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Indian Journal of Medical Microbiology, Vol. 23, No. 3, July-September, 2005, pp. 168-171 Original Article Susceptibility trends of Pseudomonas species from corneal ulcers Smitha S., Lalitha P., Prajna V.N., Srinivasan M. Department of Ocular Microbiology, Aravind Eye Hospital and PG Institute of Ophthalmology, Madurai - 625 020, Tamil Nadu Date of Submission: 26-Oct-2004 Code Number: mb05049 ABSTRACT Purpose : To assess the changing trends in the antibiotic susceptibility of Pseudomonas spp . isolated from bacterial keratitis over a nine year period with special emphasis on fluoroquinolone susceptibilities.Methods : All corneal scraping cultures positive for Pseudomonas spp. (n=585) isolated from patients with bacterial keratitis at the Aravind Eye Hospital, Madurai from1995-2003 were evaluated. Cultures were performed in liquid and solid media and susceptibility testing was done against amikacin, gentamicin, tobramycin, ciprofloxacin and ofloxacin by Kirby-Bauer disc diffusion method. Results: The susceptibility of Pseudomonas spp. was over 90% from 1995-1998 to ciprofloxacin which decreased to 83% from 1999-2003. The total number of isolates resistant to ciprofloxacin was 51(9.4%). No statistically significant increase in the number of isolates resistant to ciprofloxacin was noted. Ofloxacin showed 54% susceptibility from 1995-1998 but increased to 64% from 1999-2003. Analysis of in vitro activity of amikacin reveals that there was 43% sensitivity from 1995-1998 but later it increased to 76% from 1999-2003. In case of gentamicin, the sensitivity decreased marginally from 80% to 70% through the years. Tobramycin showed 45% sensitivity from 1995-1998 but increased to 75% from 1999-2003. Conclusions : The fluoroquinolones remain a good choice in the treatment of ocular infections, with high susceptibility of Pseudomonas spp. Among the aminoglycosides, gentamicin was found to be highly effective against Pseudomonas corneal ulcers when compared to amikacin and tobramycin. The results show a need for continuous monitoring of bacterial resistance trends. Keywords: Pseudomonas, antimicrobial susceptibilty, corneal ulcer, fluoroquinolones, aminoglycosides Bacterial keratitis is a significant cause of ocular morbidity that can result in severe visual loss.[1] For medical management of bacterial eye infections, treatment with broad spectrum antibiotics may be instituted before pathogen identification and antibiotic susceptibility tests are available.[2] Antibiotics are generally effective for treatment of most cases of bacterial ocular infections. However, holes in therapy emerge because of the frequent indiscriminate use of antibiotics, which has led to the development of resistance to many commonly used antimicrobials. [3],[4],[5] Hence, periodic susceptibility testing should be carried out as proposed by Jensen and coworkers for a period of 2-3 years to detect the resistance trends.[6] Such studies are of paramount value to health care providers who often have to select a first line antibiotic treatment without the benefit of having the microbiologic testing results . Repeated ocular infection, very large non healing corneal ulcers and those treated with antibiotics that have varied broad spectrum activities should hence be cultured to confirm the nature of organisms and to institute appropriate therapy. The general belief among ophthalmologists is that effective empiric therapy negates the need for culture confirmation. This situation is apparent in treatment of benign cases of conjunctivitis and in those keratitis cases that present with minimal corneal involvement. Patients with serious keratitis like large corneal ulcers or with a clinical picture suggestive of any unusual organisms, however should be cultured to identify the causative agent.[7] In such cases, in order to start specific therapy, it is necessary to perform meticulous laboratory investigations followed by sensitivity testing. MATERIALS & METHODS All consecutive patients with suspected Pseudomonas keratitis, who underwent a diagnostic corneal culture at Aravind Eye Hospital and Research centre from 1995-2003, were included in the study. The data reviewed in this study was retrospectively obtained from the microbiology laboratory records. Ulceration was defined as a loss of corneal epithelium with underlying stromal infiltration and suppuration associated with signs of inflammation with or without hypopyon. All patients had thoroughly undergone detailed slit lamp biomicroscopic examination by an ophthalmologist. Corneal scrapings were taken under aseptic conditions using a sterile kimura′s spatula after application of local anesthetic eye drops. The material scraped from the leading edge and the base of the ulcer was directly inoculated onto solid media (5% sheep blood agar, chocolate agar, potato dextrose agar) and incubated at 37 °C. Potato dextrose agar was incubated at 25 °C for 7 days. All the isolates were identified by routine biochemical tests. In vitro susceptibility testing was done by Kirby-Bauer disk diffusion method for all the 585 Pseudomonas isolates. Discs were purchased from Hi-media, Mumbai and were consistently tested for efficacy against ATCC strains as a general quality control procedure.[10] The results were interpreted using NCCLS standards.[11] Isolates of intermediate sensitivity were categorized as resistant as the number of intermediate isolates was insignificant when compared to the whole sample. Susceptibility testing was done against aminoglycosides - amikacin, gentamicin, tobramycin, and fluoroquinolones - ciprofloxacin, ofloxacin. The antibiotics that were tested were uniform throughout the study period.RESULTS From 1995 through 2003, 2200 culture proven cases of bacterial keratitis were obtained of which the total number of Pseudomonas isolates was 585(28.6%). The number of corneal ulcers caused by Pseudomonas through the years has been shown in [Figure - 1].The number of positive corneal cultures was not found to be consistent through the years (p = 0.001) and the average percentage of positivity was found to be 11.1%.The susceptibility trend and the percentage of sensitivity of Pseudomonas isolates to the second generation fluoroquinolones are depicted in [Figure - 2]. There were totally 573 isolates tested for ciprofloxacin and 343 