Susceptibility testing of N. gonorrhoeae
Susceptibility testeing of N. gonorrhoeae monitors the efficacy of 1st line therapy for the treatment of N.gonorrhoeae. This is monitored by a sentinnel surveilance programme - GRASP.
Background
Neisseria gonorrhoeae, the causative agent of gonorrhoea, is an obligate human pathogen, which is adept at developing and acquiring resistance to antimicrobial agents. It is a fastidious organism and therefore requires a nutritious medium and supplements, such as glucose and iron, for growth. N. gonorrhoeae is inherently a highly susceptible organism but following extensive use of agents such as penicillin and ciprofloxacin, high-level resistance and therapeutic failure is now common.
Resistance Mechanisms
Resistance to penicillin can result from the acquisition of a plasmid, which encodes for the TEM-1 ??-lactamase, that inactivates penicillin, (penicillinase-producing N. gonorrhoeae, PPNG) or the additive effect of multiple mutations, which reduce the permeability of the membrane (chromosomally-mediated resistant N. gonorrhoeae, CMRNG). Ciprofloxacin resistance is due to mutations in the DNA gyrase gene, gyrA and the topoisomerase gene IV, which are responsible for supercoiling of DNA within the bacterial cell. Resistance is usually high-level and affects all the quinolones (quinolone resistant N. gonorrhoeae, QRNG). Resistance to spectinomycin has been sporadic and currently there are no reports of resistance to the third generation cephalosporins resulting in clinical failure. Azithromycin is a macrolide used primarily for the treatment of chlamydial infections and resitance in N.gonorrhoeae has only recently emerged.
Therapy
Penicillin was the mainstay for the treatment of gonorrhoea for decades but as resistance increased, in the late 1980s, guidelines were changed to recommend ciprofloxacin, which was given orally as a single dose (500mg) and was highly effective. In the England & Wales, ciprofloxacin was the recommended first-line therapy until resistance reached 5% of all isolates in 2002 and the guidelines were revised to recommend the use of the third generation cephalosporins, cefixime (oral) or ceftriaxone (intramuscular), www.bashh.org. These are the only two cephalosporins for which there is efficacy data for use for gonorrhoea. Alternative cephalosporins such as cefuroxime and cefixime may be effective for individual patient management but have a shorter half-life and could aid the selection of resistance ( Ison et al 2004 ). Azithromycin is not a recommended first line therapy for gonorrhoea and if used should be used at a two gram dose rather than the one gram dose used for chlamydia infections. Azithromycin resistance, as shown by GRASP is still low level but increasing annually.
Susceptibility Testing
In vitro susceptibility testing is used to inform individual patient management or to provide data for surveillance programmes. Disc diffusion tests are most commonly used by diagnostic laboratories to determine the susceptibility and hence chance of therapeutic failure for infections from individual patients. The methodology is recommended by the British Society for Antimicrobial Chemotherapy ( www.bsac.org) and on occasions can be problematic because of the fastidious nature of the organism. A recent problem has been the identification of ciprofloxacin resistant N. gonorrhoeae, which show susceptibility to nalidixic acid ( Ragunathan et al, 2005 ). BSAC recommends the use of nalidixic acid discs as a reliable method for screening for ciprofloxacin resistance and has changed their breakpoints to accommodate this observation ( www.bsac.org.uk - Page 27(table 14)).
An alternative approach, particularly useful for laboratories testing small numbers of isolates, is to use Etests to determine the minimum inhibitory concentration (MIC) using the manufacturers instructions. Determination of the MIC by agar dilution is a specialist technique, which is best reserved for reference centres.
Surveillance programmes are used to:
- Characterise antimicrobial susceptibility patterns
- Correlate patient characteristics and behaviours with infections caused by resistant gonococci
- Inform the development of national and local treatment guidelines for N. gonorrhoeae.
In England & Wales, the Gonococcal Resistance to Antimicrobial Surveillance Programme (GRASP) was initiated in 2000 and is a sentinel surveillance study. Therapy for gonorrhoea is often given before the susceptibility of the infecting organism is known, and GRASP has both informed and recommended changes to national guidelines to ensure that more the 95% of infections respond to first-line therapy.
STBRL Services
STBRL offer a confirmatory service for susceptibility testing to the recommended first-line treatments, cefixime and ceftriaxone, and for azithromycin. STBRL will only test isolates found to be resistant or problematic to these antimicrobial agents, and does not offer a service for routine testing for penicillin (ampicillin), ciprofloxacin or spectinomycin.
STBRL coordinate and perform the testing for the Gonococcal Resistance to Antimicrobial Surveillance Programme (GRASP). This collects isolates from 24 laboratories and performs susceptibility testing by determination of the MIC using the agar dilution method. Patterns of antimicrobial resistance are presented in the GRASP annual reports.
Last reviewed: 29 June 2009
