Question

we need to set up some cultures on blood agar plates with antibiotics: a mixture of antibiotic-susceptible a. baumannii and antibiotic-resistant p. aeruginosa, along with various controls.
if we see a. baumannii colonies on the plate with a. baumanii and p. aeruginosa after the incubation, what could that mean?
a) a. baumannii is somehow able to aquire resistance from p. aeruginosa
b) antibiotics only work on monocultures
c) the medium was contaminated with antibiotic-resistant a. baumannii

Explanation:

• Option a: Horizontal gene transfer (acquiring resistance from another bacterium like P. aeruginosa) is a plausible mechanism for A. baumannii to gain antibiotic resistance, allowing it to grow in the presence of antibiotics.
• Option b: Antibiotics can work on mixed cultures (e.g., in clinical settings with polymicrobial infections), so this is incorrect.
• Option c: Contamination with resistant A. baumannii is a possible explanation, but the setup includes antibiotic - susceptible A. baumannii initially. However, the most biologically relevant explanation for the susceptible strain growing with a resistant strain (in the presence of antibiotics) is horizontal gene transfer (option a) or contamination (option c). But considering the experimental setup (mixing susceptible A. baumannii and resistant P. aeruginosa), the acquisition of resistance from P. aeruginosa (option a) or contamination (option c) are candidates. But among the options, option a is a key concept in microbial genetics (horizontal gene transfer), while option c is a technical error. However, if we consider the experimental design (using antibiotic - susceptible A. baumannii), seeing it grow with P. aeruginosa (resistant) in the presence of antibiotics could mean it acquired resistance from P. aeruginosa (horizontal gene transfer) or the medium was contaminated. But the question is about what “could” it mean. Option a is a valid biological explanation (acquiring resistance), option c is a technical error (contamination). But in a microbiology context, option a is a more likely biological interpretation of the result.

Answer:

a) A. baumannii is somehow able to acquire resistance from P. aeruginosa