Opa‐typing: a high‐resolution tool for studying the epidemiology of gonorrhoea

A single gonococcus possesses a family of 11 distinct and highly variable opa genes. The extensive variation and rapid evolution of the opa gene repertoire has been exploited to provide a high‐resolution typing method for studies of the short‐term transmission of gonorrhoea. The 11 opa genes are amplified with a single pair of primers by the polymerase chain reaction, digested with frequently‐cutting restriction enzymes, and the fragments are fractionated on polyacrylamide to provide an opa‐type. The method appeared to be highly discriminatory as the opa‐types of gonococci, isolated world‐wide over the last 30 years, were all different. Opa‐typing discriminated between isolates of the same auxotype/serovar class. Similarly, there were 41 opa‐types among 43 consecutive isolates from a sexually transmitted disease (STD) clinic. The two pairs of isolates from this clinic that gave the same opa‐types were identical by other criteria and may have been from unsuspected sexual contacts. With one minor exception, identical opa‐types were obtained from gonococci recovered from known sexual contacts. These results suggest that variation in the family of 11 opa genes evolves so rapidly that the opa‐types of gonococci are distinguishable, unless the isolates are from sexual contacts or a short chain of disease transmission. The identification of gonococci with identical opa‐types is therefore believed to be a good indicator that the individuals from which they were recovered were sexual partners, or part of a short chain of disease transmission.