Phenotypic resistance to life-saving antimicrobials in Gram-positive bacteria isolated from wild birds in Northern Italy: a One Health surveillance perspective.
This study highlights that 40.1% of bacterial isolates from wild birds are Gram-positive, with 17% showing multidrug-resistance, underscoring the importance of wildlife in monitoring antimicrobial resistance.
Where it sits
this study against the rest of the ll-37 corpusSummary and findings
This study investigated the phenotypic profiles of antimicrobial resistance in Gram-positive bacteria from wild birds in Lombardy, Northern Italy. A total of 205 wild birds were sampled, and 469 isolates were analyzed for resistance to Critically Important Antimicrobials. The study found that 40.1% of the isolates were Gram-positive, with multidrug-resistance detected in 17% of these isolates.
Abstract
Antimicrobial resistance (AMR) is a global health challenge, and wild birds are recognized as sentinels of environmental AMR circulation at the human-animal-environment interface. This study investigated the phenotypic profiles of AMR in Gram-positive bacteria from wild avifauna in Lombardy, Northern Italy, focusing on resistance to Critically Important Antimicrobials (CIAs) for human medicine. In total, 205 wild birds admitted to a wildlife rescue center were sampled (2024-2025). Isolates were identified by MALDI-TOF MS and characterized by disk diffusion susceptibility testing following validated international breakpoints; supplementary phenotypic tests were applied for vancomycin susceptibility (MIC strips), methicillin-resistance (MR; cefoxitin disk), high-level aminoglycoside resistance (HLAR; gentamicin disk), and beta-lactamase production (penicillin G morphology and nitrocefin test). Among 469 total isolates, 188 (40.1%) were Gram-positive, predominantly Lactobacillales (95.2%), with <i>Enterococcus</i> spp. as the dominant genus. Multidrug-resistance (MDR) was detected in 17% of Gram-positive isolates. Among Lactobacillales, resistance to rifampicin (35.7%) and tetracycline (29.6%) was most prevalent. Resistance to linezolid (6.1%) and fluoroquinolones (2.8%), authorized exclusively for human medicine, was detected in multiple <i>Enterococcus</i> species, including isolates from non-synanthropic birds. No acquired resistance to vancomycin in <i>Enterococcus</i> isolates was detected. HLAR affected 5.6% of <i>Enterococcus</i> isolates. Among Bacillales, a MR <i>Staphylococcus epidermidis</i> and a nitrocefin-confirmed beta-lactamase-producing <i>Staphylococcus saprophyticus</i> were recovered. Since sampled birds were admitted to a wildlife rescue center following injury, illness, or death, results should not be considered representative of healthy free-ranging populations. Furthermore, as characterization was exclusively phenotypic, the genetic basis of the resistance profiles detected remains to be confirmed by molecular analysis. Nonetheless, this study provides original phenotypic data across a broad taxonomic diversity of avian hosts, contributing to the evidence base for the integration of wild birds as environmental sentinels within One Health AMR surveillance frameworks.
Background
This paper addresses the issue of antimicrobial resistance (AMR) in Gram-positive bacteria found in wild birds, which are recognized as indicators of environmental AMR. Prior research has established the significance of wild birds in AMR surveillance, but specific data on the phenotypic resistance profiles in this context is limited. Understanding these profiles is crucial for assessing the impact of AMR on public health and wildlife.
Methods
The study involved sampling 205 wild birds admitted to a wildlife rescue center in Lombardy, Northern Italy, between 2024 and 2025. A total of 469 bacterial isolates were identified using MALDI-TOF MS and characterized through disk diffusion susceptibility testing. The primary outcomes included resistance profiles to Critically Important Antimicrobials, with supplementary tests for specific resistances.
Results
Among the 469 total isolates, 188 (40.1%) were identified as Gram-positive. Multidrug-resistance was detected in 17% of these Gram-positive isolates. The most prevalent resistances among Lactobacillales were to rifampicin (35.7%) and tetracycline (29.6%). Resistance to linezolid was found in 6.1% of Enterococcus species, while high-level aminoglycoside resistance affected 5.6% of Enterococcus isolates.
Interpretation
The findings indicate a notable prevalence of antimicrobial resistance among Gram-positive bacteria in wild birds, contributing to the understanding of AMR in environmental contexts. While the statistical significance of the findings is evident, the clinical implications remain uncertain due to the small sample size and the specific context of the birds sampled. The lack of genetic analysis also limits the understanding of the mechanisms behind the observed resistance.
Key findings
- 40.1% of isolates were Gram-positive, n=469.
- 17% of Gram-positive isolates exhibited multidrug-resistance.
- Resistance to rifampicin was 35.7% among Lactobacillales.
- Resistance to tetracycline was 29.6% among Lactobacillales.
- Resistance to linezolid was 6.1% in multiple Enterococcus species.
- 5.6% of Enterococcus isolates were affected by high-level aminoglycoside resistance.
Limitations
- Results should not be considered representative of healthy free-ranging populations.
- Characterization was exclusively phenotypic, lacking genetic confirmation.
- Small sample size may limit generalizability.
- Study conducted in a specific geographic area, Lombardy, Northern Italy.