• Can you think of any advantage of the MPN technique over the MF technique?
  • The exercise as performed gave you the standard total coliform count. What otherscounts can be obtained by the MF technique?
  • An egg salad sandwich was left in a warm car for 4 hours and it was found that 40,960bacterial cells were developed. How many more bacterial cells would result withadditional 4 hours of incubation? Therefore, what do your answers tell you about usingclean techniques in food preparation and storage?


  1. Can you think of any advantage of the MPN technique over the MF technique?

MPN method estimates the number of viable microorganisms in a sample based on the principle that when the degree of dilution is high enough, the culture has been diluted so much that there are many subsamples in which no growth could occur. Membrane Filtration is currently the most accurate method to count bacteria. However, the advantages of MPN technique over MF technique include that the process is simple to understand and easy to be applied, it is relatively inexpensive as the equipment involves are glassware compared to MF technique that need additional or special equipment for filtration, MPN test can use turbid sample which is impossible in MF method as turbid water interferes with the counting of coliforms and less labour intensive is needed in MPN compared to MF method (Sharath H.N., 2016). 

  • The exercise as performed gave you the standard total coliform count. What others counts can be obtained by the MF technique?

The term “total coliforms” refers to a large group of Gram-negative, rod-shaped bacteria that share several characteristics. The group includes some bacteria that may be isolated from environmental sources such as water (Jorgensen, 1979). Another count under MF technique is heterotrophic plate count includes all of the micro-organisms that are capable of growing in or on a nutrient-rich solid agar medium. Different procedure including a few factors such as incubation and temperature to enumerate bacteria that are derived principally from environmental sources. Next, thermo-tolerant or known as faecal coliforms count, being used in water microbiology to denote coliform organisms which grow at certain temperatures and ferment lactose to produce acid and gas. In practice, some organisms with these characteristics may not be of faecal origin and the term “thermo-tolerant coliform count” is, therefore, more correct and is becoming more commonly used. While for faecal streptococci in the presence of streptococci form faeces for evidence in faecal contamination. Faecal streptococci tend to persist longer in the environment than thermo-tolerant or total coliforms and are highly resistant to drying. It is, therefore, possible to isolate faecal streptococci from water that contains few or no thermo-tolerant coliforms as, for example, when the source of contamination is distant in either time or space from the sampling point. Faecal streptococci will grow in or on a medium containing certain chemical and specific temperature and they are usually being detected by the reduction of a dye (Balance, 1996).

  • An egg salad sandwich was left in a warm car for 4 hours and it was found that 40,960 bacterial cells were developed. How many more bacterial cells would result with additional 4 hours of incubation? Therefore, what do your answers tell you about using clean techniques in food preparation and storage?

In order to accurately calculate the resulting bacterial cells in the egg salad when it has been left for an additional 4 hours of incubation, 3 information are required which are the generation time of bacteria, initial number of bacteria, and the number of generation of bacteria, n. 

However, since only the incubation time and total number of bacteria are provided, assumptions of the generation time of bacteria and initial number of bacteria are made to obtain the number of generation of bacteria.

It is assumed that the initial number of bacteria is 10 and the generation time of bacteria is 20 minutes for the said 10 bacterial cells. Therefore:

????? ?????? ?? ???????? ??????? ????? ? ???????????

= 2? × ??????? ?????? ?? ????????

????? ?????? ?? ???????? ??????? ????? ? ??????????? = 212 × 40960

????? ?????? ?? ???????? ??????? ????? ? ??????????? = 167,772,160

Based on the total number of bacteria after a total of 24 generations of bacteria in the span of 8 hours of incubations which is 167,772,160, it can be observed how numerous the number of bacteria found on the egg salad after being left for a considerably long duration with a temperature optimal for bacterial growth. 

The contamination of bacteria in food can be prevented through the observation and practice of clean techniques in food preparations and storage. This is due to the fact that pathogenic bacteria can be found in various places without being observable under the naked eye such as our hands, kitchen utensils, and food packaging. To avoid these bacteria from contaminating food during food preparation, it is vital to clean items that will be in contact with the food prepared. Washing hands and surfaces often and properly helps in eliminating harmful bacteria from causing foodborne illness. It should also be noted that all prepared ingredients should be cleaned thoroughly as a method of prevention from contamination when preparing food.

When storing food, it is important to keep food in the refrigerator where the temperature is low enough to prevent the growth of bacteria; thus, keeping food safe from contamination compared to leaving food out in the open with room temperature. Keeping food covered and not exposed also factors in preventing food from being contaminated as it prevents pathogenic bacteria from gaining excess into the food when there is no part of the food exposed to the open. Besides that, keeping food covered also ensures that vectors such as flies do not land on this food and transmit pathogenic bacteria on this food.


In conclusion, it was found that the four sources of water had different levels of contamination as indicated in the colony count, coliform count, and colony formation unit in the three techniques used to enumerate viable counts of microorganisms which are Membrane Filter (MF), lawn culture method, and pour plate method. The sequence of water sources from the cleanest to the most contaminated was hot reverse osmosis, cold reverse osmosis, boiled, filtered, and then followed by unfiltered tap water. The MF method appears to provide a better yield in terms of results to enumerate viable microorganisms as compared to lawn culture and pour plate method because it utilizes 100mL of sample water compared to the latter two methods which utilize only 1mL of sample water. Hence, the MF method is a more effective method of conducting a standard analysis of water to quantify a sample water’s quality for drinking as compared to the lawn culture and pour plate method which are more suitable for analyzing small samples of water for experimental purposes, not standard analysis purposes.


Bennett, S. (2017). The Hidden Dangers of Drinking Water [Online]. Available at: [Accessed 5 March 2017]

Nabaz, H. (2013). Water microbiology [Online]. Available at: [Accessed 26  Feb. 2017]..

Sharath, Hns. (2016). Bacteriological and Chemical Analysis of Water [Online].  Available at: analysischemical-analysis-of-watersolid-phase-extraction [Accessed 26 Feb. 2017].

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