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More recently, scientists have begun to further classify gram-negative and gram-positive bacteria. They have added a special group of deeply branching bacteria based on a combination of physiological, biochemical, and genetic features. They also now further classify gram-negative bacteria into Proteobacteria , Cytophaga-Flavobacterium-Bacteroides (CFB) , and spirochetes .

The deeply branching bacteria are thought to be a very early evolutionary form of bacteria (see Deeply Branching Bacteria ). They live in hot, acidic, ultraviolet-light-exposed, and anaerobic (deprived of oxygen) conditions. Proteobacteria is a phylum of very diverse groups of gram-negative bacteria; it includes some important human pathogens (e.g., E. coli and Bordetella pertussis ). The CFB group of bacteria includes components of the normal human gut microbiota, like Bacteroides . The spirochetes are spiral-shaped bacteria and include the pathogen Treponema pallidum , which causes syphilis. We will characterize these groups of bacteria in more detail later in the chapter.

Based on their prevalence of guanine and cytosine nucleotides, gram-positive bacteria are also classified into low G+C and high G+C gram-positive bacteria . The low G+C gram-positive bacteria have less than 50% of guanine and cytosine nucleotides in their DNA. They include human pathogens, such as those that cause anthrax ( Bacillus anthracis ), tetanus ( Clostridium tetani ), and listeriosis ( Listeria monocytogenes ). High G+C gram-positive bacteria, which have more than 50% guanine and cytosine nucleotides in their DNA, include the bacteria that cause diphtheria ( Corynebacterium diphtheriae ), tuberculosis ( Mycobacterium tuberculosis ), and other diseases.

The classifications of prokaryotes are constantly changing as new species are being discovered. We will describe them in more detail, along with the diseases they cause, in later sections and chapters.

  • How do scientists classify prokaryotes?

Human microbiome project

The Human Microbiome Project was launched by the National Institutes of Health (NIH) in 2008. One main goal of the project is to create a large repository of the gene sequences of important microbes found in humans, helping biologists and clinicians understand the dynamics of the human microbiome and the relationship between the human microbiota and diseases. A network of labs working together has been compiling the data from swabs of several areas of the skin, gut, and mouth from hundreds of individuals.

One of the challenges in understanding the human microbiome has been the difficulty of culturing many of the microbes that inhabit the human body. It has been estimated that we are only able to culture 1% of the bacteria in nature and that we are unable to grow the remaining 99%. To address this challenge, researchers have used metagenomic analysis , which studies genetic material harvested directly from microbial communities, as opposed to that of individual species grown in a culture. This allows researchers to study the genetic material of all microbes in the microbiome, rather than just those that can be cultured. National Institutes of Health. “Human Microbiome Project. Overview.” http://commonfund.nih.gov/hmp/overview. Accessed June 7, 2016.

One important achievement of the Human Microbiome Project is establishing the first reference database on microorganisms living in and on the human body. Many of the microbes in the microbiome are beneficial, but some are not. It was found, somewhat unexpectedly, that all of us have some serious microbial pathogens in our microbiota. For example, the conjunctiva of the human eye contains 24 genera of bacteria and numerous pathogenic species. Q. Dong et al. “Diversity of Bacteria at Healthy Human Conjunctiva.” Investigative Ophthalmology&Visual Science 52 no. 8 (2011):5408–5413. A healthy human mouth contains a number of species of the genus Streptococcus , including pathogenic species S. pyogenes and S. pneumoniae . F.E. Dewhirst et al. “The Human Oral Microbiome.” Journal of Bacteriology 192 no. 19 (2010):5002–5017. This raises the question of why certain prokaryotic organisms exist commensally in certain individuals but act as deadly pathogens in others. Also unexpected was the number of organisms that had never been cultured. For example, in one metagenomic study of the human gut microbiota, 174 new species of bacteria were identified. J.C. Lagier et al. “Microbial Culturomics: Paradigm Shift in the Human Gut Microbiome Study.” Clinical Microbiology and Infection 18 no. 12 (2012):1185–1193.

Another goal for the near future is to characterize the human microbiota in patients with different diseases and to find out whether there are any relationships between the contents of an individual’s microbiota and risk for or susceptibility to specific diseases. Analyzing the microbiome in a person with a specific disease may reveal new ways to fight diseases.

