Total and viable count of bacteria

Introduction

• Determination of bacterial count is an important laboratory procedure used to estimate the number of microorganisms present in a sample.
• Bacterial counting helps in:
  • Assessment of contamination
  • Monitoring microbial growth
  • Evaluation of sterilization and disinfection
  • Clinical diagnosis
  • Food and water microbiology
• Bacterial counting methods are broadly divided into:
  • Total bacterial count
  • Viable bacterial count
• Total count includes both living and dead bacteria, whereas viable count includes only living bacteria capable of growth and multiplication.

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Total Bacterial Count

• Total bacterial count estimates the overall bacterial population within a sample, regardless of whether the cells are alive or dead.
• This measure is essential in contexts where bacterial biomass or general population dynamics are of interest, such as environmental studies, microbiome research, or quality control in industrial applications.

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Methods for Total Bacterial Count

Direct Microscopic Count 

• Direct Microscopic Count (DMC) is one of the simplest and oldest methods used for counting bacteria directly under the microscope.
• It is used to determine the total number of bacterial cells present in a given sample.
• This method counts all bacterial cells, including:
  • Living bacteria
  • Dead bacteria
• Because it counts both live and dead organisms, it is called a total bacterial count method.
• It is especially useful when rapid estimation of bacterial population is required without waiting for culture growth.
• DMC is commonly used in:
  • Microbiology laboratories
  • Milk bacteriology
  • Water analysis
  • Fermentation studies
  • Industrial microbiology

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Principle

• A measured volume of bacterial suspension is placed in a special counting chamber.
• The chamber contains ruled squares of known dimensions.
• Bacteria present in selected microscopic squares are counted under the microscope.
• Since the area and depth of the chamber are known, the volume represented by each square can be calculated.
• From this, the number of bacteria per milliliter of sample is determined.

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Counting Chamber Used in DMC

Common Counting Chamber

• Petroff-Hausser chamber

Structure of Counting Chamber

• It is a thick glass slide with:
  • Central ruled grid
  • Known square dimensions
  • Fixed chamber depth

Chamber Depth

• Usually 0.02 mm

Grid Arrangement

• Large square divided into many small squares for counting.

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Materials Required

• Bacterial suspension
• Petroff-Hausser counting chamber
• Coverslip
• Microscope
• Pipette
• Diluting fluid if required

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Preparation of Sample

If Sample is Too Concentrated

• Dilution is required before counting.

Common Dilution

• 1:10
• 1:100

Importance of Proper Dilution

• Prevents overcrowding of bacteria
• Improves counting accuracy

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Procedure

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Microscopic Examination

Objective Used

• Usually oil immersion objective is used.

Counting Method

• Count bacteria in multiple squares.
• Average count is taken for calculation.

Counting Rule

• Count organisms touching:
  • Upper boundary line
  • Left boundary line
• Do not count organisms touching:
  • Lower boundary line
  • Right boundary line

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Calculation of Bacterial Count

Formula

Number of bacteria per mL = Average number of bacteria per square×Dilution factor  /  Volume of one square​

Observation

• All bacterial cells visible in the selected squares are counted.
• Both live and dead bacteria are included.

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Advantages of Direct Microscopic Count

Major Advantages

• Rapid method
• No incubation required
• Immediate result obtained
• Useful for heavily contaminated samples
• Simple laboratory procedure

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Flow Cytometry

• Flow cytometry is an advanced laboratory method used to analyze physical and chemical properties of cells or particles suspended in fluid.
• It allows rapid examination of thousands of cells individually within a few seconds.
• The technique is widely used in hematology, immunology, oncology, microbiology, and transplant medicine.
• In flow cytometry, cells pass one by one through a laser beam, and signals produced are recorded and analyzed by computer.

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Principle

• Cells suspended in fluid pass through a narrow chamber in single file.
• Each cell passes through a laser beam.
• Laser light interacting with the cell produces:
  • Forward scatter (FSC)
  • Side scatter (SSC)
  • Fluorescence signals
• These signals are detected and converted into electronic data.

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Components

Fluidics System

• Transports cells in fluid stream.
• Ensures cells pass singly through laser beam.

Optical System

• Contains laser source, lenses, and filters.
• Illuminates cells and collects light signals.

Detection System

• Converts optical signals into electrical signals.
• Uses photodiodes and photomultiplier tubes.

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Procedure

1. Prepare cell suspension.
2. Add fluorescent-labeled antibodies.
3. Incubate sample.
4. Wash excess reagent.
5. Run sample through flow cytometer.
6. Analyze signals by computer.

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Data Presentation

Histogram

• Shows single parameter distribution.

Dot Plot

• Shows relationship between two parameters.

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Applications

Hematology

• Leukemia and lymphoma diagnosis

Immunology

• CD marker analysis

Oncology

• Tumor cell analysis

Microbiology

• Cell counting

Transplantation

• Immune monitoring

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Turbidity Measurement

• Turbidity measurement is a rapid method used for estimating the total number of bacteria present in a liquid culture.
• When bacteria multiply in broth medium, the medium becomes cloudy or turbid because bacterial cells remain suspended in the liquid.
• This turbidity is directly related to bacterial concentration.
• Since both living and dead bacterial cells contribute to cloudiness, this method gives total bacterial count rather than viable count.

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Principle

• Suspended bacterial cells scatter light passing through the liquid medium.
• As bacterial concentration increases:
  • Light transmission decreases
  • Light scattering increases
• The degree of turbidity is proportional to bacterial population.

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Instruments Used

Spectrophotometer

• Spectrophotometry is commonly used to measure turbidity.
• It measures optical density (OD) or absorbance.

Nephelometer

• Measures scattered light from bacterial suspension.

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Procedure

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Observation

• Clear broth indicates low bacterial count.
• Increased turbidity indicates increased bacterial count.

