Classes 1 and 2 integrons in faecal Escherichia coli strains isolated from mother-child pairs in Nigeria

Abstract

Background: Antimicrobial resistance among enteric bacteria in Africa is increasingly mediated by integrons on horizontally acquired genetic elements. There have been recent reports of such elements in invasive pathogens across Africa, but very little is known about the faecal reservoir of integron-borne genes.

Methods and findings: We screened 1098 faecal Escherichia coli isolates from 134 mother-child pairs for integron cassettes by PCR using primers that anneal to the 5' and 3' conserved ends of the cassette regions and for plasmid replicons. Genetic relatedness of isolates was determined by flagellin and multi-locus sequence typing. Integron cassettes were amplified in 410 (37.5%) isolates and were significantly associated with resistance to trimethoprim and multiple resistance. Ten cassette combinations were found in class 1 and two in class 2 integrons. The most common class 1 cassette configurations were single aadA1 (23.4%), dfrA7 (18.3%) and dfrA5 (14.4%). Class 2 cassette configurations were all either dfrA1-satI-aadA1 (n = 31, 7.6%) or dfrA1-satI (n = 13, 3.2%). A dfr cassette was detected in 294 (31.1%) of trimethoprim resistant strains and an aadA cassette in 242 (23%) of streptomycin resistant strains. Strains bearing integrons carried a wide range of plasmid replicons of which FIB/Y (n = 169; 41.2%) was the most frequently detected. Nine isolates from five different individuals carried the dfrA17-aadA5-bearing ST69 clonal group A (CGA). The same integron cassette combination was identified from multiple distinct isolates within the same host and between four mother-child pairs.

Conclusions: Integrons are important determinants of resistance in faecal E. coli. Plasmids in integron-containing strains may contribute to dispersing resistance genes. There is a need for improved surveillance for resistance and its mechanisms of dissemination and persistence and mobility of resistance genes in the community and clinical settings.

Fig 1. Plasmid replicons types detected in strains harboring only one class 1 integron cassette combination and only one replicon marker site.

Fig 2. PCR-RFLP RsaI restriction pattern types of amplified fliC genes of dfrA5 containing isolates.

Lanes 1–20: 1kb+ DNA ladder; 042, c62b, m63b, m103e, c141d, c103d, c102a, m103e, m110a, m110b, m110d, m112a, c118c, c132a, c132b, c141a, c163b, m174e, 1kb+ DNA ladder. c = Child: m = Mother.

Fig 3. fliC-RFLP of dfrA5 containing isolates (N = 59).