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Diagnosis of external ocular infections: microbiological processing and interpretation
  1. SAVITRI SHARMA
  1. L V Prasad Eye Institute, L V Prasad Marg, Banjara Hills, Hyderabad - 500 034, AP, India
    1. ANTONIO PINNA,
    2. MARIO SOTGIU,
    3. FRANCESCO CARTA
    1. Institute of Ophthalmology, University of Sassari, Sassari, Italy
    2. Department of Biomedical Sciences, Section of Experimental and Clinical Microbiology, University of Sassari, Sassari, Italy
    3. Institute of Microbiology, Università Cattolica del Sacro Cuore, Rome, Italy
    1. LEONARDO A SECHI,
    2. STEFANIA ZANETTI
    1. Institute of Ophthalmology, University of Sassari, Sassari, Italy
    2. Department of Biomedical Sciences, Section of Experimental and Clinical Microbiology, University of Sassari, Sassari, Italy
    3. Institute of Microbiology, Università Cattolica del Sacro Cuore, Rome, Italy
    1. GIOVANNI FADDA
    1. Institute of Ophthalmology, University of Sassari, Sassari, Italy
    2. Department of Biomedical Sciences, Section of Experimental and Clinical Microbiology, University of Sassari, Sassari, Italy
    3. Institute of Microbiology, Università Cattolica del Sacro Cuore, Rome, Italy

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      Editor,—I read with interest the recently published article by Pinna et al,1 and compliment the authors for bringing to light the important issue of external ocular infections associated with coagulase negative staphylococci (CoNS). Ocular microbiologists rarely pay attention to the speciation of this group of bacteria and various species involved in ocular infections are generally passed off as Staphylococcus species or CoNS. Apart from speciation, this group of staphylococci needs special attention with respect to their role in pathogenicity. Generally,S epidermidis and other CoNS along with corynebacteria and propionibacteria are normal commensals of the conjunctival sac and lids; therefore samples from the external ocular surface resulting in a light growth on primary solid culture medium like blood agar or from a thioglycolate broth, are more likely to be associated with contamination.2 In our laboratory and many others across the world, a bacterial isolate (more so a known commensal organism) from corneal scrapings or conjunctival/lid swabs is considered significant if it is consistent with the clinical signs and fulfils any one of the following criteria: (1) results of direct smear of the sample are consistent with culture; (2) the same organism is grown in more than one medium; or (3) the same organism is grown from repeated specimens. However, Pinna et al, 1 in their article, have not indicated adherence to any such criteria while selecting isolates for their study, though they have labelled the 55 isolates tested by them as “clinically significant”. Their methodology of including just two media (thioglycolate broth and Sabouraud's dextrose agar) as primary culture media also does not conform to the recommended methods of microbiological investigation of blepharitis, conjunctivitis, and keratitis.3 Though the authors did not intend to determine the pathogenicity of CoNS in external ocular infections, the methodology details provided by them can be misleading. Another concern raised by their article is the interpretation of bacterial susceptibility testing by agar disc diffusion (Kirby-Bauer method). The disc diffusion technique requires labelling of bacteria as resistant, sensitive, or intermediate. The authors have not clarified the way the “intermediate” group was dealt with, or was no such group noticed in any of the 55 isolates tested by them? Similarly, the reason for testing susceptibility to penicillin is far from clear since CoNS are known to be resistant to penicillin and penicillin is not commonly used to treat external ocular infections. Moreover, much valuable data could have been obtained by determining the minimum inhibitory concentration of the antibiotics against CoNS.

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      Editor,—We thank Dr Sharma for her interest in our article on the identification and antibiotic susceptibility of coagulase negative staphylococci (CoNS) isolated in corneal/external infections. Apart from being a common component of the normal ocular flora, CoNS may occasionally be important ocular pathogens and cause chronic blepharitis, acute conjunctivitis, and suppurative keratitis.

      As stated by Dr Sharma, a bacterial isolate from corneal scrapings or conjunctival/lid swabs is generally considered significant whenever there is (1) growth in one medium with consistent direct microscopic findings, or (2) growth of the same organism on two or more media, or (3) the same organism is grown from repeated specimens. However, when a bacterial isolate is consistent with the clinical signs, isolation of the organism even from a single medium can be considered significant. In our study, corneal scrapings for Gram stain were performed only on the patients with suppurative keratitis. In all cases the Gram stain showed the presence of grape-like clusters of Gram positive cocci. Follow up cultures performed about 12 hours after the last dose of medication showed eradication of the infecting organism in all 45 patients. According to our and other authors' experience (Leventer DB, presented at the AAO Annual Meeting, San Francisco, 1997), thioglycolate broth is an adequate, cost effective, primary culture medium for the detection of aerobic and anaerobic bacteria in external ocular infections, especially when the patients show clear signs and symptoms of infection.

      Antibiotic susceptibility tests were determined by agar disc diffusion (Kirby-Bauer method), a technique which labels bacteria as “resistant”, “intermediate”, or “sensitive”. Although we found a handful of “intermediate” isolates (Table 11), our main concern was to draw attention exclusively to the large number of “resistant” strains. Indeed, in Table 2 of the published article we reported the ratio “resistant” isolates/total isolates. Dr Sharma's criticism on this point is difficult to understand, since in a recent paper she and her co-workers included “resistant” and “intermediate” strains in a single group labelled as resistant, instead of maintaining the distinction between the two groups.

      Table 11

      Antibiotic susceptibility testing of coagulase negative staphylococci

      Susceptibility to penicillin was tested because our microbiologists are involved in a study on resistance to β lactams in CoNS isolated from different sites (blood, eye, etc). As part of this survey, penicillin resistant isolates were also tested for resistance to methicillin (data not shown).

      The Kirby-Bauer method is generally recommended for routine antibiotic susceptibility testing of bacteria. On the other hand, this method was also used extensively by Sharma and co-workers in their paper. Determining the minimal inhibitory concentration may provide more useful information, especially while testing clinically relevant antibiotics such as vancomycin, teicoplanin, and methicillin.

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