Five-year outbreak of community- and hospital-acquired Mycobacterium porcinum infections related to public water supplies

Abstract

Mycobacterium porcinum is a rarely encountered rapidly growing Mycobacterium (RGM). We identified M. porcinum from 24 patients at a Galveston university hospital (University of Texas Medical Branch) over a 5-year period. M. porcinum was considered a pathogen in 11 (46%) of 24 infected patients, including 4 patients with community-acquired disease. Retrospective patient data were collected, and water samples were cultured. Molecular analysis of water isolates, clustered clinical isolates, and 15 unrelated control strains of M. porcinum was performed. Among samples of hospital ice and tap water, 63% were positive for RGM, 50% of which were M. porcinum. Among samples of water from the city of Galveston, four of five households (80%) were positive for M. porcinum. By pulsed-field gel electrophoresis (PFGE), 8 of 10 environmental M. porcinum were determined to belong to two closely related clones. A total of 26 of 29 clinical isolates subjected to PFGE (including isolates from all positive patients) were clonal with the water patterns, including patients with community-acquired disease. Fifteen control strains of M. porcinum had unique profiles. Sequencing of hsp65, recA, and rpoB revealed the PFGE outbreak clones to have identical sequences, while unrelated strains exhibited multiple sequence variants. M. porcinum from 22 (92%) of 24 patients were clonal, matched hospital- and household water-acquired isolates, and differed from epidemiologically unrelated strains. M. porcinum can be a drinking water contaminant, serve as a long-term reservoir (years) for patient contamination (especially sputum), and be a source of clinical disease. This study expands concern about public health issues regarding nontuberculous mycobacteria. Multilocus gene sequencing helped define clonal populations.

Fig. 1.

Positive cases of M. porcinum at the UTMB from January 2005 to August 2010, charted by date of collection of first positive specimen (n = 26 [24 patients; 2 patients each had two separate episodes]).

Fig. 2.

Dendrogram of AseI digests of the two PFGE M. porcinum outbreak clones (A/A and A/B) grouped by visual reading using the criteria of Tenover et al. (32). The dendrogram prepared using the Pearson correlation method also produced two groups of closely related isolates that included patient isolates, hospital water isolates, and city water isolates (outbreak = clinical isolate). The first column includes the isolate number, the second column the isolate source, and the third column, the PFGE patterns for AseI/XbaI obtained by visual reading. Two isolates (MF-3128 and MF-3129) were run in duplicate. MF-3176, which was unrelated to the two clones by visual reading, was included as a control.

Fig. 3.

PFGE of XbaI digests of the two outbreak clones and random clinical isolates of M. porcinum. Lane 1, hospital water isolate with pattern A with AseI digests; lane 2, hospital ice machine water isolate with AseI digest pattern B; lane 3, ATCC 33776^T with a unique pattern; lanes 4 to 11, random patient isolates of M. porcinum, all of which have unique (unrelated) PFGE patterns; lane 12, lambda ladder. The letters “A” and “B” represent the two (closely related) outbreak clones; the letter “U” means unique.