Question

Why will we be testing the same potential germicides against two different kinds of bacteria? Do you expect to see any differences in the results between the two? Explain. (Note: E. coli is gram negative; S. epidermidis is gram positive)

Answer 1

Ans. A germicide can be defined as any chemical agent that kills germs (or, bacteria).

Depending on the cell wall structure, the bacteria are broadly classified into two group - Gram-negative and Gram-positive. Gram-positive bacteria has a thick peptidoglycan layer in their cell wall. Gram-negative bacteria cell wall has a thin peptidoglycan layer, but also has an LPS layer. Due to difference in structure and function of cell wall of the two groups, the same germicide may show difference in efficacy in the two groups.

Therefore, the same germicide is tested against two different kind of bacteria to evaluate –

1. If the germicide is effective against both Gram-positive and Gram-negative bacteria.

If yes, to what extent in the two groups?

2. If the germicide is effective against both Gram-positive and Gram-negative bacteria, the germicide is most likely to kill the bacteria by interfering with the cellular metabolic pathways irrespective of their Gram-nature. For example, a germicide that binds to ribosomes, or inhibit protein translation machinery, etc. would kill both types of bacteria.

3. If the germicide kills bacteria of only one group (say, Gram-negative E. coli) but not the other group (say, Gram-positive S. epidermis), the germicide is most likely to inhibit LPS synthesis or may degrade LPS.

Similarly, the germicide may only be effective against Gram-positive bacteria but has no toxic effect on Gram-negative bacteria.

# The observation of difference in result with the two bacteria would depend on the mode of action of the germicide as explained above.

If the mode of action is (2), no difference is likely to be observed.

If the mode of action is (3), a significant difference in result would be observed because the germicide kills only one group of bacteria while leaving the other unaffected.