Lab 3: Bacteriological analysis of water samples

Introduction

Water quality testing is a crucial component in the provisioning of safe drinking water and sanitation services.
The aim of this laboratory exercise is to familiarize the authors with methods for bacteriological analysis of water quality. Specifically, water quality was investigated by studying the presence and amount of coliform bacteria in the water sample. Coliform bacteria are Gram-negative and rod-shaped, and they can be used as an indicator species for faecal contamination.
The sample analysed was taken from the bottom of a dumpster of a local supermarket. The analysis included estimating presence and amount of coliform bacteria by the Most Probable Number (MPN) and membrane filtration method, Gram-type of bacteria was confirmed with a Gram-staining test and the strain of bacteria was determined using a commercial API test. In addition, the number of yeast cells in a provided sample of S. Cerevisiae was determined using cell counting in a Bürker chamber.
Given the turbidity and odour of the original sample it is expected that presence of coliform bacteria will be detected.

Method

Most Probable Number method

Method for the presumptive test according to provided lab instructions.
A complex streak from the SSLB 0.05ml tube, having produced the most amount of gas, was performed on one plate of Levine's EMB agar (selective for Gram-negative bacteria) and one plate of Endo agar (selective for Gram-negative and indicative of lactose fermentation). The EMB and Endo plates were incubated for 24 hours and the presence and color of colonies was recorded (the confirmed test). 

Membrane filtration and Gram-staining

Membrane filtration and incubation on mFC agar  was completed according to laboratory instructions. Colony counting was performed by estimating the total area covered with colonies and then counting all colonies on an area corresponding to approximately 1/4 of the total and extrapolating across the rest of the cultured area.  
A blue colony from the 10ml filtration test and a grey/pink colony from the 1ml filtration test was selected, streaked using a complex streak and incubated on LB agar plates. Two colonies from each LB plate were then Gram-stained according to laboratory instructions from Lab 2.
The bacterial concentration per 100mL was then calculated according to (\ref{membraneconc})