Understanding antibiotic resistance - 1.3 Preventing entry, increasing exit

(a) decreases the amount of antibiotic entering Gram-negative bacteria

(b) increases the amount of antibiotic entering Gram-negative bacteria

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.

Your answer is incorrect. 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.

2 Bacteria that are resistant to penicillin are likely to have:

(a) very few porin channels on their outer membrane or have replaced their porin channels with channels that exclude penicillin

(b) numerous porin channels on their outer membrane

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.

Your answer is incorrect. 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.

3 Increasing the rate of active transport of penicillin through the efflux pump would:

(a) increase the amount of penicillin in the bacterial cell

(b) decrease the amount of penicillin in the bacterial cell

Your answer is incorrect. 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.

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.

4 Bacteria that are resistant to penicillin are likely to have:

(a) efflux pumps that are unable to transport penicillin

(b) efflux pumps that transport penicillin

Your answer is incorrect. 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.

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.

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 isolates. For example, S. aureus that overexpresses multidrug-resistant efflux pumps, which transport a wide range of antibiotics, have been isolated from patients (Kosmidis et al., 2012).

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.

1.3 Preventing entry, increasing exit

Activity 1 Transporting antibiotics across the bacterial cell wall