Speed of kill plays vital role in clinical practice

Key Points

The answer may be a function of how fast anti-infectives work. Some agents work on contact; others may take several hours. Do you know how long it takes for the anti-infectives you use to work?

Although surgical technique plays a major role in patient outcomes, the therapeutic regimen a clinician chooses can have a major effect on success. We choose the best anti-infective to use based on spectrum of coverage, tolerability, and speed of kill. Speed of kill may be the most important attribute of an anti-infective. Although I hope my patients took their preoperative antibiotics as instructed, I need to know that even if they did not, the regimen begun since they arrived at my surgery center is adequate to eradicate all the bacteria on the ocular surface.

Two potential sources of bacteria can cause endophthalmitis. First, bacteria on the ocular surface at the time of surgery can be inoculated into the eye with instruments or manipulation. Second, bacteria on the surface of the eye during the first few postoperative days can be imbibed through a corneal incision until that incision becomes watertight. The most possible bacteria must be kept from the ocular surface both preoperatively and several days postoperatively.

Speed of kill most often is measured through kill curves. Kill curves track microbial suppression and growth as a function of both time and anti-infective absorption. A study presented by Terry O'Brien, MD, found that gatifloxacin ophthalmic solution 0.3% (Zymar, Allergan) killed Staphylococcus aureus and coagulase-negative Staphylococcus significantly more rapidly than moxifloxacin HCl ophthalmic solution 0.5% (Vigamox, Alcon Laboratories).¹ Gatifloxacin killed the bacterial inoculum completely at 30 minutes in 16 of 17 test strains. Once at 60 minutes, no bacterial colonies were recovered from any strain treated with gatifloxacin. At 60 minutes, moxifloxacin had reduced the bacterial inoculum in all the test strains. Complete killing within 60 minutes was observed, however, with only one of 17 strains.

With a slow-acting antibiotic, common pathogens and even more serious strains, such as methicillin-resistant S aureus (MRSA), can linger on the eye. Some anti-infectives do not work immediately, a fact that can have an impact on the time between application of the topical drop and surgery. A clinician who is unfamiliar with an anti-infective's speed of kill may be taking a risk by operating before the anti-infective has had a chance to kill the pathogens on the surface. Routine surgery would introduce pathogens capable of producing endophthalmitis.

Fluoroquinolone use

Fourth-generation fluoroquinolones have helped reduce the risk of postoperative ocular infection. New data from a large-scale clinical study by Majid Moshirfar, MD, compared gatifloxacin with moxifloxacin in the prevention of endophthalmitis after clear cornea sutureless cataract surgery.²

The study found that the incidence of endophthalmitis in patients undergoing ocular surgery was less with the group treated with gatifloxacin compared with that of the group treated with moxifloxacin. During the study period, 19,947 phacoemulsification surgeries were performed. Topical gatifloxacin was used in 16,151 (81%) patients, and moxifloxacin was used in 3,796 (19%) patients.

The incidence of acute endophthalmitis in the two groups was evaluated. The overall incidence of endophthalmitis for the entire study group was 0.07%. The incidence rate of endophthalmitis in the gatifloxacin group was 0.055% and in the moxifloxacin group was approximately twice as high at 0.13%. Although these results are not statistically significant, they are important to consider. Knowing the data on speed of kill and eradicating endophthalmitis has helped expedite patient flow at my surgery practice.

Depending on the anti-infective, the speed of kill can vary. A patient often is instructed to follow a dosing schedule up to 3 days preoperatively and receives another dose before surgery. There is no guarantee that the patient followed the regimen, so the clinician must prepare accordingly. The clinician has a short period pre-procedure to eradicate all the bacteria on the ocular surface, and he or she needs to be positive that the largest possible amounts of harmful organisms are killed.