Potential Efficacy of Streptomycin in Amikacin-resistant Mycobacterium avium-intracellulare complex Pulmonary Disease

This study suggests that streptomycin remains a potential therapeutic alternative for amikacin-resistant *Mycobacterium avium-intracellulare* complex pulmonary disease because, unlike kanamycin, it does not exhibit cross-resistance with amikacin and is associated with distinct genetic mutations.

The Gist

The Big Picture: A Broken Lock and a Spare Key

Imagine your lungs are a fortress, and a stubborn group of bacteria called MAC (Mycobacterium avium-intracellulare) has moved in and started a siege. To kick them out, doctors usually use a powerful "lock-picking" tool called Amikacin (an antibiotic).

However, the bacteria are smart. They learned how to jam the lock, making Amikacin useless. This is called drug resistance. When this happens, doctors are in a panic because they need a new tool to break the siege.

The big question this study asked was: "If the bacteria jam the Amikacin lock, does that also jam our other backup keys, like Streptomycin and Kanamycin?"

The Investigation: Testing the Keys

The researchers looked at 20 patients whose bacteria had become resistant to Amikacin. They wanted to see if these bacteria were also resistant to two other similar drugs:

1. Kanamycin (The "Twin" Key)
2. Streptomycin (The "Cousin" Key)

They tested the bacteria in a lab (like a simulation) and looked at the patients' medical records (the real-world test).

The Results: One Bad Twin, One Good Cousin

Here is what they found, using our key analogy:

1. Kanamycin is the "Twin" (It's Jammed Too)
When the bacteria learned to jam the Amikacin lock, they automatically jammed the Kanamycin lock too.

• The Analogy: Imagine Amikacin and Kanamycin are two identical keys made by the same factory. If the bacteria figure out how to break one, they break the other instantly.
• The Science: The bacteria had a specific mutation (a tiny change in their DNA code) at position 1408. This single change broke both keys.
• The Verdict: Kanamycin is useless if Amikacin doesn't work.

2. Streptomycin is the "Cousin" (It Still Works!)
Surprisingly, when the bacteria jammed the Amikacin lock, the Streptomycin lock remained perfectly fine.

• The Analogy: Streptomycin is a different key entirely. Even though it looks similar to Amikacin, it has a different shape. The bacteria's "jamming tool" for Amikacin didn't fit the Streptomycin lock.
• The Science: The bacteria that resisted Streptomycin had a different DNA mutation (at position 20). The mutation that broke Amikacin (position 1408) did not break Streptomycin.
• The Verdict: Streptomycin is still a viable weapon against these resistant bacteria.

Real-Life Proof: Two Success Stories

The researchers didn't just stop at the lab; they looked at two actual patients who had this exact problem (Amikacin-resistant bacteria).

• Both patients were given Streptomycin instead of Amikacin.
• Result: The bacteria disappeared from their sputum, their lung scans got better, and they were cured.
• This proved that the lab results matched real life: Streptomycin can save the day when Amikacin fails.

Why This Matters

For a long time, doctors were worried that if Amikacin stopped working, they would run out of options. This study is like finding a hidden spare key in the glovebox.

• Bad News: You can't use Kanamycin as a backup for Amikacin.
• Good News: You can use Streptomycin.

The Takeaway

If a patient has a stubborn lung infection that Amikacin can't kill, doctors should consider switching to Streptomycin. It's a different tool that the bacteria haven't learned to break yet.

The researchers suggest that in the future, we should test how sensitive the bacteria are to Streptomycin before giving it to patients, just to make sure it's the right key for the job. But for now, this study gives doctors a glimmer of hope and a new strategy to fight back against these tough bacteria.

Technical Summary

1. Problem Statement

• Clinical Context: Nontuberculous mycobacterial (NTM) pulmonary disease, specifically caused by Mycobacterium avium-intracellulare complex (MAC), is increasing globally and in Japan. For refractory or severe cases, aminoglycosides (AMGs) such as amikacin (AMK), streptomycin (SM), and kanamycin (KM) are critical treatment components.
• The Challenge: The emergence of AMK-resistant MAC is a major clinical concern due to poor prognosis and high mortality, particularly when co-occurring with macrolide resistance.
• The Knowledge Gap: In Mycobacterium tuberculosis, cross-resistance exists between AMK and KM, but not between AMK and SM. However, the cross-resistance profile among AMGs in MAC remains unclear. Clinicians lack data on whether SM or KM remains effective after a patient develops AMK resistance, limiting therapeutic options.

