Gain and Loss of Acquired Doxycycline Resistance in Lactiplantibacillus plantarum: An Adaptive Laboratory Evolution Study

Through an adaptive laboratory evolution study, researchers demonstrated that *Lactiplantibacillus plantarum* rapidly acquires and subsequently loses modest doxycycline resistance over 1000 generations, a process driven by specific, reversible mutations in the *rpsJ* gene encoding ribosomal protein S10.

The Gist

Imagine a tiny, helpful bacterium named Lactiplantibacillus plantarum (let's call him "Larry"). Larry is a probiotic, meaning he lives in your gut and helps you stay healthy, often found in yogurt and kimchi.

Now, imagine we put Larry in a giant, high-tech gym. But instead of lifting weights, we are training him to survive a specific challenge: a low dose of an antibiotic called Doxycycline. This is like putting a tiny, annoying fly in the room with Larry. It's not enough to kill him, but it's annoying enough to make him sweat.

Here is the story of what happened to Larry over the next five months (about 1,000 generations of his life), explained simply.

1. The Training Camp (Adaptive Laboratory Evolution)

The scientists set up two groups of Larrys:

• The Control Group: These Larrys lived in a comfortable, antibiotic-free environment.
• The Training Group: These Larrys lived in an environment with a tiny bit of Doxycycline (1/10th of the amount needed to kill them).

Every day, the scientists took a small sample of the bacteria, moved them to fresh food, and repeated the process. This is like a relay race where the bacteria pass the baton to their children every 24 hours.

The Result: After about 1,000 generations (roughly 5 months), the "Training Group" Larrys had changed. They had learned to ignore the annoying fly. They became about 4 times more resistant to the antibiotic than they were at the start. They weren't invincible, but they could handle the stress much better.

2. The "Use It or Lose It" Rule

Here is the twist. Once the scientists saw that the Training Group had become tougher, they decided to stop the training. They removed the antibiotic completely and let the bacteria live in a clean, safe environment again.

The Result: The resistance didn't stick. Within just 50 generations (a tiny fraction of the time it took to gain the resistance), the bacteria forgot how to fight the antibiotic. They went back to being their original, sensitive selves.

The Analogy: Think of it like a bodybuilder. If you stop lifting weights, your muscles don't stay huge forever; they shrink back down because maintaining that muscle takes a lot of energy. The bacteria realized, "Hey, fighting this antibiotic is hard work and costs us energy. Since the antibiotic is gone, let's stop wasting energy on it and go back to our normal, efficient lives."

3. The Secret Weapon (The Genetic "Glitch")

The scientists were curious: How did Larry learn to fight back? They looked at his DNA (his instruction manual) using a high-tech microscope called Whole Genome Sequencing.

They found the answer in a specific part of the instruction manual called the rpsJ gene.

• What is rpsJ? Imagine the bacteria's cell is a factory. The rpsJ gene is the blueprint for a specific machine part (a protein called S10) inside the factory's assembly line. This machine part helps build new proteins, which is how the bacteria grows and reproduces.
• How does the antibiotic work? Doxycycline is like a wrench thrown into the gears of this assembly line. It jams the machine, stopping the factory from working.
• The Mutation: The bacteria developed tiny "typos" (mutations) in the rpsJ blueprint. These typos changed the shape of the machine part just enough that the wrench (the antibiotic) couldn't jam the gears anymore. The factory kept running, even with the wrench nearby.

The Catch: The scientists found that these "typos" were mostly non-synonymous mutations. In plain English, this means the DNA changed in a way that altered the final protein. It's like changing a word on a sign from "Stop" to "Slow"—the sign still works, but the message is different. This change was enough to reshape the machine part so the antibiotic's wrench no longer fit, but the machine still functioned well enough to keep the factory running.

4. Why This Matters

You might wonder, "So what? It's just a yogurt bug."

