Unit 7: Microbial growth-curve determination - Subjective Questions

Definition: A microbial growth curve is a graphical representation of the change in microbial population or cell density with time in a closed culture system.

Importance in turbidimetric analysis:

• It helps determine the pattern of E. coli growth under specified laboratory conditions.
• It identifies the major growth phases: lag, log, stationary, and death phases.
• It permits estimation of growth rate, generation time, and population changes.
• It helps determine the optimum period for harvesting cells.
• It is useful for comparing the effects of nutrients, temperature, pH, antibiotics, or other environmental factors on bacterial growth.

In the turbidimetric method, cell growth is estimated indirectly by measuring the increase in optical density of the culture over time.

Principle: Bacterial cells suspended in a liquid medium scatter and absorb incident light. As the number of cells increases, the culture becomes more turbid and its optical density increases.

In this method:

• A sterile liquid medium is inoculated with a standardized culture of E. coli.
• Samples are collected at regular time intervals.
• The absorbance or optical density is measured using a colorimeter or spectrophotometer.
• Optical density is plotted against incubation time to obtain the growth curve.

Within an appropriate range, optical density is approximately proportional to cell concentration. Therefore, turbidity provides an indirect estimate of microbial biomass, although it does not distinguish between living and dead cells.

Materials required:

• Pure culture of Escherichia coli
• Sterile nutrient broth or another suitable growth medium
• Sterile culture flasks or tubes
• Sterile pipettes or micropipettes
• Inoculating loop or sterile tips
• Colorimeter or spectrophotometer
• Cuvettes
• Incubator or shaking incubator
• Sterile distilled water or uninoculated medium for blanking
• Marker, timer, and laboratory record sheet

Purpose of the main equipment:

• The incubator provides controlled temperature for bacterial multiplication.
• The shaking incubator maintains aeration and uniform distribution of cells.
• The spectrophotometer measures optical density.
• Sterile glassware and instruments prevent contamination and ensure reliable results.

Procedure:

1. Prepare sterile nutrient broth in a suitable flask.
2. Set the spectrophotometer to an appropriate wavelength, commonly around for bacterial cultures.
3. Use uninoculated sterile broth as the blank.
4. Inoculate the sterile broth aseptically with a small, known amount of E. coli culture.
5. Mix the culture thoroughly and record the initial time.
6. Incubate the culture at the required temperature, usually with shaking for adequate aeration.
7. Withdraw samples at regular intervals, such as every or minutes.
8. Mix each sample gently before measurement and transfer it to a clean cuvette.
9. Measure and record the optical density of each sample.
10. Plot optical density against time to obtain the growth curve.
11. If necessary, dilute samples whose readings exceed the linear range of the instrument and multiply the observed value by the dilution factor.

The growth curve of E. coli in a closed culture generally contains four phases:

• Lag phase: Cells adapt to the new medium, synthesize enzymes, repair cellular structures, and prepare for division. There is little increase in cell number.
• Log or exponential phase: Cells divide at a nearly constant and maximum rate. The population increases exponentially, and cells are physiologically most active.
• Stationary phase: Nutrients become limited and toxic metabolic products accumulate. The rate of cell division becomes approximately equal to the rate of cell death, so the total population remains nearly constant.
• Death or decline phase: Viable cells decrease because of nutrient depletion, accumulation of toxic substances, unfavorable pH, and other stresses.

In a turbidimetric curve, the log phase generally shows the steepest increase in optical density.

The lag phase is mainly an adaptation period, whereas the log phase represents active and balanced multiplication.

Optical density is an indirect measurement because the instrument detects light scattering or absorbance caused by suspended cells rather than counting viable cells directly.

Advantages:

• Rapid and convenient
• Does not require colony formation
• Allows repeated measurements over time
• Requires relatively small sample volumes
• Useful for constructing growth curves and monitoring cultures continuously

Limitations:

• It measures both living and dead cells.
• It cannot distinguish cells from other suspended particles.
• Readings are reliable only within the linear range of the instrument.
• Very dense cultures must be diluted.
• Cell size, shape, aggregation, and pigmentation can affect the reading.
• A calibration curve is required to convert optical density into cell concentration accurately.

