INTRODUCTION
One of the fundamental techniques of microbiology is the use of dilution theory: the
mathematical calculation of colony-forming units per millimeter (CFU/mL) of a fluid
as determined by spread plate data and consideration of the involved dilution factors
(7). In spite of student practice in the laboratory with spread plating and with calculating
CFU/mL of samples from their plate counts, my students consistently perform poorly
on examinations when asked to apply dilution theory. For example, from 2008 to 2013,
only 20%, on average, of the students tested on dilution theory were able to correctly
apply dilution theory to problem solving on a quiz.
Addressing this deficiency has been an ongoing challenge. My students initially are
introduced to dilution theory with biosafety level 1 (BSL-1) organisms to help familiarize
them with the process of pipetting, aseptic technique, and spread plating. After further
development of their skills in culturing and manipulating bacteria, I engage the students
in a fermentation-based laboratory experience where students repeatedly perform serial
dilutions and spread plating and apply dilution theory to their data, in an attempt
to improve student understanding of and practice with dilution theory. Other educators
have developed student activities related to plate counts to foster active learning.
March et al. (3) found increased student learning about dilution theory and plate
counts when students engaged in analysis of a provided data set. Serial dilution for
the enumeration of bacteriophages has been used to strengthen student understanding
of phage biology (1). Although the materials to perform this fermentation are simple
and widely available, this laboratory project also contains elements to assess changes
in microbial composition in a fermentation mixture commonly referred to as bokashi
(Japanese for “fermented organic matter”). Bokashi is a mixture of microbes resulting
from fermentation (often referred to as effective microorganisms, or EM) which is
then used to inoculate an organic substrate such as rice straw or bran (6). EM and
the resulting bokashi have been used to enhance growth of a variety of crops including
peanut and pigweed (4, 5).
PROCEDURE
Note that the microbes cultivated in this experiment fall within the BSL-2 safety
guidelines. In light of this, students must adopt the following practices during the
entirety of the experiment: the use of personal protection such as gloves and goggles,
decontamination of all microbially contaminated materials and workspace (via chemical
disinfectants or autoclaving), and proper hand hygiene. Total time for the protocol
is three weeks comprising four sessions for microbiological techniques.
Working in pairs students shake 6 g of white rice and 6 g of brown rice (both non-instant
generic brands obtained at a local grocery store) in 60 mL of distilled water for
2 minutes. The fluid is decanted into another bottle and a 1-mL portion is serially
diluted in 0.1% peptone and spread plated in triplicate (0.1 mL/plate) onto nutrient
agar (NA) and MRS agar. MRS agar is a selective media for lactobacilli (2).
All spread plates are incubated at 30°C until colonial growth is present (typically
2 days). Specific population levels of bacteria in the fermenting fluid can vary from
student group to student group so I recommend diluting the samples after each of the
three 7-day incubations, up to 106-fold, to ensure countable numbers of colonies on
the spread plates (30–300 CFU/plate is the target). Expect very few colonies on MRS
media from the initial spread plating of the rice water, but these numbers will increase
around 104-fold after the first 7-day incubation. The pH of the fluid is measured
using a pH electrode when the spread plates are set up and the remaining fluid is
incubated at 30°C for 7 days. Over the course of the experiment, the pH should drop
approximately 2 pH units. As the pH drops over the course of incubation, the relative
populations of bacteria cultured on NA versus MRS shifts, albeit in inconsistent ways.
In some student groups the populations drop 100-fold while other student groups find
an increase in bacterial levels, so I recommend plating from all dilution tubes in
order to get reliable plate counts.
After incubation of the fluid for 7 days, the previously mentioned methods of pH determination,
serial dilutions, and spread plating are repeated as described above. Cells in the
fluid are then simple stained using crystal violet to determine the frequency of cell
morphologies, and 10 mL of fluid is centrifuged at top speed in a tabletop centrifuge
to pellet bacterial cells. This pellet is suspended in 10 mL of sterile 0.85% sodium
chloride and used to inoculate GEN III microplates (Biolog, Hayward, California).
The GEN III microplate tests bacteria for the utilization of 71 different carbon sources
and sensitivity to 23 different inhibitors and pH conditions. Absorbance at 750 nm
in each well of the microplates is then determined via a plate reader after 24 hours
of incubation at 30°C.
The remaining rice water is added to 400 mL of whole milk (obtained from a local grocery
store), the pH determined, and the mixture incubated for 7 days at 30°C. After incubation,
curd formation has occurred and the supernatant of the mixture is decanted into a
bottle for simple staining, pH determination, serial dilutions with spread plating,
and microplate inoculation as described above. One mL of unsulfured molasses (Grandma’s
brand) is added to this supernatant and the mixture incubated for 7 days at 30°C.
After incubation, simple staining, pH determination, serial dilutions and spread plating,
and microplate inoculation are performed as described above for the final time. The
resulting fluid in the bottle is bokashi.
At the end of making bokashi, students are required to calculate CFU/mL fluid at each
step of the process based on the spread plate data collected and averaging the values
obtained from triplicate plating. This data can be correlated with changes in pH,
dominant cell and colony morphologies, and sensitivity/carbon utilization patterns
from the GEN III microplates. In addition to the enumerative emphasis of the methods
described above, subculturing of dominant colony morphologies can give students practice
in streak plating and bacterial identification techniques using standard techniques
not given in this exercise. Simple staining of the fermenting fluid gives the students
opportunity to practice microscopy skills. Although the use of GEN III microplates
adds additional cost, it allows for examination of the collective phenotypes of the
shifting bacterial communities over time in the fermentation fluid which can be related
to the decrease in pH of the fluid (which is typically found to decrease from 6.5
to 4.7).
SAFETY ISSUES
Nutrient agar grows a wide range of microbes, so plates and all bacteria-containing
materials should be treated with the appropriate guidelines for laboratory biosafety
level 2 (www.asm.org/images/asm_biosafety_guidelines-FINAL.pdf). Students should be
instructed in aseptic technique and proper disposal of bacterially contaminated materials.
Due to the selective nature of MRS agar, students handling these plates will most
likely be working with lactic acid bacteria. Students should be informed that the
bokashi product is not for human consumption (the strong odor should be a deterrent
for those tempted!).
CONCLUSION
The total time to produce bokashi is three weeks. The schedule allows for four weekly
serial dilutions and spread plating. I have the students spread plate in triplicate
to assess the variability of their pipetting technique. The use of both NA and MRS
agar shows the difference between nonselective and selective media on colony number
and morphology (see Fig. 1). Appendix 1 includes some questions that students can
address in their analysis of the data. All students should calculate the CFU/mL fermentation
mixture throughout the experiment, but the instructor typically assigns different
data exploration questions to different student pairs for analysis.
FIGURE 1
Spread plates (0.1 mL fluid/plate) from a bokashi fermentation mixture. (A) Colonies
on nutrient agar from the initial fermentation of rice water for 7 days. (B) Colonies
on MRS agar from the fermentation of milk/rice water for 7 days.
SUPPLEMENTAL MATERIALS
Appendix 1: Media and reagent preparation, materials needed, and questions for students