Discussion
Most Probable Number test
The results presented are consistent with a high level of coliform contamination. Coliform bacteria are characterised by being rod-shaped, Gram-negative, non-spore-forming and able to ferment lactose. The presumptive test indicates the presence and high concentration of lactofermenting bacteria. The confirmed test confirms that the bacteria present are gram-negative and lactofermenting. Together these tests are strongly indicative of high levels of coliform bacteria.
Membrane filtration method
The mFC agar used is selective for gram-negative coliform bacteria and pH indicative with lactofermenting bacteria becoming blue. The presence of both blue and non-blue colonies indicates the presence of both lactofermenting and non-lactofermenting colonies. This could either be due to the presence of both lactofermenting (coliform) and non-lactofermenting (non-coliform) bacteria on the plate or the presence of a facultatively-anaerobic species under both anaerobic and aerobic micro-conditions. This would be consistent with the API test results that indicate the presence of facultatively-anaerobic, coliform genus Entereobacter. An API test and comparison of a blue and a pink/grey colony could be used to test this hypothesis.
Furthermore the MPN coliform count of >1100cells/100mL is most consistent with interpreting all colonies as coliform (~39400 cells/100mL) than only blue colonies being coliform (20 cells/100mL).
Uncertainty in the identity of the grey/pink colonies mean the calculated coliform count is not conclusive.
API-test
While not conclusive, the API test strongly indicates the presence of either Ent. cloacae, Ent. amnigenus. Both of the genus Enterobacter, and as such coliform bacteria. The test also indicated the possible but unlikely presence of or C. sakazakii, a pathenogenic species.
Water quality conclusions
Based on the results of this lab it is concluded that the water sample in question is likely heavily contaminated with coliform bacteria at a concentration that exceeds 1100 cells/100mL, possibly of the genus Enterobacter. The contamination is supported by all tests performed, however while the membrane filtration method does indicate contamination, the level of contamination can not be concluded from the results due to uncertainty in the interpretation of the results. The MPN method is the most conclusive of the tests performed.
Bürker chamber
The calculated cellular concentration of \(6.5\cdot10^6\) cells/100mL differs from the reference value of \(3\cdot10^7\)by a factor of 4.8. Even the estimated max value of \(9.5\cdot10^6\) differs by a factor of 3.2. Possible reasons for the discrepancy may be due to chance and a small sample size, that the provided sample was not shaken until uniform concentration was reached prior to dilution or error in the provided reference value.