Pathogen or commensal: a PCR based study of ocular surface bacteria in normal and dry eyes

Purpose: To determine the ocular surface bacteria in normal and dry eye subjects and investigate whether there was any association between the bacterial population and goblet cell density (GCD). Methods: Fifty-seven normal and 34 dry eye subjects were recruited. Conventional culture, 16S rDNA PCR and DNA sequencing were used to identify bacteria. The association between reduced GCD and bacterial numbers in a sub-group of 27 subjects was assessed. Conjunctival impression cytology samples were stained with PAS and GCD graded as follows: Grade (1), >30 goblet cells/4 high power fields; (2), 15-30 GC/4HPF; (3), 5-15 GC/4HPF; (4), <5 GC/4HPF. Grades 3 and 4 indicated reduced GCD. Results: The majority of cultured bacteria were coagulase-negative Staphylococci, while molecular methods demonstrated additional atypical bacterial species. Bacterial levels were higher overall in subjects displaying reduced GCD compared to normals. A statistically significant difference (P = 0.005) was noted between mean bacterial counts in IC Grade 4 samples of dry eye subjects (35 cfu/swab) and normals (5 cfu/swab). Conclusions: Molecular analysis revealed a diverse ocular surface bacterial population with identification of potentially pathogenic bacteria presenting a diagnostic dilemma. A trend of increasing bacterial numbers with reduced GCD was observed and studies are ongoing to investigate whether bacterial colonisation of the ocular surface may alter the number and function of goblet cells. The clinical relevance of such results and whether they should prompt intervention with therapy is not yet fully determined. A fuller definition of the normal ocular flora is needed to determine bacteria implicated in ocular surface disease.