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To highlight the impact of peak height analysis on the inference to be drawn from typical, two persons mixtures (2PMs), a number of real high-quality – either "balanced" or "unbalanced" – 2PMs were subjected to a classical statistical analysis based on the likelihood of a known contributor to be therein. We calculated a LR value based on the genetic markers belonging to a real contributor (S1) and to a number of false but compatible suspects using a discrete/semi-continuous method (as implemented in a shareware of widespread adoption[LR-mix Studio]) and a quantitative approach of our own conception (built into a Microsoft Excel spreadsheet). Quantitative information conferred by the height of the peaks was irrelevant in the case of "balanced" mixtures but played a key role in eliciting correct inference from "unbalanced" 2PMs.

Objectives To analyze Staphylococcus epidermidis strains, previously tested for their virulence in a mouse model of subcutaneous infection, for various phenotypic traits (biofilm density, extracellular polysaccharide, slime-associated antigen (SAA)) and for the presence of the ica gene cluster, to determine which of these phenotypic and genotypic methods best correlates with virulence in the mouse model. Methods The quantitative biofilm assay was performed on 10 strains of S. epidermidis , comprising (1) RP62A (ATCC 35984), (2) the strongest and weakest biofilm producers in our collection, (3) a pair of phenotypic variants, and (4) a strain whose biofilm density was enhanced in iron-limited media. Biofilm density was measured after growth at 37 °C and at ambient temperature, in trypticase soy broth (TSB) with and without glucose supplementation and using both chemical and heat fixation. Strains were assayed for SAA using a double immunodiffusion method. Extracellular polysaccharide was detected by transmission electron microscopy (TEM). A 546-base-pair segment of the ica gene cluster was amplified by PCR. Results Biofilm formation in TSB, glucose-enriched TSB, extracellular polysaccharide (observed by TEM), expression of SAA and presence of the ica gene predicted virulence of nine, nine, nine, eight and eight of 10 strains, respectively. The phenotypic expression of biofilm and related properties was medium and temperature dependent. We encountered one ica -positive strain that failed to express biofilm in standard TSB at 37 °C, but was virulent in a mouse model, and another strain that lacked ica , produced biofilm and was virulent in the model. Conclusions Mouse virulence in our model can be predicted by any of the phenotypic or genotypic methods examined for ≥80% of strains. Medium and incubation conditions affect the expression of phenotypic markers by some strains. For the remaining strains, possible reasons for inconsistencies between the presence of the ica gene, phenotypic markers and mouse virulence include (1) dependence of biofilm on genes other than ica , (2) sequence differences in ica , (3) dependence of biofilm expression in vivo on strain characteristics and media used to prepare inocula for in vivo studies.