AFSL

What is microbiology test?

Microorganisms are ubiquitous in the natural environment, and they have been widely used in food manufacturing such as cheese, beer, bread, and nutritional supplement. However, certain microorganisms are pathogens, such as E. coli, Salmonella, Staphylococcus aureus, may cause discomfort even fetal harm to human body. Thus in pharmaceutical manufacturing, food and beverage and cosmetics industries, microbiology test / microbial detection is essential to quality control in order to ensure product safety.

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Common microbiology testing methods

Common analysis of microbiology testing includes multiple-tube fermentation (MPN) method, spread plate method, pour plate method and membrane filtration method.

Membrane filtration is a very widely used method for separation of coliform from liquid, and had been acknowledged by U.S. EPA since 1961.  Membrane filtration method, as compared with other methods, requires less preparation and

Microbiological analysis of Water by Membrane Filter(MF) Technique

Membrane Filtration	Multiple-tube Fermentation	Spread Plate	Pour Plate
Preparation	Agar plate	Nutrient broth	Agar plate	Agar broth (maintained at between 45~50oC)
Incubation	24 ± 2  hour	3 phases, 48 hours/phase	48 ± 3 hours	48 ± 3 hours
Consumption	Membrane, petri dish	Vials and fermentation tubes	Petri dish, inoculation hoop	Petri dish
Sample volume	High	Low	Low	Low
Heat-sensitive-bacteria-safe	Yes	Yes	Yes	No
Sample selection	Aqueous sample with low turbidity	Aqueous sample	Aqueous sample with high bacteria concentration	Aqueous sample with low bacteria concentration
Accuracy	High	Very low	Low	Low
Determination	Observation and counting	Estimation by MPN	Observation and counting	Observation and counting
Colony splitting	Yes	No	Yes	Yes
Other	• Membranes can be moved to different culture medium for incubation.• Biocide can be effectively removed.	• Able to test aqueous samples with high turbidity• Creates more wastes.	• Culture medium can be easily scratched by the hoop.• Incubation can be processed after dried.	• Colonies may exist within the culture medium, more difficult to read.

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Microbiology test procedures

Procedure of membrane filter method (NIEA E230.55B):

• The test should be carried out in a laminar flow hood.
• Preparation: Sterilize the stainless steel filtration funnel and tweezers with flame and wait till cool-down.

Test procedure:

1. Place the sterile membrane filter on the filter holder, dilute with few drops of diluent to test if the filtration equipment is settled properly.
2. Shake the samples for more than 25 times to have them finely homogenized before dilution and detection.
3. Filter 100 mL samples then rinse the funnel with 20 mL of diluent.
4. Put the membrane onto petri dish with proper culture medium. Make sure the membrane is finely attached to the medium, avoid air from entering in between.
5. Place the incubator upside down into the incubator and incubate at 35 ± 1 oC for 24 ±2 hours. Use colony counter and count the numbers of targeted colonies. Note TNTC (Too numerous to count) if there’re too many.
6. Determination: Calculate bacterial density with formula noted below. (Colony forming units/100 mL)

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Selecting microbiology testing equipment

Microbiology test (with membrane filtration) often requires sample replication and creates big amount of liquid wastes. Waste handling and sterilization in the test procedure are essential but tedious. Here we share some key points that you can consider when making decision for equipment to make your lab life easier.

1) Choose equipment that can be rapidly sterilized.
Microbiology test relavent equipment often requires sterilization with autoclave, which may take hours to complete. In order to make the whole process quicker and smoother, several sets of equipment for replacement can be prepared. Or, you may choose materials that can be rapidly sterilized. Such as stainless steel, which can be flame sanitized and be ready for use after cools down. It saves a lot of time and make the work procedure smoother especially when you have a large amount of samples to test.

Rocker SF –  Stainless Steel Filter Holder – Spin-lock design, no clamps required. – SS316 – made safe & heat resistant – Cost-effective, saves up to 46% of long-term budget.  – Eco-friendly, no plastics. Rocker Stainless Steel Manifold Rocker Vacuum Manifol Systems
	Stainless Manifold and Filter Holders

2) Easier liquid draining
Liquid waste of microbiological tests from the water samples doesn’t need additional treatment before draining to sink. So liquid collection before draining it is unnecessary. Choose equipment that can drain the liquid waste directly to sink can save you effort and time.

Rocker WaterVac Filtration System – Alligator 200: Taiwan Excellence award-winning pump – WaterVac 100: 24VDC-driven, easily portable for at-scene sample collection. – WaterVac 200: 2 samples at once. Space-saver.
	Rocker WaterVac 200 Direct Drain Filtration System

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References:

–Pour Plate and Most Probable Number tests have changed little in 50 years, while new technology has advanced Membrane Filtration Tests for microbial QC

–Analytical Methods Approved for Compliance Monitoring under the Revised Total Coliform Rule, EPA 815-B-19-007, EPA, 2019

–Difference Between Pour Plate and Spread Plate

–EQUIVALENT TESTING METHODOLOGY FOR AGRICULTURAL WATER, FDA

–ENUMERATION OF MICROORGANISMS

–Microbial Aspects of Drinking Water