By Bryce Wilmott
AIM: To calculate the quantity of observable microbial colonies on the surface of the Agar solid, as to determine the presence of microbes in consumable products i.e. yoghurt and blue vein cheese.
HYPOTHESIS: Microbial growth will be present in two of the three Agar plates (those containing the food product) due to the suspected presence of microbes, whilst the control Agar plate (containing no food products) will remain free of contamination and microbial growth.
MATERIALS:- 3x Agar plates
- 10g of berry yoghurt
- 10g of blue vein cheese
- 2 pairs of sterile gloves
- 3 zipper lock bags
- Miscellaneous items (tape, marker, label)
- 1 sterile cotton swab
METHOD:1. Prior to placing any food products into the petri dish, categorize them into the control (agar plate containing no food products) and the experimental (agar plates containing food products) via labelling. 2. Leave petri dishes inverted for a time period of 48 hours as to provide a clear illustration as to the absence of microbes and forms of contamination on the agar surface. 3. Before handling the petri dishes, equip 1 pair of sterile gloves using the sterile glove technique [figure 1]*, as to prevent the transference of epidermal microbes to the petri dish and agar plate. 4. Remove the lid of a petri dish (experimental) whilst still inverted as to reduce the likelihood of air borne microbes contaminating the agar by the gravitational settling of particles. Proceed to place either the 10g of blue vein cheese onto the lid (allowing enough to be present so the blue vein cheese touches the surface of the agar once the petri dish is closed) or 10 of yoghurt onto the surface of the agar via the sterile cotton swab.
5. Once the food product is placed into the petri dish, proceed to close the petri dish and seal the edges with tape. Dispose of sterile gloves in appropriate waste receptacle. Repeat steps 3, 4 and 5 once more so both experimental petri dishes contain food products. 6. Proceed to seal the edges of the control petri dish with tape. Place all petri dishes into individual zip lock bags as a precaution and barrier to any microbes produced by the agar microbial growth. 7. Place petri dishes (right side up) into a cold (10-15C) and dark location (minimal light exposure), which they are left in for a time period of 7 days. 8. Observe microbial growth over the waiting time period and record the quantity of bacterium and fungi at the end of the 7 day waiting period. 9. Repeat experiment to a minimum of 5 times as to acquire accurate, valid and reliable results
Experiment TypeQuantity of Colonies
Control0 fungi / 0 bacterium
Experimental No. 1 Blue Vein Cheese 0 fungi / 5 bacterium
Experimental No. 2 Activia yoghurt 4 fungi / 12 bacterium
Average Colony Quantity11/3 Fungi - 52/3 bacterium - 7 Colonies RESULTS:
DISCUSSION & CONCLUSION: This experiment was constituted by three (3) agar plates, two (2) being experimental and one (1) being a control. The aim, in conjunction with the hypothesis, described the supposed presence of microbes in consumable products, primarily in the experimental petri dishes, and the quantity of microbial colonies that have grown on the agar plate’s resultant of the food products. The results elucidated that all experimental agar plates containing food products experienced microbial growth, whilst the control agar plate experienced no microbial growth. Graphically, bacterial and fungal growth in experimental petri dish No.1 (containing blue cheese) occurred more readily than that of experimental petri dish No. 2 (containing Activia yoghurt). Bacterial growth in experimental petri dish No.1 was closely focused around the proximal vicinity of the blue vein cheese, consisting of small colonies. Fungi growth however was primarily present surrounding the bacteria or close to the petri dish wall, consisting of large furry colonies. Signs of fungal and bacterial growth in experimental petri dish No.1 occurred readily, approx. 24 hours after exposure to food product. Experimental petri dish No.2 experienced minimal bacterial growth, but to the exclusion of any fungal growth. All bacterial colonies were situated on the perimeter of the yoghurt, consisting of small lightly coloured colonies. Signs of bacterial growth were not present until approx. 72 hours. This rate of growth corresponds with the growth curve [Figure 2]* of bacterial and fungal organisms. Bacterial and fungal growth location is likely determined by the ability of the fungi to create reproductive spores that can travel long distances, whereas the bacteria lack this capability.
This graph details the growth tenancies of microbial growth, consisting of 4 phases; lag phase, exponential phase, stationary phase and death phase. The initial lag phase is where the microbes (respectively: bifidobacterium – yoghurt, brevibacterium linens & Penicillium – blue vein cheese) gather transient metals (iron, etc.) which are essential for protein production and other nutrients from their ambient environment resulting in cell enlargement. This gathering of nutrients and transient metals is a microbial mechanism by which effective and efficient binary fission can be undertaken. The exponential phase is where microbial cells begin binary fission and begin cell multiplication. This phase is additionally characterised by increased cell metabolism. The stationary phase involves minimal cell division due to the accumulation of waste products, toxic metabolites and cellular inhibitors such as antibiotics, and nutrient exhaustion creating an unfavourable environment. The death phase is characterised by cell death.
Experimental petri dish No.1 in comparison to experimental petri dish No.2 was higher in transient metals such as iron, possibly resulting in the increased bacterial growth found in the experimental petri dish No.1. Experimental petri dish No.2 showed a delayed growth rate in comparison to experimental petri dish No.1, this may be due to the inclusion of spoiling inhibitors such as pectin and locus bean gum. In the ingredients it is stated that there has been an addition of citric acid. This addition of citric acid to the yoghurt mixture causes the pH to decrease; this in turn would create an unsuitable environment for the microbes in experimental petri dish No.2. Temperature as a constant variable in this experiment was within the range of 10-15C. With prior knowledge to the optimum temperature of microbe growth but with the lack of resources to maintain the temperature of the samples I was unable to create the optimum environment for the growth of the microbes. This factor however did not impede upon the experiment due to the aim and hypothesis stating to quantitatively count the observable grown colonies. I would however alter the quantity of observations made in regards to the experiment. Whilst conducting this experiment there were colonies that began growing separately but eventually combined and became one (1) singular colony. These initial colonies were not recorded in the results; this negatively affects the accuracy of the experiment. Due to the disassociation of these initial colonies one might come to the incorrect assumption to the correct total number of fungal and bacterial colonies. The average of the results would then be correspondingly negatively affected. Repetition is an additional criterion for a reliable experiment. Experiments should be carried out and repeated a minimum of five (5) times. Microbes are found readily in the internal and external environments of biotic and abiotic structures. These fundamental organisms are essential in the decomposition of debris and tissues, and the growth of self and other organisms. Microbes have been proven to exist within and on the surface of food via the comparison of the experimental agar plates and the control agar plate. The aim and hypothesis of this experiment were proven correct by the presence of sufficient microbe colonies within the experimental group which contained food products, and in addition to the absence of microbial growth in the control group.
http://www.hsc.csu.edu.au/biology/core/better_health/9_4_1/941net.html http://archive.food.gov.uk/hea/teachers/plainenglish/part2.html http://amrita.vlab.co.in/?sub=3&brch=73&sim=1105&cnt=1
http://www.scribd.com/doc/19486853/Scientific-report-discussion-Examples http://apjcn.nhri.org.tw/server/info/books-phds/books/foodfacts/html/data/data5e.html http://www.mycotoxins.info/myco_info/field_funggrwth.html