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Christine M. Sanfilippo, Wolfgang Haas, Christine K. Hesje, Timothy W. Morris; Strain ST448 Causes the Majority of Pneumococcal Conjunctivitis Cases. Invest. Ophthalmol. Vis. Sci. 2012;53(14):83.
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Streptococcus pneumoniae is a prevalent cause of bacterial conjunctivitis (BC), especially in children. One strain in particular, ST448, has gained notoriety by causing multiple outbreaks of BC in colleges and military training facilities. In contrast, cases of BC not associated with outbreaks were generally thought to be caused by those strains that colonize the nasopharynx and that cause invasive infections. The present study was undertaken to determine the prevalence of ST448 in BC cases not associated with outbreaks.
One isolate per zip code was selected from specimens isolated during prospective, multi-center clinical BC trials conducted with besifloxacin ophthalmic suspension in 2005-07. Sequence types (ST) were determined by multi-locus sequence typing (MLST) based on sequences for aroE, gdh, gki, recP, spi, xpt, and ddl. The presence of a capsule was determined with specific antisera. Minimum inhibitory concentrations (MIC) for fluoroquinolones, azithromycin, and penicillin were determined by the broth microdilution method according to CLSI standards.
Of the 80 strains tested, 45 (56.3%) were of ST448, while the remaining 35 strains were either unique or occurred only twice. Of the latter, 19 strains represented novel MLST sequence types. All ST448 strains were unencapsulated and only 9 of the remaining strains were encapsulated (serotypes 6A, 6C, 10B, 11A, 15B, 17F, 23A, 35B). The ST448 isolates were recovered from 25 States across the US, showing no geographic preference. There was no notable difference between ST448 and the other strains in age distribution or antibiotic MIC values. Besifloxacin, moxifloxacin, and penicillin had the lowest MIC90 values with 0.12 ug/ml, while azithromycin had the highest MIC90 with 8 ug/ml.
The ST448 strain was initially discovered in 2002 as the cause of several outbreaks of BC. The data presented here demonstrate that this strain is now the cause of non-outbreak related ocular infections throughout the US, suggesting that ST448 is able to outcompete other strains in the eye.
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