Identification of Mycobacterium avium subsp. hominissuis Isolated from Drinking Water and Food

摘要

Mycobacterium avium (MA) is divided into four subspecies based primarily on host-range and consists of MA subsp. avium (birds), MA subsp. silvaticum (wood pigeons), MA subsp. paratuberculosis (broad, poorly-defined host range), and the recently described MA subsp. hominissuis (humans and swine). MA is ubiquitous in the environment and evidence suggests water is a possible source of human exposure. Routes of exposure to waterborne pathogens include ingestion, inhalation of water vapor, and ingestion of produce irrigated or washed with contaminated water. The goal of this study was to determine the specificity of MA subsp. hominissuis to humans by subspeciating human clinical isolates of MA, and also to determine the proportion of MA isolated from food and water that belongs to this subspecies. Understanding the occurrence of MA subsp. hominissuis in water and food will aid the U.S. Environmental Protection Agency in assessing the risk of human exposure to these sources. Temperature growth range, IS1245 copy number, sequencing of the 16S-23S internal transcribed spacer region or hsp65 gene and other methods have been used to differentiate subspecies hominissuis from the other subspecies. MA subsp. hominissuis can grow on Lownestein Jensen (LJ) medium supplemented with pyruvate at 45 °Cwhile bird type isolates cannot (Mijs et al., 2002). Clinical and environmental MA isolates from our collection were inoculated on this medium and allowed to incubate for up to 90 days. Ninety percent of the isolates grew at 25 °C and 67% grew at 45 °C. Isolates that grew at 45 °C also grew at 25 °C. Nearly 100% of the human isolates included in this study grew at 45 °C, suggesting they are strains of MA subsp. hominissuis. In addition, we performed PCR and sequence analysis on the 3′ end of the hsp65 gene as a means to confirm MA subsp. hominissuis identification. Human isolates had hsp65 sequences closely related to hsp65 sequences from MA subspecies hominissuis described previously. Moreover, several hsp65 sequences from environmental isolates were identical or very closely related to those of hominissuis. None of the human or environmental isolates in our study had hsp65 sequences that clustered with MA subsp. avium or paratuberculosis. These data suggest that the type of MA that is specific to humans may be more prevalent in the environment than was previously thought.