Characterization of Bacillus cereus Isolates Associated with Fatal Pneumonias: Strains Are Closely Related to Bacillus anthracis and Harbor B. anthracis Virulence Genes

Bacillus cereus is ubiquitous in nature and a common cause of emetic and diarrheal food poisoning. Most B. cereus isolates appear to be harmless, but some are considered opportunistic pathogens. In immunocompromised individuals or patients recovering from surgery, B. cereus can cause a variety of infections, including endophthalmitis, bacteremia, septicemia, endocarditis, salpingitis, cutaneous infections, pneumonia, and meningitis (19). Some strains are known to cause periodontal disease (8). Recently there have been reports of severe and sometimes fatal cases of pneumonia caused by B. cereus in apparently healthy welders (13, 20). The severity of these cases was unusual for B. cereus infections, and the patients were neither immunocompromised nor had any known underlying conditions causing susceptibility to these infections. B. cereus G9241, which was associated with severe pneumonia in a welder from Louisiana in 1994, has been well characterized, and its genome has been sequenced and analyzed (13). Genomic analysis and multilocus sequence typing (MLST) of this isolate revealed it to be closely related to B. anthracis. Several methods have shown that B. cereus isolates closely related to B. anthracis tend to be of clinical rather than environmental origin (9, 10, 11). However, it is very uncommon for B. cereus isolates, even those that are closely related to B. anthracis, to carry B. anthracis virulence plasmids (23). B. cereus G9241 carries an almost complete pXO1 plasmid, designated pBCXO1. This isolate also harbors a 218-kb circular plasmid (pBC218) and a cryptic bacteriophage (pBClin29) (13). Another unique feature of G9241 relative to other B. cereus isolates is the presence of a capsule. However, this capsule is not composed of d-glutamyl polypeptides and is not encoded by the B. anthracis cap genes normally located on the B. anthracis pXO2 plasmid. Instead, it has been hypothesized to be a polysaccharide and be encoded by a putative polysaccharide capsule biosynthetic operon located on pBC218 (13). While the presence of these plasmids in an isolate that causes severe disease similar to inhalation anthrax is intriguing, their roles, if any, in the virulence of the isolate or the presentation of disease has not yet been determined. In October and November 2003, two fatal cases of B. cereus pneumonia occurred in metal workers (a welder and a muller operator) at different locations in Texas. A detailed report of these cases and the epidemiologic investigation is in preparation (S. B. Avashia, submitted for publication). In this report, we describe the initial molecular genetic characterization of two clinical and one environmental B. cereus isolate from the investigation of these two fatal cases. We also describe how these isolates are related to each other, to B. anthracis, and to other previously characterized B. cereus and B. thuringiensis isolates.