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Resolution

Within 24 hours, the results of the diagnostic test analysis of Alex’s stool sample revealed that it was positive for heat-labile enterotoxin (LT) , heat-stabile enterotoxin (ST) , and colonization factor (CF) , confirming the hospital physician’s suspicion of ETEC . During a follow-up with Alex’s family physician, this physician noted that Alex’s symptoms were not resolving quickly and he was experiencing discomfort that was preventing him from returning to classes. The family physician prescribed Alex a course of ciprofloxacin to resolve his symptoms. Fortunately, the ciprofloxacin resolved Alex’s symptoms within a few days.

Alex likely got his infection from ingesting contaminated food or water. Emerging industrialized countries like Mexico are still developing sanitation practices that prevent the contamination of water with fecal material. Travelers in such countries should avoid the ingestion of undercooked foods, especially meats, seafood, vegetables, and unpasteurized dairy products. They should also avoid use of water that has not been treated; this includes drinking water, ice cubes, and even water used for brushing teeth. Using bottled water for these purposes is a good alternative. Good hygiene (handwashing) can also aid the prevention of an ETEC infection. Alex had not been careful about his food or water consumption, which led to his illness.

Alex’s symptoms were very similar to those of cholera , caused by the gram-negative bacterium Vibrio cholerae , which also produces a toxin similar to ST and LT. At some point in the evolutionary history of ETEC , a nonpathogenic strain of E. coli similar to those typically found in the gut may have acquired the genes encoding the ST and LT toxins from V. cholerae . The fact that the genes encoding those toxins are encoded on extrachromosomal plasmids in ETEC supports the idea that these genes were acquired by E. coli and are likely maintained in bacterial populations through horizontal gene transfer.

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Viral genomes

Viral genomes exhibit significant diversity in structure. Some viruses have genomes that consist of DNA as their genetic material. This DNA may be single stranded, as exemplified by human parvoviruses , or double stranded, as seen in the herpesviruses and poxviruses . Additionally, although all cellular life uses DNA as its genetic material, some viral genomes are made of either single-stranded or double-stranded RNA molecules, as we have discussed. Viral genomes are typically smaller than most bacterial genomes, encoding only a few genes, because they rely on their hosts to carry out many of the functions required for their replication . The diversity of viral genome structures and their implications for viral replication life cycles are discussed in more detail in The Viral Life Cycle .

  • Why do viral genomes vary widely among viruses?

Genome size matters

There is great variation in size of genomes among different organisms. Most eukaryotes maintain multiple chromosomes; humans, for example have 23 pairs, giving them 46 chromosomes. Despite being large at 3 billion base pairs, the human genome is far from the largest genome. Plants often maintain very large genomes, up to 150 billion base pairs, and commonly are polyploid, having multiple copies of each chromosome.

The size of bacterial genomes also varies considerably, although they tend to be smaller than eukaryotic genomes ( [link] ). Some bacterial genomes may be as small as only 112,000 base pairs. Often, the size of a bacterium’s genome directly relates to how much the bacterium depends on its host for survival. When a bacterium relies on the host cell to carry out certain functions, it loses the genes encoding the abilities to carry out those functions itself. These types of bacterial endosymbionts are reminiscent of the prokaryotic origins of mitochondria and chloroplasts.

From a clinical perspective, obligate intracellular pathogens also tend to have small genomes (some around 1 million base pairs). Because host cells supply most of their nutrients, they tend to have a reduced number of genes encoding metabolic functions. Due to their small sizes, the genomes of organisms like Mycoplasma genitalium (580,000 base pairs), Chlamydia trachomatis (1.0 million), Rickettsia prowazekii (1.1 million), and Treponema pallidum (1.1 million) were some of the earlier bacterial genomes sequenced. Respectively, these pathogens cause urethritis and pelvic inflammation, chlamydia, typhus, and syphilis.

Whereas obligate intracellular pathogens have unusually small genomes, other bacteria with a great variety of metabolic and enzymatic capabilities have unusually large bacterial genomes. Pseudomonas aeruginosa , for example, is a bacterium commonly found in the environment and is able to grow on a wide range of substrates. Its genome contains 6.3 million base pairs, giving it a high metabolic ability and the ability to produce virulence factors that cause several types of opportunistic infections .

Interestingly, there has been significant variability in genome size in viruses as well, ranging from 3,500 base pairs to 2.5 million base pairs, significantly exceeding the size of many bacterial genomes. The great variation observed in viral genome sizes further contributes to the great diversity of viral genome characteristics already discussed.

