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Abstract
ABSTRACTDespite dogma suggesting that lipopolysaccharide/lipooligosaccharide (LOS) was essential for viability of Gram-negative bacteria, several Acinetobacter baumannii clinical isolates produced LOS− colonies after colistin selection. Inactivation of the conserved class A penicillin-binding protein, PBP1A, was a compensatory mutation that supported isolation of LOS− A. baumannii, but the impact of PBP1A mutation was not characterized. Here, we show that the absence of PBP1A causes septation defects and that these, together with ld-transpeptidase activity, support isolation of LOS− A. baumannii. PBP1A contributes to proper cell division in A. baumannii, and its absence induced cell chaining. Only isolates producing three or more septa supported selection of colistin-resistant LOS− A. baumannii. PBP1A was enriched at the midcell, where the divisome complex facilitates daughter cell formation, and its localization was dependent on glycosyltransferase activity. Transposon mutagenesis showed that genes encoding two putative ld-transpeptidases (LdtJ and LdtK) became essential in the PBP1A mutant. Both LdtJ and LdtK were required for selection of LOS− A. baumannii, but each had distinct enzymatic activities in the cell. Together, these findings demonstrate that defects in PBP1A glycosyltransferase activity and ld-transpeptidase activity remodel the cell envelope to support selection of colistin-resistant LOS− A. baumannii.
Highlights
Despite dogma suggesting that lipopolysaccharide/lipooligosaccharide (LOS) was essential for viability of Gram-negative bacteria, several Acinetobacter baumannii clinical isolates produced LOSÀ colonies after colistin selection
The Gram-negative cell envelope is tripartite with an inner membrane, a periplasm that includes a thin peptidoglycan layer, and an outer membrane, which is enriched with surface-exposed lipopolysaccharide (LPS) or lipooligosaccharide (LOS)
A previous study found that penicillin-binding protein 1A (PBP1A) GTase activity inhibited isolation of colistin-resistant LOSÀ A. baumannii [19]
Summary
Despite dogma suggesting that lipopolysaccharide/lipooligosaccharide (LOS) was essential for viability of Gram-negative bacteria, several Acinetobacter baumannii clinical isolates produced LOSÀ colonies after colistin selection. Transposon mutagenesis showed that genes encoding two putative LD-transpeptidases (LdtJ and LdtK) became essential in the PBP1A mutant Both LdtJ and LdtK were required for selection of LOSÀ A. baumannii, but each had distinct enzymatic activities in the cell. The Gram-negative cell envelope is tripartite with an inner (cytoplasmic) membrane, a periplasm that includes a thin peptidoglycan layer, and an outer membrane, which is enriched with surface-exposed lipopolysaccharide (LPS) or lipooligosaccharide (LOS). Previous work showed that inactivation of penicillin-binding protein 1A (PBP1A) (encoded by mrcA) is a compensatory mutation that supports selection of colistin-resistant LOSÀ A. baumannii [19], but the role of PBP1A in A. baumannii physiology and how PBP1A mutation alters the cell envelope have not been reported
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