Medical Importance of Bacteria

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    Bacteria are single-celled microorganisms that lack a nuclear membrane, are metabolically active, and divide by binary fission. Medically they are a major cause of disease. Superficially, bacteria appear to be relatively simple forms of life. However, they are sophisticated and highly adaptable. Bacteria multiply at rapid rates and can utilize an enormous variety of hydrocarbon substrates, including phenol, rubber, and petroleum. These organisms exist widely in both parasitic and free-living forms. Because they are ubiquitous and have a remarkable capacity to adapt to changing environments, the importance of bacteria in every field of medicine cannot be overstated.3

    The age of the oldest living organism on Earth is between 25 million and 40 million years old. Spore of bacteria that old were resuscitated in 1995. The bacteria had been preserved in the abdomen of a 25-million-year-old stingless bee, preserved in turn in amber from the Dominican Republic. A close relative of the bacteria, Bacillus sphaericus, lives symbiotically in bees and other insects today. When food or water is scarce, some bacteria can form spores and grow protective protein shells around them. Inside their shells, the spores can dry out and remain dormant for years, millions of years.

    However, bacteria categorization as non-pathogens may change because of the adaptability of bacteria and the harmful effect of modern radiation therapy, chemotherapy, and immunotherapy on resistance mechanisms. Some bacteria previously considered to be nonpathogenic are now known to cause disease. Serratia marcescens, for example, is a common soil bacterium that causes pneumonia, urinary tract infections, and blood poisoning in compromised individuals.3

    Classification is the orderly arrangement of bacteria into groups. There is nothing scientific about classification, and different groups of scientists may classify the same organisms differently. For example, clinical microbiologists are interested in the serotype, antimicrobial resistance pattern, and toxin and invasiveness factors in Escherichia coli, whereas geneticists are concerned with specific mutations and plasmids.

    Bacterial species is a distinct organism with certain characteristic features or a group of organisms that closely resemble one another’s most important features. A species name should mean the same thing to all microbiologists, yet some definitions vary in different countries or microbiologic specialty groups. For example, the organism known as Clostridium perfringens in the United States is called Clostridium welchii in England2.

    Classification Based On Pathogenicity

    Pathogenicity is the capacity of an organism to cause disease. On this basis, bacteria can be organized into three major groups. When isolated from a patient, frank or primary pathogens are considered to be probable agents of disease (e.g., when the laboratory isolation of Salmonella spp identified the cause of the diarrheal disease from feces). Opportunistic pathogens are those isolated from patients whose host defense mechanisms have been compromised. They may be disease agents (e.g., in patients who have been predisposed to urinary tract infections with Escherichia coli by catheterization). Finally, bacteria, such as Lactobacillus acidophilus, are considered nonpathogenic because they rarely or never cause human disease. Pathogenic bacteria constitute only a small proportion of bacterial species; many nonpathogenic bacteria are beneficial to humans. For example, in the intestinal flora, they produce vitamin K. Other species participate in essential processes such as nitrogen fixation, waste breakdown, food production, drug preparation, and environmental bioremediation.


    Bacteria exhibit an incredible diversity of metabolism, structure, and role in the environment. Below you will find a representative sample of bacteria that demonstrate their medical, economic, and environmental importance.

    • Predatory Bacteria are potential therapeutic and biocontrol agents.
    • Cyanobacteria perform photosynthesis (oxygen is released) and fix nitrogen.
    • Chloroflexus are green nonsulfur bacteria that perform photosynthesis.
    • Proteobacteria
      • Acetobacter and Gluconobacter produce acetic acid (vinegar) from ethanol.
      • RhodospirillumRhodobacter, and Rhodopseudomonas perform photosynthesis; no oxygen is released.
      • EhrlichiaRickettsia, and Coxiella are intracellular parasites of humans.
      • Erysipelothrix
      • Wolbachia are symbionts of insects.
      • RhizobiumAzospirillumBeijerinkciaAzomonas, and Azotobacter perform nitrogen fixation, a process in which nitrogen (N2) in the atmosphere is converted into ammonia.
      • Agrobacterium species are pathogens of plants; Agrobacterium tumefaciens is most widely known for causing crown gall disease that affects many broad-leaved plants, including grapevines; Agrobacterium is widely used to introduce genetic material to plants.
      • Nitrobacter and Nitrosomonas perform nitrification, an important step in the nitrogen cycle in soil, where ammonia is converted to nitrate.
      • Alcaligenes perform denitrification, an important step in the nitrogen cycle in which nitrogen is returned to the atmosphere in the form of nitrogen gas.
      • NeisseriaLegionellaVibrioSalmonellaShigellaHaemophilusCampylobacterHelicobacterClostridiaStreptococcus, and Listeria are human pathogens.
      • PseudomonasStaphylococcusEnterobacterKlebsiellaSerratiaCitrobacter, and Enterococcus are opportunistic human pathogens; they cause disease only under certain conditions.
      • Escherichia and Proteus are normal intestinal bacteria of humans. Some are pathogenic e.g., P. mirabilis causes urinary tract infection. Some strains of E. coli can cause severe infections in humans and animals.
      • Bdellovibrio are predators of Gram-negative bacteria. The bacterium is also known as a “living antibiotic” since it is not the cause of any known diseases.
      • Lactobacillus produces lactic acid during fermentation; it is important in the production of fermented milk products.
    • Actinomycetes
      • Corynebacteria and Mycobacterium are human pathogens. Corynebacterium diphtheriae causes diphtheria, an upper respiratory disease mainly affecting children.
      • Propionibacterium produces propionic acid during fermentation. They are essential in the manufacture of Swiss cheese. It and can cause several infections, including the common skin disease acne vulgaris.
      • Streptomyces produce many antibiotics; they inhabit soil and are important decomposers.
    • Aeromonas are animal and human pathogens.
    • Aeromonas hydrophila is an animal and human pathogen ubiquitous in water, including chlorinated drinking water and groundwater.
    • Coliform bacteria cause infections, which may lead to severe diarrheal disease.
    • Chlamydia are intracellular parasites of humans.
    • Pantoea are plant, animal, and human pathogens.
    • Spirochaetes are human pathogens.

