In this article we will discuss about the role of cellular biology in eukaryotic interaction.
By 1980s it became clear that bacterial infection may result in human death. Development of antibiotic resistance in bacteria continued resulting in the emergence of new bacterial diseases. In 1940s, Staphylococcus aureus was discovered as penicillin-resistant commensal bacterium. Now this bacterium is resistant to all forms of penicillin hence known as methicillin (some multiple)- resistant S. aureus (Fig. 27.2).
Different strains of E. coli causing diarrhoea through cell-to-cell interaction or producing many toxins (may be newly discovered) are given in Table 27.2. The E. coli strain 0157 also called huburger bug is most common in public. It causes food poisoning (mainly haemolytic uraemic syndrome) resulting in death of the sufferers.
On an average human body consists of about 1013 eukaryotic cells but supports bacteria on the epithelial surface. But less is known about normal microflora of humans in spite of 90% bacterial population on surface of human body. For therapeutic benefits, attempts were made for years to modify them.
A fermented milk product called Yakult® has been prepared which is most popular in changing microflora of the human colon. By using such product, normal microflora can be changed and new bacterial population can be established which promote host nutrition or increase resistance to infection in body.
Preparation of such microfloral bacteria is called probiotics. Now capsules of bacterial preparation are also available in Indian Market and easily used by patients.
Table 27.2 : Different strains of E.coli causing diarrhoea.
During the 1980s, due to rapid development, the two branches, molecular biology and cell biology were much advanced. During 1990s, application of these methodologies to microbiology enabled the emergence of cellular microbiology.
You know that bacteria are regarded as complex and beautifully adaptable organisms which respond to varied environmental conditions, whereas multicellular organisms depend on intercellular communication. Earlier, presence of such communication was not thought to operate in bacteria because bacteriologists use to grow them in axenic culture on artificial substrates lacking any signals.
Now it is established fact that bacteria respond through cell-to-cell signalling mechanism called quorum sensing (see preceding section for detail). This ability determines the maintenance of cell density and also switch on or switch off particular genes.
Quorum sensing was earlier discovered in marine bacterium. Vibrio fischeri establishing symbiotic association in light organs of certain marine fishes. But its operation in other bacteria is also known. It provides a virulence mechanism in bacteria and permits them to increase their number without inducing the virulence genes.
(i) Signal Molecules:
Bacteria respond to signals coming from eukaryotic host cells. Example of the mammalian signal molecules are serotonin, catecholamines, insulin and cytokines (e.g. interleukin 1 and 2, tumour necrosis factor and transforming growth factor alpha).
Kapreylants and Kell (1996) suggested a term ‘micro endocrinology’ to denote prokaryotic-eukaryotic communication. Besides, bacteria produce cytokines (i.e. local cell-to-cell regulatory molecules) which regulate the production of local hormone. Cytokines also control immune responses in vertebrates.
(ii) Small Peptides:
Antibacterial small peptides are also important in the interaction of bacteria with host cells. These peptides bind the bacterial cell wall, form pore like structure and results in loss of intracellular fluids. Hence, these kill both Gram-positive and Gram-negative bacteria and provide a defence to us. Hundreds of such peptides have been discovered from organisms including bacteria also.
(iii) Adhesins:
The intimacy of contacts between bacteria and the host cells has been emphasised by cellular microbiology. The first step of infection is the adherence of bacterial cell to host cell. Bacteria produce adhesins which selectively bind them to suitable host cell. Many surface molecules (e.g. lipids, glycolipids, carbohydrates and proteins) are present on host cell which form ligands with bacterial adhesins.
Adhesins trigger cytokine production which causes inflammation to check the entry of pathogens. As a result, adhesion bacteria release a number of specific proteins into cytoplasm of target cell. This influences the behaviour of eukaryotic cells. Shigella flexneri and Salmonella typhimurium use this mechanism to steal the cytoskeletal machinery of the host and facilitate the bacterium to enter the host cell.
The protein kinases and phosphatases are injected into the host cell. Bacteria participate ‘in biomolecular dialogue’ as the host cells. Ability of bacteria to bind and enter the cell and bio molecular dialogue between bacterium and host cell are described in Sections C and D.
(iv) Exotoxins:
Exotoxins are the major chemical weapon of the infectious bacteria. A large number of exotoxins have been discovered. They act on host cells in various ways. Cell biologists are using exotoxins as probes of cell function. Using exotoxins in understanding of cell signalling has now been determined. Besides, exotoxins are being used as therapeutic agents.
With the development of new molecular techniques, understanding of bacteria-host cell interaction has become clear. Following these techniques you can identify and analyse the genes which get induced both in bacteria and host cell during infection. Some of these new techniques are listed in Table 27.3.
Table 27.3 : Techniques of molecular biology which determine genes involved in bacterial infection.
A new branch called genomics has emerged which deals with complete DNA sequencing of bacteria. Genomes of some bacteria have completely been sequenced (Table 27.4). On the other hand, a question has always been asked what genes are switched on or switched off when a pathogen comes in contact of host cell.
To answer this question several techniques have been developed. One of such techniques is the in vitro expression technology which can identify genes that are switched on by the bacterium in the host cell.
Differential display polymerase chain reaction (DD-PCR) identifies genes expressed under different conditions i.e. cultured conditions and within the cells. Signature-tag mutagenesis technique can detect the genes which are involved in infection process. 