Key concepts and summary

  • Prokaryotes are unicellular microorganisms whose cells have no nucleus.
  • Prokaryotes can be found everywhere on our planet, even in the most extreme environments.
  • Prokaryotes are very flexible metabolically, so they are able to adjust their feeding to the available natural resources.
  • Prokaryotes live in communities that interact among themselves and with large organisms that they use as hosts (including humans).
  • The totality of forms of prokaryotes (particularly bacteria) living on the human body is called the human microbiome, which varies between regions of the body and individuals, and changes over time.
  • The totality of forms of prokaryotes (particularly bacteria) living in a certain region of the human body (e.g., mouth, throat, gut, eye, vagina) is called the microbiota of this region.
  • Prokaryotes are classified into domains Archaea and Bacteria.
  • In recent years, the traditional approaches to classification of prokaryotes have been supplemented by approaches based on molecular genetics.

True/false

Among prokaryotes, there are some that can live in every environment on earth.

True

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Fill in the blank

When prokaryotes live as interacting communities in which one population benefits to the harm of the other, the type of symbiosis is called ________.

parasitism

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The domain ________ does not include prokaryotes.

Eukarya

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Pathogenic bacteria that are part of the transient microbiota can sometimes be eliminated by ________ therapy.

antibiotic

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Nitrogen-fixing bacteria provide other organisms with usable nitrogen in the form of ________.

ammonia

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Short answer

Compare commensalism and amensalism.

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Give an example of the changes of human microbiota that result from medical intervention.

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Questions & Answers

what is parasitic helminths
Sadiya Reply
list three categories of symbiotic relationships.
mary Reply
what's the difference between microbial intoxication and infectious diseases
dranimva Reply
microbial intoxication results when a person ingests a toxin or a poisonous substance that has been produced by a microbe while infectious disease results when a pathogen colonize the body and subsequently cause disease.
mary
weighing balance is needed in lab of microbiology for weighing?
amna Reply
It is mainly used for media preparation and product testing purpose.
Nakaweesi
thank you for your joining
nuur
what is microbiology
Shamsuddeen Reply
study of living organisms that are too small to be visible with naked eye
Marah
yes
Ahmed
microbiology is the study of microscopic organisms, either unicellular(single cell), multicellular( cell colony), or acellular (lacking cell).
munachimso
is the word Atypical or a typical bacteria. .am confused pliz help
MUWANGUZI Reply
typical bacteria
Zulpha
okay thank you what does that mean
MUWANGUZI
atypical means that it has some characters from bacteria not all characters ...but tybical means that it has all the characters that bacteria have
Reham
thank you Zulpha
MUWANGUZI
some examples please
MUWANGUZI
thank you Reham
MUWANGUZI
1) typical bacteria contain a cell wall whereas atypical bacteria usually do not contain a cell . 2) typical bacteria can be either Gram-positive or Gram-negative while atypical bacteria remain colorless with Gram staining. 3) cells of typical bacteria are large ,while cells of the atypical small
Marah
Example of atypical : Mycoplasma pneumoniae , chlamydophila pneumoniae , legionella
Marah
what is micro biology
Jauharah Reply
is the study of organisms which can't be viewed by our necked eyes
Egumat
Because it preexisting causing secondary infection after collateral damage of normal microbota
Rafaa Reply
combinations of drugs that can't be taken together and why
Grace Reply
Antagonism: the combined action is less than that of the more effective agent when used alone). All these effects may be observed in vitro (particularly in terms of bactericidal rate) and in vivo والله اعلم
Lenovo
....fermentros have 1-15litre capacity
AMAR Reply
which of the following microorganisms are classified as eukaryotic?
Semugab Reply
how do I see the list?
Melissa
what are the choices?
Melissa
dear tell us the choices
MUWANGUZI
mr semugab give us the list please
Nambi
Fungi
munachimso
what are the new discoveries of microorganisms
Ezibon Reply
bacteriology,viriology,micrology
Egumat
understanding of contributed to attempts to treat contain disease
james Reply
what is a bacteria
ROSE Reply
yes tell us
Judith
a bacteria is the largest living organisms on the planet Earth
Egumat
oh, sure. we have also one big here nearby!
Maxim
Bacteria is a large group of single-cell microorganisms. some causes infections and disease in animals and humans..
munachimso
do you mean procariots?
Maxim
Types of Respiratory diseases Pneumonia, Bronchitis, Asthma, Lung cancer, Emphysema.
Enoch Reply
Practice MCQ 3

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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