• The Good News: This shows that when we stop using antibiotics, bacteria can quickly lose their resistance. This is a hopeful sign for public health. It suggests resistance isn't always permanent; it can be reversible.
• The Bad News: It proves that even "safe" bacteria (probiotics) can evolve resistance if we expose them to antibiotics, even in small amounts. If we take antibiotics for a stomach bug, we might accidentally be training our good gut bacteria to become resistant, too.
• The Future: The scientists are now thinking, "What if we could give probiotics a little bit of this training before we give them to patients?" Imagine a probiotic pill that has already been trained to survive the antibiotic you are taking, so it can keep helping your gut while the antibiotic kills the bad bacteria.

Summary

The scientists played a game of "survival of the fittest" with a helpful gut bacteria.

1. Pressure: They exposed the bacteria to a low dose of antibiotics.
2. Adaptation: The bacteria evolved a tiny genetic tweak to survive.
3. Reversal: When the pressure was removed, the bacteria quickly dropped the tweak because it was too expensive to keep.
4. Mechanism: The tweak happened in a specific gene (rpsJ) that controls how the bacteria builds proteins, making the antibiotic's "wrench" less effective.

It's a story about how life is flexible, how it adapts to survive, and how quickly it can return to normal when the threat is gone.

Technical Summary

1. Problem Statement

Probiotics, particularly Lactiplantibacillus plantarum (formerly Lactobacillus plantarum), are widely used in fermented foods and clinical settings to support gut health. However, the widespread use of antibiotics, specifically tetracyclines like doxycycline (DOX), poses a risk of selecting for antibiotic resistance in these beneficial bacteria. While resistance mechanisms in pathogenic bacteria are well-documented, the dynamics of resistance acquisition and reversibility in probiotics under sublethal antibiotic pressure remain poorly understood. Specifically, it is unclear if probiotics can rapidly evolve resistance to DOX, what genetic mechanisms drive this adaptation, and whether such resistance is stable once the selective pressure is removed.

2. Methodology

The researchers employed an Adaptive Laboratory Evolution (ALE) approach to investigate these questions using the probiotic strain L. plantarum ATCC 14917 (Lp14917).

• Experimental Design:

  • Initial Setup: The Minimal Inhibitory Concentration (MIC) for DOX was determined to be ~30 µg/mL. The ALE experiment exposed three replicate cultures of Lp14917 to a sublethal concentration of 3 µg/mL (1/10 MIC) of doxycycline. Three parallel control cultures were grown without antibiotics.
  • Duration: The experiment ran for approximately 1,000 generations (5 months) with daily propagation.
  • Reversibility Phase: After generation 1000, doxycycline was removed from the media, and the cultures were propagated for an additional ~375 generations to observe if resistance was lost.
  • Archiving: Samples were frozen ("archived") every ~50 generations to create a "time machine" for retrospective analysis.

• Phenotypic Assays:

  • Fitness Tests: Every ~100 generations, cultures were subjected to microbroth dilution assays in 96-well plates with a gradient of DOX concentrations.
  • Quantification: Growth was measured via optical density ($OD_{600}$). Data was fitted to a four-parameter Hill model to calculate the $IC_{50}$ (concentration inhibiting 50% of growth) for each generation.

• Genomic Analysis:

  • Whole Genome Sequencing (WGS): Genomic DNA was extracted from archived samples at generations 0, 350, 750, 1000, and 1200. Sequencing was performed on an Illumina NovaSeq6000 (PE150).
  • Bioinformatics: Reads were mapped to the reference genome (GCA_000143745.1). Single Nucleotide Variants (SNVs) were detected using GATK and filtered for quality.
  • Verification: Specific variants in the rpsJ gene were verified using colony PCR and Sanger sequencing.

