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Description and Significance
L. plantarum is a gram positive bacterium that is found in a
variety of niches. These niches include dairy, meat, and much vegetable
fermentations, it is also found in the human gastrointestinal tract. It
is a facultative heterofermentative lactic acid bacterium that utilizes
an extensive range of fermentatable carbon sources. Lactic acid
bacteria are Gram-positive and they are non-spore forming, fermentative
bacteria that grow anaerobically. The main function of these bacteria
is the fermentative conversion of sugars present in raw materials into
lactic acid. L. plantarum also produces anti-microbial peptides
and exopolysaccharides. It has the ability to maintain a pH gradient
between the inside and outside of the cell in the presence of large
amounts of acetate or lactate. (5) L. plantarum
is one of the most common microbes used as a silage inoculant. Silage
is a fermented fodder that can be fed to ruminants or used as a biofuel
feedstock for anaerobic digesters. (6)
L. plantarum is currently being explored to convert lignocellulosic biomass to biofuel and bioproducts. Current research into this idea is looking at a strain of L. plantarum which has certain genes inactivated to eliminate undesirable fermentation products. (9)L. plantarum is also able to degrade cassava raw starch. Its ability to degrade raw starch is useful because it could potentially be used as a starter in certain traditional fermentation processes. (6) There are also potential uses for L. plantarum to be used in treatments of certain wastewater due to its ability to degrade phenolic compounds, such as those in olive mill wastewaters. (2)
The ability of the this microbe to adapt and thrive in a range of environments, its ability and capacity to be genetically manipulated, as well as its ability to ferment and degrade different materials makes L. plantarum a very interesting and important bacteria to study.
Genome Structure
L. plantarum has one of the largest genomes among lactic acid
bacteria. In its circular chromosome it contains 3,308,274 base pairs.
The genome was sequenced by using whole genome sequencing as assembly
approach. The overall GC content of its chromosome is 44.5%, the
plasmids tend to have a lower percent GC content.
Putative biological functions have been given to 2,120 of the predicted
proteins. One particular interesting region of the chromosome is the
213-kb region from 3,072,500 – 3,28,500, which encodes proteins for
sugar transport, metabolism, and regulation. This region has a lower
percent CG content (41.5%), leading researchers to believe that many of
these genes have been acquired by horizontal gene transfer. (7)
L. plantarumhas three plasmids, pWCFS101, pWCFS102, and pWCFS103. The plasmid sizes are as follows: pWCFS101 contains 1,917 bp, pWCFS102 contains 2,365 bp, and pWCFS103 contains 36,069 bp. Plasmid pWCFS101 is believed to contain replication proteins. Plasmid pWCFS102 is believed to contain replication proteins as well as proteins that function as copy number controls. Plasmid pWCFS103 contains genes that are predicted to be involved in arsenate and/or arsenite resistance as well as cadmium resistance; it also has genes that are believed to encode replication proteins, resolvases, DNA-damage-inducible proteins, and oxidases. L. plantarum contains two apparently complete prophage genomes, as well as some prophage remnants. (8)
The L. plantarum chromosome reveals that this microbe has a major focus on carbon catabolism. The sequence of its chromosome also supports its extreme flexibility, versatility, and ability to adapt to different environmental conditions. (7)
Cell Structure, Metabolism and Life Cycle
L. plantarum has a rod shaped structure with rounded ends.
This microbe is a gram positive bacteria meaning there is a high
concentration of peptidoglycan in the cell wall, and lack an
outercellular membrane. The organism is also Auxotrophic meaning that it
synthesizes few organic compounds, when it has the ability to break
down sugars and pyruvate. L. plantarum is also a facultative
heterofermentative lactobacilli microorganism, this means that the
organism takes carbon from sugars and pyruvate and the byproduct is
either alcohol or lactic acid. This process happens in an aerotolerant
environment meaning that oxygen is not present. When oxygen is present
it is released as hydrogen peroxide which can be used as a weapon that
kills off other bacteria. Do to the inability to handle oxygen the
organism uses a manganese dependent process. This process uses metal as a
pseudo catalase and lowers oxygen concentration which is favorable to
the aerotolerant environment. Sugar is a key source of energy for the
microorganism to degrade. (3)
During the degradation of sugar carbon is released and becomes a source of energy for the lactobacillus plantarum. When Lactobacillus plantarum is exhibiting a pyruvate metabolism it is similar to homolactic fermentation. This happens when growth occurs on glucose which is degraded to pyruvate though an EMP pathway. Once the pyruvate is formed it is converted to d and l-lactate though sterospecific lactate dehydrogenase enzymes (3).
Ecology and Pathogenesis
L. plantarum can be found in many different environments, most
commonly it is isolated from plant material and the human
gastrointestinal tract. (5) Researchers believe that the sequence of the L. plantarum genome has certain features that allow this microbe to be versatile and adaptive to different environments. (7) It can grow in-between 15-45 °C and can grow at pH levels of 3.2 and greater. This versatility allows L. plantarum
isolates to be found in human saliva, fermenting dairy products, plant
material, silage, and even certain waste waters. It gains its energy
through the fermentative conversion of sugars to lactic acid, as long as
is able to go through this process, most environments will allow the
growth of this microbe. Experts believe that the high number of
regulatory genes causes this microorganism to be so adaptable. The most
common habitat is in a protein enriched environment such as dairy
because of its primary protein-degradation which produces peptides. A
study showed that there are 144 N-terminals which can be used for
peptidase cleavage. Another key part to describe the adaptability of
this microorganism is its ability to perform horizontal gene transfer.
This process is accomplished though natural competition, bacteriophage
infection and more. Lactobacillus plantarum can perform these
transformations because it can bind DNA and uptake that DNA. (7).
Recently L. plantarum has been identified as a probiotic. Probiotics are non-pathogenic microorganisms that can have a positive impact on human health when they are digested. They are becoming a very popular dietary supplement to many people, especially those who have gastrointestinal problems. (1) In this case, L. plantarum can be considered a human symbiont. When the probiotics are ingested regularly it is possible that the composition of microflora in the intestinal tract can be manipulated. This manipulation may allow an improvement of microbe balance, stabilization of digestive enzyme patterns, and immunomodulation by activating and regulating mucosa-associated and systemic immune system responses. The microflora found in the intestinal tract are thought to provide protection from pathogens. Some companies currently sell bottles containing L. plantarum as a probiotic to help with intestinal problems including IBS and IBD, stating that these bacteria help to "Balance the Intestinal Ecosystem." (4)
Along with its possible use as a probiotic, there is another very important use for L. plantarum. Gram positive bacteria have been researched for a long time for their possible ability to convert lignocellulosic biomass to biofuels and bioproducts. A current goal with this research is to genetically manipulate certain lactic acid bacteria to convert agricultural biomass into ethanol as well as other value-added products. L. plantarum is a widely studied species and has now become a model for genetic manipulations of lactic acid bacteria. Current studies are looking at the induction of different mutations into the chromosome to eliminate undesired products that come with fermentation.
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