Simplifying Collection of Corneal Specimens in Cases of Suspected Bacterial Keratitis

Abstract

Identification of the causative organisms in suspected bacterial keratitis traditionally involves collecting multiple corneal scrapes, which are plated directly onto different solid agar culture media. Difficulties have been reported with this practice, so the development of a simpler diagnostic method in suspected bacterial keratitis would be useful. It is unclear whether a single corneal scrape sent to the microbiology laboratory in a liquid transport culture medium (indirect method) is as reliable for the diagnosis of bacterial keratitis as inoculation of multiple scrapes directly onto agar plates (direct method). To investigate this, bacterial recovery was assessed following transfer and transport of different concentrations and types of bacteria from an artificially contaminated surgical blade into brain heart infusion (BHI). Bacterial recovery rates between the proposed (indirect) and standard (direct) method were then compared after the in vitro inoculation of pig corneas and following specimen collection in patients with presumed bacterial ulcerative keratitis. Recovery of bacteria from contaminated surgical blades was found to be the same from both solid and liquid culture media. There was no significant difference in the numbers of positive cultures from solid (direct) and liquid (indirect) culture media, both in the experimental pig cornea inoculation study (P ‫؍‬ 0.34) and in experiments with patients with clinical infections (P ‫؍‬ 0.4), with an 85.2% agreement between methods (kappa ‫؍‬ 0.61, P < 0.0001). In conclusion, therefore, the collection of two corneal scrapes, one used for Gram staining and the other transported in BHI followed by plating and subculturing in an enrichment medium, provides a simple method for the investigation of presumed bacterial keratitis.

Investigating Infectious Keratitis

“…If no laboratory-skilled assistant can be present at the time of sampling, basic microbiologic training should be given to the ophthalmologist, and fresh media should be provided as required. Data showing a rate of definitive diagnosis (70.4%) that is among the highest of those published (1,2,3) are the basis for this encouragement.…”

“…070, 2003). Similarly to Kaye et al (2), Nicola et al found that 55% of patients had received an antibiotic treatment before being evaluated in our center, and this clearly affected the recovery of an etiologic agent (45% versus 80%, with and without previous treatment, respectively [P Ͻ 0.01]). Moreover, in some cases, the pathogens were detected in only one of the inoculated media, and it was not unusual that very few colonies (fewer than five) grew on agar plates.…”

“…In a recently published study, Kaye et al (2) compared two approaches to evaluating bacterial keratitis: the conventional method, in which samples are obtained by taking several corneal scrapes and placing them on different media and slides (direct method, as described by Kaye et al), and a proposed method (indirect), in which only one scrape is placed into a tube containing brain heart broth for ulterior processing at the laboratory. Their experiments with in vitro suspensions of ocular pathogens and with enucleated pig eyes inoculated with the same organisms showed that both methods led to similar bacterial recoveries.…”

Molecular characterization of corneal ulcers causing Staphylococcus aureus

“…Then samples were incubated at 37°C for 24 hours for propagation. 12,13 Isolation and identification of pathogenic bacteria After incubation, a loopful of each enriched culture was streaked on CLED agar and Nutrient agar plates were incubated at 37°C for 24 hours. Colonies with different morphological characters and Gram's characters were selected and inoculated on respective selective media viz.…”

Do Pathogens from Corneal Scraping Show Their True Colours Better in Bactec Bottles?

“…However, even if a specimen of the corneal infiltrate is obtained for microbiological investigation using traditional method, subsequent growth and identification of pathogens occurs only in average of 50.47% cases (Kratz et al 2006). The traditional method involves the use of multiple specimens scraping with direct inoculation onto enriched solid culture agar plates (Kaye et al 2003). Studies were conducted to study the suitability of other transport medium and commercially available broth culture medium to increase positive yields (Kratz et al 2006;Kaye et al 2003).…”