Abstract: This laboratory exercise demonstrates the effect of garlic and orange extracts on the growth of Escherichia coli, Enterobacter aerogenes and Bacillus cereus.

Introduction: Various spices have been used throughout history as an alternative approach to the preservation and enhancement of foods and for medicinal purposes. Garlic contains several active chemical ingredients, which have an antimicrobial property. Orange rind extract contains a chemical called citronella, which is used to repel insects. These two spices were used arbitrarily to test their effect on three species of bacteria.

Background: E. coli is found in the lower part of the intestine of warm-blooded animals. Many, but not all members may show opportunistic pathogenic infections in humans. E. coli is a Gram - negative bacteria. E. aerogenes can be found in the feces of man and other animals, sewage, soil, water, and dairy products. E. aerogenes is also Gram - negative. B. cereus tends to occur in chains and has a dull or frosted glassy appearance and an undulating margin. It is Gram - positive and has been associated in cases of mass food poisoning when extensive multiplication has occurred in food products.

Materials: Three petri dishes with nutrient agar, prepared pure cultures of E. coli, E. aerogenes and B. cereus, [Note: As an alternative to pure cultures a soil sample suspended in distilled H2O, rain water or pond water can be used to obtain various strains of bacteria. The disadvantage to this method is that the culture generated will be unknown. Further microbiological techniques not covered in this exercise must be employed for purposes of identification.], three sterile pipettes, mortar and pestle, 95% ethanol, garlic cloves, orange peel, or any other spice extract, three disposable eye droppers, glass rod spreader, filter paper.

Procedure: Set up a sterile workstation. Macerate several garlic cloves or desired spice extract in a mortar and pestle with distilled water or ethanol. Add some of the liquid to a sterile filter paper disc, made with a hole punch and allow discs to dry. If you use pure bacterial strains, plate them on nutrient agar petri dishes that are labeled and dated. If you use other sources such as soil and water samples place them in a test tube. Allow the sediments to settle to the bottom and pipette only the liquid portion. Inoculate the bacteria by spreading with a sterile glass rod that has been passed through a flame. Place the filter paper discs containing the spice extract on the surface of the inoculated agar. Let stand at room temperature for 24 hours. Observe the results and compare with Figures 1 and 2.

Results: E. coli and E. aerogenes (Figure 1.) were not inhibited by the garlic or the orange extract. B. cereus (Figure 2.) growth was enhanced in orange extract but inhibited with the garlic extract.

Conclusion: E. coli and E. aerogenes are both Gram - negative bacteria and have a higher concentration of lipids in the cell wall. This leads to the conclusion that the high concentrations of lipids in the cell wall do not inhibit the growth of the bacteria. B. cereus is a Gram - positive bacteria and contains techoic acid in the peptidoglycan cell wall and is therefore inhibited by both of the spice extracts.

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