Speciation of Coagulase Negative Staphylococcus Causing Bacterial Keratitis : Indian Journal of Ophthalmology

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Letters to the Editor

Speciation of Coagulase Negative Staphylococcus Causing Bacterial Keratitis

Kaliamurthy, J PhD; Geraldine, P PhD; Thomas, Philip A MD, PhD

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Indian Journal of Ophthalmology: Apr–Jun 2005 - Volume 53 - Issue 2 - p 144-145
doi: 10.4103/0301-4738.16187
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Dear Editor,

We congratulate Manikandan et al1 for their speciation of coagulase-negative staphylococci (CoNS) causing keratitis, but wish to offer a few comments:

  1. The authors have rightly listed various ocular conditions in which CoNS have been implicated, with the prominent exclusion of endophthalmitis; CoNS are, in fact, the predominant causes of early postoperative endophthalmitis.2
  2. Capsule formation is not a salient feature of staphylococci; however, staphylococci possess the enzyme catalase, and the catalase test is important in differentiating staphylococci (catalase-positive) from streptococci (catalase-negative).3
  3. White, non-haemolytic colonies on blood agar (type of blood not mentioned) were considered presumptive CoNS; however, colony pigmentation and hemolysis are very variable characteristics.3 Instead, all “pin-head sized” (1-2 mm in diameter) colonies on blood agar could have been deemed potentially staphylococci (irrespective of pigmentation or haemolysis); Gram stain, and catalase and coagulase tests could have been done on these colonies.
  4. Since the article is centered on the importance of CoNS, some information on how the coagulase test was performed would have helped. Ideally, the slide test (for bound coagulase) and tube test (for free coagulase) should have been done for all isolates, although the tube test result is more important.
  5. The criteria used to consider the significance of the growth appear to be inadequate. Even growth of 2 to 3 colonies of virulent corneal pathogens (Pseudomonas aeruginosa, Streptococcus pneumoniae and Staphylococcus aureus) is considered significant, but Staphylococcus epidermidis and other CoNS are significant only if isolated on multiple media or in abundance.
  6. According to the paper, “antibiotic susceptibility testing was performed by following the method of Kirby-Bauer disc diffusion method” (no reference provided) while the “susceptibility of the organisms to these antibiotics was based on NCCLS standards”. This appears contradictory, since the size of the discs used and the interpretation of the zone diameters achieved differ in the two methods.
  7. Susceptibilities to lomefloxacin, sparfloxacin and gatifloxacin, fluoroquinolones which have good in vitro activity against Gram-positive cocci and which are commercially available in India as eye drop preparations (unlike vancomycin, cefotaxime, and cefazolin), could have been determined.
1. Manikandan P, Bhaskar M, Revathy R, John RK, Narendran K, Narendran V. Speciation of coagulase-negative Staphylococcus causing bacterial keratitis Indian J Ophthalmol. 2005;53:59–60
2. Endophthalmitis Vitrectomy Study Group. . Results of the Endophthalmitis Vitrectomy Study: a randomized trial of immediate vitrectomy and intravenous antibiotics for the treatment of postoperative bacterial endophthalmitis Arch Ophthalmol. 1995;113:1479–96
3. Baron EJ, Finegold SM Bailey & Scott's Diagnostic Microbiology. 19908th ed St.Louis, Missouri The C.V. Mosby Company
© 2005 Indian Journal of Ophthalmology | Published by Wolters Kluwer – Medknow