With the development of cephalosporin resistant N. gonorrhoeae(NG), there are few options available for the treatment of this sexually transmitted infection. As a participant in the Gonorrhea Isolates Surveillance Project (GISP), our New Orleans site was aware of local isolates with decreased susceptibility to azithromycin and cephalosporins. This prompted our study of in vitro susceptibility testing of NG to ertapenem (ERT), a broad spectrum carbapenem antibiotic that can be administered in a single intramuscular dose. Five previous studies have investigated the susceptibility of NG to ERT. The most notable one performed in the UK showed that as ceftriaxone minimum inhibitory concentrations (MICs) rose, MICs to ERT fell. The MIC ranges for ERT in these studies were 0.002 µg/L to 0.5 µg/L.
Using the E-test method, we performed susceptibility testing on GISP isolates with confirmed NG via direct visualization on modified Methylene Blue/Gentian Violet stain and positive culture. All viable isolates from our site from 2014 were tested (n=150). Antibiotic E-test strips were applied for ERT, ceftriaxone, azithromycin, and gentamicin. The MICs for each isolate were recorded and compared to the established Clinical Laboratory Standard Institute (CLSI) values.
Of the 150 isolates tested MICs for ERT ranged from < 0.02 to 0.19 µg/L, and 98% had MICs of < 0.12 µg/L. which is considered the optimal susceptible range. Gentamicin exhibited MICs ranging from 0.25 to 8 µg/L, which is within the susceptible range. Results for azithromycin were poor with eight isolates having an MIC of > 2 µg/L. All isolates were susceptible to ceftriaxone with MICs < 0.0125 µg/L. The two isolates which had relatively higher MICs to ERT compared to the proposed breakpoints of ≤0.12 also had higher MICs to ceftriaxone of 0.94. Since this is a conservative breakpoint the relative MICs found are lower overall for ERT suggesting that NG may be susceptible to this range.
We demonstrated that NG is very susceptible to ERT in vitro and shows promise as a possible therapy. Our findings also support the 2015 CDC STD Treatment Guidelines that azithromycin should not be used alone in the treatment of NG. The next phase of our research will look at activity of ERT against additional cephalosporin resistant strains.
M. J. Nsuami, None
J. Hebert, None
S. Taylor, None
See more of: Poster Abstract Session