Thus, optical density is best used for estimating biomass and following trends in growth.

Correct use of the instrument involves:

• Selecting a wavelength suitable for bacterial turbidity measurements, commonly near .
• Allowing the instrument to warm up according to the manufacturer's instructions.
• Using a clean, unscratched cuvette.
• Filling the cuvette sufficiently and removing bubbles from the optical path.
• Wiping the outside of the cuvette before placing it in the holder.
• Zeroing or blanking the instrument with sterile uninoculated medium.
• Keeping the cuvette orientation consistent for every reading.
• Mixing samples uniformly before measurement.
• Diluting samples that give readings above the linear range.
• Recording the time and optical density immediately after each measurement.

These precautions improve precision and reduce errors caused by contamination, bubbles, fingerprints, or inconsistent sampling.

During the exponential phase, bacterial growth can be expressed as:

where is the initial population, is the population after time , and is the number of generations. Rearranging gives:

The generation time, , is the time required for one generation:

Therefore:

When optical density is proportional to cell concentration, optical density values may be used in place of population values:

This calculation should be made only using readings from the exponential phase and within the linear range of the instrument.

The generation time is the time required for one complete cycle of cell division or for the microbial population to double under specified conditions.

It is significant because:

• It provides a quantitative measure of the growth rate.
• A shorter generation time indicates faster multiplication.
• It allows comparison of growth under different media or environmental conditions.
• It helps identify the exponential phase of the growth curve.
• It is useful in industrial microbiology, clinical microbiology, and antibiotic studies.

For a culture growing exponentially, the generation time can be calculated from two optical density readings using:

Only data from the exponential phase should be used for this calculation.

Steps for plotting:

1. Arrange the observations in a table containing sampling time and optical density.
2. Place incubation time on the horizontal axis.
3. Place optical density on the vertical axis if a standard turbidity curve is required.
4. Plot each time and optical density pair accurately.
5. Join successive points with a smooth curve rather than connecting unrelated points with sharp lines.
6. Identify the lag, log, stationary, and death phases.
7. For a more accurate analysis of exponential growth, plot against time.
8. Determine the slope of the straight-line portion of the semi-logarithmic plot.
9. Use the exponential-phase data to calculate growth rate or generation time.

A properly labeled graph should include the title, axes, units, scale, data points, and phase designations.

Optical density versus time:

• Uses a linear scale for optical density.
• Shows the overall pattern of the growth curve clearly.
• Displays lag, log, stationary, and decline phases.
• The exponential phase appears as a curved, steeply rising region.

Logarithm of optical density versus time:

• Uses the logarithm of optical density on the vertical axis.
• Exponential growth appears approximately as a straight line.
• The slope can be used to calculate the specific growth rate.
• It is more suitable for determining generation time and comparing growth rates.

Thus, the ordinary plot is useful for visualizing the complete growth pattern, whereas the semi-logarithmic plot is more useful for quantitative analysis of exponential growth.

The uninoculated medium is used as a blank to establish the baseline absorbance of the growth medium and the cuvette.

It corrects for:

• Natural color or turbidity of the medium
• Absorption by dissolved nutrients
• Light scattering caused by the medium
• Minor optical effects of the cuvette

After blanking, the measured optical density primarily represents turbidity caused by the bacterial cells. If the blank is not used, the recorded values may be artificially high, leading to an incorrect estimation of cell growth. The same type of medium used for culturing should be used as the blank.

Precautions:

• Use a pure and freshly prepared culture of E. coli.
• Maintain aseptic conditions throughout the experiment.
• Use sterile medium, pipettes, tubes, and cuvettes.
• Maintain a constant incubation temperature and shaking speed.
• Use equal sample volumes and fixed sampling intervals.
• Mix the culture gently before removing each sample.
• Avoid bubbles, fingerprints, scratches, and droplets on the cuvette.
• Blank the instrument with uninoculated medium.
• Take readings within the linear range of the instrument.
• Dilute highly turbid samples and apply the dilution factor.
• Return samples to the culture only if sterility and experimental design permit it.
• Record time and optical density immediately.
• Dispose of cultures using appropriate biosafety procedures.

