Sample Lab Report (the data and microorganisms presented here are fictional)

The Effect of Temperature on the Growth of Three Species of Bacteria

Joan McKearnan

Abstract

Living organisms are limited in their growth and reproduction by environmental conditions.  I explored the effect temperature would have on the growth of three bacteria species (Alba polarensis, Vastus intermedius and Thermophilus caliente).  I hypothesized that the bacteria would not grow under all temperatures and that they would all grow optimally at 37°C.  The three species were cultured in tryptic soy broth at four different temperatures (4°C, 23°C, 37°C, and 60°C) for 48 hours.  Growth was estimated on a scale of 0-4 with 4 being the most growth.  A. polarensis grew best at 4°C, but not above 23°C. V. intermedius grew best at 37°C, but not at 4°C or 60°C.  T. caliente grew best at 60°C, but not below 37°C.  Temperature did limit the growth of the three bacterial species, but only V. intermedius grew best at 37°C.  The optimal temperature for growth may vary among species.

Introduction

Few, if any, living organisms can survive under all possible environmental conditions on Earth (Dei 2001).  Organisms are limited by the environmental conditions by which habitats they can inhabit.  These limitations are set by interactions between the environment and the organism's physiology.  Each species has their own range of conditions under which they can thrive, grow, and reproduce.  This range is called their tolerance range and they can not survive below the lower limit of the range and above the upper limits of the range.  Within the range, there is usually a set of values under which growth and reproduction is at a maximum; this is called the optimum (Talaro and Talaro 1999).

Temperature is one of those environmental parameters that limit the growth of species, including bacteria (Harley and Prescott 1999).  My question for this experiment was: Does temperature limit the growth of bacteria, and, if so, what is the optimum temperature for bacterial growth?  My hypothesis was that temperature can limit the growth of bacteria and that the optimum growth would be around 37°C.  I have several reasons for my hypotheses. First, I know that heat is used as a sterilization technique and therefore extreme heat must kill bacteria.  Second, I also know that the bacteria that grow in polar habitats are different than those that grow in temperate climates (NSF 1999).  Third, I believe the optimum will be around 37°C because that is our body temperature and I know that several bacteria, especially pathogenic species, grow in the human body.  Fourth, many enzymes also have an optimal temperature under which they catalyze reactions at the fastest rate and that is frequently at 37°C.  The rate of growth and reproduction of organisms is determined by the speed by which enzymes catalyze reactions (Campbell et al. 2009).

Procedure

I used three bacteria for my study: Alba polarensis, Vastus intermedius , and Thermophilus caliente.  Each bacterium was inoculated into four test tubes of tryptic soy broth.  One test tube of each bacterium was incubated at 4˚C, another tube at 23˚C, a third tube at 37˚C, and the fourth tube at 60˚C.  After 48 hours, the growth in each tube was estimated on a visual scale of 0-4, with 4 being the most growth. If the first part of my hypothesis is correct, the bacteria will not grow at the same rate at all temperatures.  If the second part of my hypothesis is correct, all three species will grow best at 37˚C.

Results

The bacterial growth was not constant amongst the different temperatures. A. polarensis grew best at 4°C and moderately at 23˚C.  It did not grow at the higher temperatures.  V. intermedius grew moderately at 23°C and best at 37˚C, but not at cold or hot temperatures.  T. caliente did not grow at cold and room temperatures, but moderately at 37°C and very well at 60˚C (Table 1).  Based on the temperatures used in this experiment, the optimum temperature for A. polarensis was 4˚C, for V. intermedius  was 37˚C, and for T. caliente was 60˚C (Figure 1).

Table 1. Growth of bacteria at different temperatures.  Growth is measured on a scale of 0-4 with 4 being highest.

Discussion

My central question was, does temperature limit the growth of bacteria, and, if so, what is the optimum temperature for bacterial growth?  I hypothesized that the growth of bacteria could be limited by temperature and that optimal temperature for all species would be 37˚C.  The first hypothesis was supported and was demonstrated by the different temperatures at which bacteria did not grow.  For example, A. polarensis did not grow in the warmer temperatures.  The second hypothesis was not completely supported.  The three bacteria I used showed variation among the species.  V. intermedius did grow best at 37˚C, but the other two had optimal temperatures different than 37˚C.  A. polarensis grew best at 4˚C and T. caliente at 60˚C.  While some species of bacteria may have an optimal temperature at 37˚C, there are others who grow better at cooler and warmer temperatures which mean that they may favor habitats where those temperatures are more commonly experienced.

The experiment could better explore my central question by using more bacteria species to see what is the most common optimal temperature and by using more temperatures to more precisely determine optima.  V. intermedius may have grown best at 37˚C in this experiment, but it may also grow better at other temperatures not used in this experiment.  Also, the experiment could better determine the variation within a species by using replicates and other strains of the same species.  Future experiments could look at other species to determine their optima and also explore what are the molecular mechanisms that determine temperature tolerance ranges. 

Literature Cited

Dei, J.C.  2001.  Personal Communication.

Campbell

Harley, J. P. and L. M. Prescott.  1999.  Laboratory exercises in microbiology, 4th ed. WCB McGraw-Hill Co. Boston, MA. 491 pp.

National Science Foundation.  1999.  NSF News: Bacteria may thrive in Antarctic Lake.  Online: http://www.nsf.gov/od/lpa/news/press/99/pr9972.htm

Talaro, K.P. and A. Talaro.  1999.  Foundations in microbiology, 3rd ed.  WCB McGraw-Hill     Co. Boston, MA. 873 pp.

last revised in 2007