1. What is the purpose of aseptic technique in microbiology lab? How can you ensure aseptic

technique when handling cultures and specimens?

- The purpose of aseptic technique is to prevent contamination of cultures, specimens,

equipment, and the environment by microorganisms. Aseptic technique can be ensured by

following proper procedures such as wearing gloves, lab coats, and eye protection, disinfecting

the work area and equipment before and after use, flaming the inoculating loop or needle,

opening culture tubes or plates near the flame, and avoiding contact with sterile surfaces or

materials.

2. What are the differences between gram-positive and gram-negative bacteria? How can you

identify them using gram staining?

- Gram-positive bacteria have a thick layer of peptidoglycan in their cell wall that retains the

purple dye (crystal violet) after decolorization with alcohol. Gram-negative bacteria have a thin

layer of peptidoglycan and an outer membrane that contains lipopolysaccharides, which are

washed away by alcohol and stain pink with the counterstain (safranin). Gram staining involves

four steps: applying crystal violet, iodine, alcohol, and safranin to a heat-fixed smear of bacteria

and observing the color under a microscope.

3. What are the advantages and disadvantages of using selective and differential media in

microbiology lab? Give an example of each type of media and explain how they work.

- Selective media are designed to inhibit the growth of certain types of bacteria while allowing

the growth of others. Differential media are designed to distinguish between different types of

bacteria based on their metabolic or biochemical characteristics. The advantages of using these

media are that they can help isolate and identify bacteria based on their growth patterns,

morphology, or color changes. The disadvantages are that they may not be specific enough to

differentiate between closely related bacteria or may not detect some bacteria that have atypical

reactions. An example of selective media is mannitol salt agar (MSA), which contains a high

concentration of salt that inhibits most bacteria except staphylococci, and also contains mannitol

and phenol red that indicate whether the bacteria can ferment mannitol (yellow colonies) or not

(red colonies). An example of differential media is blood agar, which contains sheep blood that

shows whether the bacteria can produce hemolysins that lyse red blood cells (beta-hemolysis:

clear zones around colonies; alpha-hemolysis: greenish zones around colonies; gamma-

hemolysis: no zones around colonies).