 A graph showing genome sizes. Viruses have genomes that range from 1.7x10 to the 2nd bp to 2.5x10 to the 6th bp. Bacteria have genomes that range in size from 10 to the 5th to 10 to the 7th. One example is E. coli which ranges from 4.6 to 5.6 x 10 to the 6th bp. Fungi have genomes that range from 10 to the 6th to 10 to the 8th bp. Saccharomyces cerevisiae (yeast) has a genome of 1.2 x 10 to the 7th bp. Plants and animals have genomes that range from 10 to the 6th to 10 to the 11th bp. Mammals range from 10 to the 9th to 10 to the 10th bp. Humans have a genome of 3 x  10 to the 9th.
There is great variability as well as overlap among the genome sizes of various groups of organisms and viruses.

Key concepts and summary

  • The entire genetic content of a cell is its genome .
  • Genes code for proteins, or stable RNA molecules, each of which carries out a specific function in the cell.
  • Although the genotype that a cell possesses remains constant, expression of genes is dependent on environmental conditions.
  • A phenotype is the observable characteristics of a cell (or organism) at a given point in time and results from the complement of genes currently being used.
  • The majority of genetic material is organized into chromosomes that contain the DNA that controls cellular activities.
  • Prokaryotes are typically haploid, usually having a single circular chromosome found in the nucleoid. Eukaryotes are diploid; DNA is organized into multiple linear chromosomes found in the nucleus.
  • Supercoiling and DNA packaging using DNA binding proteins allows lengthy molecules to fit inside a cell. Eukaryotes and archaea use histone proteins, and bacteria use different proteins with similar function.
  • Prokaryotic and eukaryotic genomes both contain noncoding DNA , the function of which is not well understood. Some noncoding DNA appears to participate in the formation of small noncoding RNA molecules that influence gene expression; some appears to play a role in maintaining chromosomal structure and in DNA packaging.
  • Extrachromosomal DNA in eukaryotes includes the chromosomes found within organelles of prokaryotic origin (mitochondria and chloroplasts) that evolved by endosymbiosis. Some viruses may also maintain themselves extrachromosomally.
  • Extrachromosomal DNA in prokaryotes is commonly maintained as plasmids that encode a few nonessential genes that may be helpful under specific conditions. Plasmids can be spread through a bacterial community by horizontal gene transfer.
  • Viral genomes show extensive variation and may be composed of either RNA or DNA, and may be either double or single stranded.

True/false

Within an organism, phenotypes may change while genotypes remain constant.

True

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Noncoding DNA has no biological purpose.

False

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Fill in the blank

Plasmids are typically transferred among members of a bacterial community by ________ gene transfer.

horizontal

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Short answer

What are some differences in chromosomal structures between prokaryotes and eukaryotes?

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How do prokaryotes and eukaryotes manage to fit their lengthy DNA inside of cells? Why is this necessary?

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What are some functions of noncoding DNA?

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In the chromatin of eukaryotic cells, which regions of the chromosome would you expect to be more compact: the regions that contain genes being actively copied into RNA or those that contain inactive genes?

