The Many Uses for Bacterial Ghosts

Bacterial ghosts (BGs) are empty cell envelopes or membrane structures derived from bacterial cells after a controlled removal of their cellular content, including the cytoplasm and genetic material. The bacteria are usually Gram-negative. The process of creating bacterial ghosts typically involves the induction of controlled cell lysis, leaving behind only the cell envelope or outer membrane. This empty shell retains the original structure of the bacterial cell, i.e. its intact cellular morphology. This includes surface proteins and other components, but lacks the live cellular content. The retained surface molecules will be antigens especially  pathogen-associated molecular patterns (PAMPs). You would expect to find lipopolysaccharides (LPS), peptidoglycan, flagellae, monophosphoryl lipid A (MPL), adhesins and a host of immune-related stimulating elements from what is natural bacteria (Langemann et al., 2010). It can never be described as an artificial bacterial system.

Given that bacterial ghosts have retained their bacterial antigenicity and adhesivity, they are recognised by immune cells. Having this property means they are readily taken up by the mucosal cells and so doing will begin conferring mucosal immunity as well other forms of immunity -cellular and humoral (Abtin et al., 2010).

Creating a Bacterial Ghost

The creation of bacterial ghosts involves a controlled removal of the cellular content while preserving the structural integrity of the cell envelope. The most commonly used method for creating bacterial ghosts is the “E gene-based” or “Suicide Gene System” approach. The E gene, also known as the lysis gene, encodes a bacteriophage (virus that infects bacteria) protein responsible for causing cell lysis. The most well-known lysis gene is the PhiX174 E gene.

Here’s a general overview of the process:

1. Selection of Host Bacteria: Choose a bacterial strain suitable for ghost production. Typically, Gram-negative bacteria like Escherichia coli (E. coli) are used due to their well-understood genetics and the availability of suitable vectors.
2. Cloning the Lysis Gene: Clone the lysis gene (E gene) into a plasmid vector. This plasmid also contains a temperature-sensitive origin of replication, which means it can replicate at lower temperatures but not at higher temperatures. The plasmid is introduced into the host bacteria.
3. Culturing the Host Bacteria: Grow the transformed bacteria at a permissive temperature (lower temperature) to allow the replication of the plasmid carrying the lysis gene. This phase allows the bacteria to propagate and produce copies of the plasmid.
4. Induction of Lysis Gene Expression: Once a sufficient bacterial population has been established, shift the temperature to a non-permissive temperature (higher temperature) to prevent further plasmid replication. This change in temperature induces the expression of the lysis gene.
5. Cell Lysis and Ghost Formation: The expressed lysis gene product, usually a bacteriophage protein, causes controlled lysis of the bacterial cells. This process results in the removal of cellular content, leaving behind only the empty cell envelope or bacterial ghost.
6. Purification of Bacterial Ghosts: Purify the bacterial ghosts by removing any remaining cellular debris. This can be done through centrifugation and other purification techniques.

The resulting bacterial ghosts retain the original structure of the cell envelope, including surface proteins and other components. These empty structures can then be used for various biotechnological applications, such as vaccine development, drug delivery, and diagnostic tools.

It’s important to note that the specific details of the method may vary depending on the bacterial species, the lysis gene used, and the intended application of the bacterial ghosts. Researchers continually refine and optimize the process to enhance the efficiency and safety of bacterial ghost production.

These bacterial ghosts have gained significance in biotechnology for several reasons:

1. Vaccine Development: Bacterial ghosts can be used as carriers for antigens to develop vaccines. By incorporating pathogenic antigens into the bacterial ghost structure, they can stimulate an immune response without the risk of causing disease, as the ghosts are devoid of the pathogenic genetic material. This approach can enhance the safety and efficacy of vaccines.
2. Drug Delivery Systems: Bacterial ghosts can be engineered to deliver drugs or therapeutic agents. The empty bacterial envelopes can be loaded with specific drugs or molecules and then targeted to specific cells or tissues. This targeted drug delivery system can minimize side effects and improve the effectiveness of treatments.
3. Bioremediation: Bacterial ghosts can be used in bioremediation processes to remove or detoxify environmental pollutants. By engineering the bacterial ghosts to bind to or absorb specific contaminants, they can be employed to clean up polluted environments.
4. Diagnostic Tools: Bacterial ghosts can serve as diagnostic tools. Surface proteins or antigens from pathogens can be incorporated into the bacterial ghosts, allowing for the development of diagnostic tests or assays for the detection of specific diseases.
5. Studying Cell Surface Interactions: Bacterial ghosts provide a platform for studying cell surface interactions. Researchers can investigate how different molecules interact with the surface proteins of bacterial ghosts, which can provide insights into cellular processes and the development of new therapies.
6. Biotechnological Research: Bacterial ghosts are useful in various biotechnological research applications, serving as a model system to study cell structure, protein localization, and other cellular processes without the need for live bacteria.

The controlled production of bacterial ghosts has opened up opportunities for their application in various fields, offering a safe and versatile platform for biotechnological advancements.

References

Abtin, A.; Kudela, P.; Mayr, U.B.; Koller, V.J.; Mildner, M.; Tschachler, E.; Lubitz, W. (2010) Escherichia coli ghosts promote innate immune responses in human keratinocytes. Biochem. Biophys. Res. Commun. 400, pp. 78–82

Langemann, T.; Koller, V.J.; Muhammad, A.; Kudela, P.; Mayr, U.B.; Lubitz, W. (2010) The Bacterial Ghost platform system: Production and applications. Bioeng. Bugs 1, pp. 326–336.