Enumeration of Coliforms in Food (Essay Sample)

Enumeration of Coliforms in Food
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PART 1
The Purpose of the Most – Probable – Number (MPN) in Enumeration of Organisms
The most probable number (MPN) technique is valuable for the evaluation of quantitative microbial burden if inoculation for colony forming is not specified. MPN is applied in the estimation of the concentration of viable microbes in a medium by a way of replicate liquid growth media in 10 fold series dilutions. The method is valuable when evaluating the concentration of viable microorganisms when plate counting methods is not possible due to the interference of particulate matter in a given sample (Sutton, 2010).
Given replicate dilutions one tube may receive a sample containing viable cells and thus turn turbid while its replicate may not get any bacterial cell in its sample. This is an indicator of the initial concentration of bacterial cells in the sample. This is the basic concept behind the MPN which relies on presence of growth versus no growth principle. Accuracy can be optimized by diluting the inoculums followed by a comparison of recoveries in all tubes along the dilution series (Sutton, 2010).
MPN assumes that microorganisms distribute themselves randomly in a sample and that dilutions are acutely achieved. This is not always the case since some organisms exist in colonies in broth and pipettes may sometimes deliver inaccurate volumes (Sutton, 2010).
Advantages of MPN
This list is obtained from Janalta Interactive Inc., 2015.
1 The MPN method provides an ease for interpretation by use of observatory means or by gas emission
2 The toxins present in the sample are diluted
3 The method is effective when analyzing several samples for example sediments, mud among others.
Disadvantages of MPN
This list is obtained from Janalta Interactive Inc., 2015.
1 Results take a long time to come by
2 Results obtained may be so accurate
3 The method demands for provision of more glassware and media requirements
4 There are high chances for occurrences of false positives.
PART 2
Eosin Methylene Blue agar (EMB agar) is composed of two dyes namely eosin and methylene blue.  The medium inhibits the growth of Gram-positive organisms.  However, it is selective for the growth of Gram-negative species. The media is only selective for Gram negative coliforms due to the presence of bile salts which kills other bacteria which are not coliforms (Madigan, Martinko & Parker, 2006).
E. coli and other lactose-fermenting organisms generate a black precipitate when grown on an EMB medium.  These colonies are either black or contain dark centers while their peripheries are transparent or colorless.  Non-lactose fermenting bacteria for instance Proteus sp., Shigella sp. and Salmonella sp. among others materialize as pink or uncolored colonies.  In that case EMB is considered to be a differential media as far as lactose fermentation is concerned. In general EBM is both selective as well as a differential medium for coliforms (Madigan et al., 2006).
The primary use of EMB is to distinguish between lactose fermenting bacteria form non- lactose fermenters. It gives a color indicator to help one distinguish between the two groups of organisms. It is applied in a medical laboratory to help characterize different kinds of pathogenic bacteria (Madigan et al., 2006).
PART 3
Chromogenic agars have ability to selectively and identify microorganisms by specifically changing their colors. Their color change property is useful for the morphological identification of a specific species. They contain different chemicals and infusions. Chromogenic media are utilized by microbiologists in order to isolate and identify specific species of pathogenic bacteria (Long, 2010).
Hospitals and clinical facilities utilizes chromogenic agar in order to immediately and positively identify pathogenic organism from their patients as necessary. Specialized chromogenic agar is utilized for this purpose for example the MRSA identification agar for the Methicillin –resistant – Staphylococcus aureus. MRSA causes toxic shock syndrome which is difficult to diagnose due to the fact that other bacteria usually contaminates the media. But specialized media is definite for the diagnosis of a certain species (Long, 2010). .
Chromogenic media are useful in the fact that it leaves no doubt about the absence or the presence of a certain bacteria in question. They are also useful in the inhibition of the growth of undesired microorganism. It is utilized in the isolation along with the identification of Gram – negative bacteria of medical importance such as Neisseria spp (Long, 2010).
PART 4
Fecal coliforms are good indicators of fecal contamination....