Research Article | Vol. 6, Issue 3 | Journal of Clinical Immunology & Microbiology | Open Access |
Reem Amine1*, Heleen DeCory2, Daniel F Sahm3, Penny Asbell4
1Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina, United States
2Trilogy Writing and Consulting GmbH, Durham, North Carolina, United States
3IHMA, Schaumburg, Illinois, United States
4Bioengineering Department, University of Memphis, Memphis, Tennessee, United States
*Correspondence author: Reem Amine, Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina, United States; Email: [email protected]
Citation: Amine R, et al Antibiotic Resistance of Intraocular Pathogens from the ARMOR Study: 2009 -2022. J Clin Immunol Microbiol. 2025;6(3):1-5.
Copyright© 2025 by Amine R, et al. All rights reserved. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
| Received 20 October 2025 | Accepted 03 November, 2025 | Published 11 November, 2025 |
Abstract
Purpose: Endophthalmitis is a rare but serious complication of ocular procedures. This study analyzed in-vitro antibiotic resistance profiles of intraocular isolates collected over 13 years (2009-2022) as part of the Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) study, the U.S. multicenter surveillance study.
Methods: Presumed endophthalmitis Isolates from aqueous and vitreous humor were collected from participating ARMOR centers and sent to an independent laboratory for susceptibility testing. Minimum Inhibitory Concentrations (MICs) were determined per Clinical and Laboratory Standards Institute (CLSI) methodology.
Results: A total of 370 intraocular isolates were collected. Methicillin resistance was observed in 45% (33/73) of Staphylococcus aureus and 41% (93/231) of coagulase-negative Staphylococci (CoNS), with multidrug resistance prevalent among methicillin-resistant strains. Gram-negative isolates showed minimal resistance, with Haemophilus influenzae displaying no resistance to tested drugs.
Conclusion: Resistance was prevalent among Staphylococci, with multi-drug resistance common in MR isolates; resistance was uncommon in Gram-negative organisms. These findings provide insights to inform antibiotic selection for endophthalmitis prophylaxis and treatment.
Keywords: Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR); Methicillin Resistance; Gram-Negative; Multi-Drug Resistance
Introduction
Endophthalmitis is a rare but serious complication of ocular surgery and intraocular injections, typically resulting from the perioperative introduction of a patient’s own bacterial flora [1]. Prophylactic use of perioperative antibiotics may reduce the risk of endophthalmitis. However, the emergence of antibiotic-resistant bacteria complicates the effectiveness of such measures [1,2]. The Antibiotic Resistance Monitoring in Ocular microorganisms (ARMOR) study is the only prospective, multicenter, nationwide surveillance program specifically designed to monitor antibiotic resistance in ocular bacterial pathogens [3,4]. In this study, we analyzed in-vitro antibiotic resistance profiles of intraocular isolates collected over a 13-year period (2009 to 2022) as part of the ARMOR study.
Methodology
Isolates of Staphylococcus aureus, Coagulase-Negative Staphylococci (CoNS), Streptococcus pneumoniae, Pseudomonas aeruginosa and Haemophilus influenzae were collected from participating centers of the ARMOR study. These isolates were sent to an independent laboratory for species confirmation and susceptibility testing. Minimum Inhibitory Concentrations (MICs) were determined for up to 16 antibiotics across 10 antibiotic classes using guidelines outlined by the Clinical and Laboratory Standards Institute (CLSI) [5]. Based on CLSI interpretive criteria, isolates were categorized as susceptible, intermediate or resistant, when available [6]. Staphylococcus species were further classified as Methicillin-Resistant (MR) or Methicillin-Susceptible (MS) based on oxacillin resistance or susceptibility. Gentamicin breakpoints were utilized to determine resistance or susceptibility of staphylococcal isolates to tobramycin. For this analysis, bacterial pathogens isolated from intraocular specimens, specifically Aqueous Humor (AqH) and Vitreous Humor (ViH), were included.
