Anthrax, caused by the bacterium Bacillus anthracis, is a disease of historical and current importance that is found throughout the world. The basis of its historical transmission is anecdotal and its true global population structure has remained largely cryptic. Seven diverse B. anthracis strains were whole-genome sequenced to identify rare single nucleotide polymorphisms (SNPs), followed by phylogenetic reconstruction of these characters onto an evolutionary model. This analysis identified SNPs that define the major clonal lineages within the species. These SNPs, in concert with 15 variable number tandem repeat (VNTR) markers, were used to subtype a collection of 1,033 B. anthracis isolates from 42 countries to create an extensive genotype data set. These analyses subdivided the isolates into three previously recognized major lineages (A, B, and C), with further subdivision into 12 clonal sub-lineages or sub-groups and, finally, 221 unique MLVA15 genotypes. This rare genomic variation was used to document the evolutionary progression of B. anthracis and to establish global patterns of diversity. Isolates in the A lineage are widely dispersed globally, whereas the B and C lineages occur on more restricted spatial scales. Molecular clock models based upon genome-wide synonymous substitutions indicate there was a massive radiation of the A lineage that occurred in the mid-Holocene (3,064–6,127 ybp). On more recent temporal scales, the global population structure of B. anthracis reflects colonial-era importation of specific genotypes from the Old World into the New World, as well as the repeated industrial importation of diverse genotypes into developed countries via spore-contaminated animal products. These findings indicate humans have played an important role in the evolution of anthrax by increasing the proliferation and dispersal of this now global disease. Finally, the value of global genotypic analysis for investigating bioterrorist-mediated outbreaks of anthrax is demonstrated.
Bacillus anthracis is a bacterial pathogen of great importance, both historically and in the present. This study presents data collected from several investigations and indicates that B. anthracis virulence is associated with the clonality and virulence of plasmids pXO1 and pXO2. Guinea pigs vaccinated with Anthrax Vaccine Adsorbed were challenged with 20 B. anthracis isolates representative of worldwide genetic diversity. These same isolates were characterized with respect to plasmid copy number by using a novel method of quantitative PCR developed for rapid and efficient detection of B. anthracis from environmental samples. We found that the copy numbers for both pXO1 and pXO2 differed from those in previously published reports. By combining the data on survival, plasmid copy numbers, and clonality, we developed a model predicting virulence. This model was validated by using a randomly chosen set of 12 additional B. anthracis isolates. Results from this study will be helpful in future efforts to elucidate the basis for variation in the virulence of this important pathogen.
A test of 25 genetically diverse isolates of Bacillus anthracis was conducted to determine their susceptibility to seven clinically relevant antimicrobial agents. Etest strips (AB BIODISK, Solna, Sweden) were used to measure the MICs for the isolates. Using the National Committee for Clinical Laboratory Standards MIC breakpoints for staphylococci, three isolates were found to be resistant to penicillin and five were found to be resistant to cefuroxime. The penicillin-resistant isolates were negative for -lactamase production. Continued surveillance of B. anthracis field isolates is recommended to monitor antimicrobial susceptibility.The etiologic agent of anthrax, Bacillus anthracis, causes an acute disease, primarily of herbivores, that is transmissible to humans. Susceptible animals are primarily infected by the spores formed during the vegetative state of the bacteria. Spores may be found in soil contaminated by diseased animals or in diseased animal products, such as hair, wool, hides, and bones. The importance of treatment of the disease in humans has been underscored by the bioterrorism events of October 2001 in the United States. Ciprofloxacin was the antimicrobial agent of choice for prophylactic treatment after exposure to the spores of B. anthracis that were used in the bioterrorism events. Penicillin, traditionally the drug of choice for treatment, is still recommended in other parts of the world despite reports of penicillin resistance (2, 3). The Centers for Disease Control recommended ciprofloxacin, penicillin, and doxycycline for the treatment of human anthrax and for use as a prophylactic measure prior to and during the October event (1). In the past, streptomycin, penicillin, gentamicin, and chloramphenicol have also been recommended (7).Other articles concerning B. anthracis and antimicrobial susceptibility have been written: two over 10 years ago and one within the past year. The first paper, published in 1990 by Lightfoot et al., determined the antimicrobial susceptibility of 70 isolates of B anthracis to penicillin, amoxicillin, cefuroxime, gentamicin, streptomycin, erythromycin, tretracycline, chloramphenicol, and ciprofloxacin by agar dilution (6). Penicillin resistance and -lactamase production were noted in two isolates. This paper was quickly followed in 1991 by a paper by Doganay and Aydin (2). They tested 22 B. anthracis isolates against 27 antimicrobial agents by agar dilution and reported that 19 isolates showed resistance to the five broad-spectrum cephalosporins tested. The isolates were shown to be sensitive to the other antimicrobials tested. These two papers were very basic in their approach and reporting. The latest paper, by Mohammed et al., tested 65 isolates with nine antimicrobial agents by using the National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method (7). The results obtained from 50 of the isolates were compared to those obtained with Etest strips. One isolate was penicillin resistant, and no statistically significant difference was ...
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