Plasmid content is generally homogenous between genospecies
Borrelia afzelii and B. bavariensis both differed
significantly in plasmid numbers between Europe and Asia, with EuropeanB. afzelii (two-sided unpaired t-test, p = 0.03) and AsianB. bavariensis (p < 0.001) having significantly
more plasmids in comparison to the other geographic population (Figure
3A). Borrelia garinii isolates did not differ in overall plasmid
number between Asia and Europe (p = 0.08) but had significantly
fewer plasmids in comparison to both B. afzelii populations
(Asian, p = 0.003; European, p < 0.001) and to
Asian B. bavariensis (p < 0.001) (Figure 3A).
When we look at the absolute plasmid number for a population, defined as
the number of unique plasmid types present in at least one isolate from
the population, only European B. bavariensis showed a lower
absolute plasmid number (black circle; Figure 3A) in comparison to the
other species such as B. garinii . Borrelia garinii also
had on average lower plasmid numbers per isolate (comparable to EuropeanB. bavariensis ), but the absolute number of plasmid types present
in the population (i.e. diversity of plasmid types) is comparable toB. afzelii and Asian B. bavariensis (Figure 3A).
Based on the plasmid presence/absence matrix for all samples (n = 136),
we further ran a multi-dimensional scaling (MDS) analysis to test if
plasmid content corresponds to factors such as continent (i.e. vector)
or genospecies. Plasmid content appears more homogenous between Asian
isolates (Figure 3B) versus European isolates, which display clusters
based on genospecies (Figure 3B). This could result from European
isolates representing a subset of available plasmid combinations which
are all present in the Asian populations. Even so, no plasmid types were
more frequently associated with factors such as genospecies or geography
(Figure S2). It is important however to note, as Borrelia can
lose plasmids due to long-term culturing (G. Margos et al., 2017). Many
human isolates (Table S1) have been potentially kept in culture longer
suggesting that the current plasmid results could be biased even though
sequencing focused on low passage isolates (<10 passages).
Tick isolates did indeed have on average higher plasmid content (Table
S6; mean: 1.19; 95% CI: 0.16, 2.22) suggesting that this bias may be
present for our human isolates.