Question

3) We used two methods, turbidity and spread plates, to determine bacterial growth. What is a disadvantage of turbidity over spread plate? 4) We used two methods, turbidity and spread plates, to determine bacterial growth. What is a disadvantage of spread plate over turbidity?

Answer 1

Turbidity measurement is generally performed by measuring the density of bacterial growth using normal spectrophotometer. Generally, bacteria are grown in nutrient or luria broth and incubate for 24 hrs (depends on the growth cycle of bacteria). Later, the density of cultured broth is measured by using spectrophotometer at 600 nm, which works on principle of Beer- Lambert law.

The disadvantages of this measurement are:

We cannot differentiate between different species (if it is mixed culture). Even we cannot identify different sub species present within same sample.

If the inoculated sample or broth is contaminated with other bacteria, we will not get accurate results.

We cannot calculate how many exact colonies are present in sample (if compared to spread plate).

We cannot make exact cell count because the cells tend to change their cell size and shape at different cell stages.

The major disadvantage is we cannot differentiate between dead and live cells because both scatter light in spectrophotometer and show reading  

Whereas in spread plate technique, we do serial dilution of the sample and inoculate particular dilution onto the plate. After growth of bacteria, we can determine colony morphology and also identify different species present in same sample (Environmental sample). we can also count bacterial colonies. Spread plate technique is widely used to determine the bacterial diversity and growth in environmental samples.

The major limitations are:

It time consuming compared to turbidity (we have to serial dilute the sample and inoculate;whereas, in turbidity measurement we can inoculate concentrated sample directly).

Strict aerobes will grow properly, whereas microaerophiles will take time to grow.

The proper dilutions should be taken and pipette errors should be minimized.

It will be difficult to enumerate if colonies were overgrown and form clumps.