<i>Acinetobacter baumannii</i> represses type VI secretion system through a manganese-dependent small RNA-mediated regulation

Type VI secretion system (T6SS) is utilized by many Gram-negative bacteria to eliminate competing bacterial species and manipulate host cells. <i>Acinetobacter baumannii</i> ATCC 17978 utilizes T6SS at the expense of losing pAB3 plasmid to induce contact-dependent killing of competitor microbes, resulting in the loss of antibiotic resistance carried by pAB3. However, the regulatory network associated with T6SS in <i>A. baumannii</i> remains poorly understood. Here, we identified an Mn²⁺-dependent post-transcriptional regulation of T6SS mediated by a bonafide small RNA, AbsR28. <i>A. baumannii</i> utilizes MumT, an Mn²⁺-uptake inner membrane transporter, for the uptake of extracellular Mn²⁺ during oxidative stress. We demonstrate that the abundance of intracellular Mn²⁺ enables complementary base pairing of AbsR28-<i>tssM</i> mRNA (that translates to TssM, one of the vital inner membrane components of T6SS), inducing RNase E-mediated degradation of <i>tssM</i> mRNA and resulting in T6SS repression. Thus, AbsR28 mediates a crosstalk between MumT and T6SS in <i>A. baumannii</i>.IMPORTANCESmall RNAs (sRNAs) are identified as critical components within the bacterial regulatory networks involved in fine regulation of virulence-associated factors. The sRNA-mediated regulation of type VI secretion system (T6SS) in <i>Acinetobacter baumannii</i> was unchartered. Previously, it was demonstrated that <i>A. baumannii</i> ATCC 17978 cells switch from T6- to T6+ phenotype, resulting in the loss of antibiotic resistance conferred by plasmid pAB3. Furthermore, the derivatives of pAB3 found in recent clinical isolates of <i>A. baumannii</i> harbor expanded antibiotic resistance genes and multiple determinants for virulence factors. Hence, the loss of this plasmid for T6SS activity renders <i>A. baumannii</i> T6+ cells susceptible to antibiotics and compromises their virulence. Our findings show how <i>A. baumannii</i> tends to inactivate T6SS through an sRNA-mediated regulation that relies on Mn²⁺ and retains pAB3 during infection to retain antibiotic resistance genes carried on the plasmid.