Characterization of bacteriophages from tox-containing, non-toxigenic isolates of Corynebacterium diphtheriae

Diphtheria

Diphtheria is a potentially fatal infection mostly caused by toxigenic Corynebacterium diphtheriae strains and occasionally by toxigenic C. ulcerans and C. pseudotuberculosis strains. Diphtheria is generally an acute respiratory infection, characterized by the formation of a pseudomembrane in the throat, but cutaneous infections are possible. Systemic effects, such as myocarditis and neuropathy, which are associated with increased fatality risk, are due to diphtheria toxin, an exotoxin produced by the pathogen that inhibits protein synthesis and causes cell death. Clinical diagnosis is confirmed by the isolation and identification of the causative Corynebacterium spp., usually by bacterial culture followed by enzymatic and toxin detection tests. Diphtheria can be treated with the timely administration of diphtheria antitoxin and antimicrobial therapy. Although effective vaccines are available, this disease has the potential to re-emerge in countries where the recommended vaccination programmes are not sustained, and increasing proportions of adults are becoming susceptible to diphtheria. Thousands of diphtheria cases are still reported annually from several countries in Asia and Africa, along with many outbreaks. Changes in the epidemiology of diphtheria have been reported worldwide. The prevalence of toxigenic Corynebacterium spp. highlights the need for proper clinical and epidemiological investigations to quickly identify and treat affected individuals, along with public health measures to prevent and contain the spread of this disease.

Recent Outbreaks of Diphtheria in Dibrugarh District, Assam, India

Diphtheria is still a significant child health problem in countries with low immunization coverage. Reports of diphtheria in adult population are also increasing. Here we describe three recent outbreaks of diphtheria in Dibrugarh district, Assam in two consecutive years. The study was undertaken in Assam Medical College & Hospital, Dibrugarh after the diagnosis of two Diphtheria cases in the month of September and October 2015 and another in January 2016. Outbreak investigation was done after defining operational definition and throat swabs were collected from thirty three (33) individuals including three (3) index cases and thirty (30) close contacts. Diagnosis was done by clinical findings, direct microscopy, bacteriological culture and in-house designed multiplex Polymerase Chain Reaction (PCR) of the isolates for the expression of Corynebacterium diphtheriae specific rpoB gene and tox gene. Out of the 10 confirmed cases, 2 and 7 were in the first two outbreaks while only one in the third outbreak respectively. All the cases were of age > 10 years, unimmunized or partially immunized. The overall mortality was 20%. PCR results revealed all the culture positive isolates to be tox gene positive. Diphtheria is a resurgent problem in our region with a significant age shift towards adult.

Genomics and proteomics of mycobacteriophage patience, an accidental tourist in the Mycobacterium neighborhood

Newly emerging human viruses such as Ebola virus, severe acute respiratory syndrome (SARS) virus, and HIV likely originate within an extant population of viruses in nonhuman hosts and acquire the ability to infect and cause disease in humans. Although several mechanisms preventing viral infection of particular hosts have been described, the mechanisms and constraints on viral host expansion are ill defined. We describe here mycobacteriophage Patience, a newly isolated phage recovered using Mycobacterium smegmatis mc(2)155 as a host. Patience has genomic features distinct from its M. smegmatis host, including a much lower GC content (50.3% versus 67.4%) and an abundance of codons that are rarely used in M. smegmatis. Nonetheless, it propagates well in M. smegmatis, and we demonstrate the use of mass spectrometry to show expression of over 75% of the predicted proteins, to identify new genes, to refine the genome annotation, and to estimate protein abundance. We propose that Patience evolved primarily among lower-GC hosts and that the disparities between its genomic profile and that of M. smegmatis presented only a minimal barrier to host expansion. Rapid adaptions to its new host include recent acquisition of higher-GC genes, expression of out-of-frame proteins within predicted genes, and codon selection among highly expressed genes toward the translational apparatus of its new host.

IMPORTANCE

The mycobacteriophage Patience genome has a notably lower GC content (50.3%) than its Mycobacterium smegmatis host (67.4%) and has markedly different codon usage biases. The viral genome has an abundance of codons that are rare in the host and are decoded by wobble tRNA pairing, although the phage grows well and expression of most of the genes is detected by mass spectrometry. Patience thus has the genomic profile of a virus that evolved primarily in one type of host genetic landscape (moderate-GC bacteria) but has found its way into a distinctly different high-GC environment. Although Patience genes are ill matched to the host expression apparatus, this is of little functional consequence and has not evidently imposed a barrier to migration across the microbial landscape. Interestingly, comparison of expression levels and codon usage profiles reveals evidence of codon selection as the genome evolves and adapts to its new environment.

Nontoxigenic corynebacterium diphtheriae: an emerging pathogen in England and Wales?

Confirmed isolates of nontoxigenic Corynebacterium diphtheriae in England and Wales increased substantially from 1986 to 1994. Ribotyping of 121 isolates confirmed in 1995 showed that 90 were of a single strain isolated exclusively from the throat; none had previously been identified in toxigenic strains from U.K. or non-U.K. residents. The upward trend in nontoxigenic C. diphtheriae probably represented increased ascertainment, although dissemination of a particular strain or clone may have been a factor.

Biology and molecular epidemiology of diphtheria toxin and the tox gene

Diphtheria toxin (DT) is an extracellular protein of Corynebacterium diphtheriae that inhibits protein synthesis and kills susceptible cells. The gene that encodes DT (tox) is present in some corynephages, and DT is only produced by C. diphtheriae isolates that harbor tox+ phages. The diphtheria toxin repressor (DtxR) is a global regulatory protein that uses Fe2+ as co-repressor. Holo-DtxR represses production of DT, corynebacterial siderophore, heme oxygenase, and several other proteins. Diagnostic tests for toxinogenicity of C. diphtheriae are based either on immunoassays or on bioassays for DT. Molecular analysis of tox and dtxR genes in recent clinical isolates of C. diphtheriae revealed several tox alleles that encode identical DT proteins and multiple dtxR alleles that encode five variants of DtxR protein. Therefore, recent clinical isolates of C. diphtheriae produce a single antigenic type of DT, and diphtheria toxoid continues to be an effective vaccine for immunization against diphtheria.

Use of molecular subtyping to document long-term persistence of Corynebacterium diphtheriae in South Dakota

Enhanced surveillance of patients with upper respiratory symptoms in a Northern Plains community revealed that approximately 4% of them were infected by toxigenic Corynebacterium diphtheriae of both mitis and gravis biotypes, showing that the organism is still circulating in the United States. Toxigenic C. diphtheriae was isolated from five members of four households. Four molecular subtyping methods-ribotyping, multilocus enzyme electrophoresis (MEE), random amplified polymorphic DNA (RAPD), and single-strand conformation polymorphism-were used to molecularly characterize these strains and compare them to 17 archival South Dakota strains dating back to 1973 through 1983 and to 5 isolates collected from residents of diverse regions of the United States. Ribotyping and RAPD clearly demonstrated the household transmission of isolates and provided precise information on the circulation of several distinct strains within three households. By MEE, most recent and archival South Dakota strains were identified as closely related and clustered within the newly identified ET (electrophoretic type) 215 complex. Furthermore, three recent South Dakota isolates and eight archival South Dakota isolates were indistinguishable by both ribotyping and RAPD. All of these molecular methods showed that recent South Dakota isolates and archival South Dakota isolates were more closely related to each other than to the C. diphtheriae strains isolated in other parts of the United States or worldwide. The data also supported the improbability of importation of C. diphtheriae into this area and rather strongly suggest the long-term persistence of the organism in this region.