¹For this purpose, the independent sampling sizes for Europe and the USA were taken as n₁ and n₂, the number of individuals within a given shared-type "x" was k₁ and k₂, and in this case, the representativeness of the two samples was p₁=k₁/n₁ and P₂=k₂/n₂, respectively. To assess if the divergence observed between p₁ and p₂ was due to sampling bias or the existence of two distinct populations, the percentage of individuals (p₀) harboring shared-type "x" in the population studied was estimated by the equation p₀= k₁+k₂/n₁+n₂=n₁p₁+n₂p₂/n₁+n₂. The distribution of the percentage of shared-type "x" in the sample sizes n₁ and n₂ follows a normal distribution with a mean p₀ and a standard deviation of and respectively, and the difference d=p₁-p₂ follows a normal distribution of mean p₀-p₀=0 and of variance σ_d²=σ_p1²+σ_p2² = p₀q₀/n₁+p₀q₀/n₂ or σ_d²=p₀q₀ (1/n₁+1/n₂). The two samples being independent, the two variances were additive; the standard deviation σ_d= was calculated, and the homogeneity of the samples tested was assessed using the quotient d/σ_d=p₁-p₂/. If the absolute value of the quotient d/σ_d<2, the two samples were considered to belong to a same population (CI 95%) and the variation observed in the distribution of isolates for given shared types could be due to a sampling bias. Inversely, if d/σ_d>2, then the differences observed in the distribution of isolates for given shared types were statistically significant and not due to potential sample bias.