for ofloxacin. Out of the 573 cases, 522 isolates were susceptible to the drug while the others (n = 51, 9.4%) were found to be resistant. The susceptibility was over 90% from 1995-1998 for ciprofloxacin but decreased to 83% from 1999-2003 (p < 0.001). Although there was an increase in the number of isolates resistant to ciprofloxacin, a large no. of the isolates were still sensitive. With respect to ofloxacin, out of the 343 isolates, 217 were found to be susceptible and 126 isolates were resistant. The susceptibility was 54% from 1995-1998 but increased to 64% from 1999-2003 (p < 0.0001). The susceptibility trend and percentage of sensitivity to aminoglycosides is depicted in [Figure - 3]. Analysis of in vitro activity of amikacin reveals that out of 402 isolates tested, 326 were susceptible and the remaining 76 isolates were resistant. There was 43% sensitivity from 1995-1998 but later it increased to 76% from 1999-2003 (p = 0.003). In case of gentamicin, out of the 581 isolates tested, 444 were susceptible and 137 were found to be resistant. The sensitivity decreased slightly from 80% to 70% through the years (p < 0.001). In case of tobramycin, out of 414 isolates, 286 were susceptible and the remaining 128 were found to be resistant. The susceptibility trend showed 45% sensitivity from 1995-1998 which increased to 75% from 1999-2003 (p < 0.001). DISCUSSION Bacterial keratitis is an ophthalmic emergency that needs immediate institution of treatment. In absence of laboratory diagnosis the initial therapy is usually broad spectrum intensive treatment. Specific therapy should be based on laboratory data which identifies the causative agent and provides antibacterial susceptibility results. However, it must be noted that these are in vitro results and one limitation of using in vitro susceptibility as a surrogate for in vivo effectiveness is that antibiotic sensitivities do not always mirror the clinical response to an antibiotic.[3],[12],[13],[14] Drug levels attainable in cornea are considerably higher than the serum and the disk diffusion method tends to overestimate resistance for ocular pathogens from a clinical standpoint. Because the commercial antibiotic preparations are believed to contain too low a concentration of antibiotic for effective treatment of keratitis concurrent use of two different antibiotics in higher concentrations prepared from standard parenteral products is often practised. Antibiotics like gentamicin, amikacin or cefazolin are used in fortified forms. But with flouroquinolones like ciprofloxacin or ofloxacin commercial preparation are effective. The emergence of antibiotic resistant ocular isolates has always been a concern. [15],[16],[17],[18] Results from the present study support this and indicate shifting trends of antibiotic susceptibility in case of Pseudomonas spp., which accounts for 26% of the total bacterial isolates of keratitis.Pseudomonas is a virulent corneal pathogen associated with rapid, liquefactive necrosis of the cornea.[19],[20] Results from this study show that Pseudomonas is most susceptible to gentamicin, ciprofloxacin and ofloxacin. Analysis of the aminoglycoside susceptibility trend shows that there was a decrease in the rate of sensitivity of amikacin from 1995 to 1998. This may be because the number of isolates tested against amikacin was less in the above years and amikacin was not included in the panel of antibiotics administered at that period. Gentamicin has been found to be highly effective against Pseudomonas spp. Ciprofloxacin, a second generation fluoroquinolone, also appears to be effective in treatment of Pseudomonas keratitis. A similar study conducted in India also showed that ciprofloxacin is highly effective against gram negative pathogens like Pseudomonas spp.[21] Fluoroquinolones are concentration dependent antibiotics and when bacteria are exposed to sub lethal concentration, resistance can be induced.[7] It has been said that following routine systemic use clinically significant resistance to ciprofloxacin may develop among isolates of P.aeruginosa . A recent series, which examined the conjunctival and corneal isolates of Pseudomonas , revealed a significant increase in ciprofloxacin resistance.[15] This increasing resistance in systemic isolates suggests that increasing resistance to this drug can be expected in ocular infections. In this study, 51 isolates (9.0%) were found to be resistant to ciprofloxacin out of 573 isolates tested for ciprofloxacin. A statistically significant linear increase in the number of isolates resistant to ciprofloxacin (p < 0.001) was not noted in our study unlike the study by Sharma et al .[5] Recent studies have also shown an increased in vitro resistance of ocular isolates to fluoroquinolones. [3],[4],[5] In 1996, Knauff et al and Bower et al reported 91.7% and 82.3% overall susceptibility of ocular isolates to ciprofloxacin but gram negative organisms showed 100% susceptibility in this series.[15], [22] Another study in India reported that 30.7% of 1558 isolates were resistant to ciprofloxacin by in vitro susceptibility testing.[3] Ofloxacin was also found to be effective against Pseudomonas spp. with over 50% sensitivity. Resistance of gram negative organisms to ciprofloxacin and ofloxacin has been noted in other locations in United States and in India for P.aeruginosa.[1],[5],[23] There are reports of Pseudomonas isolated from hospital acquired respiratory tract infections, which showed less than 50% susceptibility to ciprofloxacin and azetreonam. These results confirm the need for continuous monitoring of bacterial resistance trends.[24] In conclusion, this study analysed the second most common cause of corneal ulcer in our population and the antibiotics analysed especially the fluoroquinolones are the prescribed and commonly used in our community apart from the aminoglycosides like gentamicin. We believe that the fluoroquinolones remain a good choice in the treatment of gram negative ocular infections. These antibiotics are effective but should be used judiciously to avoid bacterial resistance to them and to ensure their future potency. REFERENCES
Copyright 2005 - Indian Journal of Medical Microbiology The following images related to this document are available:Photo images[mb05049f2.jpg] [mb05049f1.jpg] [mb05049f3.jpg] |
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