INTRODUCTORY BIOLOGY 

(BS 130) CASE STUDY                                                      Name________________

Introduction

Rhizopus species are the most common etiologic agents of zygomycosis. Host symptoms range from allergic responses to cutaneous infections to systemic disease with invasion of the central nervous system, arterial blood vessels, lungs, gastrointestinal tract, and other parts of the body (Ribes, J. A., Vanover-Sams, C. L., and Baker, D. J. 2000). Although the actual number of species of Rhizopus is still unclear, the genus was divided into four main groups by Liou et al (2007) to include R. oryzae, R. microsporus, R. lyococcus and R. stolonifer. R. oryzae is the most frequently isolated species in infections, followed by R. microsporus. Rarely are other species involved (Ribes, J. A., Vanover-Sams, C. L., and Baker, D. J. 2000). R. stolonifer can cause allergic alveolis but lacks the ability to invade body tissue since it cannot grow above 33 C. Morphological characterisitics and growth temperature have traditionally been used to differentiate species of this genus, although there is variation in expected ranges reported (Larone, D. H. 1993; Ribes, J. A., Vanover-Sams, C. L., and Baker, D. J. 2000; Schipper 1984). Molecular techniques are now also being used in the formation of groups (Liou et al 2007). Since all of these species can be found in dust and thus can contaminate laboratory media, it is important to differentiate these species at least for epidemiological studies.

Table 1. Temperature and sporangium characteristics used for speciation of the genus Rhizopus.(Larone, D. H. 1993; Ribes, J. A., Vanover-Sams, C. L., and Baker, D. J. 2000; Liou et al 2007)                                                                                             

The Problem

It can be seen that the reported ranges for sporangia diameters vary even for the same species. In practice, ranges for sporangia diameters may be even greater. Thus, how reliable are these measurements for species differentiation? We will examine this question as we attempt to solve the following case.

A swab from the nasal cavity of a female patient suspected of suffering from zygomycosis was cultured on laboratory media. A second plate containing a known culture of R. stolonifer is used for comparison. However, the technician grew both plates at room temperature (25ºC). A mold identified as a Rhizopus species grew on the plate from the patient. Can it can now be speciated using sporangia diameters to determine if the isolate is R. stolonifer or another species of Rhizopus? Using a calibrated ocular micrometer, the diameters of 10 different sporangia from each plate were determined (see Table 2). Analyze the data using Excel’s non-matched pairs student's t test and F test to determine if there is any difference between the two cultures and answer the questions regarding the experiment.

Table 2. Measurements of sporangia diameters for known and unknown Rhizopus cultures.

Questions

1. What is the experimental treatment in this study?

2. What is the control group in this study?

3. For statistical analysis, a null hypothesis is stated. A null hypothesis  is one of no difference between the control and experimental treatments. State a specific null hypothesis for this experiment.

4. For statistical analysis, an alternative hypothesis is also stated. An alternative hypothesis states that there is a difference between  the control and experimental treatments. Write a specific alternative  hypothesis for this experiment.

5. What were the results for your F test? Did you use the t test for equal variances or the t test for unequal variances? Are the two groups of data significantly different at the alpha = 0.05 level? At the alpha = 0.01 level? At the alpha = 0.001 level? How many times would you expect incorrect results at the alpha = 0.001 level?.

6. What is your statistical conclusion? Explain.

7. What is your biological conclusion (are they the same species or not)? Explain.

8. Study these concepts and prepare for a quiz on this material next week (1st perid week 2)

Concepts

Project

1. Each student pair will recieve a problem set to analyze using the student's t-test. (1st perid week 2)

2. Your goal will be to answer the following questions regarding your problem:

a. What is the experimental treatment in this study?

b. What is the control group in this study?

c. For statistical analysis, a null hypothesis is stated. A null hypothesis  is one of no difference between the control and experimental treatments. State a specific null hypothesis for this experiment.

d. For statistical analysis, an alternative hypothesis is also stated. An alternative  hypothesis states that there is a difference between the control and experimental treatments. Write a specific alternative  hypothesis for this experiment.

e. What were the results for your F test? Did you use the t test for equal variances or the t test for unequal variances? Are the two groups of data significantly different at the alpha = 0.05 level? At the alpha = 0.01 level? At the alpha = 0.001 level?

f. What is your statistical conclusion? What is your chance of being incorrect?

g. What is your biological conclusion?    Explain.

3. Bring in your work for the first period of week three. These should be typed using
Microsoft Word and you will need two copies, one for yourself and one to hand in. Be
ready to correct any misconceptions during class time.

4. Study these concepts and prepare for a quiz on this material next week (1st period
week 4)

References

Larone, D. H. 1993. Medically important fungi- a guide to identification. 2nd ed.      American Society for
     Microbiology: Washington, D. C.