Chapter 4 : Physiology of Enterococci

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Enterococci are medically important bacteria that also happen to comprise part of the normal human intestinal flora. This chapter reviews an expanding literature on enterococcal physiology, emphasizing those topics that have been more thoroughly studied: central carbon metabolism, respiration, ion transport, pyrimidine and folate pathways, stress responses, and the metabolism of reactive oxygen species. The newly completed genome databases for and speeds up progress in many areas under current investigation. In addition, these databases should catalyze new studies on metabolism. These resources are used here to help identify putative genes in known or suspected metabolic pathways and spur additional interest in the many fascinating and unusual aspects of physiology for these medically important organisms. A wide variety of carbohydrates can be fermented by enterococci. Essential components of the respiratory chain-de-methylmenaquinone, cytochrome , fumarate reductase, and FF-ATP synthase are described in the chapter. Oxidation of lactate for energy is also potentially linked to respiratory components. Growth on lactate is well characterized for , but surveys of other enterococci are lacking. For enterococci, investigators have commonly used (formerly ) to study ion transport mechanisms. Iron is an essential nutrient for aerotolerant microorganisms like enterococci. In a survey of enterococci, several strains were identified that produced two or three different siderophores. Except for folate, lipoic acid, demethylmenaquinone, and hematin, little is known about this topic for enterococci. Enterococci are potent producers and scavengers of reactive oxygen species.

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4

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Aromatic Amino Acid Biosynthesis
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Figure 1

Phosphoenolpyruvate phosphotransferase and catabolite repression. Phosphorylation of phosphocarrier protein (HPr) by enzyme I (EI) at histidine residue 15 forms HPr(hisl5-P) and couples sugar uptake to sugar phosphorylation through carbohydrate-specific enzymes II (EIIs). Phos-phorylated sugars are channeled into catabolic pathways (ED, Entner-Doudoroff; EMP, Embden-Meyerhof-Parnas; and PP, pentose phosphate). HPr(hisl5-P) also regulates glycerol metabolism through glycerol-3-phosphate kinase. The bifunctional HPr kinase/phosphatase enzyme (HprK) regulates HPr activity through phosphorylation at conserved serine residue 46 to form HPr(ser46-P), which is unable to phosphorylate EIIs. HPr(ser46-P) also activates catabolite repression through the catabolite control protein A (CcpA) and up-regulates inducer expulsion through phosphatase II (Pase II, hashed arrow). HprK phosphatase is attenuated by ATP (hashed arrow).

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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Figure 2

Glycerol metabolism. Glycerol uptake in occurs through an energy-independent diffusion facilitator. Enzymes for glycerol dissimilation include: GlpK, ATP-dependent glycerol kinase; GlpO, HO-producing L-α-glycerophosphate oxidase; NAD-dependent glycerol dehydrogenase; and dihydroxyacetone kinase. A putative NADH:quinone oxidoreductase regenerates NAD using membrane-associated demethylmenaquinone (DMK). In the presence of fumarate, demethylmenaquinol (DMKH2) is oxidized by fumarate reductase. GlpK is activated by enzyme I-activated histidine-containing protein (HPr[hisl5-P]) of the phosphoenolpyruvate phosphotransferase system and allosterically inhibited by fructose 1,6-bisphosphate (FBP). GlpK and GlpO are expressed only under aerobic or microaerophilic conditions.

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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Figure 3

Pyruvate metabolism. Circled compounds represent substrates for pyruvate synthesis. Major end-products are boxed. E. faecalis genes coding enzymes for reactions, cofactors, and gradients are shown: , pyruvate kinase; , malic enzyme; , citrate lyase; ACP, acyl carrier protein (y-subunit), , oxaloacetate decarboxylase; ΔμNa, transmembrane sodium gradient; , pyruvate decarboxylase; fdred, reduced ferredoxin; fdox, oxidized ferredoxin; , pyruvate dehydrogenase complex with El, E2, and E3 subunits; TIP, thiamine pyrophosphate; , L-(+)-lactate dehydrogenase; , α-acetolactate synthase; , a-acetolactate decarboxylase; , pyruvate formate-lyase; , aldehyde-alcohol dehydrogenase; , acetokinase; , phosphoacetyltransfer-ase. Exact stoichiometry is not indicated, and reaction details are omitted for clarity.

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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Figure 4

Deiminase catabolism. Arginine and agmatine pathways both lead to a high-energy carbamoyl phosphate intermediate. A third reaction forms ATP from carbamoyl phosphate. Specific, gradient-dependent antiporters provide import and export of substrates and products. Genes coding enzymes for reactions are shown: , arginine deiminase; , oirrithine carbamoyltrans-ferase; and , arginine-induced carbamate kinase. Genes for agmatine deiminase, putrescine carbamoyltransferase, and agmatine-induced carbamate kinase have not been identified.

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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Figure 5

Respiration. Conceptualized model of E. faecalis respiratory components. A putative transporter facilitates hematin uptake for incorporation into cytochrome bd (CydAB). Cytosolic reducing equivalents are transferred to demethylmenaquinone (DMK) through a putative NADH:quinone oxido-reductase. Fumarate reductase (FrdABCD) and cytochrome bd are terminal demethylmenaquinol (DMKH2) oxidases that generate succinate from fumarate, and HO from O, respectively. Cytochrome bd translocates one proton per electron to establish a proton motive force. FF-ATPsynthase couples proton movement into the cell to formation. A putative -lactate:quinone oxidoreductase for lactate oxidation is not shown.

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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Figure 6

Copper metabolism. The CopZ copper chaperone is central to copper homeostasis. Cu is extracellularly reduced to Cu by a putative Cu-reductase prior to import via a P-type ATPase termed CopA. CopZ transports bound Cu to target proteins and the CopY repressor that controls cop expression. CopB is a P-type ATPase that accepts Cu from CopZ for export.

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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Image of Figure 7
Figure 7

Superoxide production. Model of Oˉ production by demethylmenaquinone. Cytosolic reducing equivalents transfer to demethylmenaquinone (left) through oxidoreductases forming demethylmenaquinol (right). Normally, reduced demethylmenaquinone binds terminal quinol oxidases such as fumarate reductase or cytochrome bd. In the absence of fumarate or hematin, extracellular Oˉ as a by-product through the univalent reduction of O by stabilized semiquinone radicals (middle). Under acidic conditions Oˉ spontaneously dismutes into HO and, in the presence of transition metals like iron or copper, catalytically forms damaging hydroxyl radicals (-OH).

Citation: Huycke M. 2002. Physiology of Enterococci, p 133-175. In Gilmore M, Clewell D, Courvalin P, Dunny G, Murray B, Rice L (ed), The Enterococci. ASM Press, Washington, DC. doi: 10.1128/9781555817923.ch4
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