Final answer:
The likely mechanism of antibiotic resistance observed, where adding protons increases the intrabacterial drug concentration, is decreased drug efflux pump activity. Efflux pumps, when operating less efficiently in an acidic environment, allow more antibiotic to accumulate inside the bacterium, indicating this as the resistance mechanism.
Step-by-step explanation:
The information provided indicates that the resistant bacteria exhibit decreased intracellular concentrations of the antibiotic. When protons are added to the growth media, intrabacterial drug concentrations increase, suggesting that the mechanism involved is likely related to changes in pH affecting drug accumulation. The probable mechanism is decreased drug efflux pump activity.
Efflux pumps are known to actively transport antimicrobial drugs out of the cell. In an acidic environment, which can be simulated by adding protons, these pumps may be less active or efficient, allowing more of the antibiotic to accumulate inside the bacteria. Therefore, when protons are added to the media, decreasing the pH, it may hinder the activity of efflux pumps, leading to an increase in the intracellular concentration of the antibiotic.
This mechanism is distinct from other resistance strategies such as drug degradation (where resistance genes code for enzymes that chemically modify or destroy the drug), target modification (where genetic changes in the targets prevent drug binding), and decreased drug uptake (where changes in the cell's membrane structures prevent the entry of the drug). Given the observed effects of proton addition, the most likely mechanism for the resistance observed by the scientist is decreased drug efflux pump activity.