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Interpretation

• Higher optical density indicates greater bacterial concentration.
• Optical density is compared with standard calibration curve to estimate bacterial count.

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Calculation

• Optical density value is converted into bacterial concentration using standard reference curve.

 

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Viable Bacterial Count

• Viable bacterial count is the method used to determine the number of living bacteria present in a sample.
• Only bacteria capable of growth and multiplication are counted.
• Dead bacteria are not included in this method.
• The result is expressed as CFU/mL (Colony Forming Units per milliliter).
• This method is important because each visible colony on culture medium is assumed to arise from one viable bacterial cell or a group of viable cells.

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Methods for Viable Bacterial Count

Plate Count Method

Principle

• A bacterial sample usually contains very large numbers of bacteria, so direct plating may produce overcrowded colonies.
• Therefore, serial dilution of the sample is performed first.
• A measured quantity of diluted sample is inoculated on nutrient agar.
• During incubation, viable bacteria multiply and form visible colonies.
• Colonies are counted, and bacterial concentration is calculated.

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Materials Required

• Sterile nutrient agar plates
• Sterile dilution tubes
• Sterile pipettes
• Inoculating spreader
• Incubator
• Bacterial sample

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Serial Dilution Before Plate Count

Why Serial Dilution is Required

• To reduce bacterial concentration.
• To obtain countable colonies.

Common Dilutions

• 10⁻¹
• 10⁻²
• 10⁻³
• 10⁻⁴

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Procedure of Plate Count Method

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Types of Plate Count Method

1. Pour Plate Method

Procedure

• 1 mL diluted sample is placed in sterile petri dish.
• Molten agar at 45°C is added.
• Plate is gently rotated.
• After solidification, incubate.

Result

• Colonies develop:
  • Inside agar
  • On agar surface

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2. Spread Plate Method

Procedure

• Small volume of diluted sample is placed on agar surface.
• Sterile spreader distributes sample evenly.
• Plate is incubated.

Result

• Colonies form only on surface.

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Incubation

Temperature

• Usually 37°C for clinical bacteria

Duration

• 18–24 hours or according to organism

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Colony Counting

Ideal Countable Range

• 30–300 colonies per plate

Why This Range is Used

• Less than 30 colonies → statistically unreliable
• More than 300 colonies → overcrowding and merging

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Calculation of Viable Count

Formula

CFU/mL = Number of colonies × Dilution factor / Volume inoculated

Observation

• Each separate colony is counted.
• Mixed colony types may indicate contamination.

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Interpretation

• More colonies indicate higher viable bacterial count.
• Colony morphology may also provide preliminary identification.

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Advantages of Plate Count Method

Major Advantages

• Counts only living bacteria
• Highly accurate
• Widely accepted standard method

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Most Probable Number (MPN) Method

Principle

• A sample is serially diluted and inoculated into multiple broth tubes.
• After incubation, tubes are examined for bacterial growth.
• The number of positive tubes in each dilution series is compared with standard MPN statistical tables.
• From this pattern, the most probable bacterial count is estimated.

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Why It Is Called Most Probable Number

• The exact number of bacteria cannot be directly counted.
• Statistical probability is used to estimate the most likely number of viable organisms present.

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Materials Required

• Sterile broth tubes
• Durham tubes (for gas detection if coliform testing)
• Pipettes
• Sample for testing
• Incubator

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Procedure

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Arrangement of Tubes

Common Three-Tube Method

• First set: 10 mL sample
• Second set: 1 mL sample
• Third set: 0.1 mL sample

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Observation

Positive Tube Shows

• Turbidity
• Gas formation
• Color change depending on medium

Negative Tube Shows

• Clear broth without reaction

Interpretation

• Positive tube pattern is written as three-digit combination.

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Membrane Filtration

Principle

• A measured volume of liquid sample is passed through a membrane filter with very small pores.
• The membrane usually has pore size 0.45 µm, which retains bacteria.
• After filtration, the membrane is placed on nutrient agar or selective medium.
• Viable bacteria retained on the membrane grow into visible colonies during incubation.

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Membrane Filter Used

Characteristics

• Thin sterile membrane
• Uniform pore size

Common Pore Size

• 0.45 micrometer

Function

• Retains bacteria while liquid passes through.

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Apparatus Required

• Filtration unit
• Vacuum pump
• Membrane filter
• Sterile forceps
• Nutrient agar plate

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Procedure

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Observation

• Colonies appear directly on membrane surface after incubation.
• Each colony represents one viable organism retained on filter.

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Calculation

Formula

CFU per mL = Number of colonies counted  /  Volume of sample filtered

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Advanced Methods and Viability Distinction Techniques

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Practical Applications in Various Fields

Clinical Microbiology

• Used for bacterial count in:
  • Urine samples
  • Blood cultures
  • Sputum samples
  • Wound specimens
• Helps determine infection severity and monitor treatment response.

Water Analysis

• Used to assess microbial safety of drinking water.
• Common methods:
  • Most Probable Number (MPN)
  • Membrane filtration
• Detects coliform bacteria indicating fecal contamination.

Food Microbiology

• Used to detect bacterial contamination in:
  • Milk
  • Meat
  • Vegetables
  • Processed foods
• Ensures food safety and shelf-life monitoring.

Pharmaceutical Industry

• Used for sterility testing of:
  • Injections
  • Syrups
  • Tablets
  • Intravenous fluids

Dairy Industry

• Bacterial count determines milk quality and hygiene during processing.

Fermentation Industry

• Used to monitor bacterial growth during production of:
  • Antibiotics
  • Yogurt
  • Organic acids

Research Laboratories

• Used for microbial growth studies and experimental analysis.

Public Health Microbiology

• Helps monitor outbreaks and environmental contamination.