labs 17&18: antiseptics, disinfectants, and antibiotic effectiveness using kirby-bauer method

Sepsis

the growth of any microorganism

bacteriostatic

chemicals which inhibit the growth of bacteria

bactericidal

chemical that kills bacteria

disinfectant

used on non living surfaces

antiseptics

used on living tissues

zone of inhibition

to evaluate the effectiveness,

diameter of the zone

millimeter (mm)

Mueller Hinton agar (MHA)

bacteria are inoculated onto a special agar

Satellite Colonies

is when the inhibition is no longer in effect it causes this effect.

Do all antiseptics and disinfectants work equally well against all bacteria? Explain.

No. Different antiseptics/disinfectants vary in effectiveness depending on the type of bacteria. Some bacteria (like Gram-negative or spore-formers) are more resistant due to their cell wall structure, enzymes, or ability to form endospores.

If the zone of inhibition for E. coli is 12 mm for A and 4 mm for B, which is more effective?

Disinfectant A is more effective because it has a larger zone of inhibition, meaning it inhibited bacterial growth more.

Why is Mueller-Hinton agar used for Kirby-Bauer testing?

It allows consistent diffusion of antibiotics and supports good bacterial growth without interfering with results, making it reliable and standardized.

Resistant

Bacteria are not affected; antibiotic won’t work.

Susceptible

Bacteria are inhibited/killed; antibiotic is effective.

Intermediate

Partial effect; may work at higher doses or in certain conditions.

Broad-spectrum

Work against many types of bacteria (Gram + and -).

Narrow-spectrum

Target specific types of bacteria.

Why do different bacterial species respond differently to the same antibiotic?

They have different structures (cell walls, membranes), metabolic pathways, and resistance mechanisms (like enzymes or efflux pumps).

What do you need to evaluate effectiveness of the seven antibiotics?

You need to measure the zone of inhibition (in mm) and compare it to standard charts to determine susceptibility.

What do satellite colonies indicate?

They indicate that some bacteria are resistant or able to grow in low antibiotic concentrations, often due to enzyme production that inactivates the antibiotic nearby.