Acinetobacter baumannii Is Dependent on the Type II Secretion System and Its Substrate LipA for Lipid Utilization and In Vivo Fitness

Abstract

Gram-negative bacteria express a number of sophisticated secretion systems to transport virulence factors across the cell envelope, including the type II secretion (T2S) system. Genes for the T2S components GspC through GspN and PilD are conserved among isolates of Acinetobacter baumannii, an increasingly common nosocomial pathogen that is developing multidrug resistance at an alarming rate. In contrast to most species, however, the T2S genes are dispersed throughout the genome rather than linked into one or two operons. Despite this unique genetic organization, we show here that the A. baumannii T2S system is functional. Deletion of gspD or gspE in A. baumannii ATCC 17978 results in loss of secretion of LipA, a lipase that breaks down long-chain fatty acids. Due to a lack of extracellular lipase, the gspD mutant, the gspE mutant, and a lipA deletion strain are incapable of growth on long-chain fatty acids as a sole source of carbon, while their growth characteristics are indistinguishable from those of the wild-type strain in nutrient-rich broth. Genetic inactivation of the T2S system and its substrate, LipA, also has a negative impact on in vivo fitness in a neutropenic murine model for bacteremia. Both the gspD and lipA mutants are outcompeted by the wild-type strain as judged by their reduced numbers in spleen and liver following intravenous coinoculation. Collectively, our findings suggest that the T2S system plays a hitherto-unrecognized role in in vivo survival of A. baumannii by transporting a lipase that may contribute to fatty acid metabolism. IMPORTANCEInfections by multidrug-resistant Acinetobacter baumannii are a growing health concern worldwide, underscoring the need for a better understanding of the molecular mechanisms by which this pathogen causes disease. In this study, we demonstrated that A. baumannii expresses a functional type II secretion (T2S) system that is responsible for secretion of LipA, an extracellular lipase required for utilization of exogenously added lipids. The T2S system and the secreted lipase support in vivo colonization and thus contribute to the pathogenic potential of A. baumannii.

Pathogenic Acinetobacter: from the Cell Surface to Infinity and Beyond

“…Specifically, mice intranasally infected with the gspD mutant strain had ϳ2-log lower bacterial burdens in both the lungs and the spleen after 36 h, compared to both the parental strain and the complemented mutant. In A. baumannii ATCC 17978, Johnson et al identified a lipase, LipA, secreted in a T2SS-dependent manner that was required for growth on medium containing lipids as a sole carbon source (72). Mutants with mutations in lipA or the T2SS structural gene gspD were less competitive than the wild-type strain in a mixed-infection murine model of bacteremia.…”

The Secrets of Acinetobacter Secretion

“…Subsequently, the T2SS of A. baumannii ATCC 17978, a commonly used laboratory strain, was shown to be active [22]. Mutagenesis of the assembly ATPase, gspE1 , or secretin, gspD , in this strain abolished secretion of a putative lipase, LipA, identifying it as a T2SS effector.…”

“…Biochemical characterization of LipA showed it possessed serine hydrolase activity, capable of hydrolyzing 4-nitrophenyl myristate. LipA was required for growth in minimal media supplemented with long-chain fatty acids as the sole carbon source, indicating an important role in nutrient acquisition [22]. Furthermore, the T2SS and T2SS-dependent lipid utilization were required for competitive colonization of a neutropenic murine model of infection [22].…”

“…LipA was required for growth in minimal media supplemented with long-chain fatty acids as the sole carbon source, indicating an important role in nutrient acquisition [22]. Furthermore, the T2SS and T2SS-dependent lipid utilization were required for competitive colonization of a neutropenic murine model of infection [22]. Interestingly, A. baumannii ATCC 17978 encodes two assembly ATPase orthologs, where gspE1 is required for LipA secretion under laboratory conditions and gspE2 is dispensable [22].…”

“…Furthermore, the T2SS and T2SS-dependent lipid utilization were required for competitive colonization of a neutropenic murine model of infection [22]. Interestingly, A. baumannii ATCC 17978 encodes two assembly ATPase orthologs, where gspE1 is required for LipA secretion under laboratory conditions and gspE2 is dispensable [22]. The gene encoding inner membrane platform protein GspN is absent from many bacterial species with functional T2SS and is not required for secretion of the type II pullulanase in Klebsiella oxytoca [23, 24].…”