Efficacy of a Novel Ophthalmic Antimicrobial Drug Combination Toward a Large Panel of Staphylococcus aureus Clinical Ocular Isolates From Around the World

Abstract

Purpose: Staphylococcus aureus is a leading cause of keratitis requiring urgent antimicrobial treatment. However, rising antibiotic resistance has rendered current ophthalmic antibiotics increasingly ineffective. First, a diverse, ocular S. aureus strain set was evaluated for resistance to six commonly used ophthalmic antibiotics. Next, a recently discovered antimicrobial drug combination containing polymyxin B/trimethoprim + rifampin that displayed impressive efficacy towards S. aureus in both in vitro and in vivo studies was evaluated as a potential novel keratitis therapeutic through testing this combination's efficacy against the clinical strain set.Methods: 163 S. aureus isolates were collected commercially or from the University of Rochester, Flaum Eye Institute. The minimum inhibitory concentrations (MICs) of moxifloxacin, levofloxacin, vancomycin, erythromycin, tobramycin, rifampin and polymyxin B/trimethoprim (PT) were determined for the entire strain set to establish the incidence of resistance to current treatment options among a contemporary clinical isolate set and compared to the performance of PT + rifampin.Results: Among all 163 isolates tested, high rates of antibiotic resistance were found toward erythromycin (69% resistance), moxifloxacin (33%), levofloxacin (40%) and tobramycin (17%). Conversely, the entire strain set, including multi-drug resistant (MDR) isolates, was sensitive to PT + rifampin, demonstrating the potency of this combination. Conclusions:We establish that antibiotic resistance is pervasive among clinical S. aureus isolates, underscoring the concern for the effectiveness of current ophthalmic antibiotics. The drug combination of PT + rifampin, however, eradicated 100% of isolates tested, demonstrating the

Genomics of Staphylococcus aureus ocular isolates

“…One hundred and thirteen (69%) were classified as methicillin-sensitive (MSSA) while 50 strains (31%) were classified as methicillinresistant (MRSA). Multi-drug resistance, as defined by resistance to 3 or more classes of antibiotics, was previously determined to be 24% among the ocular isolates in this study [47].…”

“…). A detailed description of this strain set has been previously published [47], and its core characteristics are presented in Table 1. The majority of isolates were collected from North America (n = 110, 67%), followed by Europe (n = 30, 18%), South America (n = 9, 6%), Middle East (n = 9, 6%), Asia (n = 3, 2%) and Africa (n = 2, 1%).…”

“…Methicillin resistance. In the entire S. aureus ocular strain set, 50 isolates were previously classified as methicillin-resistant (MRSA) and 113 methicillin-sensitive (MSSA) based on minimum inhibitory concentration testing to oxacillin [47]. There was considerably more diversity of STs among MSSA strains with 37 individual strain types represented.…”

Staphylococcal Enterotoxins Promote Virulence in Bacterial Keratitis

“…IHMA70, an ocular isolate that encodes enterotoxins sed, sej, sek, seq, and ser , and IHMA104, an ocular isolate that is devoid of enterotoxins, were purchased from International Healthcare Management Associates (Schaumburg, IL, USA) and are part of a larger strain collection described previously. 17 Comparator non-ocular strains NE1809 (USA300Δ selX ) and NE1787 (USA300Δ srtA ) were obtained from the Nebraska Transposon Library. 43 Overnight cultures were prepared by inoculation of a single colony into 5 mL brain-heart infusion (BHI) broth (Becton Dickenson, Franklin Lakes, NJ, USA) and incubated overnight (37°C, 200 rpm).…”

Antimicrobial Drug–Drug Interactions in the Treatment of Infectious Keratitis

“…As a first step, the MICs of a panel of commonly used ophthalmic antimicrobial drugs (9 antibiotics: vancomycin, gentamicin, moxifloxacin, polymyxin B-trimethoprim, erythromycin, tobramycin, ceftazidime, cefazolin, and rifampin; 3 antifungals: natamycin, amphotericin B, and voriconazole; and 2 antiacanthamoeba agents: chlorhexidine and PHMB) were determined for both S. aureus and P. aeruginosa (Table 1). Of note, while rifampin is not currently used in ophthalmic practice, it has been recently shown to significantly improve the activity of polymyxin B/trimethoprim when used in combination and may represent a novel keratitis therapeutic, [23][24][25][26] thus was included for further study. Among antibiotics, S. aureus was found to be sensitive to cefazolin (MIC = 1.08 6 0.520 mg mL 21 ), erythromycin (0.474 6 0.126 mg mL 21 ), gentamicin (1.21 6 0.555 mg mL 21 ), moxifloxacin (0.096 6 0.040 mg mL 21 ), polymyxin B/trimethoprim (1.43 6 0.452 mg mL 21 ), rifampin (0.0136 6 0.0037 mg mL 21 ), tobramycin (0.68 6 0.680 mg mL 21 ), and vancomycin (0.962 6 0.329 mg mL 21 Regarding the antifungal agents amphotericin B, natamycin, and voriconazole, none displayed antimicrobial activity when tested individually against either S. aureus or P. aeruginosa.…”