Ciprofloxacin resistance and tolerance of Pseudomonas aeruginosa ocular isolates

Tolerance to antibiotics may occur due to changes in bacterial growth patterns and can be a precursor to development of resistance. However, there is a lack of information on the ability of ocular bacteria isolates to develop tolerance. This paper explores the tolerance to 8 different antibiotics of 61 microbial keratitis isolates of Pseudomonas aeruginosa from Australia and India using the MBC/MIC ratio, with tolerance defined by a ratio ≥ 32, and tolerance to ciprofloxacin by an agar diffusion assay.

Antibiotics used were ciprofloxacin, levofloxacin, gentamicin, tobramycin, piperacillin, imipenem, ceftazidime and polymyxin B. Isolates were sourced from microbial keratitis infections in Australia and India. Minimum bactericidal and minimum inhibitory concentration (MBC and MIC) were obtained using broth microdilution and compared to breakpoints from the Clinical Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) to determine bacterial susceptibility. Tolerance was assessed as MBC/MIC ≥ 32. An alternative method for tolerance detection (TD) was assessed with 13P. aeruginosa sensitive isolates by agar disk diffusion assay of ciprofloxacin followed by application of glucose to the agar and observation of re-growth of colonies.

Thirty-three isolates were resistant to imipenem, 20 to ciprofloxacin, 14 to tobramycin and piperacillin, 12 to levofloxacin and ceftazidime, 8 to gentamicin, and 5 to polymyxin B. The percentage of strains resistant to levofloxacin (7 vs 30 %; p = 0.023), gentamicin (0 vs 24 %; p = 0.005) and tobramycin (4 vs 33 %; p = 0.004) was significantly greater in isolates from India.On average, strains from India exhibited notably greater MIC and MBC values compared to strains obtained from Australia. Out of 61 isolates, none displayed an MBC/MIC ratio ≥ 32. However, three sensitive isolates had low tolerance, nine had medium tolerance and one had high tolerance to ciprofloxacin with the TDtest.

This study used two methods to determine whether P. aeruginosa strains could show tolerance to antibiotics. Using the MBC/MIC criteria no strain was considered tolerant to any of the eight antibiotics used. When 13 strains were tested for tolerance against ciprofloxacin, the most commonly used monotherapy for keratitis, one had high tolerance and nine had medium tolerance. This demonstrates the capacity of P. aeruginosa to develop tolerance which may result in therapeutic failures if inappropriate dosing regimens are used to treat keratitis.