The factors that underlie pathogenicity differences between subtypes of the enteric parasite, Cryptosporidium parvum, are not well defined. A new study by Huang et al. identifies a C. parvum ABC transporter that confers resistance to a toxic gut microbial metabolite.

Increasing the dose or serial passage of the avirulent IOWA-AZ isolate in immunocompromised mouse strains did not affect pathogenicity. To determine whether functional variants in specific genes could modulate infectivity, the authors generated genetic crosses between fluorescently tagged AUB and IOWA-AZ parasites and carried out forwards genetic analyses. Specifically, bulk segregant analysis was performed on pools of progeny parasites grown in immunocompromised mice under permissive (antibiotic treatment) or restricted (no antibiotic treatment) conditions. Genomic loci were identified on chromosomes 1, 6 and 7 that were positively associated with the virulent AUB parent. Of particular note was a significant quantitative trait locus on chromosome 7, which had four genes with polymorphisms between AUB and IOWA-AZ. The authors decided to focus on cgd7_4520 as a candidate gene on chromosome 7, owing to the presence of two non-synonymous SNPs.