A novel mechanism of ceftolozane-tazobactam resistance in Pseudomonas aeruginosa mediated by L2 β-lactamase

This study identifies a novel mechanism of high-level ceftolozane-tazobactam resistance in *Pseudomonas aeruginosa* mediated by the acquisition of multiple copies of an *L2* β-lactamase gene (*blaL2*) and a truncated regulator, which confers resistance independently of its canonical AmpR regulator.

The Gist

The Big Picture: A New Super-Weapon for Bacteria

Imagine Pseudomonas aeruginosa as a notorious, tough-as-nails burglar that loves to break into hospitals and infect patients. For a long time, doctors had a very effective "super-lock" to keep this burglar out: a powerful antibiotic combination called Ceftolozane-Tazobactam (C/T). It was like a master key that could pick almost any lock the burglar had.

However, in this study, researchers found a new trick the burglar learned. They discovered that this specific strain of bacteria had stolen a new, rare weapon from a different type of germ (a neighbor called Stenotrophomonas maltophilia) and used it to break the "super-lock."

The Story of the Patient: A Case of Mistaken Identity

The story starts with a patient who had been in and out of the hospital with serious infections.

• Two months ago: The doctors found the "burglar" (bacteria) in his lungs. They tested it, and it was still scared of the super-lock (C/T). The doctors treated him, and he got better.
• Two weeks later: The patient got sick again. This time, the bacteria in his bone infection was completely immune to the super-lock. It was like the burglar had learned how to pick the lock instantly.

The doctors were confused. Usually, bacteria become resistant by making small mutations to their own DNA (like changing the shape of their own tools). But when they sequenced the DNA of this new, super-resistant bacteria, they found something shocking: it wasn't just a mutation. The bacteria had acquired a brand new gene that it didn't have before.

The "Heist": Stealing a Gene from a Neighbor

The researchers found that the bacteria had acquired a gene called blaL2.

• The Analogy: Think of Pseudomonas and Stenotrophomonas as two different families living in the same neighborhood (the human body or the environment). Usually, they keep to themselves. But in this case, the Pseudomonas burglar somehow snuck into the Stenotrophomonas house and stole a specific blueprint for a specialized enzyme (a protein that acts like a pair of molecular scissors).
• The Weapon: This enzyme (L2 beta-lactamase) is designed to cut up antibiotics. In its original home (Stenotrophomonas), it's very good at destroying many drugs. When the Pseudomonas stole it, it suddenly gained the ability to chop up Ceftolozane-Tazobactam.

The "Copy-Paste" Glitch: Why It Got So Strong

Here is the wildest part of the story. The researchers found that this stolen gene wasn't just sitting there once.

• The Analogy: Imagine the bacteria didn't just steal one blueprint; it photocopied it five times and pasted all five copies into its own instruction manual, one right after the other.
• The Result: Because it had five copies of this "scissor gene," it was producing a massive amount of the enzyme. It was like having five factories churning out scissors instead of one. This allowed the bacteria to destroy the antibiotic so quickly that the drug couldn't work at all.

The "Broken Remote": A Surprising Discovery

Usually, when bacteria have a gene like this, they also have a "remote control" (a regulator gene) that turns the gene on only when they see the antibiotic.

• The Twist: In this stolen package, the "remote control" was broken (truncated). It was cut short and couldn't function.
• The Surprise: The researchers expected the bacteria to be confused without the remote. Instead, they found that the bacteria had learned to run the factory 24/7, regardless of whether the antibiotic was there or not. The broken remote didn't stop the production; it just meant the bacteria was constantly armed and ready to fight, even before the drug arrived.

What Did the Scientists Do to Prove It?

To make sure this stolen gene was actually the cause of the resistance, the scientists played a game of "genetic Jenga":

1. Adding the Gene: They took a harmless bacteria and gave it just one copy of this stolen gene. Suddenly, the harmless bacteria became slightly resistant to the drug.
2. Removing the Gene: They took the super-resistant bacteria from the patient and deleted the stolen gene. Suddenly, the bacteria became weak again and the drug worked!
3. The Conclusion: This proved that the stolen gene was the only reason the bacteria was so tough.

