Novel approaches in bacterial keratitis

Bacterial keratitis is a serious ophthalmic disease with significant visual morbidity. The aims of this thesis are to investigate (1) the prevalence and type of bacterial virulence factors in the two major species causing keratitis; Staphylococcal aureus and Pseudomonas aeruginosa and (2) the pharmacodynamics and pharmacokinetics of meropenem, a potentially novel antimicrobial for use in keratitis. The prevalence, genetic diversity and clinical relevance of the lukSF-PV gene, encoding the bacterial toxin Panton Valentine Leukocidin, were investigated in S. aureus, isolated from cases of bacterial keratitis in the UK. Patients with lukSF-PV+ve S. aureus were found to be associated with a trend to worse clinical outcome and more surgical interventions, with an effect unrelated to minimum inhibitory concentrations (MICs). This suggests that lukSF-PV may be an important virulence factor in S. aureus associated keratitis. The genetic characteristics of P. aeruginosa isolates collected from patients with bacterial keratitis were compared between two time periods; 2003-04 and 2009-10 using an Array Tube genotyping system. 71% of keratitis-associated P. aeruginosa isolates clustered together, with no evidence for major variations in the distribution of clone types between the two time periods. The “core keratitis cluster” was found to be related to the P. aeruginosa eccB clonal complex, which is associated with adaptation to survival in environmental water. This suggests that adaptation to environmental water is a key factor in the ability of P. aeruginosa to cause keratitis. Further analysis of clinical data and studies involving additional sets of patients for verification of this hypothesis will provide a clearer picture. Pharmacodynamic properties of commonly used and potentially novel antimicrobials were determined by calculating MICs against isolates from patients with keratitis. Antimicrobial combinations were investigated for synergy or antagonism against isolates of S. aureus and P. aeruginosa using E-Tests. Meropenem was identified as a potentially novel antimicrobial for use in keratitis, offering broad-spectrum cover against both Gram-positive and -negative microorganisms. Meropenem was also shown to show synergistic properties when used in combination with other commonly used antimicrobials. In light of these results, the antimicrobial meropenem was chosen to undergo further pharmacokinetic studies. MTT and Live Dead toxicity assays were performed on human keratocytes and human corneal epithelial cells treated with meropenem. Meropenem was found to have low toxicity against these cells. Corneal penetration of meropenem was assessed in human cadaver corneo-scleral discs mounted onto artificial anterior chambers. The concentration of meropenem in the aqueous estimated by HPLC and disc diffusion bioassay exceeded the MIC90 of E. coli in all 18 corneas tested after the 45 minutes sample point

Recurrent bacterial keratitis

Purpose.: To investigate the rate of recurrent bacterial keratitis, associated bacteria, and surgical intervention.Methods.: Patients with suspected bacterial keratitis were identified from microbiological requests over a 16-year period between 1995 and 2010. Recurrences and number of surgical interventions were analyzed according to bacterial type.Results.: A total of 2418 patients were included, of whom 2124 (87.84%) had only one episode of keratitis, 294 (12.15%) at least two, 88 (3.63%) at least three, 40 (1.65%) at least four, and 22 (0.91%) five or more episodes. The bacterial isolation rate was 35.74% (SD 9.41%), increasing to 56.01% in patients with two or more episodes. There was an increase in the isolation of Staphylococcus aureus with increasing number of episodes (P = 0.008), and S. aureus occurred more commonly in patients with recurrent disease due to the same bacterial group (P = 0.04). Patients whose recurrent keratitis was associated with S. aureus had a higher rate of requiring subsequent corneal transplantation (7 of 10) compared to those with Enterobacteriaceae (2 of 7), Pseudomonas aeruginosa (2 of 4), streptococci (2 of 5), or coagulase-negative staphylococci (none of 8) (P = 0.02).Conclusions.: S. aureus is particularly associated with recurrent keratitis. Identification and treatment of the possible source of the infection may be necessary to reduce the risk of recurrent disease. The potential for the autocthonous S. aureus colonizing the nasopharynx or conjunctiva or lid margin to be a reservoir for recurrent keratitis suggests that decolonization of S. aureus could be considered as a potential intervention in those patients with recurrent disease