In this lab practical the students will analyze samples of water from the sea, a well, a pond and also a sample of drinking water.

A. Total aerobic bacteria.
To count the total aerobic bacteria, 0.1 ml will be inoculated into 2 nutrient agar plates and they will be incubated at 22ºC and 37ºC.

B. Total coliform bacteria E.coli, enterococcus, Clostridium perfringens and Pseudomonas.
Fractions of 100 ml of each water sample will be filtered through a membrane filter of  0.45 microns in pore size and the filter will be inoculated in plates containing:

1. MacConkey agar to determine the total coliform bacteria (red colonies).
2. Levine agar to count the E. coli (shiny metallic green colonies).
3. KAA agar to count the enterococcus (colonies surrounded by the black halos).
4. Perfringens agar to count the Clostridium perfringes (black colonies).
5. Cetrimide agar to count Pseudomonas (green colonies)
6. MSA agar to count Staphylococcus aureus (yellow colonies)

The microorganisms in a sample of water are determined by means of the membrane filtration technique (see video) that is carried out in many laboratories, because it differentiates water that is suitable and unsuitable for consumption from a bacteriological point of view in the fastest, cheapest and simplest way by using the most probable number technique. It can be used in the analysis of water with low contamination levels. A known volume of water is filtered (e.g. 100 ml) thorough a sterile membrane of  0.45 mm. This membrane is put on a selective culture medium plate (different according to the type of microorganism to be detected) and it is normally incubated for  24 hr. at 37ºC.

C. Filamentous fungi and yeast.

The filamentous fungi and yeasts will be determined by inoculating a Sabouraud culture medium with chloramphenicol, either using the filtration technique (when it is suspected that there is a small amount of fungi), or by diluting the homogenate and by inoculating aliquots of each one of them (when it is suspected that there is a high number).

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