Comparative in-vitro activities of topical antibiotics against conjunctival isolates

J Antimicrob Chemother 1999; 43: 611–612

Jabed Iqbala,*, Mizanur Rahmanb, M. Shahidul Kabirb and Ahmed J. Sharifc

a Medical Service Department, Square Pharmaceuticals Ltd, Dhaka b Department of Microbiology, National Diagnostic Network, Diabetic Association of Bangladesh,Dhaka c Department of Ophthalmology, Dhaka Medical College Hospital, Dhaka, Bangladesh

Sir,

The present study was undertaken to evaluate the in-vitro activities of commonly usedophthalmic antibiotic preparations against external ocular pathogens.

The patients were residents of Dhaka City with clinical diagnoses of acute bacterialconjunctivitis who had not received systemic or intra-ocular antibiotics in the preceding 4weeks. The diagnosis was based on the presence of conjunctival hyperaemia and one or both of apurulent exudate or crusting on the eyelids. Conjunctival swabs were processed at a centrallaboratory according to standard laboratory techniques and the isolates obtained afterovernight culture were identified by standard procedures. Isolates were regarded as pathogensif they met one of the following criteria: Gram-negative bacillus; ß-haemolytic streptococcus; Streptococcus pneumoniae; >=10 colonies of an {alpha}-haemolytic streptococcus or Staphylococcus aureus; or >=100 colonies of a coagulase-negative staphylococcus (CoNS). 1

The susceptibilities of the isolates were determined by the disc diffusion method accordingto recommendations of the National Committee for Clinical Laboratory Standards (NCCLS). 2 The medium used was Mueller–Hinton agar, supplemented with 5% sheep blood forstreptococci, or chocolate agar for Haemophilus spp. The following antibiotics were tested: tetracycline (30 µg), chloramphenicol (30 µg), gentamicin (10 µg), trimethoprim/ sulphamethoxazole (25 µg), penicillin (10 µg), erythromycin (15 µg), ciprofloxacin (30 µg), lomefloxacin (30 µg), bacitracin (10 µg), tobramycin (10 µg) and neomycin (30 µg). All of the discs were obtained from Oxoid, Unimed Co. Ltd (UK) and the isolates werecategorized as susceptible or resistant according to inhibition zone diameter breakpointsrecommended by the NCCLS. 2

Ninety-eight presumed pathogens were recovered. S. aureus was the most common isolate (accounting for 62% of the pathogens), followed by CoNS (24%), although the roles of these organisms as causes of acute conjunctivitis are controversial. The susceptibilities of the isolates are summarized in the Table. The ranking of the antibiotics interms of percentage susceptible strains (in descending order) was as follows: bacitracin; gentamicin; ciprofloxacin; tobramycin; lomefloxacin; chloramphenicol; trimethoprim/sulphamethoxazole; neomycin; tetracycline; erythromycin; and penicillin. While chloramphenicol is the most commonly used ophthalmic antibiotic in Bangladesh, followed by ciprofloxacin and gentamicin (S. Hossain, personal communication), it was active against fewer isolates than the latter two drugs. Ciprofloxacin was active against a marginally higherpercentage of isolates than lomefloxacin and, of the aminoglycosides tested, gentamicin was active against the highest percentage of strains.


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Table. Susceptibilities of 98 external ocular pathogens to various antibiotics available as topical formulations
 
The results of this study are in accord with published data from Pakistan 3 which also showed that bacitracin was the most active antibiotic tested (91% of isolates being susceptible); moreover, the percentages of strains that were susceptible to tetracycline, chloramphenicol, gentamicin and neomycin were comparable to those reported here. Whilst the absence of comprehensive published data concerning the antibiotic susceptibilities of ocular pathogens precludes drawing accurate conclusions about trends in susceptibility/resistance, the results of this limited study suggest that no currently available topical formulation is active in vitro against all of the common pathogens. Only one antibiotic combination, trimethoprim/sulphamethoxazole, was evaluated by us and this agent was not superior to the single antibiotics tested. In conclusion, we have demonstrated that bacitracin is the most broad-spectrum agent currently available in Bangladesh as empirical treatment of patients with external ocular infections.

Acknowledgments

We are grateful to the Diabetic Association of Bangladesh for providing financial support for this study. We would also like to thank Mr Shazeed Hossain of Square Pharmaceuticals Ltd, Dhaka,for providing the unpublished data referred to in this article.

Notes

* Correspondence address. Medical Service Department, Square Pharmaceuticals Ltd, Square Center, 48 Mohakhali CA, Dhaka 1212, Bangladesh. Tel: +880-2-887729; Fax: +880-2-888608/9; E-mail: square{at}bangla.net Back

References

1 . Jacobsen, J. A., Call, N. B., Kasworm, E. M., Dirks, M. S. & Turner, R. B. (1988). Safetyand efficacy of topical norfloxacin versus tobramycin in the treatment of external ocularinfections. Antimicrobial Agents and Chemotherapy 32, 1820–4.[ISI][Medline]

2 . National Committee for Clinical Laboratory Standards. (1993). Performance Standards for Antimicrobial Disc Susceptibility Tests—Fifth Edition:Approved Standard M2-A5. NCCLS, Villanova, PA.

3 . Woodland, R. M., Darougar, S., Thaker, U., Cornell, L., Siddique, M., Wania, J. et al. (1992). Causes of conjuctivitis and keratoconjuctivitis in Karachi, Pakistan. Transactions of the Royal Society of Tropical Medicine and Hygiene83 , 317–20.





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