Resistant strain emerges
During the course of the study, Dr. Kim noted 1 patient showed conclusive and compelling evidence of emergence of a resistant strain in the nasopharynx.
Using pulse gel electrophoresis, the patient at baseline had one specific species of S. aureus cultured from the nasopharynx. Pulse gel electrophoresis involves taking DNA from the bacterial strain and subjecting it to restriction enzymes that cleave the DNA into different fragments to get a “fingerprint” that then identifies certain species.
“This strain was pansensitive to all 16 antibiotics,” Dr. Kim said. “The same strain was re-cultured on visit 1 and visit 2. In visit 3, we didn't pick up the strain.”
However, by visit 4, there was a “very clear emergence of a different strain of S. aureus,”2 Dr. Kim added. This strain, unlike the baseline strain, showed resistance to moxifloxacin, and to “all the other fluoroquinolones and to several other antibiotics, including clindamycin.”
Dr. Kim said that at baseline, there was “surprisingly and alarmingly, already substantial levels of resistance to macrolides, third-generation fluoroquinolones, and fourth-generation fluoroquinolones.”
An analysis of the antibiotic susceptibility of all the coagulase-negative Staphylococci (CoNS), isolated from the conjunctiva from visits 1-4 (excluding baseline cultures), found the antibiotic susceptibility of 48 CoNS from treated eyes exposed to fluoroquinolones had a clear pattern of increasing resistance to the third- and fourth-generation fluoroquinolones compared to the fellow control eye.
In the 17 CoNS that were isolated from treated eyes exposed only to azithromycin, a different pattern emerged showing increasing resistance to macrolides and decreasing resistance to the fluoroquinolones.