2. Methodology

The study employed a multi-faceted approach combining retrospective clinical analysis, in vitro experimental evolution, and genomic sequencing.

• Study Design: A single-center, retrospective observational study conducted at Fukujuji Hospital, Japan (Oct 2021 – March 2025).
• Patient Cohort: 20 patients with MAC pulmonary disease (MACPD) who acquired AMK resistance via rrs gene mutations.
• Susceptibility Testing:
  • MIC Determination: Minimum Inhibitory Concentrations (MICs) for AMK, SM, and KM were measured using broth microdilution in two media: Cation-adjusted Mueller–Hinton broth (CAMHB) and Middlebrook 7H9 broth.
  • Comparative Analysis: Paired MIC values for SM and KM were compared before and after the acquisition of AMK resistance. Statistical equivalence was assessed using the two one-sided test (TOST) with a margin of ±1 log₂ step.
• In Vitro Resistance Generation:
  • A drug-susceptible M. avium strain (63A) was subjected to serial passage on agar plates containing increasing concentrations of SM and KM to generate resistant strains (designated 63S and 63K).
  • MICs were re-tested for these generated strains.
• Genomic Analysis:
  • Whole-genome sequencing (WGS) was performed on the clinical isolates and the in vitro generated resistant strains (63S and 63K) to identify specific resistance-associated mutations.
• Clinical Correlation: Two specific cases where SM was used clinically after AMK resistance was confirmed were analyzed for treatment outcomes.

3. Key Contributions

• First Direct Comparison: This is the first study to systematically evaluate cross-resistance between AMK, SM, and KM in MAC by comparing MICs before and after AMK resistance acquisition.
• Genotype-Phenotype Correlation: The study links specific rrs gene mutations to phenotypic resistance profiles in MAC, distinguishing between AMK/KM resistance and SM resistance.
• Validation of SM Utility: It provides evidence that SM may serve as a viable salvage therapy for AMK-resistant MAC, a scenario where treatment options are currently extremely limited.

4. Key Results

• Streptomycin (SM) vs. Amikacin (AMK):
  • No Cross-Resistance: There was no significant increase in SM MIC values following the acquisition of AMK resistance. The mean difference in log₂-transformed MICs was near zero (p > 0.05), and the values fell within the predefined equivalence margin.
  • Genetic Divergence: In vitro generated SM-resistant strains (63S) harbored a mutation at position 20 of the rrs gene. This is distinct from the positions associated with AMK resistance (1408 and 1491).
  • Clinical Success: In two clinical cases where patients developed AMK resistance, SM treatment (MIC ≤16 µg/mL) resulted in culture negativity and radiological improvement.
• Kanamycin (KM) vs. Amikacin (AMK):
  • Strong Cross-Resistance: All isolates developed AMK resistance showed a uniform and dramatic increase in KM MICs to >256 µg/mL.
  • Shared Mutations: AMK-resistant and KM-resistant isolates shared mutations at position 1408 of the rrs gene.
• Methodological Findings:
  • MIC concordance between CAMHB and 7H9 broth was high for AMK and KM but low for SM. However, the study recommends using CAMHB for SM testing to align with CLSI standards for AMK.

5. Significance and Implications

• Therapeutic Strategy: The findings suggest that streptomycin is a potential therapeutic alternative for patients with AMK-resistant MACPD, provided the strain retains susceptibility (MIC ≤16 µg/mL). This offers a crucial lifeline for patients with limited options, especially those with macrolide-resistant MAC.
• Avoidance of Kanamycin: The study confirms strong cross-resistance between AMK and KM in MAC. Consequently, KM should likely be avoided in patients with confirmed AMK resistance, as it is unlikely to be effective.
• Genetic Markers: The identification of rrs position 20 as a potential marker for SM resistance (distinct from AMK/KM markers at 1408/1491) aids in the development of rapid genotypic drug susceptibility testing (DST) for MAC.
• Guideline Impact: These results support the inclusion of SM in treatment regimens for refractory MAC and highlight the need for standardized MIC testing (using CAMHB) to guide clinical decision-making.

Limitations: The study is limited by its small sample size (n=20), single-center design, and the fact that the SM-resistant strain was generated in vitro rather than isolated directly from a clinical failure. Multi-center validation is recommended.