The present case involved suppurative keratitis caused by
S. lugdunensis in an elderly patient, and the characteristic findings of our case are as follows: (1) the contributing factors are unknown, (2) the condition can lead to severe suppurative keratitis, (3) the clinical course is rapid, and (4) the infectious corneal ulcer showed susceptibility to treatment with antibiotics. The representative causative bacteria of suppurative keratitis are
Streptococcus pneumoniae and
Pseudomonas aeruginosa [
6,
7], and suppurative keratitis progresses rapidly. Therefore, in the treatment of suppurative keratitis, it is important that empiric therapy be initiated without waiting for the results of a bacterial culture. The antibacterial spectrum of antibiotics is taken into consideration when conducting empiric therapy; when the causative bacterium is presumed to be a gram-positive coccus such as
Streptococcus pneumoniae, a treatment combining cephem and fluoroquinolone antibacterial agents in an ophthalmic solution is selected, and when the causative bacterium is presumed to be a gram-negative bacillus such as
Pseudomonas aeruginosa, a treatment combining aminoglycoside and fluoroquinolone antibacterial agents in an ophthalmic solution is selected. In the present case, the clinical examination revealed severe suppurative keratitis; therefore, empiric therapy was conducted presuming that the causative bacterium was
Pseudomonas aeruginosa. However, the efficacy of the gentamicin and ofloxacin treatment used in this case will need to be verified.
S. lugdunensis was previously isolated from a patient with suppurative keratitis [
5], suggesting it should be recognized as a causative organism of suppurative keratitis. The results of drug-sensitivity tests conducted on the clinical isolates in the present case showed no drug resistance, but low sensitivity to gentamicin, which was used in the empiric therapy. In addition, β-lactamase-producing strains of
S. lugdunensis have been isolated from abscesses and surgical wounds [
3], suggesting that certain strains may exhibit drug resistance similar to that of methicillin-resistant
S. aureus. Therefore, before conducting an empiric therapy, scrapings of the lesions are recommended to identify the causative organism through smear testing and bacterial isolation and culture, and the antibiotic therapy may need to be revised in accordance with the test results. The small amount of corneal scraping specimens that can be collected limits the choice of microbiological methods that can be exploited in diagnosis. Accordingly, we performed gram staining using a smear and a chocolate agar culture in this examination. Chocolate agar is a bacterial isolation medium, but may also be able to isolate fungi such as
Candida and
Fusarium in about 5 days of culture (data not shown). Therefore, at our institution, we directly apply a corneal scraping specimen to chocolate agar as a screening test for infectious keratitis when a small amount of specimen is obtained.