Horizontal gene transfer

Horizontal gene transfer (HGT) allows the transfer of genetic material between bacteria belonging to the same generation (that it is not offspring), representing a main driver for bacterial adaptation and evolution (Averhoff et al., 2021). HGT mechanisms are particularly important in the context of antimicrobial resistance dissemination. Conjugation, transduction, and natural transformation are the main known HGT mechanisms and a fourth mechanism, vesicle-mediated transfer, has been recently recognized (Figure 1). All these four mechanisms of HGT have been reported in Acinetobacter baumannii, including in the context of antimicrobial resistance dissemination (Da Silva & Domingues, 2016). Despite conjugation is assumed to be the most common HGT mechanism among bacteria, it does not seem the major driving force in acquisition of exogenous DNA in this species, as most sequenced plasmids lack transfer and mobilization genes (Fondi et al., 2010).

Natural transformation

Natural transformation is known to occur in the Acinetobacter genus for a long time. In 2010, natural competence was described for the first in A. baumannii (Ramirez et al., 2010). Recently, natural transformation was suggested as the main HGT mechanism involved in antibiotic resistance dissemination in A. baumannii (Averhoff et al., 2021; Godeux et al., 2022). Different studies have determined the conditions that affect transformability of A. baumannii, but most results seem to be strain dependent. A. baumannii is able to uptake both plasmid and chromosomal DNA (Wilharm et al., 2013; Traglia et al., 2016; Godeux et al., 2018). A recent study shows that A. baumannii can efficiently capture long DNA fragments secreted by living cells (Godeux et al., 2022).

Transformation can occur at different pH levels (Traglia et al., 2016; Godeux et al., 2018) and competence can be induced by the presence of calcium ions (Traglia et al., 2016) and sub-inhibitory concentrations of some antibiotics (Quinn et al., 2018). Presence of albumin in the induction of competence seems contradictory, with studies showing both the increase (Traglia et al., 2016; Martinez et al., 2018) and decrease (Godeux et al., 2018) of transformation. Osmolarity has also shown to not affect (Traglia et al., 2016) or to influence (Domingues et al., 2019; Hu et al., 2019) transformability. Natural transformation is less efficient in liquid medium (Traglia et al., 2016) than in semisolid (Domingues et al., 2019) and solid medium (Godeux et al., 2018).

Type IV pili are essential to transformation in A. baumannii (Wilharm et al.,2013; Vesel and Blokesch, 2021), with piliation and transformability peaking at the exponential growth phase (Vesel and Blokesch, 2021). Several genes involved in natural transformation of A. baumannii have been recently identified (Vesel and Blokesch, 2021; Hu et al., 2022).

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