In addition, two other glucose-catabolizing pathways are found in bacteria: the oxidative pentose phosphate pathway (hexose monophosphate shunt), (Fig. 4-3) and the Entner-Doudoroff pathway, which is almost exclusively found in obligate aerobic bacteria (Fig. 4-4). The highly oxidative Azotobacter and most Pseudomonas species, for example, utilize the Entner-Doudoroff pathway for glucose catabolism, because these organisms lack the enzyme phosphofructokinase and hence cannot synthesize fructose 1,6-diphosphate, a key intermediate compound in the glycolytic pathway. (Phospho-fructokinase is also sensitive to molecular O2 and does not function in obligate aerobes). Other bacteria, which lack aldolase (which splits fructose-1,6-diphosphate into two triose phosphate compounds), also cannot have a functional glycolytic pathway. Although the Entner-Doudoroff pathway is usually associated with obligate aerobic bacteria, it is present in the facultative anaerobe Zymomonas mobilis (formerly Pseudomonas lindneri). This organism dissimilates glucose to ethanol and represents a major alcoholic fermentation reaction in a bacterium.
EXCLUSIVE Intermediate Stories For Reproduction 2 L A Hill
For most microbial fermentations, glucose dissimilation occurs through the glycolytic pathway (Fig. 4-1). The simple organic compound most commonly generated is pyruvate, or a compound derived enzymatically from pyruvate, such as acetaldehyde, α-acetolactate, acetyl SCoA, or lactyl SCoA (Fig. 4-5). Acetaldehyde can then be reduced by NADH + H+ to ethanol, which is excreted by the cell. The end product of lactic acid fermentation, which occurs in streptococci (e.g., Streptococcus lactis) and many lactobacilli (e.g., Lactobacillus casei, L. pentosus), is a single organic acid, lactic acid. Organisms that produce only lactic acid from glucose fermentation are homofermenters. Homofermentative lactic acid bacteria dissimilate glucose exclusively through the glycolytic pathway. Organisms that ferment glucose to multiple end products, such as acetic acid, ethanol, formic acid, and CO2, are referred to as heterofermenters. Examples of heterofermentative bacteria include Lactobacillus, Leuconostoc, and Microbacterium species. Heterofermentative fermentations are more common among bacteria, as in the mixed-acid fermentations carried out by bacteria of the family Enterobacteriaceae (e.g., Escherichia coli, Salmonella, Shigella, and Proteus species). Many of these glucose fermenters usually produce CO2 and H2 with different combinations of acid end products (formate, acetate, lactate, and succinate). Other bacteria such as Enterobacter aerogenes, Aeromonas, Serratia, Erwinia, and Bacillus species also form CO2 and H2 as well as other neutral end products (ethanol, acetylmethylcarbinol [acetoin], and 2,3-butylene glycol). Many obligately anaerobic clostridia (e.g., Clostridium saccharobutyricum, C. thermosaccharolyticum) and Butyribacterium species ferment glucose with the production of butyrate, acetate, CO2, and H2, whereas other Clostridum species (C. acetobutylicum and C. butyricum) also form these fermentation end products plus others (butanol, acetone, isopropanol, formate, and ethanol). Similarly, the anaerobic propionic acid bacteria (Propionibacterium species) and the related Veillonella species ferment glucose to form CO2, propionate, acetate, and succinate. In these bacteria, propionate is formed by the partial reversal of the Krebs cycle reactions and involves a CO2fixation by pyruvate (the Wood-Werkman reaction) that forms oxaloacetate (a four-carbon intermediate). Oxaloacetate is then reduced to malate, fumarate, and succinate, which is decarboxylated to propionate. Propionate is also formed by another three-carbon pathway in C. propionicum, Bacteroides ruminicola, and Peptostreptococcus species, involving a lactyl SCoA intermediate. The obligately aerobic acetic acid bacteria (Acetobacter and the related Gluconobacter species) can also ferment glucose, producing acetate and gluconate. Figure 4-5 summarizes the pathways by which the various major fermentation end products form from the dissimilation of glucose through the common intermediate pyruvate.
Theoretically, T. gondii can be passed between intermediate hosts indefinitely via a cycle of consumption of tissue cysts in meat. However, the parasite's lifecycle begins and completes only when the parasite is passed to a feline host, the only host within which the parasite can again undergo sexual development and reproduction.[29] 2ff7e9595c
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