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Chapter 8*: SANITARY INSPECTION AND MICROBIOLOGICAL WATER QUALITY - page 16 / 52

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positive and negative tubes in the confirmatory phase of the technique is generally used to determine the MPN of the target bacteria by using tables of positive and negative tube reactions (WHO/UNEP, 1994a; APHA/AWWA/WPCF, 1995).

The major advantages of the MPN technique are (Fujioka, 1997):

  • It will accept both clear and turbid samples.

  • It inherently allows the resuscitation and growth of injured bacteria.

  • The results may be recorded by personnel with minimal skill.

  • Minimal preparation time and effort are required to start the test, and therefore

processing of samples can be initiated at any time of the day.

By contrast the MPN technique may also have several disadvantages, such as:

  • The total time, labour, material and costs required to analyse one sample.

  • The substantial increase in reagents, tubes, incubation space and cleanup

requirements when multiple samples need to be analysed or when the sample volume must be increased to 100 ml.

  • The multiphase nature of the technique, each phase requiring a 24 hour or 48 hour

incubation period.

  • The fact that MPN is a simple estimated number, while the true number (95 per cent

confidence limit) may show extreme variation from the MPN.

The choice of precision level of the technique (using 3, 5 or 10 tubes of each dilution) depends on the required detection sensitivity, because the total volumes analysed by each are 33.3, 55.5, and 111 ml, respectively. Miniaturised MPN methods with 96 incubation wells (e.g. ISO 1996a,b) are more precise than traditional five-tube tests with three descending decimal dilutions and equivalent to membrane filtration (Hernandez et al., 1991, 1995). The existing standardised procedures for the MPN technique are given in Tables 8.3 and 8.4.

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