PBP1A and LdtJ transpeptidase activities are crucial for cell envelope homeostasis and counteract the selection of colistin resistance in Acinetobacter baumannii.

Simultaneous inactivation of LdtJ and mrcA in Acinetobacter baumannii severely impairs growth and outer membrane homeostasis and yields the highest recovery frequency of colistin-resistant, LOS-deficient variants.

• Why it matters: The study addresses how peptidoglycan 4-3 and 3-3 transpeptidation interact to maintain envelope stability and constrain emergence of colistin resistance when LOS is lost.
• What they did: Genetic and functional interplay was tested by inactivating LdtJ and mrcA, assessing growth, morphology, viable counts, outer membrane homeostasis, and peptidoglycan cross-linking; co-immunoprecipitation tested PBP1A–LdtJ association.
• The result: The ldtJ mrcA mutants showed reduced overall cross-linking and shorter glycan chains, and PBP1A associated with LdtJ; these mutants also exhibited the highest recovery frequency of colistin-resistant, LOS-deficient variants versus wild type or single mutants.