    Archea And Bacteria

    Archea and bacteria are extensively similar. In terms of their cell structure, Archea is indistinguishable from Gram-positive bacteria. Within prokaryotes, only these two groups of organisms are bounded by a single unit lipid membrane. Most of the metabolic pathways which make up the vast majority of any organism’s gene repertoire, are common between Archea and bacteria.

    The majority of the genes that indicate Archea to be different from bacteria are information transfer processes, such as those responsible for DNA replication, transcription, and protein synthesis.

    Archea does not contain typical bacterial DNA polymerases, such as helicase or other proteins involved in different DNA replication stages. The RNA polymerase are the same in Archea and bacteria, but the archeal enzyme also contains several smaller subunits not found in bacteria. Archea also differs from bacteria in having a small number of unique r-proteins as well as many translation initiation factors 5.

    Role of Bacteria In Cancer

    Combined, the “non-you” cells in your body outnumber “the you” cells by about 10:1. And if some sadistic scientist were to grind up and sequence all the DNA in every cell in and on your body, only about 2% of the genetic material would be human. The rest is microbes.

    It has been hypothesized that infectious agents predispose a person to cancer development under the influence of several genetic and metabolic factors. It has not yet been determined how infectious agents can cause cancer without being contagious. The suspected mechanisms by which bacteria may be involved in cancer are the establishment of chronic infection and the production of toxins, which alter normal cell cycle and trigger uncontrolled cell growth.

    It has been further hypothesized that bacteria are capable of horizontal DNA transfer to human genes, followed by transformation and mutation of host cells. During the analysis of the human genome by the International Human Genome Sequencing Consortium, they found that at least 113 genes entered in vertebrate genomes by horizontal gene transfer. About 40 genes were found to be exclusively shared by humans and bacteria that offer a good possible example of horizontally transferred genes.

    The idea that certain bacteria are capable of causing cancer is supported by various studied relationships of bacterial infections with cancer. The best studied relationship between bacterial infection and cancer is Helicobacter pylori with gastric and MALT lymphoma. Besides, individuals infected with Salmonella typhi have several times the increased risk of developing gallbladder carcinoma. Chlamydia pneumoniae infection has been implicated in several chronic lung diseases; persistently elevated C. pneumoniae antibody titers have been found in lung cancer patients. Several bacterial species are linked with chronic infection of the colon, increasing the risk of colon cancer. These bacterias include Escherichia coli and Streptococcus bovis.

    Not every bacteria-induced transformation or chronic infection or production of mutagenic products causes cancer on its own. However, the right mixture of all factors might turn out to be a fatal combination that enables the accumulation of irreversible mutations that lead to cancer.7

    Learn about extremophiles and the world’s toughest bacterium.

    Gram-Negative Bacteria

    Gram-Positive Bacteria


    Video Credits: Lecturio Medical
    Image Credits: WikiImages


    1. DNA Vaccines. DNA Delivery With Attenuated Intracellular Bacteria. Joachim Fensterle and Stefan H.E. Kaufmann.
    2. Medical Microbiology. The University of Texas Medical Branch at Galveston.
    3. Introduction to Bacteriology. Charles P. Davis, Gail Woods, David Niesel. The University of Texas Medical Branch at Galveston.
    4. Molecular Biology of the Cell. Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter.
    5. Microbial phylogeny and evolution: concepts and controversies. Jan Sapp.
    6. The Search for Infectious Causes of human Cancers: Where and Why Nobel Lecture, December 7, 2008 by Harald zur Hausen.
    7. Bacterial infections associated with cancer: possible implication in etiology with special reference to lateral gene transfer by Abdul Arif Khan & Abhinav Shrivastava
    8. Blue Genes and Polyester Plants: 365 More Suprising Scientific Facts, Breakthroughs, and Discoveries


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