<< Chapter < Page Chapter >> Page >

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.

Go back to the previous Clinical Focus box.

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

Got questions? Get instant answers now!

Noncoding DNA has no biological purpose.

False

Got questions? Get instant answers now!

Fill in the blank

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

horizontal

Got questions? Get instant answers now!

Short answer

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

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

What are some functions of noncoding DNA?

Got questions? Get instant answers now!

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?

Got questions? Get instant answers now!

Questions & Answers

why are there so many long words and is there simplified versions of these words?
Ethan Reply
What is the different between abiogensis and biogensis
RAPHERA Reply
Abiogenesis is the process by wich living organisms arises from a nonliving matter while Biogenis organisms arises from living matter
Nji
Abiogenesis deals with the law of spountaneous generation which talks about organisms arising from non-living organic matters while biogenesis deals with organisms arising from living organic matters
Ademola
What are the importance of mircobiology
RAPHERA
mechanisms of microbiology
Erneet Reply
What is immunizations
RAPHERA Reply
the action of making a person or animal immune to infection, typically by inoculation.
Pup
pls someone should help with ANTIMICROBIAL MNEMONICS
Shuaib Reply
Aminoglycosides: Charasteristics of Aminoglycosides Nymonics; (AMINO)  Active against Aerobic gram negative Organisms Mechanism Of resistance is by Modifying enzymes Inhibit protein synthesis by binding to 30s ribosomal subunit Nephrotoxic Ototoxic Members  Of Aminoglycosides: Nymonics: (ST
Najeebul
Please can some briefly describe the germ theory?
Michael Reply
hyy all
Shuaib
pls someone should help with ANTIMICROBIAL MNEMONICS
Shuaib
which drug can be used to treat gonococus?
Lambon Reply
Madication - Antibiotic " and Gonorrhoea can be treat with Antibiotic
Maryam
Penicillin -An antibiotic
Michael
Ciprofloxacin tablets also an antibiotic
Michael
Ofloxacin is also antibiotics
Ikenna
yes
Elli
sory no fot gonococus
Elli
oofer gang roblox memes
Ethan
what are eukaryotes
Kakande Reply
eukaryotes simply refers to a cell who does not have a definite nucleus.
Franca
Eukaryotic cell has a true nucleus
arfa
what is right answer ?
krishna
I can remember now truly that eukaryotes have true nucleus.I was referring to procaryotes.
Franca
Eukaryotes have got true nuclues
Ede
they have a nucleus, larger than prokaryotes
Alanna
The eukaryotes cell are found in advance organisms.
Franca
Eukaryotes are multicellular organism which has a membrane bounded structure mostly found in animals and some plant cells
KINGSFORD
bacteria growth curve
sani Reply
No bacteria's don't grow curvey. Curve is a shape for a bacteria not growth
KINGSFORD
What is the difference between eukaryotes and prokaryotes?
KINGSFORD Reply
What's a multicellular organism?
KINGSFORD
multicellular organisms is an organism composed of many cells which are to varying degree integrated and independent.
Ruth
Boiling of milk to kill germs is called?
Kasigwa
pasteurization
Aiyedun
pasteurization
Katta
kadha
Katta
pasteurization
Gomathi
what is microscopic
monish
Pasteurization
Hosy
what are Flora
Hosy
you mean like the normal flora?
Tenucharity
pasteurization has to do with killing of the harmful microbes present for safe consumption. Eg. pasteurized yoghurt beside the bacteria that are supposed to be present, there may be harmful ones that have been killed
Tenucharity
Can someone explain flora
KINGSFORD
what are the normal Flora on the ff anatomical sites? Skin, nose, ear,oral cavity, intestines and the vagina
Hosy
what is cell?
Anita Reply
the cell is the functional and structural unit of the body
Ruth
yes ur right
Kasigwa
who add me
Wali
hy
Katta
hy
Shivangi
basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life.
Haru
hi
Ruth
hi
Haru
coming Oct 2 I have a exam on microbiology
Haru
okay all the best
Ruth
hi
sachin
hi
Ruth
plz bro send question of microbiology
umesh
structural and functional unit of the life
monish
this is not the correct answer
Alanna Reply
why
Hayat
why not
Zaajid
what is the correct answer?
vannessa
where is question
Zaajid
ask you question
Zaajid
What are mitochondria
Hayat
what is mitochondria
Hayat
who is the father of microbiology
T-
not
Hayat
Antonie Van Luweenhoek
Ravikumar
Mitochondria is the powerhouse of the Cell. And is the cell organelle which helps cell in generation of energy by Cellular Respiration
Ravikumar
Mitochondria are threadlike structuresbounded by double membrane,and serves as the power house of the cell
Felicity
can I get notes on introduction to microbiology just started I have no idea please
rita
Mitochondria is a membrane bounded structure, it's sausage in shape and has inner and outer membrane. The inner membrane is folded at the surface called cristae which project to form matrix. They function as the power house for the cell.
KINGSFORD
Rita are you a medilab student?
KINGSFORD
the father of microbiology is Luis pasture
Able
Erythromycin--------(+++) Cefuroxine ------------(++++) Chloramphenicol --(++) Vancomycin ---------(++++) please what does the above result means. under urine culture and sensitivity?
Kwame
francesco redi was the first to challenge spontaneous generation not pasteur
Alanna
Rita are you a medilab student? no please am a public health student
rita
kwame Evans this result ll mean that the bacterial in the urine can be remedy / killed by this antibacterial (cefuroxime and the rest are antibiotics
rita
Rita Appiah thanks.
Kwame
contact backstage
Emmanuel
what is a vacual
Star Reply
What is microbiology
Nji Reply
it is the study of microscopic organisms and basically their relation to the environment
Tenucharity
what is elements
Erneet Reply
an Element is a substance that can not be further decomposed by ordinary chemical means.
vannessa

Get the best Microbiology course in your pocket!





Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Microbiology' conversation and receive update notifications?

Ask