Volume 6, Spring 2008 - Saddleback College

Fall 2007 Biology 3A Abstracts
zone will represent a majority of pathogens present
throughout the beach. This study hypothesizes that
three days is an insufficient amount of time following
a rainstorm for beach goers to return to the water due
to elevated bacteria levels at the beach.
Materials and Methods
Seven sterilized one hundred milliliter
bottles were used to obtain samples of water
collected from a storm drain run-off site located in
San Clemente, California on January 28 through
February 3 2008. The samples of water were
collected in the surf zone directly in front of the runoff.
The samples were then kept in the refrigerator
until multi-tube fermentation tests could be
performed three weeks later.
Nine lactose fermentation tubes, 3 triple
strength and six single strength, were arranged for
each of the seven samples collected. 10 milliliters,
one milliliter, and 0.1 milliliters of sample were
transferred into each of the three lactose tubes,
respectively, using sterile techniques (flaming the
inoculation loop before transferring the sample).
After a period of 24 hours of incubation at 37 o C, the
tubes were observed for gas and acid production.
Acid production in each tube is recorded as a positive
presumptive test when the lactose color changed from
green to yellow. Gas production, if present, was seen
as a small amount of gas captured in the Durham
tubes at the bottom of the large test tubes.
Finally, the most probable number (MPN)
was determined using an MPN index for various
combinations of positive and negative results for a
change in acid. From the MPN index, 95%
confidence intervals were also determined for the
sample.
Results
Days with a large amount of rainfall
corresponded with positive results for acid
production (Figure 1). The day with the most
significant amount of rainfall had the highest MPN
number and most acid production. This day also had
Rainfall (in.)
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
1/27/2008*
1/28/2008
1/29/2008
1/30/2008
1/31/2008
2/1/2008
2/2/2008
2/3/2008*
Figure 1. Total rainfall Date (in.) (D/M/Y) for each of seven days
samples were collected. Days with asterisk (*)
correspond to days showing positive presumptive tests
55
for acid production.
Saddleback Journal of Biology
Spring 2008
the highest variability determined by the size of the
95% confidence interval.
Gas production and acid production did not
correspond with the presence or absence of one or the
other in a single tube (Table 1). Additionally, gas
and acid production were not prevalent in one
strength tube versus another.
Date 10 mL
3X
lactose
1 mL
1X
lactose
0.1 mL
1X
lactose
MPN
1/27/2008 +/+ -/- -/-