Why Does This Matter?

This discovery is a wake-up call for doctors and scientists:

• Bacteria are creative: They don't just mutate slowly; they can steal entire "super-weapons" from other species.
• Rare but dangerous: This specific gene (blaL2) is very rare in Pseudomonas, but when it shows up, it can make the bacteria nearly impossible to treat with our current best drugs.
• The future: As we use more antibiotics, we might see more of these "heists." Doctors need to be aware that if a patient isn't getting better with Ceftolozane-Tazobactam, the bacteria might have stolen a new tool, and they might need to switch to a different type of drug (like Ceftazidime-Avibactam, which this specific thief couldn't break).

In short: A tough hospital germ stole a "super-scissor" from a neighbor, copied it five times, and learned to run it non-stop, making it immune to one of our best antibiotics. The scientists found the thief, identified the stolen tool, and proved that removing the tool makes the germ vulnerable again.

Technical Summary

1. Problem Statement

Pseudomonas aeruginosa is a major cause of difficult-to-treat (DTR) infections, particularly in critically ill patients. Ceftolozane-tazobactam (C/T), a combination of a novel cephalosporin and a $\beta$-lactamase inhibitor, has been a cornerstone therapy for multidrug-resistant (MDR) P. aeruginosa. However, resistance to C/T is emerging.

• Current Knowledge: The primary mechanism of C/T resistance in P. aeruginosa involves mutations in the chromosomal bla$_{AmpC}$ gene (leading to overexpression or structural changes) or overexpression of efflux pumps.
• The Gap: While rare, novel mechanisms of resistance are appearing. This study investigates a specific clinical case where a patient developed high-level C/T resistance (>256/4 µg/mL) despite the absence of known bla$_{AmpC}$ mutations, suggesting an alternative, unidentified resistance mechanism.

2. Methodology

The study employed a multi-faceted approach combining clinical epidemiology, genomics, molecular biology, and microbiology:

• Clinical Case Analysis: Longitudinal tracking of P. aeruginosa isolates from a single patient with chronic osteomyelitis and pneumonia over several months. Isolates included a susceptible strain (PA-NM-055, collected 2 months prior) and a highly resistant strain (PA-NM-086, collected during the index admission).
• Whole Genome Sequencing (WGS):
  • Short-read (Illumina): Used for de novo assembly and variant calling.
  • Long-read (Oxford Nanopore): Used to resolve complex genomic regions, specifically tandem repeats.
  • Comparative Genomics: Compared PA-NM-086 against PA-NM-055 and public databases (NCBI) to identify acquired resistance genes and sequence types (ST).
• Genetic Characterization:
  • Identified the presence of the bla$_{L2}$ gene (typically found in Stenotrophomonas maltophilia) and a truncated regulator gene (ampR$_{L2}^{tr}$).
  • Analyzed the genomic context, revealing a 9,034 bp element containing bla$_{L2}$, ampR$_{L2}^{tr}$, and other resistance genes (tetracycline, aminoglycoside) repeated in tandem.
• Functional Validation:
  • Exogenous Expression: Cloned bla$_{L2}$ into reference strains (PA14, PAO1) using an IPTG-inducible plasmid to assess its direct impact on C/T Minimum Inhibitory Concentrations (MICs).
  • Gene Deletion: Created a deletion mutant ($\Delta$bla$_{L2}^{act}$) in a clinical isolate (PS2045) containing a single copy of the gene to confirm the gene's role in resistance.
  • Regulator Analysis: Deleted the truncated regulator (ampR$_{L2}^{tr}$) to determine if it controlled bla$_{L2}$ expression.
• Phenotypic Testing: Determined MICs for C/T and other $\beta$-lactams using broth microdilution and E-test strips.
• Transcriptomics: Used RT-qPCR to measure bla$_{L2}$ and bla$_{AmpC}$ expression levels under induced (imipenem) and uninduced conditions.

