A simple catheter disk model system was used to study the development in vitro of mixed species biofilms of Candida albicans and Staphylococcus epidermidis, two organisms commonly found in catheter-associated infections. Two strains of S. epidermidis were used: a slime-producing wild type (strain RP62A) and a slime-negative mutant (strain M7). In mixed fungal-bacterial biofilms, both staphylococcal strains showed extensive interactions with C. albicans. The susceptibility of 48-h biofilms to fluconazole, vancomycin and mixtures of the drugs was determined colorimetrically. The results indicated that the extracellular polymer produced by S. epidermidis RP62A could inhibit fluconazole penetration in mixed fungal-bacterial biofilms. Conversely, the presence of C. albicans in a biofilm appeared to protect the slime-negative staphylococcus against vancomycin. Overall, the findings suggest that fungal cells can modulate the action of antibiotics, and that bacteria can affect antifungal activity in mixed fungal-bacterial biofilms.

Deciphering the contribution of genetic instability in somatic cells is critical to our understanding of many human disorders. Myotonic dystrophy type 1 (DM1) is one such disorder that is caused by the expansion of a CTG repeat that shows extremely high levels of somatic instability. This somatic instability has compromised attempts to measure intergenerational repeat dynamics and infer genotype-phenotype relationships. Using single-molecule PCR, we have characterized more than 17 000 de novo somatic mutations from a large cohort of DM1 patients. These data reveal that the estimated progenitor allele length is the major modifier of age of onset. We find no evidence for a threshold above which repeat length does not contribute toward age at onset, suggesting pathogenesis is not constrained to a simple molecular switch such as nuclear retention of the DMPK transcript or haploinsufficiency for DMPK and/or SIX5. Importantly, we also show that age at onset is further modified by the level of somatic instability; patients in whom the repeat expands more rapidly, develop the symptoms earlier. These data establish a primary role for somatic instability in DM1 severity, further highlighting it as a therapeutic target. In addition, we show that the level of instability is highly heritable, implying a role for individual-specific trans-acting genetic modifiers. Identifying these trans-acting genetic modifiers will facilitate the formulation of novel therapies that curtail the accumulation of somatic expansions and may provide clues to the role these factors play in the development of cancer, aging and inherited disease in the general population.

Myotonic dystrophy type 1 (DM1) is one of the most variable inherited human disorders. It is characterized by the involvement of multiple tissues and is caused by the expansion of a highly unstable CTG repeat. Variation in disease severity is partially accounted for by the number of CTG repeats inherited. However, the basis of the variable tissue-specific symptoms is unknown. We have determined that an unusual Dutch family co-segregating DM1, Charcot-Marie-Tooth neuropathy, encephalopathic attacks and early hearing loss, carries a complex variant repeat at the DM1 locus. The mutation comprises an expanded CTG tract at the 5'-end and a complex array of CTG repeats interspersed with multiple GGC and CCG repeats at the 3'-end. The complex variant repeat tract at the 3'-end of the array is relatively stable in both blood DNA and the maternal germ line, although the 5'-CTG tract remains genetically unstable and prone to expansion. Surprisingly though, even the pure 5'-CTG tract is more stable in blood DNA and the maternal germ line than archetypal DM1 alleles of a similar size. Complex variant repeats were also identified at the 3'-end of the CTG array of approximately 3-4% of unrelated DM1 patients. The observed polarity and the stabilizing effect of the variant repeats implicate a cis-acting modifier of mutational dynamics in the 3'-flanking DNA. The presence of such variant repeats very likely contributes toward the unusual symptoms in the Dutch family and additional symptomatic variation in DM1 via affects on both RNA toxicity and somatic instability.

The European Union Seventh Framework Programme.

Myotonic dystrophy type 1 (DM1) is a multisystem disorder, caused by expansion of a CTG trinucleotide repeat in the 3'-untranslated region of the DMPK gene. The repeat expansion is somatically unstable and tends to increase in length with time, contributing to disease progression. In some individuals, the repeat array is interrupted by variant repeats such as CCG and CGG, stabilising the expansion and often leading to milder symptoms. We have characterised three families, each including one person with variant repeats that had arisen de novo on paternal transmission of the repeat expansion. Two individuals were identified for screening due to an unusual result in the laboratory diagnostic test, and the third due to exceptionally mild symptoms. The presence of variant repeats in all three expanded alleles was confirmed by restriction digestion of small pool PCR products, and allele structures were determined by PacBio sequencing. Each was different, but all contained CCG repeats close to the 3'-end of the repeat expansion. All other family members had inherited pure CTG repeats. The variant repeat-containing alleles were more stable in the blood than pure alleles of similar length, which may in part account for the mild symptoms observed in all three individuals. This emphasises the importance of somatic instability as a disease mechanism in DM1. Further, since patients with variant repeats may have unusually mild symptoms, identification of these individuals has important implications for genetic counselling and for patient stratification in DM1 clinical trials.