Phospholipid Transportation in Gram-negative Bacteria

Bacteria are ubiquitous, around us and in our bodies and they can either be beneficial or harmful. Harmful bacteria cause infectious diseases that make us sick.

Diagnosis of bacterial illness elicits the type of bacteria that causes us illness. These diagnostic tests are geared towards identifying the type of bacteria as this would enable advise on the type of treatment. Gram stain is a microbiological technique used in the diagnosis of diseases. It was named in honor of Hans Christian Gram who developed the technique in 1880. His discovery was that certain bacteria did not take on the purple dye during staining and hence were not visible in the purple color when viewed under microscope, this was the basis for the name Gram-negative bacteria.

The simple reason behind their inability to take on the dye comes from the nature of their enveloped cell wall. This cell wall was made up of tightly packed sugars and cemented with an outer membrane. An understanding of bacteria’s ability to build this barrier is an essential step in bio engineering to maneuver their defense. The Lipopolysaccharide (LPS) is the major component of this cell wall it helps to increase the mechanical strength of the Gram-negative cell envelope and in this way is able to form a coating that prevents toxic molecules, such as antibiotics from entering the cell. On another note LPS is a potent toxin that can cause severe illness when it is released from the bacterial outer membrane or secreted by the cell.

LPS in small amounts results in bacterial cell rupture while in large amounts leads to high toxicity levels especially if poorly assembled. Three essential membrane proteins are required to monitor the biosynthesis, transport and assembly of LPS. This is a complex problem for bacteria because potentially dangerous LPS made within the cell must be transported across the cell wall to reach the outer membrane. This must be balanced against the manufacture and transport of phospholipids that make up other components of the Gram-negative bacteria membrane.

Transport of phospholipids to the outer membrane is a mystery. It is believed that phospholipids are able to flow back and forth between the bacterium’s inner cell membrane and its outer membrane at zones of contact. Another believe is the existence of a passive mode of transport.

A study to identify proteins involved in trafficking phospholipids between the inner and outer membranes was conducted. One approach was the use of mutant strains of bacteria that increases the rate at which phospholipids flow from the inner membrane to the outer membrane. During deprivation of nutrients, these bacteria were observed to experience shrinking, inner membrane rupture and eventual cell death due to an inability to manufacture new phospholipids for the inner membrane as replacement for that expended in the outer membrane. Additional mutations in these bacteria in other to look for mutated genes that affect how quickly the bacteria die after nutrient withdrawal was performed.

Using next-generation sequencing, the finding was that disruption of the gene yhdP slowed phospholipid transport.

This was proof that the gene encoded a protein required for phospholipid transport between the inner and outer cell membrane, it isn’t clear how yhdP is able to affect this process. It is hoped that clues might be found in similarities to other proteins with known functions. An instance of these is the mammalian protein required for the formation of a channel that transports phospholipids across membranes. Further suggesting that yhdP might form a hydrophobic channel between the inner and outer membrane through which phospholipids flow.

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