Antimicrobial Sensitivity Profile of Chlamydia trachomatis isolates from Croatia in McCoy Cell Culture System and Comparison with the Literature.

BACKGROUND

Chlamydia trachomatis (C. trachomatis) is the most common bacterial agent of sexually transmitted infections around the world, but susceptibility testing of this pathogen is rarely pursued due to its intracellular niche. The principal aims of this research were to determine in vitro sensitivity profile of urogenital chlamydial strains isolated from Croatian patients and to compare obtained concentration values of different antimicrobial drugs mutually and with the literature.

METHODS

Forty strains of C. trachomatis isolated during 2010-2012 at the National Reference Laboratory for Chlamydia and two reference strains were subjected to susceptibility testing in 96-well microtiter plates containing McCoy cell monolayers. Minimal inhibitory concentration (MIC) and minimal chlamydicidal concentration (MCC) were determined for azithromycin, doxycycline, and levofloxacin. Briefly, strains were inoculated on McCoy cells, followed by addition of serially diluted antimicrobial drugs. Upon incubation, growth of C. trachomatis was detected using fluorescein-conjugated antibody to the lipopolysaccharide genus antigen under the inverted fluorescent microscope.

RESULTS

All chlamydial strains were susceptible to the antibiotics tested (MIC < 4 pg/mL), thus the pattern of homotypic or heterotypic resistance has not been found. MCC values were equal or 1-5 dilutions higher than MIC values. Statistically significant differences in the effectiveness of antimicrobial agents in vitro have been proven. Significant correlation has been found for MCCs in the case of two antimicrobial pairs: azithromycin and levofloxacin, and doxycycline and levofloxacin. Comparison of medians for different clinical samples did not reveal any significant difference.

CONCLUSIONS

Although resistant strains have not been found in this study, several literature reports of unsuccessfully treated genitourinary infections caused by C. trachomatis require our alertness for possible discovery of resistant strains. Considering the overall antibiotic burden worldwide, pursuing this kind of research is crucial in order to detect possible decreased susceptibility (or even resistance) of chlamydial strains, despite the laborious and time-consuming methodology.