The Inhibitory Effects Of A Me Essay

, Research Paper

The inhibitory effects of a mercury compound, silver nitrate, and copper sulfate on Bacillus cereus, Escherichia coli, Micrococcus luteus, and Pseudomonas putida

Introduction

Certain heavy metals can have harmful or inhibitory effects on microorganisms, a process known as oligodynamic action; for example, some heavy metal ions accumulate in cellular proteins leading to protein denaturation and ultimately cell death (Biology 108 lab manual 2001). A consequence of this is that heavy metals can affect organisms higher up in the food chain, a process called biological magnification. One particular heavy metal, mercury, has an unusual ability to be concentrated in living tissues (Brock and Madigan 1991). In addition to the dubious effects of heavy metals, certain bacteria can have harmful effects on organisms. In May of this year, E. coli O157:H7 was blamed for the deaths of 7 inhabitants of Walkerton, Ontario (Mossman 2001). Therefore, in addition to studying heavy metal effects, it is also important to study bacteria in an attempt to learn how they may be controlled. This experiment attempts to accomplish this.

Recent research in the area of oligodynamic action has focused on the injury by heavy metals in Escherichia coli (Cenci et al. 1985) and the resistance to heavy metals by Pseudomonas aeruginosa (De Vicente et al. 1990). In this study, the inhibitory effects of a mercury compound, silver nitrate, and copper sulfate on Bacillus cereus, Escherichia coli, Micrococcus luteus, and Pseudomonas putida were examined. In particular, the zone of inhibition for each heavy metal in relation to each bacterium was examined. A zone of inhibition can be defined as the diameter of the area where no bacterial growth occurs around a paper disk impregnated with the heavy metal solution. The results of this experiment are expected to show that heavy metal treatments affect the growth of bacterial cultures in a negative way; more specifically, it is felt that the mercury solution will create the largest zone of inhibition. This is based on “general knowledge” that mercury is a toxic heavy metal. Additionally, it is expected that certain bacterial species will not be as sensitive to heavy metal compounds. Once again, based on “general knowledge”, it is felt that Escherichia coli will be the most resistant of the bacterial species.

Methods and Materials

The methods and materials for this lab are outlined on Pages 28 – 30 and 49 – 52 of the Biology 108 Lab Manual (2001). No changes were made to the experimental method.

Results

Table 1 outlines the summarized results for the experiment.

Table 1: The average diameters of the zones of inhibition created by the three different heavy metals on the four different species of bacteria.

Organism Average Diameter of the Zone of Inhibition (mm)

Mercury Compound Silver Nitrate Copper Sulfate Control

Bacillus cereus 34.4 15.5 slight 0.0

Escherichia coli 24.5 13.2 11.2 0.0

Micrococcus luteus 24.2 12.5 0.0 0.0

Pseudomonas putida 27.5 15.0 slight 0.0

As shown by Table 1, the mercury compound affected Bacillus cereus the most followed by Pseudomonas putida. Escherichia coli and Micrococcus luteus were affected by approximately the same amount but to a lesser degree than the other two bacteria. Also indicated by Table 1 is the trend that the silver nitrate solution was less effective at inhibiting bacterial growth as compared to the mercury compound. For this heavy metal, Bacillus cereus and Pseudomonas putida were relatively affected the same amount. Also, Escherichia coli and Micrococcus luteus were affected the same amount but to a lesser degree than the other two. The overall trend of metal effectiveness in preventing bacterial growth is outlined in Table 2.

Table 2: The ability of each heavy metal to prevent bacterial growth.

Heavy metal compound Effectiveness of the heavy metal compound in preventing growth in all 4 of the bacterial species (mm)

Mercury Compound 27.7

Silver Nitrate 14.1

Copper Sulfate 2.8

Control 0.0

As indicated by the data in Table 2, the mercury compound had the largest effect on all 4 bacterial species. The silver nitrate solution had an intermediate effect, and the copper sulfate solution had a slight effect. The control data show that the control cultures had no effect on bacterial growth.

Table 1 also shows that some bacteria such as Escherichia coli and Micrococcus luteus are more resistant to heavy metals than other bacteria. This trend is indicated by the smaller zones of inhibition. This trend of increased resistance of certain bacteria is outlined in Table 3.

Table 3: The sensitivity of each of the 4 bacterial species to the 3 heavy metal solutions.

Bacterial Species Sensitivity of each of the bacterial species (mm)

Bacillus cereus 25.0

Escherichia coli 18.8

Micrococcus luteus 18.4

Pseudomonas putida 21.2

As outlined by Table 3, Escherichia coli and Micrococcus luteus are the most resistant (or most insensitive) of the all bacterial species. The data indicate they are approximately equally resistant. Pseudomonas putida is the 3rd most resistant, and Bacillus cereus is the least resistant. To provide further data for analysis, Ta

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