Knowledge of the ocular surface microbiome can help the specialist recognise when the risk factors for intraocular infection e at determination of antibiotic prophylaxis more effective.
Ocular microbiome diversity and vertical stratification
Our organism is inhabited by a large and diverse community of microbial species: the human microbiomethe study of which is fundamental to understanding the pathophysiology of various diseases. However, while the Human Microbiome Project initially focused on the analysis of the gastrointestinal and urogenital tracts, the skin, and the oral and nasal mucosa, an emerging field of research is focusing on the ocular microbiome.
The ocular surface microbiome refers to the resident, non-pathogenic micro-organisms colonising the conjunctiva and corneawith the exception of the eyelid, whose microbes are instead considered part of the skin microbiome. Several studies have shown that the epithelial cells of the ocular surface are able to respond selectively, producing pro-inflammatory cytokinesto specific components of the ocular pathogenic bacteriaand instead do not respond to non-pathogenic bacteria, thus supporting the colonisation of a true microbiome.
Analysing the composition of the ocular microbiome but it is still in its initial stepsearly studies using microbiological culture techniques, reported a less diverse profile than that recently discovered using the new molecular and sequencing techniques. In fact, whereas until a few years ago the ocular surface microbiome seemed to be dominated by Gram-positive bacteria and a few Gram-negative bacteria, as well as some fungal strains, genomics has now revealed a considerable diversity in the ocular surface microbiomewith an average of 221 species of bacteria per subject.
Consistent with microbiome data from other areas of the body, such as the epidermis, the gender composition of the ocular microbiome appears to have a vertical stratification. Indeed, by swabbing the ocular surface with light pressure, one can isolate opportunistic and environmental micro-organisms, which represent transient species on the ocular surface. Conversely, by performing a 'deeper' swab, one can isolate staphylococci, Cornyebacteriae, Proteobacteria. Thus, one swabbing comprehensive and at different levels is necessary to achieve a Precise characterisation of ocular surface microbiome diversity.
The case: Impact of the ocular microbiome on the occurrence of keratitis induced by Pseudomonas aeruginosa
P. aeruginosa is one of the Gram-negative pathogens most frequently isolated from bacterial keratitis, a severe condition that can rapidly progress towards descemetocele formation to corneal perforation and endophthalmitis. This condition requires an appropriate laboratory approach to ensure antibiotic therapy aimed at overcoming the resistance of P. aeruginosa to common disinfectants, which combined with its adhesion capacity facilitates its survival in the eye.
Recently, some authors have compared the contribution of the ocular surface microbiome in regulating the possibility of P. aeruginosa to induce infectious keratitis. The results of the study, recently published in PLOS Pathogenhave shown that the presence of a healthy ocular microbiome strengthens the innate ocular immune barrierby significantly increasing the concentrations of immune effectors in the tear film, including IgA and complement proteins. The authors also conducted experiments in vivo on Swiss Webster (SW) mice, generally resistant to keratitis induced by P. aeruginosa, which instead became susceptible once their ocular microbiome was altered. Protective immunity was subsequently re-established by colonising the ocular surface of the mice with coagulase-negative Staphylococci previously isolated from conjunctival swabs. Thus, these data underline the role of the microbiome in regulating ocular sensitivity to keratitisand are particularly important in light of the increasing isolation of strains of P. aeruginosa multi-resistant (multi drug resistant - MDR) in patients with eye infections.
In this regard, the phenotypic characterisation of P. aeruginosa MDR isolated from a patient with Stevens-Johnson syndrome, The VRFPA04 strain showed a highly resistant and virulent nature and caused significant loss of visual acuity in the patient despite appropriate antibiotic therapy. VRFPA04 was indeed resistant to beta-lactam antibiotics (penicillins, cephalosporins, carbapenems), aminoglycosides, quinolonics, and sensitive only to aztreonam and fourth-generation cephalosporins. Comparative genome analyses also revealed that the virulence determinants acquired by this strain could derive from related bacteria, but of environmental origin, underlining how the spread of MDR strains in an ocular setup can permanently alter already circulating bacterial genotypes.
Bibliography
- Lu LJ et al. Human Microbiota and OphthalmicDisease. Yale journal of biology and medicine. 2016; 89: 325-330.
- Murugan M et al. Unravelling genomic and phenotypic nature of multidrug-resistant (MDR) Pseudomonas aeruginosa VRFPA04 isolated from keratitis patient. Microbiological Research. 2016; 193: 140-149.
- Kugadas A et al. Impact of Microbiota on Resistance to Ocular Pseudomonas aeruginosa - Induced Keratitis. PLOS Pathogens. 2016.
Dr. Carmelo Chines
Direttore responsabile
English 
Italian