These measures reduce contamination, sampling error, instrument error, and variation in growth conditions.

A prolonged lag phase may occur for several reasons:

• The inoculum is old, damaged, or taken from the stationary phase.
• The inoculum size is too small.
• The composition of the new medium differs substantially from the previous medium.
• The incubation temperature, pH, or osmotic conditions are unsuitable.
• The culture has been exposed to antibiotics, disinfectants, radiation, or other stress.
• The medium lacks an essential nutrient or growth factor.
• The cells require time to synthesize enzymes needed to use a new substrate.
• The culture was not mixed or aerated adequately.
• Contamination or incorrect incubation conditions affected growth.

During lag phase, cells are metabolically active but are primarily adapting and preparing for division. A prolonged lag phase therefore does not necessarily indicate that the cells are dead.

The stationary phase is defined mainly by the absence of a significant increase in viable cell number, not necessarily by an immediate absence of turbidity.

Optical density may continue to increase because:

• Cells may enlarge without dividing.
• Non-viable cells and cellular debris still scatter light.
• Cell aggregation can alter light scattering.
• Some cells may continue dividing while others die, causing biomass to remain high.
• Intracellular storage materials may accumulate.
• The instrument may detect suspended debris and lysed-cell components.

Therefore, optical density measures total suspended biomass rather than viable cell count. Viable plate counts or another direct method may be required to confirm the stationary and death phases.

Use the generation-time equation:

Given:

• minutes

The ratio of the final to initial optical density is:

Since , the culture has undergone generations. Therefore:

Answer: The generation time of the culture is minutes, assuming that optical density is proportional to cell concentration and both readings were obtained during the exponential phase.

Optical density and viable cell count are related because an increase in the number of suspended bacterial cells generally causes greater scattering of light. During the early and middle exponential phases, optical density may show an approximately linear relationship with cell concentration within a suitable range.

However, the relationship is not universally direct because:

• Optical density measures total particles, including dead cells and debris.
• Viable cell count measures only cells capable of forming colonies.
• Cell size and shape change during growth.
• Cell clumping causes one group of cells to behave optically as a single particle.
• Very dense cultures produce readings outside the linear range.

To establish the relationship, serial dilutions can be plated for viable counting while corresponding optical density readings are recorded. A calibration curve can then be prepared by plotting viable cell count against optical density.

Sources of error and control measures:

• Contamination: Use aseptic technique and sterile materials.
• Unequal sampling intervals: Use a timer and fixed sampling schedule.
• Cell settling: Mix the culture gently before sampling.
• Cell clumping: Use consistent mixing and avoid excessive agitation that may damage cells.
• Incorrect blanking: Blank with the same uninoculated medium.
• Dirty or scratched cuvettes: Use clean, clear cuvettes and wipe their outer surfaces.
• Air bubbles: Remove bubbles before taking a reading.
• Readings above the linear range: Dilute the sample and multiply by the dilution factor.
• Temperature variation: Maintain a constant incubation temperature.
• Inadequate aeration: Use an appropriate flask-to-volume ratio and controlled shaking.
• Inaccurate pipetting: Calibrate pipettes and use proper pipetting technique.
• Instrument drift: Warm up and periodically check the spectrophotometer.

Careful standardization of culture conditions and measurement procedures improves reproducibility.

Samples should be diluted when their optical density exceeds the linear measuring range of the colorimeter or spectrophotometer.

Reasons:

• In very dense cultures, multiple scattering of light occurs.
• The instrument response is no longer proportional to cell concentration.
• The reading may appear falsely low or become unreliable.
• Dilution places the sample within the validated linear range.

After measurement, the original optical density is calculated as:

For example, if a sample is diluted tenfold and gives an optical density of , the estimated original optical density is:

The dilution method and factor must be recorded carefully.