Results
Intraocular Isolates
A total of 370 intraocular isolates were collected from 44 participating sites across 22 states (aqueous, n=96; vitreous, n=274). Among the aqueous humor isolates, there were 27 Staphylococcus aureus isolates, 45 coagulase-negative Staphylococci (CoNS) isolates, including 38 Staphylococcus epidermidis, 13 Streptococcus pneumoniae isolates, 6 Pseudomonas aeruginosa isolates and 5 Haemophilus influenzae isolates. The vitreous humor isolates included 46 Staphylococcus aureus isolates, 186 CoNS isolates (of which 150 were Staphylococcus epidermidis), 20 Streptococcus pneumoniae isolates, 13 Pseudomonas aeruginosa isolates and 9 Haemophilus influenzae isolates.
In-vitro Antibiotic Resistance Profiles
Oxacillin/Methicillin Resistance (MR) was identified in 45% (33/73) of Staphylococcus aureus isolates and 41% (93/231) of CoNS isolates (Fig. 1). Of the MR Staphylococcus aureus (MRSA), 85% were resistant to ciprofloxacin and azithromycin and out of MRCoNS isolates, 70% were resistant to ciprofloxacin and azithromycin (Fig. 2). Of S. pneumoniae isolates, 39% (13/33) were resistant to each for azithromycin and penicillin (Fig. 3). P. aeruginosa and H. influenzae isolates exhibited little to no antibiotic resistance.
Besifloxacin demonstrated significantly lower MIC90 values (minimum inhibitory concentration that inhibits 90% of isolates) compared to other fluoroquinolones, particularly for MR isolates and its efficacy was comparable to vancomycin. Against S. aureus, the MIC90 values for besifloxacin, moxifloxacin and ciprofloxacin were 2, 8 and 64, respectively, with vancomycin at 1, indicating a 4- to 32-fold lower value for besifloxacin. Similarly, for MRSA, the MIC90 values were 2, 16 and 256, respectively (vancomycin= 1), showing besifloxacin’s values were 8- to 128-fold lower. In the case of CoNS, the MIC90 for besifloxacin was 4, compared to 32 for moxifloxacin and 64 for ciprofloxacin (vancomycin= 2), representing an 8- to 16-fold reduction. For MRCoNS, besifloxacin’s MIC90 was also 4, while moxifloxacin and ciprofloxacin were 64 (vancomycin= 2), demonstrating a 16-fold lower value (Table 1).
High rates of in-vitro concurrent multidrug resistance, defined as resistance to three or more antibiotic classes, were observed in 41% (30/73) of S. aureus isolates and 40% (93/231) of CoNS isolates, with particularly elevated rates among Methicillin Resistant (MR) strains (MRSA, 38.4%; MRCoNS, 29.9%) (Fig. 4).
Antibiotic | S. aureus | MRSA | CoNS | MRCoNS |
Besifloxacin | 2 | 2 | 4 | 4 |
Moxifloxacin | 8 | 16 | 32 | 64 |
Gatifloxacin | 8 | 32 | 32 | 64 |
Ciprofloxacin | 64 | 256 | 64 | 64 |
Levofloxacin | 64 | 256 | 256 | 256 |
Ofloxacin | 16 | 128 | 32 | >256 |
Vancomycin | 1 | 1 | 2 | 2 |
MIC90 Minimum inhibitory concentration that inhibits 90% of isolates (μg/mL); MR: Methicillin-Resistant | ||||
Table 1: In-vitro MIC90 results of different antibiotics for methicillin-sensitive and methicillin-resistant staphylococcus (S. aureus and CoNS).
Figure 1: Oxacillin/Methicillin Resistance (MR) was identified in 45% (33/73) of Staphylococcus aureus isolates and 41% (93/231) of CoNS isolates.
Figure 2: Resistant to ciprofloxacin and azithromycin.
Figure 3: Resistant to each for azithromycin and penicillin.
Figure 4: Resistant among Methicillin Resistant (MR) strains (MRSA, 38.4%; MRCoNS, 29.9%).
