained in aseptic technique.
(d) Equipment should meet the specifications shown below [3,5,15]:
• Balances for weighing samples should be capable of weighing to 0.1 g or less.
• Accuracy for addition of diluent to the sample during preparation of the homogenate should be ±5% of the target volume or weight.
• Accuracy of volumes for dilution fluid used for preparation of decimal dilutions should be ±2% of the target volume.
• Accuracy of sample volumes used in preparation of decimal dilutions and inoculation of media should be ±5% of the target volume. In order to achieve this accuracy the use of fixed volume calibrated displacement pipettors and sterile disposable tips should be considered.
• pH meters should be capable of measuring to an accuracy of ±0.1 pH units with a minimum measurement threshold of 0.01 pH units.
• Incubators and water baths should be capable of maintaining a stable temperature which is evenly distributed to within ±1°C for incubators and within ±0.5°C for water baths.
• The accuracy of temperature measurement should be four times greater than the requested accuracy; for example for a requested accuracy of ±2°C, the measurement accuracy should be ±0.5°C.
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• Loading of incubators should ensure adequate air circulation. Plates, tubes and bottles should not be in contact with the sides of the incubator.
Bottles, tubes and stacks of plates should not be in contact with one an-other and plates should not be stacked more than six high.
Laboratory accreditation
One of the essential criteria for many laboratories is accreditation by an external body to a recognized standard. This may be to an internationally recognized standard such as ISO 17025 [16] or to an industry-based standard. In order to achieve accreditation the laboratory must demonstrate that it is working to the requirements of a documented quality system. A key element of accreditation is the demonstration of full traceability of results, which is also a prerequisite for samples that are likely to proceed to legal action. Testing should only be under-taken by appropriately trained staff or under their direct supervision, and full training records are necessary. In order to demonstrate competency the labora-tory will need to undertake a programme of internal quality assurance testing and participate in an external scheme for proficiency testing. Such a scheme is available from the PHLS.
Food external quality assurance (EQA) schemes run by the PHLS are:
• The Standard Scheme — suitable for European official laboratories and all other laboratories offering examination for pathogens and microbial enumerations.
• The Extended Scheme — suitable for public health and other laboratories offering a wide range of examinations.
• The Shellfish Scheme — suitable for laboratories offering examination of raw bivalve molluscs for end product testing or classification of shellfish harvesting beds.
• The Dairy Scheme — suitable for all laboratories offering examination of dairy products (pathogen-free option available).
• The Non-Pathogen Scheme — suitable for all laboratories offering tests for aerobic colony counts, indicator and spoilage organisms that prefer not to introduce pathogens onto their premises.
• The Flexible Schedule — suitable for laboratories that require a limited number of samples which may be chosen from more than one of the above EQA schemes.
Further information is available from: PHLS Food EQA Schemes, Food Safety Microbiology Laboratory, PHLS Central Public Health Laboratory, 61 Colindale Avenue, London NW9 5HT. Tel: 0208 2004400; Fax: 0208 2008264; E-mail:
References
1 Food Standards Agency. Food Safety Act 1990. Code of Practice No. 7: Sampling for Analysis or Examination. London: Food Standards Agency, 2000.
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2 Local Authorities Coordinating Body on Food and Trading Standards (LACOTS).
Guidance on Food Sampling for Microbiological Examination. London: LACOTS, 2002.
3 ISO 7218 (BS 5763 Part 0). Microbiology of Food and Animal Feeding Stuffs — General Rules for Microbiological Examinations. Geneva: International Organization for Standardization (ISO), 1996.
4 Great Britain. Statutory Instrument No. 2463. The Food Safety (Sampling and Qualifica-tions) Regulations 1990. London: HMSO, 1990.
5 BS EN ISO 6887-1. Microbiology of Food and Animal Feeding Stuffs — Preparation of Test Samples, Initial Suspension and Decimal Dilutions for Microbiological Examina-tion. Part 1: General Rules for the Preparation of the Initial Suspension and Decimal Dilutions. Geneva: International Organization for Standardization (ISO), 1999.
6 ISO/CD 6887-2. Part 2. Specific rules for the preparation of the initial suspension and decimal dilutions of meat and meat products. In preparation.
7 ISO/CD 6887-3. Part 3. Specific rules for the preparation of the initial suspension and decimal dilutions of fish products. In preparation.
8 ISO/CD 6887-4. Part 4. Specific rules for the preparation of the initial suspension and decimal dilutions of products other than milk and milk products, meat and meat products, fish products. . In preparation.
9 ISO 8261. Milk and Milk Products — Preparation of Test Samples and Dilutions for Mi-crobiological Examination. Geneva: International Organization for Standardization (ISO), 2001.
10 Department of Health. Guidelines for the Safe Production of Heat Preserved Foods.
London: HMSO, 1994.
11 Rees JAG, Bettison J, eds. Processing and Packaging of Heat Preserved Foods. London:
Blackie and Son Ltd, 1990.
12 Footitt RJ, Lewis AS. The Canning of Fish and Meat. Glasgow: Blackie Academic and Professional, 1994.
13 ISO 13369. Microbiology of food and animal feeding-stuffs — Horizontal method for the determination of water activity. Geneva: International Organization for Stan-dardization (ISO), 2000.
14 Troller JA, Scott VN. Measurement of water activity (aw) and acidity. In: Vanderzant C, Splittstoesser DF, eds. Compendium of Methods for the Microbiological Examination of Foods, 3rd edn. Washington, D.C.: American Public Health Association, 1992.
15 Peterz MEG. Temperature in agar plates and its influence on the results of quantitative microbiological food analyses. Int J Food Microbiol 1991; 14: 59–66.
16 ISO/IEC 17025. General Requirements for the Competence of Testing and Calibra-tion Laboratories. Geneva: InternaCalibra-tional OrganizaCalibra-tion for StandardizaCalibra-tion (ISO), 1999.