High frequency of Nichols-like strains and increased levels of macrolide resistance in Treponema pallidum isolates from Buenos Aires, Argentina

Abstract Globally, 94% of Treponema pallidum subsp. pallidum (TPA) clinical isolates belong to the SS14-like group and 6% to the Nichols-like group, with a prevalence of macrolide resistance of 90%. Our goal was to determine whether local TPA strain distribution and macrolide resistance frequency have changed significantly since our last report, which revealed that Argentina had a high frequency of Nichols-like strains (27%) and low levels of macrolide resistance (14%). Swab samples from patients with suspected syphilis were collected during 2015-2019 and loci TP0136, TP0548, TP0705 were sequenced in order to perform multilocus sequence typing. Strains were classified as Nichols-like or SS14-like. The presence of macrolide resistance-associated mutations was determined by examination of the 23S rDNA gene sequence. Of 46 typable samples, 37.0% were classified as Nichols-like and 63.0% as SS14-like. Macrolide resistance prevalence was 45.7%. Seven allelic profiles were found, five were SS14-like and two were Nichols-like. The frequency of Nichols-like strains increased between studies (26.8% vs. 37.0%, p=0.36). A dramatic increase was found in the frequency of macrolide resistant isolates between studies (14.3% vs. 45.7%, p=0.005). Our results are in agreement with international tendencies and underscore the need to pursue further TPA molecular typing studies in South America.