Identification of Virulent Capnocytophaga canimorsus Isolates by Capsular Typing

Abstract

Capnocytophaga canimorsus is a dog oral commensal that causes rare but severe infections in humans. C. canimorsus was recently shown to be endowed with a capsular polysaccharide implicated in resistance to the innate immune system of the host. Here, we developed the first C. canimorsus capsular serotyping scheme. We describe nine different serovars (A to I), and this serotyping scheme allowed typing of 25/25 isolates from human infections but only 18/52 isolates from dog mouths, indicating that the repertoire of capsules in the species is vast. However, while only three serovars (A, B, and C) covered 88% of the human isolates tested (22/25), they covered only 7.7% of the dog isolates (4/52). Serovars A, B, and C were found 22.9-, 14.6-, and 4.2-fold more often, respectively, among human isolates than among dog isolates, with no geographical bias, implying that isolates endowed with these three capsular types are more virulent for humans than other isolates. Capsular serotyping would thus allow identification of virulent isolates in dogs, which could contribute to the prevention of these infections. To this end, we developed a PCR typing method based on the amplification of specific capsular genes.

Capsular serovars of virulent Capnocytophaga canimorsus are shared by the closely related species C. canis and C. cynodegmi

“…According to the 16S rDNA phylogenetic analysis, these six strains were, thus, reclassified as C. canis (Supplementary Table S1 ). We then subjected the 96 C. canimorsus sensu stricto strains to the PCR test designed to detect the three A, B, and C capsular serovars 14 . These analyses showed that 87 of the 96 strains were positive and could be further typed as either A, B, or C by serovar-specific PCRs (Table 1 and Supplementary Fig.…”

“…In summary, 86 of the 96 C. canimorsus strains tested (89.58%) belonged to either serovars A (30/96, 31.3%), B (28/96, 29.2%), or C (28/96, 29.2%). Next, we tested all strains by PCRs detecting the D and E capsular types 14 . According to these analyses, seven strains belonged to serovar D and none to serovar E (Table 1 and Supplementary Fig.…”

“…Recently, we reported the discovery of a capsular polysaccharide at the surface of C. canimorsus cells, composed of the same polysaccharide units as the lipooligosaccharide O-antigen 13 . While the repertoire of capsular serovars is extensive among strains isolated from dog mouths, three serovars, named A, B, and C, are predominant, worldwide, among human infections 14 . Here, to further investigate the correlation between capsular serovars and virulence, we serotyped the Capnocytophaga strains obtained from the Culture Collection University of Gothenburg (CCUG), the largest collection of strains of Capnocytophaga spp.…”

Rapid Diagnosis of <em>Capnocytophaga canimorsus</em> Septic Shock in an Immunocompetent Individual Using Real-Time Nanopore Sequencing

“…The use of rapid sequencing and quantification technologies also allowed for additional characterization of the bacterial isolate, confirming the suspected etiological link between the dog bite and clinical presentation. Genomic analyses showed the isolate to be serotype D, which is not commonly found in clinical cases and thus not considered to be highly virulent [6]. The low virulent serovar status is unusual given the patient also did not have significant risk factors for C. canimorsus infection, such as immunosuppression, asplenia, heavy alcohol use, or elderly age [8,[10][11][12].…”

“…The patient and dog C. canimorsus genomes were sequenced through a combination of MinION and Illumina sequencing, generating complete genomes which were nearly indistinguishable (Supplementary Materials). A computational analysis of virulence markers[6] indicated the patient isolate was of the Serotype D lineage. No antibiotic resistance markers were identified within the patient isolate genome, which fit the fully susceptible antibiotic profile obtained as part of the culture-based diagnostic investigation (Supplementary Materials).…”