Question

what is purpose of bacteria isolation and why is it necessary for medical diagnosis? Describe two different plates (How do they work and what information do they give you about bacteria)

Answer 1

In microbiology, the term isolation means the separation of a bacterial strain from a population of living microbes, as present in the environment(eg: water or soil flora) or from living beings in the skin , oral cavity or gut , to identify the microbes.

Isolation of bacteria includes several steps –

• Specimen collection
• Preservation and transportation of specimen
• Microscopic examination of sample

Various methods are used for the isolation of bacteria culture. Important ones being culture on solid or liquid media and automated system.

Isolation is done to get pure bacterial cultures. Pure culture is necessary in the study of physiology, morphology, biochemical characteristics, and susceptibility to antimicrobial agents of a bacterial strain. This will help in the exact medical diagnosis when we correlate it with clinical history.

There are two main ways to isolate organisms.

1. Streaking for isolation on agar plate.
2. The pour plate method.

Agar plate is a Petri dish which contains agar as solid growth medium and nutrients. It is used to culture microorganisms. Sometimes selective compounds are added to the culture plate to influence growth, eg:antibiotics.

Blood agar

Blood agar is a an enriched medium which is used to culture the bacteria or microbes that do not grow easily.

It contain mammalian blood (sheep or horse) at a concentration of 5–10%. BAPs also contain meat extract, sodium chloride, tryptone, and agar. It acts as a differential media used to isolate fastidious organisms and it detect hemolytic activity.

• β-Hemolytic activity shows complete digestion of RBC contents surrounding that colony. Eg:Streptococcus haemolyticus.
• α-Hemolysis causes incomplete lysis of the RBC (the cell membrane is intact) . It appears green or brown colour, hemoglobin to methemoglobin conversion is the cause for this colour. An example is Streptococcus viridans.
• γ-Hemolysis (or nonhemolytic) means there is no hemolytic activity.

Chocolate agar

Chocolate Agar is actually lysed blood agar. The name is derived from the chocolate-brown color of the medium which is due to RBC lysis. Chocolate agar is used for isolation of fastidious organisms, such as Haemophilus influenzae.

The composition of chocolate agar is same as blood agar. The only difference is when preparing chocolate agar, the RBCs are lysed.

It is used for the short-term storage of microbes such as N.meningitidis, S.pneumoniae, and H.influenzae for up to 1 week.

1. Neisseria meningitidis: Growth is greyish, non-hemolytic, convex, round moist, smooth, glistening colonies with clearly defined edge.
2. Neisseria gonorrhoeae: Colonies appear as pinkish-brown and translucent, have smooth consistency and defineite margins, they are usually 0.5-1 mm diameter in size
3. Haemophilus influenzae: Non-hemolytic colonies which are opaque cream-to-grey in appearance.