Enoyl-Acyl Carrier Protein Reductase I (FabI) Is Essential for the Intracellular Growth of Listeria monocytogenes

Abstract

Listeria monocytogenes is a facultative intracellular Gram-positive bacterium that is the primary cause of the disease listeriosis (1). The environmentally hardy and ubiquitous L. monocytogenes is ingested through contaminated food products and can cause noninvasive diseases, such as gastroenteritis, or invade through the intestinal epithelium to cause central nervous system infections or bacteremia. L. monocytogenes poses a major medical threat for pregnant women and immunocompromised patients. Yearly outbreaks of L. monocytogenes in a variety of food products from fruits to ice cream underscore the need to understand the requirements for L. monocytogenes infection and discover new methods to inhibit the growth of L. monocytogenes.The phospholipids of L. monocytogenes are composed of branched-chain fatty acids synthesized by the L. monocytogenes type II fatty acid synthesis system. Decreasing the synthesis of branchedchain fatty acids by the genetic deletion of the branched-chain ␣-keto acid dehydrogenase compromises the environmental survival and intracellular pathogenesis of L. monocytogenes (2-6). The pathways for fatty acid synthesis, phospholipid synthesis, and exogenous fatty acid incorporation of L. monocytogenes are predicted to be the same as those for the phylogenetically related and wellcharacterized Staphylococcus aureus system (Fig. 1). Fatty acids are synthesized using a prototypical type II bacterial fatty acid synthesis, with 2-methylbutyryl-coenzyme A (CoA) as the primer, to make branch-chain fatty acids (3,5,(7)(8)(9)). An acyl-acyl carrier protein (ACP) of the appropriate length is used as the acyl donor by the PlsX/Y/C system to make phosphatidic acid from glycerol-3-phosphate (10, 11). Exogenous fatty acids are incorporated via the fatty acid kinase system (12)(13)(14). An unusual feature of L. monocytogenes fatty acid synthesis is the presence of four genes that may encode enoyl-acyl carrier protein reductases, including a FabI, a FabL, and two FabK isoforms, rather than the single FabI found in S. aureus and a variety of other bacterial pathogens. The FabL isoform was discovered in the phylogenetically related Bacillus, which also has a FabI (15). Both proteins function in fatty acid synthesis in Bacillus. The FabK isoform is a flavoprotein unrelated to FabI that was originally discovered in Streptococcus pneumoniae (16). The Enterococcus faecalis genome encodes both FabI and FabK, but FabI is responsible for the bulk of fatty acid synthesis while FabK plays a minor role in supporting fatty acid synthesis (17).The goal of this study was to identify which of the putative L. monocytogenes reductase genes code for functional enoyl-acyl carrier protein reductases, assess their contribution to supporting type II bacterial fatty acid synthesis, and determine their role in planktonic and intracellular growth. Complete inhibition of FabI using the FabI selective inhibitor AFN-1252 reduced the fatty acid synthesis of L. monocytogenes strain 10403S by 80% and reduced the growth rate of L. monocytog...

Exogenous fatty acid metabolism in bacteria

“…No combination of exogenous fatty acids can rescue FASII inhibition in S. aureus without genetic modification to the reference organism [11,16]. Host fatty acids cannot rescue the FabI inhibition of intracellular Listeria monocytogenes , and exogenous fatty acid combinations cannot replace endogenous synthesis in extracellular cultures [10]. Exogenous fatty acids cause only a dose dependent, partial inhibition of fatty acid synthesis in these bacteria [11].…”

“…Exogenous fatty acids cause only a dose dependent, partial inhibition of fatty acid synthesis in these bacteria [11]. Malonyl-CoA levels remain elevated and endogenous fatty acids made up half or more of the acyl chains in phospholipid synthesis even in presence of exogenous fatty acids [10,11]. The inhibition of the remaining FASII activity causes accumulation of short chain acyl-ACP and depletion of free ACP [16].…”

“…Decreasing the ratio of branched-chain fatty acids severely decreased the extracellular growth rate of Staphylococcus and Listeria , while restoring the ratio of branched-chain fatty acids restored the growth rate [31,32]. Decreasing the content of branched-chain fatty acids also inhibited intracellular pathogenesis and proliferation in a mouse peritonitis model [10,74,75]. Branched-chain amino acids are precursors to branched-chain fatty acids, and the increased import of branched-chain amino acids correlated with increased virulence while the decreased import of branched-chain amino acids correlated with decreased virulence [76].…”

Host Fatty Acid Utilization by Staphylococcus aureus at the Infection Site

“…Although this may be an issue with some groups of bacteria (11,29), it is not generally correct. FabI inhibitors are effective against Listeria monocytogenes (31), Chlamydia trachomatis (32,33), Neisseria gonorrhoeae (6), and S. aureus (11) in the presence of exogenous FAs. S. aureus acetyl-CoA carboxylase mutants (ΔaccD) are grown in the laboratory in medium supplemented with branched-chain FAs (11) but fail to establish a blood infection in mice (34), suggesting that host FAs alone cannot support growth.…”

Small-Molecule Inhibition of the C. difficile FAS-II Enzyme, FabK, Results in Selective Activity

“…29,32 Similarly, FabK has demonstrated resistance to FabI inhibitors like triclosan, which are consequently inactive against strains carrying FabK. 22,27,45,46 Lastly, the ability of bacterial strains carrying two enoyl reductases to survive the deletion of one isozyme suggests that both enzymes would need to be dually inactivated to inhibit the growth of such organisms. 21 This data justifies the investigation of FabK as a narrow-spectrum target for selective anti-difficile therapy.…”