In vitro evaluation of the antibacterial activity of Fluorescein® 0.5% eye drops.

Abstract

In patients with a severe corneal abscess (CA), a microbiological sample is usually taken to identify the causative microbe and adapt local antibiotic therapy. The frequency of positive CA cultures ranges from 21% to 81% according to various studies (Ancele et al. 2009; Darugar et al. 2011). Increasing the culture positivity rate is therefore key to improving the management of patients with CA. Several studies have shown the antibacterial activity of local anaesthetics used in ophthalmology, with the risk of a negative culture being produced (Labetoulle et al. 2002; Pelosini et al. 2009). However, there have been no studies looking at the antibacterial activity of Fluorescein®, despite the fact that it is used in clinical practice to diagnose CA. The objective of our study was to carry out an in vitro evaluation of the antibacterial activity of Fluorescein® 0.5% and the main anaesthetic eye drops used in clinical practice. The main microbes found in cases of CA were tested using clinical isolates: Staphylococcus epidermidis, Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus pneumoniae, Moraxella lacunata, Moraxella catarrhalis, Serratia marcescens and Corynebacterium macginleyi. The following eye drops were tested: Fluorescein® 0.5%, Oxybuprocaine® 0.4% and Tetracaine® 1% (Fluorescein Faure® single-dose 0.5% from SERB, Oxybuprocaine Hydrochloride® single-dose from THEA and Tetracaine® single-dose 1% from THEA). Minimum inhibitory concentration (MIC) determination was performed in a 96-well plate by microdilution in a liquid growth medium, following the European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines. For increased reliability, the tests were performed three times. We showed antibacterial activity from Fluorescein® on Moraxella Gram-negative cocci (M. catarrhalis and M. lacunata). The MIC for M. lacunata was 625 mg/l (1/8 dilution) and 1250 mg/l (1/4 dilution) for M. catarrhalis. No antibacterial activity was shown on the other microbes (MIC > 2500 mg/l). We observed no antibacterial activity from Oxybuprocaine® in the presence of P. aeruginosa and S. marcescens (MIC > 2000 mg/l). However, a MIC was determined for all the other microbes, from 250 mg/l for M. catarrhalis up to 2000 mg/l for S. aureus and S. epidermidis. Finally, a MIC was determined for all the microbes in the presence of Tetracaine®. These MIC were systematically lower than with Oxybuprocaine®, ranging from 312 mg/l for C. macginleyi and M. catarrhalis up to 2500 mg/l for P. aeruginosa. The MIC results are summarized in Table 1. This is the first study to evaluate the antibacterial activity of Fluorescein®. Our MIC results for the local anaesthetics were similar to those of previous studies (Kleinfeld & Ellis 1966; Labetoulle et al. 2002; Pelosini et al. 2009). It should be noted that in clinical practice, Fluorescein® is very often used in combination with a local anaesthetic (Darugar et al. 2011). An additional analysis was therefore carried out using the combinations of Fluorescein®-Oxybuprocaine® and Fluorescein®-Tetracaine® (Table 1). No interaction was detected for the Fluorescein®-Tetracaine® combination. However, the Fluorescein®-Oxybuprocaine® combination appeared to have a synergistic effect on Gram-positive cocci (Staphylococci and Pneumococci). Finally, with any cases of severe CA, the patient's eye should be routinely rinsed with sterile saline solution before samples are taken, in order to remove the fluorescein and local anaesthetic and avoid producing a negative culture.