Novel Sequence Type in Bacillus cereus Strains Associated with Nosocomial Infections and Bacteremia, Japan

Abstract

Bacillus cereus is associated with foodborne illnesses characterized by vomiting and diarrhea. Although some B. cereus strains that cause severe extraintestinal infections and nosocomial infections are recognized as serious public health threats in healthcare settings, the genetic backgrounds of B. cereus strains causing such infections remain unknown. By conducting pulsed-field gel electrophoresis and multilocus sequence typing, we found that a novel sequence type (ST), newly registered as ST1420, was the dominant ST isolated from the cases of nosocomial infections that occurred in 3 locations in Japan in 2006, 2013, and 2016. Phylogenetic analysis showed that ST1420 strains belonged to the Cereus III lineage, which is much closer to the Anthracis lineage than to other Cereus lineages. Our results suggest that ST1420 is a prevalent ST in B. cereus strains that have caused recent nosocomial infections in Japan.

Keywords: Bacillus cereus; Japan; bacteremia; bacteria; enteric infections; food safety; multilocus sequence typing; nosocomial infections; phylogenetic analysis; pulsed-field gel electrophoresis; repetitive-element PCR.

Figure 1

PFGE of the Tokyo strains of Bacillus cereus isolates, Japan. The 80% similarity cutoff for PFGE cluster typing is shown as a vertical line in the phylogenetic tree. Red letters and numbers represent samples isolated from patients with bacteremia. Rep-PCR cluster type and MLST sequence types are also shown. The Tokyo_ID31 strain was not analyzed. Scale bar indicates percent similarity. MLST, multilocus sequence typing; PFGE, pulsed-field gel electrophoresis; Rep-PCR, repetitive-element PCR.

Figure 2

Pulsed-field gel electrophoresis (PFGE) results of the ST1420 strains of A) Tochigi strains and B) Tottori strains of Bacillus cereus isolates, Japan. The 80% similarity cutoff for PFGE cluster typing is shown as a vertical line in the phylogenetic tree. Names of strains of sequence type 1420 are indicated in red. The Tochigi_ID31_#3 was not analyzed. ID, identification. Scale bars indicate percent similarity.

Figure 3

Multilocus sequence typing (MLST)–based phylogenetic trees of strains and STs of Bacillus cereus isolates, Japan. Reference sequences were obtained from the MLST database (https://pubmlst.org). Definitions of clades and lineage names followed those of Priest et al. (9). A) Phylogenetic tree of isolates from patients with bacteremia. Blue indicates Tokyo strains, red indicates Tochigi stains, orange indicates Tottori strains, and green indicates Kochi strains. B) Phylogenetic tree of STs detected in Tokyo and Tochigi strains. Blue indicates STs detected in Tokyo strains, and red indicates STs detected in Tochigi strains. Scale bars indicates nucleotide substitutions per site. Ba, B. anthracis; Bc, B. cereus; ID, identification; ST, sequence type.