Abstract
Purpose :
Calcium alginate (CA) swabs are recommended for culturing corneal ulcers, but cotton tip applicators (CTA) are less expensive and more readily available. Whether one is absolutely better than the other is unknown. We compared the efficacy of each swab in recovering ocular surface bacteria using a novel in vitro model under moistened and dry swabbing conditions.
Methods :
Ex vivo pig eyes were de-epithelialized and incubated at room temperature to allow for growth of bacteria on the ocular surface over time. The corneas were swabbed at 4 hour increments from 0 to 24 hours and plated on blood agarose plates. Colonies were counted to create a growth curve. It was determined that swabbing the corneas after six hours of incubation allowed for enough growth of colonies that could be easily counted and was reproducible. Next, 40 pig eyes incubated together were evenly divided into two groups; 20 eyes were swabbed with thioglycolate-moistened or dry CA swabs, and the remaining were swabbed similarly with CTA. The colonies on each blood agarose plate were counted, groups were averaged and a 2-tail T-test was used for statistical analysis.
Results :
Overall, there is a statistically significant difference in number of colonies obtained after swabbing the ocular surface with moistened versus dry swabs (51.9 +/- 13.2 vs 28.5 +/- 13.8, respectively; p-value <0.0001). Moreover, swabbing with a moistened CTA yielded more bacteria than dry CTA (56.1 +/- 11.5 vs 29.9 +/- 14.8, respectively; p-value <0.0001); this held true for wet versus dry CA as well (47.6 +/- 13.7 vs 27.2 +/-13.0, respectively; p-value <0.0001). Furthermore, there was a statistically higher number of colonies obtained using moistened CTA over moistened CA (56.1 +/- 11.5 vs. 47.6 +/- 13.7, respectively; p-value 0.0399).
Conclusions :
Moistened CTA swabs are superior to moistened CA in isolating bacteria from the ocular surface, but a moistened CA is better than swabbing with a dry CA swab or dry CTA. Regardless of which is used, it is important that all swabs be moistened with a medium such as thioglycolate to increase the recovery of bacteria from the ocular surface.
This is a 2021 ARVO Annual Meeting abstract.