Microbial Quantification Methods and Fermentation Tests in Microbiology

Serial Dilutions and Plate Counts

Measures viable/live microorganisms by growing them into colonies on agar.

Membrane Filtration Plate Counts

Measures viable/live microorganisms by trapping them on a membrane filter and growing them on agar.

Metabolic Activity Measures

Measures microbial activity such as oxygen consumption, ATP production, or acid production; indicates living/active cells.

Visual Examination / Direct Microscopic Count

Counts cells directly under a microscope; detects total cells, including live and dead cells.

Electronic Cell Counters

Uses an electronic instrument to count cells; generally measures total cells, including live and dead cells.

Turbidity Measurements

Measures the cloudiness of a microbial suspension; greater turbidity generally indicates more cells.

Dry Weight Measurements

Measures the total mass of dried microbial cells; detects total biomass, including live and dead cells.

Viable Cell Measurement Methods

Serial dilution/plate counts, membrane filtration, and metabolic activity measurements.

Total Cell Measurement Methods

Direct microscopic counts, electronic cell counters, turbidity measurements, and dry weight measurements.

Colony Forming Unit (CFU)

A viable microorganism or group of microorganisms capable of producing one colony on an agar plate.

CFU vs. Individual Cell

One CFU does not necessarily equal one cell because a colony can develop from a group or cluster of cells.

CFU/mL

An estimate of the concentration of viable microorganisms in a liquid sample.

Serial Dilution

A process of repeatedly diluting a microbial sample by a known dilution factor.

1:10 Dilution

One part sample is combined with nine parts diluent, producing a 10⁻¹ dilution.

Dilution Factor

The reciprocal of the dilution used to calculate the original concentration.

CFU/mL Formula

Number of colonies × dilution factor ÷ volume plated in mL.

Probiotics

Live microorganisms that provide a health benefit when consumed in adequate amounts.

Probiotics and Health

Probiotics can help support a healthy microbial community, especially in the digestive system.

Fermentation

A metabolic process that breaks down sugars and allows cells to regenerate NAD⁺ while producing fermentation products.

Basic Fermentation Process

Glucose → glycolysis → pyruvate → fermentation products.

Acidic Fermentation

A fermentation pathway that produces acidic end products.

Alcoholic Fermentation

A fermentation pathway that produces alcohol and carbon dioxide.

Common Fermenting Microorganisms

Escherichia coli, Lactobacillus, Streptococcus, and Saccharomyces are examples of microorganisms capable of fermenting sugars.

Yeast Fermentation

Yeast can ferment sugars to produce ethanol and carbon dioxide.

Phenol Red Carbohydrate Broth

A culture medium used to determine whether a microorganism can ferment a specific carbohydrate.

Phenol Red

A pH indicator used to detect acid production during carbohydrate fermentation.

Positive Phenol Red Test

Yellow broth indicates that the carbohydrate was fermented and acid was produced.

Negative Phenol Red Test

Red or pink broth indicates that the carbohydrate was not fermented and significant acid was not produced.

Yellow Phenol Red Broth

Indicates positive carbohydrate fermentation and acid production.

Red Phenol Red Broth

Indicates negative carbohydrate fermentation and no significant acid production.

Durham Tube

A small inverted tube used to detect gas production during fermentation.

Bubble in Durham Tube

Indicates that gas was produced during fermentation.

Yellow Broth + Bubble

Indicates positive fermentation with both acid and gas production.

Red Broth + No Bubble

Indicates negative fermentation with no detectable gas production.

Plate Count

Measures viable/live cells.

Membrane Filtration

Measures viable/live cells.

Metabolic Activity

Indicates living/active cells.

Microscopic Count

Measures total cells, including live and dead cells.

Turbidity

Measures the total microbial population based on cloudiness.

Dry Weight

Measures total microbial biomass, including live and dead cells.

Yellow = Positive

Yellow phenol red broth means acid was produced from carbohydrate fermentation.

Red = Negative

Red phenol red broth means no significant acid was produced.

Bubble = Gas

A bubble in the Durham tube indicates gas production.

CFU Calculation

Colonies × dilution factor ÷ volume plated.