MLSB is the medium of choice in the UK and Europe for the isolation and enumeration of coliform bacteria, and is the medium that we are most familiar with using. However, it is not the only medium that can be used with the Delagua kit. Any of the broth media for the isolation and enumeration of total coliforms, thermotolerant coliforms or E.coli by the membrane filtration method can be used with the kit. Before using a different type of medium, check the manufacturer's instruction for preparation and use of the medium, and interpretation of the results. The instructions provided in the users' manual will not apply to these other types of media.
In 2001 we changed the type of battery charger that is supplied with the kit. The two types of battery charger are easy to distinguish from each other: the old battery charger is light grey, large, heavy and has two LEDs; the new charger is black, small, light and has a single LED.
There are some important differences between the operation of the two chargers that you should be aware of:
Thermotolerant coliforms produce yellow colonies when grown on Membrane Lauryl Sulphate Broth (MLSB). The yellow colour is the result of a pH change in the medium that is caused by the synthesis of acids during the growth of the bacteria. It is important to note that the yellow colour will fade and change on cooling and standing; thus, colonies should be counted WITHIN 15 MINUTES of being removed from the incubator. The picture below shows the appearance of thermotolerant coliforms on MLSB.
The recommendations for counting given in the text books and standard methods are that you should count all yellow colonies, however faint, and irrespective of size. In practice, most thermotolerant coliform colonies will be between 2 and 3 millimetres in diameter. Do not count colonies of any other colour; however, you should make a note on your results sheet if the petri dish is covered with a large number (greater than 100) of pink colonies. The growth of these non-coliform bacteria can obscure the growth of the coliform bacteria.
This description applies to the growth of thermotolerant coliforms on MLSB only. If you are using any other type of growth medium, consult the manufacturer's instruction sheet for a description of the characteristics of positive colonies.
All orders are normally despatched within 1-3 working days providing all items are available.
Sterilising the equipment in the field presents some practical difficulties and must be carried out using simple methods. The most appropriate is the use of methanol, which is described in the manual. We appreciate, however, that methanol may not be readily available locally, and the cost of shipping from the UK can be prohibitively expensive. Furthermore, for some countries existing trade embargoes may forbid the export of methanol. Where these restrictions apply, alternative methods for disinfecting the filtration apparatus may be used. But please note, we would strongly recommend reverting to the use of methanol as soon as supplies become available.
The filter funnel, the filter head (the aluminium block that holds the bronze filter support disc) and the sample cup can be disinfected by immersion in boiling water. Leave the equipment immersed for at least three minutes; ideally 10 minutes. Remove the equipment from the water using forceps, or some other instrument, to avoid scalding yourself. Do not use a pressure cooker, steam steriliser or autoclave to sterilise the equipment. Both the filter head and the filter funnel will be damaged at sterilisation temperatures.
If it is not practical to boil water while you are in the field, you can significantly reduce the risk of cross-contamination between samples by using a clean tissue to dry the inside of the filter funnel after the sample has been filtered. Rinse the inside of the filter funnel with the next sample before assembling the filtration apparatus. The bronze support disc is protected from contamination by the filter membrane and, provided that it is not accidentally contaminated during use, the risk of cross-contamination from the disc is low. The same procedure can be used to clean the sample cup. However, we would recommend that at the end of each day's use the equipment is disinfected by immersion in boiling water for 10 minutes before storage.
We would welcome any feedback about your experiences with these methods, or suggestions that you may have for alternative disinfection procedures that we can share with other users. Please use the feedback form on this web site.
Observation
When trying to calibrate the 44°C incubator, the temperature rises to approximately 49°C and then suddenly falls to approximately 37°C before once again rising to 49°C. If the kit is left switched on, the incubator continues to cycle between these two temperatures.
Fault
The incubator is fitted with a thermal cut-out switch that prevents the temperature rising above 50°C. This safety switch is inactivated once the temperature falls below 35°C. However, because the switch is connected to the outside of the incubator, the switch heats up and cools slightly faster than the core temperature.
The apparent fault is caused by the incubator temperature having been accidentally adjusted to above 50°C.
Remedy
Reduce the temperature of the incubator by turning the adjuster screw two turns in a clockwise direction. This should reduce the temperature of the incubator by approximately 8°C. Switch on the incubator and check the temperature after one hour (this is sufficient time for the incubator temperature to stabilise). If the temperature is still cycling between 49°C and 37°C, turn the temperature adjuster screw one more turn in a clockwise direction.
Once the incubator temperature has stabilised, follow the procedure for recalibrating the temperature. The incubator calibration section of the manual can be downloaded and viewed as a PDF file using the link in the right-hand column.
The simple answer is no. Methanol is the only alcohol that produces formaldehyde gas as a by-product of combustion in a low oxygen atmosphere. Neither ethanol nor methylated spirit can be used to give the same result.
If you do not have a supply of methanol, please refer to the question about alternative disinfection strategies for advice on how to proceed.
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