A case of Streptococcus gallolyticus subsp. gallolyticus infective endocarditis with colon cancer: identification by 16S ribosomal DNA sequencing

Characterization of TelE, a T7SS LXG Effector Exhibiting a Conserved C-Terminal Glycine Zipper Motif Required for Toxicity

Streptococcus gallolyticus subsp. gallolyticus (SGG) is an opportunistic bacterial pathogen strongly associated with colorectal cancer. Here, through comparative genomics analysis, we demonstrated that the genetic locus encoding the type VIIb secretion system (T7SSb) machinery is uniquely present in SGG in two different arrangements. SGG UCN34 carrying the most prevalent T7SSb genetic arrangement was chosen as the reference strain. To identify the effectors secreted by this secretion system, we inactivated the essC gene encoding the motor of this machinery. A comparison of the proteins secreted by UCN34 wild type and its isogenic ΔessC mutant revealed six T7SSb effector proteins, including the expected WXG effector EsxA and three LXG-containing proteins. In this work, we characterized an LXG-family toxin named herein TelE promoting the loss of membrane integrity. Seven homologs of TelE harboring a conserved glycine zipper motif at the C terminus were identified in different SGG isolates. Scanning mutagenesis of this motif showed that the glycine residue at position 470 was crucial for TelE membrane destabilization activity. TelE activity was antagonized by a small protein TipE belonging to the DUF5085 family. Overall, we report herein a unique SGG T7SSb effector exhibiting a toxic activity against nonimmune bacteria. IMPORTANCE In this study, 38 clinical isolates of Streptococcus gallolyticus subsp. gallolyticus (SGG) were sequenced and a genetic locus encoding the type VIIb secretion system (T7SSb) was found conserved and absent from 16 genomes of the closely related S. gallolyticus subsp. pasteurianus (SGP). The T7SSb is a bona fide pathogenicity island. Here, we report that the model organism SGG strain UCN34 secretes six T7SSb effectors. One of the six effectors named TelE displayed a strong toxicity when overexpressed in Escherichia coli. Our results indicate that TelE is probably a pore-forming toxin whose activity can be antagonized by a specific immunity protein named TipE. Overall, we report a unique toxin-immunity protein pair and our data expand the range of effectors secreted through T7SSb.

Gut microbiota, inflammation and colorectal cancer

Although genes contribute to colorectal cancer, the gut microbiota are an important player. Accumulating evidence suggests that chronic infection and the ensuing inflammation contributes to tumor initiation and tumor progression. A variety of bacterial species and tumor-promoting virulence mechanisms have been investigated. Significant advances have been made in understanding the composition and functional capabilities of the gut microbiota and its roles in cancer. In the current review, we discuss the novel roles of microbiota in the progression of colon cancer. Although microbiota technically include organisms other than bacteria e.g., viruses and fungi, this review will primarily focus on bacteria. We summarize epidemiological studies of human microbiome and colon cancer. We discuss the progress in the scientific understanding of the interplay between the gut microbiota, barrier function, and host responses in experimental models. Further, we discuss the potential application in prevention, diagnosis, and therapy of colon cancer by targeting microbiota. We discuss the challenges lie ahead and the future direction in studying gut microbiome in colon cancer to close the gap between the basic sciences and clinical application.

Neonatal invasive Streptococcus gallolyticus subsp. pasteurianus infection with delayed central nervous system complications

Group D streptococci are known to cause newborn septicemia and meningitis, but the Streptococcus bovis group strains rarely cause serious neonatal infections in Korea. Central nervous system (CNS) complications of neonatal S. bovis group infection have rarely been reported. In adults, S. bovis group strains cause bacteremia and endocarditis, and are associated with gastrointestinal malignancy. However, only a few studies have reported meningitis and septicemia in infants. Here, we describe a case of bacteremia and meningitis due to Streptococcus gallolyticus subsp. pasteurianus with a delayed CNS complication in an infant. A 28-day-old male infant was admitted to the hospital with a 1-day history of fever. Cultures of blood, cerebrospinal fluid, and urine showed the presence of S. bovis group strain-S. gallolyticus subsp. pasteurianus. He was discharged after 21 days of intravenous ampicillin and cefotaxime administration. Two weeks later, he was readmitted with a fever and short episodes of tonic-clonic movements. Brain magnetic resonance imaging showed marked bilateral frontal subdural effusion. He was discharged after 31 days of antibiotic therapy, and no neurological sequelae were observed at the 9-month follow-up. In conclusion, we present a rare case of neonatal S. gallolyticus subsp. pasteurianus infection causing urinary tract infection, septicemia, meningitis, and delayed CNS complications. This case emphasizes the need for physicians to be aware of S. bovis infection in infants.

Infective endocarditis caused by Streptococcus gallolyticus subspecies pasteurianus and colon cancer

A 56-year-old man presented to our department with a 2-month history of fever and chills. He had received a mitral valvuloplasty 3 years ago. He had been administered levofloxacin for 2 months. We discontinued levofloxacin and repeated the blood cultures. Bacterial blood cultures were positive and transoesophageal echocardiography revealed vegetation attached to the posterior mitral leaflet. We started the patient on intravenous antibiotic therapy for infectious endocarditis by Streptococcus gallolyticus subspecies pasteurianus. A colonoscopic screening revealed adenomatoid intracellular carcinoma. Previous studies have reported a weak association between colorectal cancer and Streptococcus bovis biotype II/2, which includes S gallolyticus subspecies pasteurianus; however, the rate is notably higher than the rate of colorectal cancer as indicated by positive faecal occult-blood test results. We conclude that colonoscopies should be routine while scanning for colorectal cancer in all patients with S bovis bacteraemia, regardless of the subspecies.

Development of a multilocus sequence typing scheme for Streptococcus gallolyticus

Streptococcus gallolyticus is often found as a member of the normal gut microflora in various animals. However, it has been reported to cause mastitis in cattle, septicaemia in pigeons, and meningitis, septicaemia and endocarditis in humans. However, little is known about the epidemiology and crucial virulence factors of S. gallolyticus. To help address these issues, we developed a multilocus sequence typing (MLST) scheme for S. gallolyticus. Seven housekeeping gene fragments were sequenced from each of 58 S. gallolyticus isolates collected from diverse origins and sources. The MLST scheme had good discriminatory ability. The 63 strains, including the 5 whole genome sequenced strains examined, resolved into 57 sequence types (STs), with 52 STs represented by only a single strain. With respect to the identification of S. gallolyticus subspecies (i.e. S. gallolyticus subsp. gallolyticus, S. gallolyticus subsp. pasteurianus and S. gallolyticus subsp. macedonicus), the results of biochemical tests and DNA-DNA hybridization were in high concordance with those of the MLST scheme. The MLST scheme developed in this study may be a useful tool capable of replacing the conventional methods used for S. gallolyticus subspecies identification. The results of this study suggest that the biology and virulence of two pathogenic S. gallolyticus subspecies (i.e. S. gallolyticus subsp. gallolyticus and S. gallolyticus subsp. pasteurianus) are very different. The MLST scheme offers researchers a valuable typing tool that will promote further investigation of the epidemiology of S. gallolyticus.