Variations in virulence and molecular biology among emerging strains of Clostridium difficile

Abstract

Clostridium difficile is a Gram-positive, spore-forming organism which infects and colonizes the large intestine, produces potent toxins, triggers inflammation, and causes significant systemic complications. Treating C. difficile infection (CDI) has always been difficult, because the disease is both caused and resolved by antibiotic treatment. For three and a half decades, C. difficile has presented a treatment challenge to clinicians, and the situation took a turn for the worse about 10 years ago. An increase in epidemic outbreaks related to CDI was first noticed around 2003, and these outbreaks correlated with a sudden increase in the mortality rate of this illness. Further studies discovered that these changes in CDI epidemiology were associated with the rapid emergence of hypervirulent strains of C. difficile, now collectively referred to as NAP1/BI/027 strains. The discovery of new epidemic strains of C. difficile has provided a unique opportunity for retrospective and prospective studies that have sought to understand how these strains have essentially replaced more historical strains as a major cause of CDI. Moreover, detailed studies on the pathogenesis of NAP1/BI/027 strains are leading to new hypotheses on how this emerging strain causes severe disease and is more commonly associated with epidemics. In this review, we provide an overview of CDI, discuss critical mechanisms of C. difficile virulence, and explain how differences in virulence-associated factors between historical and newly emerging strains might explain the hypervirulence exhibited by this pathogen during the past decade.

Fig 1

Overview of C. difficile infection and outcomes. In a typical scenario, a patient is exposed to C. difficile spores, although a small number of cases appear to be community acquired. Alterations in the normal flora by antibiotic therapy likely support a niche for the bacterium in the gastrointestinal tract. The order of exposure to spores and antibiotic use required for clinical illness has not been established. The disease can range in severity from subclinical to clinical disease with systemic complications. Typically, rehydration and appropriate antibiotic therapy result in resolution of symptoms. The boldness of the arrows reflects the fact that most patients recover fully from the disease, while some will relapse, and death is the least frequent outcome.

Fig 2

Comparison of C. difficile pathogenicity loci between historical and hypervirulent strains. The schematics depict the 19.6-kb PaLoc of historical C. difficile (HIST) (top) and hypervirulent C. difficile (HV/Ep) (bottom). DNA homology, protein similarity, and protein identity are based upon reference strains CD630 (GenBank accession no. NC_009089.1) and BI1 (GenBank accession no. NC_017179.1). TcdC protein similarity and identity calculations are not trivial, as truncations of tcdC vary among hypervirulent/epidemic (HV/Ep) isolates, resulting in final products ranging from 61 to 226 amino acids (aa).

Fig 3

Comparison of virulence-associated factors between historical and hypervirulent C. difficile strains. A cartoon of C. difficile summarizing the differences between historical (Hist) and hypervirulent/epidemic (HV/Ep) strains is shown. Historical strains contain full-length tcdC but lack the binary toxin. Additionally, they are generally sensitive to fluoroquinolones, have a positive aggregation phenotype, and are nonmotile. Hypervirulent strains lack a full-length tcdC gene and have the opposite phenotype of historical strains for the above virulence factors.