Date of Degree


Document Type


Degree Name

Master of Science (MS)




David E. Starkey


Carlos A. Garcia


Michael A. Maguigan


Soil samples were examined outside Bonilla Science Hall on the campus of the University of the Incarnate Word in San Antonio, Texas, at three time points between the calendar years 2016 and 2017. These time points correspond to Fall, Winter, and Spring. Samples were taken at two topsoil depths, 1 cm and 4 cm, to determine if there is a difference in bacterial load or distribution across the time frame examined.

Soil samples were diluted and plated on nutrient agar plates in order to identify unique colony morphologies. A total of 132 distinct morphological isolates were identified and sequenced. Of the 132 distinct colonies identified, sequences were generated for 95. These 95 colonies were found to represent 2 genera, Bacillus and Arthrobacter. The six unique Bacillus species identified among the isolates were: subtilis, cereus, megaterium, niancini, pumilis. A single unidentified species of Arthrobacter was also discovered.

B. subtilis and B. cereus were present for all time points and depths. B. megaterium was present at all time points and depths with the exception of time point 3.4. Upon comparison of 1cm and 4 cm depths at each time point, bacterial load was found to decrease by as much as 91%. If similar depths are compared across time points, bacterial load was also found to decrease with the exception of time point 1.4 vs. 2.4, or 3.4, which show an increase in bacterial load. In terms of bacterial diversity, the only comparison that demonstrated a similar distribution of bacteria was a comparison between time points 1.1 and 1.4. Macronutrient concentration and soil temperature were sampled at time point 3 and found to differ across depths at this time point. This suggests that these factors play a role in the bacterial diversity present at this time point. Overall, the results of this study suggest that the topsoil is a dynamic environment and the distribution and load of bacteria can differ significantly across both depth and time.