In Vitro Susceptibility Test of Sitafloxacin against Resistant Gram-Negative Bacilli Isolated from Thai Patients by Disk Diffusion Method
Visanu Thamlikitkul MD*, Surapee Tiengrim MSc*
Affiliation : * Division of Infectious Diseases and Tropical Medicine, Department of Medicine, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand
Objective : To determine a correlation of minimum inhibitory concentration (MIC) of sitafloxacin determined by agar dilution
method with inhibition zone diameter of sitafloxacin determined by disk diffusion method, and to determine inhibition zone,
diameter breakpoints of sitafloxacin against resistant gram-negative bacilli isolated from Thai patients.
Material and Method: The study bacteria were 332 clinical isolates of gram-negative bacilli including ESBL-producing E.
coli, ESBL-producing K. pneumoniae, P. aeruginosa and A. baumannii. Each isolate of the present study bacteria was tested
for minimum inhibitory concentration (MIC) of sitafloxacin by agar dilution method and inhibition zone diameter of sitafloxacin
by disk diffusion method.
Results : The MICs and inhibition zone diameters of sitafloxacin against gram-negative bacilli were well correlated (correlation
coefficient -0.926, p-value <0.001). The inhibition zone diameter >15 mm had the least total error for determining susceptibility
to sitafloxacin based on MIC value of sitafloxacin but the inhibition zone diameter >16 mm had less false susceptibility than
that of >15 mm when compared with sitafloxacin MIC <2 mg/l that was considered susceptible. The inhibition zone diameter
>19 mm had the least total error for determining susceptibility to sitafloxacin based on MIC value of sitafloxacin but the
inhibition zone diameter >18 mm had less false susceptibility than that of >19 mm when compared with sitafloxacin MIC <1
mg/l that was considered susceptible.
Conclusion : For the susceptibility test of sitafloxacin against resistant gram-negative bacilli by disk diffusion method, the
inhibition zone diameter >16 mm and >18 mm seem to be the appropriate breakpoints for susceptibility for resistant gram-
negative bacilli isolated from urine and blood, respectively, since the serum concentration of sitafloxacin is rather low
whereas the urinary concentration of sitafloxacin is much higher.
Keywords : Sitafloxacin, Inhibition zone, Minimum inhibitory concentration, Gram-negative bacilli, Susceptibility
