Microbiology:Testing for Bacteria

Linda Wolf

Glencoe High School

SWRP Teacher for 12 years

Pathogens Pathogens are organisms capable of

causing disease The following are some of the “bad guys”:

Protozoa: Giardia, Cryptosporidium Bacteria: Salmonella typhi, Legionella,

Shigella, Vibrio cholerae, Vibrio vulnificus Virus: Hepatitis, Polio

Sizes Bacteria are 2 - 4 µm Viruses are 0.02 - 0.09 µm

For reference: 106 microns (or micrometer, µm) = 1 meter 1000 µm = 1 mm

Testing for Pathogens Direct testing for pathogens is impractical Pathogens are usually found in low

numbers Can’t survive for very long outside the

warm confines of a human or animal body Too many methods are too sophisticated

and expensive

Indicator Bacteria Some bacteria can be good indicators of

human pollution – the source for most pathogens

Bacteria present in sewage pollution Survive longer than pathogens Easily detectable

Common Indicator Bacteria Total Coliforms Fecal Coliforms E. coli Enterococci

E. coli

Total Coliforms

Fecal Coliforms

Total coliforms Rod-shaped, gram negative bacteria Ferment lactose at 35°C Found in intestinal tracts of cold and

warm-blooded animals Group members: Escherichia, Klebsiella,

Enterobacter, Serratia, Citrobactera, Edwardsiella

Fecal coliforms Subset of Total coliform group Present in sewage and indicate possibility

of human pathogens Distinguished from Total coliform by

ability to ferment lactose at 44.5°C Group members: E. coli and Klebsiella (not

always fecal often associated with paper, textile & pulp waste)

Fecal coliforms Common in the intestines of both warm

and cold-blooded animals If fecal coliforms are present it is presumed

that human or animal excrement is present Diseases such as typhoid fever, hepatitis,

gastroenteritis, dysentery and ear infections can be contracted in water with high Fecal coliform levels

E. coli Escherichia coli is a specific species within

the Fecal coliform group Specific to intestines of mammals and

other warm blooded animals Only specific strains (i.e. O157:H7) are

pathogenic According to EPA, is the best indicator of

health risks from water contact recreation

Enterococci Survives in salt water More human specific Found primarily in the intestinal tract of

warm-blooded animals Used in some states as indicator organism

in estuarine and marine waters

Bacterial Measurement Membrane Filtration Methods

Quantify bacteria numbers by filtering water, growing bacteria, and counting

Most Probable Number Methods Estimate bacterial numbers based upon a

color change or amount of gas produced through a specific bacterial metabolic process

Membrane Filtration Known volume of water is filtered through

a filter (0.45 µm) that is capable of trapping all bacteria

Filter transferred to Petri dish containing growth media

Individual bacterial cells will grow on the filter into visible colonies in 24 hours

Membrane Filtration m-ColiBlue24 broth Due to the metabolism of the bacteria on

the media: Blue colonies indicate E. coli Red colonies indicate other Total coliform

bacteria E. coli turn blue from the action of β-

glucuronidase enzyme on 5-bromo-4-chloro-3-indolyl-Beta-B-glucuronide

Procedure Collect water in a sterile container Filter water within 6 hours* Place sample in cooler if taking to lab

*6 hours is standard holding time, but samples should definitely be filtered within 24 hours

Prepare plates Determine amount of water to filter

5 plates for each site For each site label one plate 0 mL for a

“blank”# plates Label Volume

1 0 mL 10 mL sterile

2 10 mL 10 mL sample

2 30 mL 30 mL sample

Prepare Plates Label bottom of plate with:

Date and time Sampling site Volume to be filtered

Use sterile forceps to place sterile absorbent pad in each plate, if plates don’t already have them

Place about 2 mL of broth on each pad, using either a sterile pipette or by shaking and pouring ampule

Filter Samples Sterilize forceps, place membrane filter on

filter holder Use sterile water for small samples of

water (1 mL) to wet the filter Pump until most of water is through filter Release pressure Sterilize forceps and place filter grid-side-

up on the absorbent pad

Plates Put cover on plate Leave upright until all plates are filtered Incubate upside down for 24 hours in an

incubator at 35° C

Calculating Results Count the blue and red colonies on each

plate Blue colonies are E. coli Red + Blue = Total Coliforms If there are greater than 200 colonies report

that plate as TNTC (Too numerous to count)

Most accurate platesThe best are when the colony counts are in the

range of: 20 – 80 colonies per plate for E. coli, and 50 – 200 for Total coliforms

Calculating Results Standard Units = CFU/100 mL (Colony Forming

Units) Average colony counts x 100 = CFU/100 ml Volume Filtered (mL) If fewer than 20, estimate CFU/ 100 ml using all

plates.Add total number of colonies and total volumeTotal colony counts x 100 = CFU/100 ml

Total mL filtered

Other problems If over 200, but colonies are clearly

countable, use the same general formula.

Conflicting colony counts:

go with the smaller sample size

0 mL filtered 1 mL filtered

10 mL filtered 30 mL filtered

24 colonies counted

6 colonies counted

0 colonies counted

67 colonies counted

The 10 mL plate would be used for calculating CFU/100 mL:

24 / 10 x 100 = 240 CFU/100 mL

0 mL filtered

30 mL filtered

1 mL filtered

23 colonies counted

10 mL filtered

18 colonies counted

0 colonies counted

52 colonies counted

1 mL plate has more colonies that 10 mL plate.

Possible problem(s):• mislabeled plate• contaminated

apparatus

The 1 mL plate would be used for calculating CFU/100 mL:

23 / 1 x 100 = 2300 CFU/100 mL

0 mL filtered 0 mL filtered

1 mL filtered 30 mL filtered

23 colonies counted

6 colonies counted

7 colonies counted

Estimate or report as Too Numerous to Count (TNTC)

Possible problem: finger on filter or contaminated forceps

Possible problem: “sterile” water not sterilized

Possible problem: filter not wetted with sterilized water before filtering low volume sample – sample concentrated in one area of filter.

Example of plate with more than 200 colonies. Colonies could be counted or estimated, and results flagged as “estimate”.

The sterile water “blank” or 0 mL plate is a quality control measure – bacterial growth on the blank makes the other plate counts suspect.

Water Quality Standards In Oregon, based upon contact recreation 126 CFU/100 mL for 5 samples within a 30

day period 406 CFU/100 mL for a single sample