Abstract

The purpose of this experiment was to explore the correlation between cold and warm loving bacteria (psychrotrophs and mesophiles, respectively) and the temperature changes that occur in Lake Michigan. Our hypothesis states that as the temperature gets colder the ratio of cold to warm loving bacteria will increase and that as bacterial life in the test tubes increases, the transmittance of the broth will decrease. We took water samples and inoculated them in nutrient broths, taking percent transmittance before and after incubation. In correlation graphs between the two sets of data, it was shown that the transmittance did decrease after incubation, indicating bacterial growth. The transmittance, or growth of bacteria, was not significantly different between the two types of bacteria.

Key Words

Psychotrophs, mesophiles, bacteria, Lake Michigan, temperature changes, field research

Introduction

There are different classifications among the bacterial world; two of those categories are mesophiles and psychrotrophs (Buchanan and Gibbons 1974). These two types are defined as bacteria that grow well in a warmer (above 37° C) climate and bacteria that grow well in a colder (around 10°C) environment, respectively. These bacteria have a difficult time growing in the other’s temperature; thus we assumed that any growth within our specific incubation temperature is of the bacterial group we are selecting for, i.e.—growth within the 10°C incubator is of psychrotrophs only. We chose to explore whether Lake Michigan experiences a change in the ratio of these two groups as the weather gets colder over the fall and approach of winter. Our hypothesis was to try to measure whether the psychrotrophs grow in number as the mesophiles decrease, relative to the temperature decrease over the months September and October. After our field data was gathered, a comparison of the inferred amount of bacteria in each of the groups, represented by percent transmittance, will show us if our hypothesis is supported. Beaches along Lake Michigan have been repeatedly closed due to increased levels of E. coli, a bacteria harmful to humans (Yonkman 2002). For that reason a special interest should be taken to investigate the levels of bacteria in Lake Michigan.

Methods

First, we autoclaved 16 500-mL glass bottles and their cork stoppers, for the retrieval of our samples. Water was taken from 4 locations in the South Shore Marina, Lake Michigan. One bottle was used for one sample from one of the four spots. The first location was about 2 meters from the shore on a boating dock, while the other three locations were 6 meters apart on the shore. We retrieved the water samples, every seven days starting 27 September and ending 18 October. Great care was taken not to contaminate the bottles prior to or after gathering the samples by using sterile gloves while taking and handling the samples. Upon returning to Alverno College we quickly transported the samples to the microbiology laboratory. There the samples were transferred and inoculated into sterile nutrient broths. We transferred two milliliters of each sample into a test tube of broth, with a total of four tubes per site. Two tubes per sample were put into a 37°C incubator and the other two are put into a 10°C incubator. The photospectrometer was set to zero using a sterile nutrient broth, and readings were recorded prior to incubating the samples. A week later, the producing broths were removed from incubation and readings of the percent transmittance were taken again for comparison with the prior week’s readings.

Results

The samples incubated in 37⁰ Celsius had an average transmittance of 9.13% with a standard deviation of 0.006 after one week of growth. The samples incubated in 10⁰ Celsius had an average transmittance of 80.09% with a standard deviation of 0.027 after one week of growth. There are four graphs, each showing a comparison of one week worth of growth for both the psychotrophs and the mesophiles.

These graphs show comparisons, throughout the weeks, of a single sites growth. While these help to show a short-term difference, they aren’t too helpful in displaying the overall changes throughout the season as related to temperature. Thus we made four more graphs to show these changes.

The trend that is shown in these graphs is in no way consistent to what we would have needed to see for our hypothesis to be supported. If there were a way to have continued these measurements over a longer period of time, a correlation might have become apparent.

Discussion

Our hypothesis that transmittance would decrease as bacterial growth increased was supported and this was statistically significant at the 95 percent confidence level. However, our hypothesis that cold loving bacteria would increase as the weeks progressed was not supported. The percent transmittance of the psychrotrophic bacteria only decreased, on average, 19.91 percent, while the percent transmittance of the mesophilic bacteria decreased 90.87 percent. The small amount of growth by the psychotrophs may be due to the large number of mesophiles. There might be some form of competition between the two types of bacteria. Also, the water temperature of Lake Michigan in the months of September and October is not as cold as it is in the months of December or January. These later winter months could be the time when psychrotrophs are most abundant because the mesophiles cannot survive in lower temperatures, below 5^oCelsius (Banwart 1979). If more time were allowed for this experiment, we would like to continue to culture the samples and find what types of bacteria are living in Lake Michigan. We know that E. coli live in the lake, and it may be helpful to learn what other species live there as well (Yonkman 2002).

Literature Cited

Banwart, G.J. 1979. Basic Food Microbiology. Westport, Conn.: AVI. Chapter 4, Factors that affect microbial growth in food; 25-32 pp.

Buchanan, R.E. and Gibbons, N.E., editors. 1974. Bergey’s Manual of Determinating Bacteriology. Williams and Wilkins Company: Baltimore. 962-63 pp.

Yonkman, D. 2002. “E. coli prompts Lake Michigan beach closing.” The Holland Sentinal Online. Available Online: http://www.hollandsentianl.com/stories/082102/loc_082102002.shtml.