1.3 Preventing entry, increasing exit
Antibiotics are only effective if they can reach their target. Preventing antibiotics from reaching their target is the final mechanism of antibiotic resistance that you will look at this week.
As you should recall from Week 2, the cell wall protects bacteria from osmotic and mechanical damage. To reach their targets inside the cell, antibiotics must cross this cell wall. In Activity 1 you will look at the mechanisms that antibiotics use to cross this bacterial cell wall.
Activity 1 Transporting antibiotics across the bacterial cell wall
First, watch the following animation which describes how antibiotics are transported across the bacterial cell wall.

Transcript: Video 1 Animation of the mechanisms of transport of antibiotics across the membrane.
Now answer the following questions.
a.
(a) decreases the amount of antibiotic entering Gram-negative bacteria
b.
(b) increases the amount of antibiotic entering Gram-negative bacteria
c.
(c) has no effect on the amount of antibiotic entering Gram-negative bacteria.
The correct answer is a.
a.
Your answer is correct. Most antibiotics cannot cross the outer membrane of Gram-negative bacteria and therefore enter the cell via porin channels. Decreasing the number of porin channels will decrease the amount of antibiotic entering the bacteria.
a.
(a) very few porin channels on their outer membrane or have replaced their porin channels with channels that exclude penicillin
b.
(b) numerous porin channels on their outer membrane
c.
(c) replaced their porin channels with channels that selectively transport penicillin.
The correct answer is a.
a.
Your answer is correct. If an antibiotic cannot reach its target, bacteria will be resistant to its action. Decreasing the expression of porins, or replacing them with channels that cannot transport the antibiotic, will prevent the antibiotic from crossing the outer membrane and reaching its target, therefore these bacteria will be resistant.
a.
(a) increase the amount of penicillin in the bacterial cell
b.
(b) decrease the amount of penicillin in the bacterial cell
c.
(c) have no effect on the amount of penicillin in the bacterial cell.
The correct answer is b.
b.
Your answer is correct. Efflux pumps actively transport antibiotics out of the bacterial cell. Therefore, increasing transport through these channels will decrease the amount of antibiotic inside the cell.
a.
(a) efflux pumps that are unable to transport penicillin
b.
(b) efflux pumps that transport penicillin
c.
(c) no efflux pumps.
The correct answer is b.
b.
Your answer is correct. If an antibiotic cannot reach its target, bacteria will be resistant to its action. Actively transporting antibiotics out of the cell decreases their concentration inside the cell, so that they cannot build up to a high enough concentration to exert the effect on their target.
Increasing active transport by expressing more efflux pumps that can actively transport the antibiotic out of the cell decreases the amount of antibiotic inside the cell and prevents it from acting on its target.
As you should now appreciate, bacteria can prevent antibiotics from reaching their target by decreasing the permeability of their outer membrane or by actively transporting antibiotics out of the cell (Activity 1). Both decreased porin expression and increased efflux pump expression have been reported in antibiotic-resistant clinical
You will look at an example of how altering porin expression contributes to antibiotic resistance in the case study at the end of this week.