6. Microorganisms that produce antibiotics are unaffected by the same antibiotic. One
mechanism to explain this is:
a. Microorganisms export the antibiotic using efflux pumps as soon as it exceeds a
certain intracellular concentration
b. Antibiotics are made in parts and are assembled after being transported to the
extracellular environment.
c. Antibiotics are always produced in inactive forms and get activated by the enzymatic
action of the target cell
d. The active pharmacophore of the bactericidal antibiotic remains bound to antibodies
while they are intracellular

Correct answer: a. Microorganisms export the antibiotic using efflux pumps as soon as it exceeds a certain intracellular concentration.​

Why option (a) is correct

Many antibiotic‑producing microbes (especially actinomycetes like Streptomyces) possess efflux pumps that actively export the antibiotic out of the cell, keeping the intracellular concentration below toxic levels. These membrane transporters recognize the antibiotic (or its precursor) and pump it into the extracellular environment, providing self‑resistance while still allowing high extracellular levels that inhibit competitors.​

In several producers, an inactive precursor of the antibiotic can induce expression of the efflux pump before the fully active molecule accumulates, so the cell is already protected when active antibiotic is formed. This fits the idea in option (a): rapid export as soon as intracellular concentration rises, which is a well‑documented resistance mechanism in antibiotic‑producing bacteria.​

Why option (b) is incorrect

Option (b): “Antibiotics are made in parts and are assembled after being transported to the extracellular environment.”

• Antibiotics are typically synthesized as full molecules (sometimes as inactive precursors/prodrugs) via biosynthetic pathways inside the cell, not as “parts” that are later assembled outside.​

• While many antibiotics are secreted only after biosynthesis, the assembly steps are enzymatic and intracellular, not extracellular Lego‑style assembly of fragments.​

Therefore, this mechanism does not accurately describe the main reason for self‑protection in producers and is not a recognized general strategy.

Why option (c) is incorrect

Option (c): “Antibiotics are always produced in inactive forms and get activated by the enzymatic action of the target cell.”

• Some antibiotics are indeed produced as prodrugs or less active precursors that are later activated, but this is not universal (“always”).​

• Activation usually occurs via producer enzymes or environmental factors, not by specific enzymes of the target cell; many targets do not carry out any activation step at all.​

Thus, the absolute wording (“always” and “by the enzymatic action of the target cell”) makes this option incorrect as a general explanation.

Why option (d) is incorrect

Option (d): “The active pharmacophore of the bactericidal antibiotic remains bound to antibodies while they are intracellular.”

• Antibiotic‑producing bacteria do not rely on antibodies for self‑protection; antibodies are components of the adaptive immune system of higher animals, not bacterial proteins.​

• Known microbial resistance mechanisms include efflux pumps, enzymatic inactivation, target modification and metabolic shielding, but not antibody binding.​

Therefore, this option is biologically inappropriate for bacteria and clearly incorrect.

SEO‑friendly introduction

Antibiotic producing microorganisms must avoid being killed by the very antibiotics they secrete, so they evolve precise self resistance mechanisms to survive. One of the most important of these mechanisms is the use of efflux pumps that rapidly export the antibiotic, keeping intracellular levels below toxic thresholds while still harming competing microbes.​

Summary table of options