Growth kinetics within Balb/c mice after IV infection with three L. monocytogenes strains. (A) A tail vein injection containing an equal ratio of three L. monocytogenes strains (EGDe, 10403S and F2365) was administered into 15 Balb/c mice with a total inoculum of 2 × 104 CFU per 100 μl. Each of the strains was tagged with a different antibiotic marker. On each subsequent day, 5 mice were sacrificed, with the spleens and livers enumerated on different BHI agars as described in the materials and methods. Bars correspond to; green (EGDe::pIMC3kan), red (10403S::pIMC3ery) and grey (F2365::pIMC3tet). Data are presented as mean CFU per organ with standard deviation. Statistical analyses were conducted using the one sample T-test (see additional file 1) measuring the raw CFU differences between two strains to calculate the mean difference per organ over 5 mice. The relative virulence ratio (RVR) (B) was calculated from the data presented in (A) as described in figure legend 2. Symbols correspond to; green circle (EGDe::pIMC3kan), red diamond (10403S::pIMC3ery) and grey triangle (F2365::pIMC3tet). Statistical analyses were conducted using the one sample T-test measuring the RVR differences between two strains to calculate the mean difference of 5 organs (see additional file 2). The calculated P-values are presented, with values below 0.05 considered significant (P < 0.05 (*), P < 0.005 (**) and P < 0.0005(***)). (C) The competitive index score relative to EGDe (set as 1) was calculated by dividing the percentage of the strain (10403S or F2365) within an organ by the percentage value obtained for EGDe. The CI score per organ were plotted (with the same symbols as in (B)) and the mean CI written below. A CI score of 1 denotes no difference in the virulence compared to EGDe (for CI calaulation see additional figure 3). Underlined scores denote were statistical significance from (B) was observed.