We appreciate the comments made on our article “Speciation of coagulase negative Staphylococcus causing bacterial keratitis”.1 The ocular conditions listed in our article are conditions, where isolation of CoNS is not considered significant. But in postoperative endophthalmitis the pathogenic significance of CoNS is established beyond doubt. We enumerated the ocular conditions where CoNS is only implicated and its pathogenecity is disregarded. Capsule formation is not a salient feature but usually mentioned to complete the morphological description of staphylococcal species. However, the capsule/slime production is important in Staphylococcus epidermidis and other CoNS for colonizing foreign bodies, for antibiotic resistance and some authors regard them as a very significant attribute of pathogenecity.2
Catalase is done to differentiate between Staphylococcus and Streptococcus. But most of the biochemical tests including catalase test require these organisms to be grown on nutrient agar, and especially catalase should not be done from colonies on blood agar. Hence we have grown them on nutrient agar. Streptococcus will not be able to grow on nutrient agar, as this is nutritionally demanding bacteria. Here the need for a catalase test is obviated but can still be done to differentiate other gram-positive cocci from Staphylococci.
The colony characters with Gram stain results should definitely differentiate between Staphylococcus and Streptococcus. Staphylococcus produces opaque colonies but Streptococcus produces only transparent colonies. Hence the confusion between Staphylococcus aureus and CoNS colonies, and biochemical tests (including the coagulase test) will differentiate between the two. The coagulase test was done as per standard procedures described in Mackie & Mccartney′s practical medical microbiology.2
The inadequacy of the inclusion criteria has been correctly pointed out. We have restructured the inclusion criteria in the continuing study. However, in the pilot study itself most of our strains showed confluent growth and growth in more than one media.
The disc diffusion was done as described in NCCLS performance standards3 and the results were interpreted according to the same approved standard, which was based on the method originally described by Kirby, Bauer, Sherris and Turck.
In the pilot study we have included antibiotics of common use. The newer antibiotics may be included in subsequent phases.
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. Collee JG, Fraser AG, Marmion BP, Simmons A Mackie & Mccartney's Practical Medical Microbiology. 199614th ed Chruchill Livingstone
3. Villanova PA. Performance standards for antimicrobial disk susceptibility tests. In: National Committee for Clinical Laboratory Standards. Approved standard. 7th edition. National Committee for Clinical Laboratory Standards 2000;20(No 1):M2–A7