Broad-spectrum antibacterials such as the ophthalmic fluoroquinolones are powerful weapons for treating and preventing potentially sightthreatening infections – but only if prescribed rationally. In just a few years, some common ocular pathogens have become less susceptible to once standard therapies. Studies have shown that the prevalence of antibiotic resistance can vary widely by region and may be linked to outpatient consumption of antibiotics. 1,2 The sheer number of bacteria exposed to oral antibiotics is so large that it is a statistical certainty that some bacteria will develop resistance to antibiotics. The greater the exposure of bacteria to a drug over years of clinical use, the more ‘selection pressure’ there is for resistance to emerge to that drug. While the operating assumption is that treatment-limiting resistance to all antibiotics will develop over time, this is true only if resistant bacteria survive to pass on resistance genes.3
Most resistant ocular pathogens emerge as a result of widespread systemic antibiotic use, not specifically from ophthalmic antibiotic use. However, information on local, regional and global antibiotic resistance can be an important resource for ophthalmologists, especially when susceptibility data are available from ocular infections. Programmes that systematically monitor antibacterial susceptibility/resistance patterns provide critical information for the rational use of antibacterials. In vitro susceptibility testing of large numbers of isolates from multiple laboratories, compiled over time, can detect and track emerging resistance.
In susceptibility testing, minimum inhibitory concentrations (MICs) are interpreted as ‘susceptible’ or ‘resistant’ according to published standards. These interpretations are useful for predicting which drugs might be effective against a particular pathogen. Large-scale susceptibility surveillance has documented the potential durability of fluoroquinolone usefulness in the form of levofloxacin, which continues to exhibit in vitro activity against common ocular pathogens despite a decade of widespread oral and intravenous use for systemic illnesses. The durability of this antibacterial activity of levofloxacin has most recently been documented by nationwide surveillance programmes in the US.
TRUST and Ocular TRUST
Tracking Resistance in the US Today (TRUST) is the largest continuous active surveillance network of its kind. Conducted annually since 1996, TRUST has tested susceptibility in more than 70,000 isolates from all infection sources, collected from over 200 hospitals across the US. Ocular TRUST is a substudy of TRUST that collects and tests isolates specifically from ocular infections submitted by participating eye centres and community hospitals. Both programmes use the same methodology of testing by a central, independent laboratory. The results of the first Ocular TRUST survey have recently been reported.4,5
Ocular TRUST 1 – Staphylococcus aureus Susceptibility
In the inaugural year of Ocular TRUST (October 2005 to June 2006), participating centres across the US submitted nearly 200 ocular Staphylococcus aureus isolates for susceptibility testing, most of which (>80%) were methicillin-susceptible (MSSA).4 The greatest in vitro activity against MSSA (>92% susceptibility) was seen with tobramycin and trimethoprim. Two antibacterials – polymyxin B and penicillin – were almost completely inactive against MSSA (<10% susceptibility), and azithromycin was only moderately active (54% susceptibility). The four fluoroquinolones tested – ciprofloxacin, gatifloxacin, levofloxacin and moxifloxacin – were indistinguishable in terms of MSSA susceptibility (80–81%).