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Questions & Answers

write the life cycle of HIV
Firomsa Reply
describe the internal and external structure of prokaryotic cell in terms of there appearance and functions
Lenia Reply
compare and contrast similar structures found in prokaryotic and eukaryotic cells
Lenia
control of microorganisms
ISTEFAZUL Reply
how can a doctor treat a person affected by endospore forming bacteria in his/her wound?
Mambo Reply
a 28 years old woman come to your clinic with complain of fever painful genital blisters which express clear fluid when ruptured burning sensation around the bristers what is the diagnosis?
Ramadhani Reply
genital herpes caused by a virus called herpe simplex virus
Doris
what is the reference between selective medium and differential medium?
Tony Reply
the micro flora of air is transient why
Princess
Hello.... Am new here
essien Reply
welcome essien
Muhammad
welcome essien
Beka
classification of gram positive
lissa Reply
classify gram positive
lissa
catalase test is done to differentiate between staph and strep. as staph is catalase positive while strep is catalase negative then staph is differentiate further on coagulase positive and coagulase negative. staph aureus is coagulase positive while staph epidermidis is coagulase negative.
Muhammad
gram positive staph is further differentiated on sensitivity test and manitol salt fermentation test while gram positive strep is differentiated on hemolysis pattern
Muhammad
more about gramm positive
lissa
please
lissa
ok
Blessing
am back any gist
Blessing
taxonomy
Rahul
can i use the graph of bacterial growth for my master's thesis?
Christoph Reply
facultative anaerobic bavteria gives uniform turdibity in nutrient broth why?
Princess Reply
oils and waxes are not sterilized in autoclave
Princess
what is the function of paraffin
Muhammad
@ MAHI because they can grow all over the media from surface to the bottom as they can utilize oxygen or conduct fermentation in its absence.
Online
2.@MAHI Autoclave uses steam under pressure. The steam cannot penetrate through oil and wax. Thus, dry heat sterilization is preferred than autoclave .
Online
thnku
Nadiya
how nutirent agar can converted into blood agar
Princess
thank you
Princess
take a nutrient agar ........and add 5ml blood and put in it ...........!
Nadiya
nutrient agar +blood 5ml+ distilled water =blood agar .
Nadiya
mixed wellllll
Nadiya
hi
Rahul
why agar is not a neutrient source?
Shimul
because it's a general purpose agar only use for growing bacteria
Richa
way is antigen
opaleye
what is antigen
opaleye
antigen is a foreign body that cause activation of antibody?
ASNAKE
Antigen is a toxin or other foreign substance which induces an immune response in the body, especially the production of antibodies.
Shimul
thanks for that
opaleye
what is pharmacology
Luyenu
a science that studies about the drug
ASNAKE
hello ever one
Sayid
y
Abdul
Work hard
Anigor
y
Abdul
thanks alot
Luyenu
What is the major different between gram positive and gram negative bacteria.
Asikur
bacteria kya hai
Mala
content of their cell wall
Doris
koi mujhse basic microbiology ki study k Lea book suggest kro .
Richa
Harley prescott
Vikas
what are the five predisposing factors of opportunistic infections
Asher Reply
1. long term exposure to diseases, that the body immunity weakens.
prosper
2. Nature Of some diseases to weaken body immunity such as, AIDS.
prosper
3. Use of immuno-suppresive drugs.
prosper
anyone... need a hand here 🙋😞
prosper
Low immune is the major cause eg for kaposis sarcoma
Davismith
suitable conditions e.g temp,
Muhammad
what os gnormal flora
kifayat Reply
morphological and genetic classification of bacteria
Potter Reply
what is the morphogical
Bupe
h
Sudhir
morphologycal is defined as relating to the branch of biology that deals with the form of living organisms,and with relationship between their structures.
Nadiya
relating to the form or structure of things .
Nadiya
what is the meaning of lessions
oluwa
i am frim Afghanistan.
wahidullah Reply
iam from iraq
Haidar
and i'm from pakistan
Muhammad
Nice
Amina
what's about the qualification
Muhammad
Hlo
Asiya
Hmm
Number
Almustafa Muhammad from Nigeria
Mujtafa
am ustaz Abdulsalam from Ghana
ustaz
am Bupe Chifita from Zambia
Bupe
what is microbiology
Bupe
is the study of invisible micro organism
Mujtafa
microbiology is the study of very too small organisms which we can't see with naked eyes ......in other words ....study of microorganism is called microbiology.
Nadiya
Hello! How Can Microorganisms Be Isolated From The Skin?.. 5 ways if possible. (briefly)
prosper
from skin swabbing is the prefer method worldwide
Muhammad
thanks Muhammad Nauman very much!
prosper
hi
Meek
hi Meek
prosper
are there any other ways pals?
prosper
hi prosper
Meek
hello Meek Mild.
prosper
Any question or ideas, Meek?😊
prosper
and if a patient have any blister or abscess on the skin then first cut the blister or abscess. inside material is taking by syringe.
Muhammad
in case of fingernail or toenails simply cut a small piece of nail and culture on the appropriate media at a required temperature and time
Muhammad
Again, thanks Mr. Muhammad! I appreciate the help.
prosper
you welcom prosper 😊
Muhammad
what are the media use for skin swab?
Sieh-Mlanwin
blood and macckonkey mostly usededia for skin bacteria
Muhammad
used media*
Muhammad
@ SIEH mostly fungal infection occurs in skin so SDA ( Sabouraud Dextrose Agar) or MEA ( Malt extract agar) are common.
Online
thanks
Sieh-Mlanwin
Microbiologicam source of vitamins
Bachi Reply
what do you mean?
Md
that's example?
Md
hi everyone
Muhammad
such as. c/s media
Md
hmm hlw
Md
just tell me about that ?
Md
where are u from
Muhammad

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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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