Are the new trends in resistance patterns in conjunctivitis and microbial keratitis a concern for contact lens wearers? In particular, what is the status of MRSA-related infection risk in CL wearers?
To understand the impact of methicillin-resistant Staphylococcus aureus (MRSA) as it relates to bacterial keratitis, “it’s useful to take a step back and look at what exactly MRSA means and where it came from,” says Aaron Bronner, OD, a staff optometrist at the Pacific Cataract and Laser Institute of Kennewick, Wash.
MRSA colonies result from environmental stress brought on by beta-lactam antibiotics. “In these populations, a mutation to the cell wall protein—which drugs like penicillin, methicillin and cephalosporins bind to and act against—rendered these drugs ineffective by eliminating their target,” says Dr. Bronner.
In addition to being transmitted vertically from parent cell to offspring, along bacterial cell lineages, these specific mutations are readily transmitted horizontally by plasmids to non-descendant lines of bacteria, Dr. Bronner says, thereby widening the spread of resistance.
Although MRSA species are widely able to resist the effects of many conventional antibiotics, they were less efficient colonizers than many non-MRSA Staph. species. As a result, these particular species would not routinely cause pathology—unless such traits were selected for as a result of antibiotic use.
Due to this selectivity, “until the 1990s MRSA was primarily a hospital-borne disease,” known more formally as hospital acquired (HA)-MRSA, says Dr. Bronner. “However, beginning in the 1990s, some MRSA strains became more effective colonizers that were able to compete with non-MRSA strains, and so began to show up in the community in individuals with no history of hospitalization or antibiotic use.” This is the phenomenon of so-called community acquired (CA)-MRSA.
According to Dr. Bronner, these CA-MRSA isolates are “more virulent and effective growers than HA-MRSA.” Despite this fact, they lost some of their resistance during the transformation. As a result, the CA-MRSA isolates are more susceptible to antibiotics.
“As far as resistance within other organisms goes, MRSA and methicillin-resistant Staph. epidermidis (MRSE) are discussed with great frequency because they are the primary causative organisms in severe ophthalmic disease,” adds Dr. Bronner. A number of other important pathogens, such as Pseudomonas aeruginosa and Streptococcal species, are showing resistance patterns as well.
Staph. species causing ocular disease have been shown to exhibit resistance in dramatically increasing numbers. “In fact, MRSA species may replace methicillin-susceptible Staph. aureus (MSSA) as the dominant cause of Staph.-related eye disease—if this has not already occurred,” says Dr. Bronner. Studies currently suggest that between one-third and two-thirds of all Staph.-related eye infections are the result of MRSA populations.1,2
Pertaining specifically to contact lenses, despite the widely recognized increase in incidence of gram-negative etiologies in the setting of contact lens use in the US, Staph. species are still either the first or second most encountered source of contact lens-associated bacterial keratitis, according to Dr. Bronner, so it stands to reason that such patients will have greater exposure to resistant Staph. species as well.
“As MRSA is becoming more common among all types of bacterial keratitis, it should be presumed that it will likely become equally more common among contact lens-associated disease,” says Dr. Bronner. “Because of the widening distribution of MRSA—as well as the generally prominent role Staph. aureus has in ocular disease overall—it can be expected that MRSA infections will become the primary Staphylococcal etiology of both bacterial keratitis as a whole, as well as contact lens-associated bacterial keratitis.”
1. Asbell PA, Sahm DF, Shaw M, Draghi DC, Brown NP. Increasing prevalence of methicillin resistance in serious ocular infections caused by Staphyloccocs auereus in United States: 2000 to 2005. J Cataract Refract Surg. 2008; 34: 814-8.
2. Hsiao CH, Chuang CC, Tan HY, Ma DH, Lin KK, Chang CJ, Huang YC. Methicillin-resistant Staphyloccus aureus ocular infection: a 10 year hospital based study. Ophthalmology. 2012; 119: 522-7.