3. Key Contributions

• Discovery of a Novel Mechanism: The study identifies the acquisition of the L2 $\beta$-lactamase (a class A serine $\beta$-lactamase native to S. maltophilia) as a novel mechanism conferring C/T resistance in P. aeruginosa.
• Cross-Species Transfer Evidence: It provides genomic evidence of the horizontal transfer of a chromosomal resistance element from S. maltophilia to P. aeruginosa, a rare event given the chromosomal nature of the gene in the donor species.
• Regulator Independence: The study demonstrates that in P. aeruginosa, the L2 $\beta$-lactamase confers resistance independently of its canonical transcriptional regulator (ampR$_{L2}$). The acquired regulator was truncated and non-functional, yet the enzyme was expressed constitutively.
• Copy Number Effect: The research highlights a correlation between the copy number of the bla$_{L2}$-containing genetic element and the degree of resistance (MIC), with the highly resistant isolate containing five tandem copies.

4. Key Results

• Genomic Findings:
  • The resistant isolate (PA-NM-086) belonged to ST235, while the earlier susceptible isolate (PA-NM-055) was ST111, indicating a replacement by a different high-risk clone.
  • PA-NM-086 contained five tandem copies of a 9,034 bp genetic element. This element included bla$_{L2}$, a truncated ampR$_{L2}$ (507 bp vs. 867 bp in S. maltophilia), and other resistance genes.
  • No mutations were found in the native bla$_{AmpC}$ gene or efflux pump regulators that could explain the resistance.
• Phenotypic Impact:
  • Exogenous Expression: Introducing bla$_{L2}$ into PA14 and PAO1 increased C/T MICs by approximately 4-fold (2 doubling dilutions).
  • Deletion: Deleting bla$_{L2}$ in the clinical isolate PS2045 (1 copy) reduced the C/T MIC from ~4 µg/mL to ~0.75 µg/mL, confirming the gene's contribution.
  • Copy Number Correlation: Isolate PS2046 (2 copies) had a C/T MIC >256 µg/mL, while PS2045 (1 copy) had an MIC of ~4 µg/mL. PA-NM-086 (5 copies) also had an MIC >256 µg/mL.
• Regulation:
  • Unlike in S. maltophilia, where bla$_{L2}$ is induced by $\beta$-lactams via AmpR, bla$_{L2}$ expression in the P. aeruginosa isolates was constitutive and unaffected by imipenem induction.
  • Deletion of the truncated ampR$_{L2}^{tr}$ gene did not alter C/T MICs, confirming the regulator is non-functional in this context.
• Spectrum of Activity: The L2 $\beta$-lactamase conferred resistance to aztreonam, cefepime, ceftazidime, and piperacillin/tazobactam but did not confer resistance to carbapenems (imipenem, meropenem, doripenem) in the tested strains.

5. Significance

• Clinical Implications: This mechanism explains cases of C/T treatment failure where standard genomic screens for AmpC mutations or ESBLs are negative. Clinicians should be aware that C/T resistance can arise via the acquisition of foreign $\beta$-lactamases like L2.
• Therapeutic Targeting: The finding that L2 confers resistance independently of AmpR suggests that targeting the AmpR regulator (a proposed strategy for S. maltophilia) would be ineffective against P. aeruginosa strains expressing L2.
• Evolutionary Insight: The study underscores the environmental reservoir of resistance genes. Both P. aeruginosa and S. maltophilia are ubiquitous in soil and water (and co-colonize cystic fibrosis lungs), facilitating rare but clinically significant horizontal gene transfer events.
• Surveillance: The rarity of this event (found in only ~12 of ~46,000 public genomes) suggests it is currently an emerging threat rather than a widespread one, but its potential for amplification (via tandem duplication) warrants close surveillance.

In conclusion, this paper defines a new, rare, and clinically significant pathway for ceftolozane-tazobactam resistance in P. aeruginosa, driven by the cross-species acquisition and amplification of the S. maltophilia L2 $\beta$-lactamase, operating independently of its native regulatory system.