Salt vs. Bacteria: The Effects of Salt Concentrations on Bacteria in Santa Ana River Water

Overview

Objectives/Goals The purpose of this project is to see if the salinity level of Santa Ana River water samples has an effect on bacterial growth, particularly to see if a lower salinity level increases the level of bacteria (esp. potentially harmful E.coli & other coliforms). Methods/Materials Collect samples from 2 locations of the Santa Ana River & mix equal parts for standard river water. Keep 1 standard 100ml sample. Dilute 1 sample by 1/2 & one by 1/4 using distilled water. Concentrate samples by 3X, 11X, & 31X by adding NaCl to standard samples. Measure all samples for TDS (salinity proxy) using a multi-parameter tester. In a lab add Colilert media to each sample & shake. Pour samples into Quanti-tray & incubate for 24 hrs. Count total coliform & fluorescing E.coli using a black light. Calculate results using IDEXX Quanti-tray MPN table. Results All samples were positive for coliform bacteria & E.coli bacteria. All samples had very high levels of coliform bacteria with the 1/4 dilution being 2400MPN/100ml (MPN=most probable #) & all other samples being >2400MPN/100ml. The sample diluted by 1/4 had the least amount of E.coli, 1/4 the amount found in the standard river sample. The E.coli found in the 1/2 dilution through the 11X concentration remained within a range of 60MPN/100ml. The bacterial level at the 11X concentration began to decline slightly while the 31X concentration had an even greater decrease. If the bacterial concentrations for the 1/4 & 1/2 dilution samples were multiplied (adjusted) by 4 (for the 1/4 dilution) & by 2 (for the 1/2 dilution), then the 1/4 sample's bacterial concentration would be 384MPN (4 X 96MPN) (close to the standard river water concentration, 378MPN) & the 1/2 sample would be higher, 656MPN (2 X 328MPN). Conclusions/Discussion Because there is little difference in the amount of bacteria from the 1/2 dilution to the 11X concentration of NaCl, it appears that the E.coli bacteria can tolerate salinity levels within a broad range above & below the standard river sample. It appears that NaCl is necessary in some concentration to promote bacterial growth & that in the 1/2 concentration [151.7ppm] the NaCl is closer to being optimal for coliform growth. By repeating the experiment with the 31X concentration of NaCl, I found that the increasing salinity levels do appear to have negative effects on bacteria. I believe an even greater increase would result in a greater decrease in bacteria.

Competition history

• CSEF 2011 Microbiology (General) · Entry J1504

Resources

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