3. Key Results

A. Phenotypic Evolution of Resistance

• Acquisition: Over 1,000 generations, the DOX-exposed cultures evolved a modest but significant increase in resistance. The $IC_{50}$ increased from an initial 18.4 µg/mL to 70.7 µg/mL (a ~4-fold increase).
• Reversibility: Upon removal of doxycycline at generation 1000, resistance was lost rapidly. Within just 50 generations (approx. 1 week), the $IC_{50}$ of the previously exposed cultures dropped back to ~19 µg/mL, nearly identical to the original wild-type and control cultures. This indicates the resistance trait is reversible and likely carries a fitness cost in the absence of the antibiotic.

B. Genomic Mechanisms (WGS Analysis)

• Variant Identification: WGS of the generation 1000 cultures revealed 16 distinct SNVs across 15 genes. Crucially, these variants were absent in the original culture (Gen0) and same-generation controls.
• The rpsJ Gene: The most significant findings were mutations in the rpsJ gene, which encodes the ribosomal protein S10 (a component of the 30S ribosomal subunit).
  • Specific Mutations: Four non-synonymous SNVs were identified in rpsJ: H56Y, K57M, K57I, and S94N.
  • Location: These mutations cluster in a flexible loop (residues 50–60) of the S10 protein that interacts with the 16S rRNA helix h31, adjacent to the tetracycline binding site (A-site).
  • Dynamics: The H56Y mutation appeared early (Gen 350) and persisted. The K57M mutation was prevalent in Gen 350 and 750 but was largely lost by Gen 1200 after antibiotic removal.
• Other Variants: Other SNVs were found in genes related to transcription (e.g., bglG antiterminator), cell wall biogenesis (lysM, glnA), and nucleotide metabolism, but rpsJ variants were the most consistent with the resistance phenotype.

C. Verification

• Sanger sequencing confirmed the presence of H56Y, K57M, and K57I variants in specific colonies.
• Notably, some variants detected by WGS (e.g., S94N) were not found in all Sanger-sequenced colonies, suggesting population heterogeneity or low-frequency variants within the culture.
• No rpsJ variants were found in the Gen0 or control cultures.

4. Key Contributions

1. First ALE Study on L. plantarum and Tetracyclines: This is the first reported ALE experiment demonstrating the evolution of doxycycline resistance in L. plantarum under sublethal pressure.
2. Demonstration of Reversibility: The study provides empirical evidence that antibiotic resistance in probiotics can be rapidly lost (within ~50 generations) once the selective pressure is removed, suggesting that resistance is not genetically fixed and may impose a fitness cost.
3. Mechanistic Insight: The study identifies rpsJ mutations (specifically in the S10 ribosomal protein) as a primary driver of modest DOX resistance in this species, distinct from the more common efflux pump (tet) or ribosomal protection (tetM) mechanisms often found in other bacteria.
4. Methodological Framework: The combination of quantitative fitness assays (Hill model fitting) with time-resolved WGS provides a robust framework for tracking the evolution and reversion of resistance traits.

5. Significance and Implications

• Probiotic Safety: While the resistance acquired was modest (~4-fold), the ability of probiotics to evolve resistance under sublethal antibiotic exposure raises concerns about their use in patients undergoing antibiotic therapy.
• Horizontal Gene Transfer: The study notes that the resistance mechanisms observed (chromosomal rpsJ mutations) are less likely to be horizontally transferred compared to plasmid-borne resistance genes (like tet genes), potentially limiting the spread of resistance to pathogens.
• Therapeutic Applications: The findings suggest that "pre-adapting" probiotics to specific antibiotics (via ALE) could be a strategy to protect them during antibiotic treatment, ensuring their survival and efficacy in restoring gut microbiome diversity. However, the rapid loss of resistance upon removal of the drug implies that such adapted strains might need continuous selection or encapsulation to maintain the trait in the gut.
• Evolutionary Dynamics: The rapid reversion of resistance highlights the plasticity of bacterial genomes and the high fitness cost associated with rpsJ mutations in the absence of the antibiotic.

In conclusion, this study elucidates that L. plantarum can rapidly acquire and lose doxycycline resistance via specific ribosomal protein mutations, offering critical insights into the evolutionary stability of resistance in beneficial bacteria.