Discussion
In this analysis of presumed endophthalmitis isolates, in-vitro antibiotic resistance was notably prevalent among Staphylococci, with CoNS demonstrating the highest resistance rates. Staphylococci, particularly MR strains, frequently exhibited multidrug resistance. In contrast, resistance was uncommon among Gram-negative organisms. In general, the findings are consistent with those of other studies evaluating in-vitro antibiotic resistance among endophthalmitis isolates and prior reporting on endophthalmitis isolates collected in the ARMOR study [7]. These and other studies have shown that methicillin resistance and fluoroquinolone resistance among Staphylococci is particularly high. Furthermore, one study found 61% of S. epidermidis isolates were resistant to levofloxacin and 50% to moxifloxacin. Another study found that 34.2% of CoNS isolates were resistant to ciprofloxacin with cross-resistance observed among other fluoroquinolones. Findings from the ARMOR study provide valuable surveillance data that could guide antibiotic selection for infection prophylaxis and the treatment of intraocular following surgery. The notably high MIC90 values of moxifloxacin against MRCoNS (64 mg/mL) and MRSA (16 mg/mL) in the current study warrant caution when considering its use in dropless cataract surgery via intravitreal delivery or intracameral injection as a prophylactic measure. While vancomycin remains effective against all tested staphylococcal isolates, its limited availability as a topical formulation and its reserved use for infections resistant to other antibiotics reduce its practicality for prophylactic purposes. For ocular surface prophylaxis, selecting an antibiotic with the highest tear Cmax (maximum concentration) to MIC90 ratio for most ocular bacteria is optimal, ensuring effective antimicrobial activity [4]. This study has some limitations that should be considered when interpreting the results. One limitation is the application of systemic breakpoints to interpret Minimum Inhibitory Concentrations (MICs). Antibiotic concentrations are more likely higher following topical ocular treatment than after systemic therapy and the application of systemic breakpoints may therefore overestimate antibiotic resistance [4]. Additionally, information on diagnosis and etiology was not collected, making it unclear what proportion of presumed endophthalmitis isolates originated from perioperative eyes versus those from intravitreal injections, other procedures or open globe injuries. In addition, these data do not provide insight into species prevalence for bacterial endophthalmitis, as ARMOR participating sites were instructed to submit a predetermined number of isolates for each bacterial species.
Conclusion
In summary, the ARMOR surveillance study provides key information on antibiotic resistance of ocular isolates and initial guidance on treatment. Clinical evaluation is, however, always needed for optimal patient care. Isolate surveillance studies can provide information on trends to help guide treatment and lead to efforts for new antibiotic discovery.
Conflict of Interest
Dr Asbell reported serving as a consultant and receiving personal fees from Alcon, Kao, Medscape, Perrigo, Santen, ScientiaCME, Senju and Shire and serving on advisory boards for Allakos, Allergan, Bausch + Lomb (a division of Bausch Health US, LLC), Dompé, Kala, Novaliq, Novartis, Regeneron Pharmaceuticals and Sun Pharmaceuticals outside the submitted work. Dr Amine has no conflicts of interest.
Funding/Support
The Antibiotic Resistance Monitoring in Ocular Microorganisms (ARMOR) study was funded by Bausch + Lomb (a division of Bausch Health US, LLC).
References
Reem Amine1*, Heleen DeCory2, Daniel F Sahm3, Penny Asbell4
1Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina, United States
2Trilogy Writing and Consulting GmbH, Durham, North Carolina, United States
3IHMA, Schaumburg, Illinois, United States
4Bioengineering Department, University of Memphis, Memphis, Tennessee, United States
*Correspondence author: Reem Amine, Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina, United States;
Email: [email protected]
Reem Amine1*, Heleen DeCory2, Daniel F Sahm3, Penny Asbell4
1Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina, United States
2Trilogy Writing and Consulting GmbH, Durham, North Carolina, United States
3IHMA, Schaumburg, Illinois, United States
4Bioengineering Department, University of Memphis, Memphis, Tennessee, United States
*Correspondence author: Reem Amine, Department of Ophthalmology, Duke University School of Medicine, Durham, North Carolina, United States;
Email: [email protected]
Copyright© 2025 by Amine R, et al. All rights reserved. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Amine R, et al Antibiotic Resistance of Intraocular Pathogens from the ARMOR Study: 2009 -2022. J Clin Immunol Microbiol. 2025;6(3):1-5.
