Answers to Questions from the Classroom
Here is where I (we) will attempt to answer questions I couldn't answer in class. You can also post questions here.
Please add any answers or additional information you find too!
How does Salmonella get into chicken eggs?
According to the CDC, this is due to Salmonella infections of the chicken ovaries. The bacteria is on the egg before the shell forms!
What are the white structures in Spirillum volutans?
In the lab we looked at slides of Spirillum volutans. Did you notice lighter staining regions within the cell? Those are inclusion bodies, they are storage particles. They are sometimes also called volutin, hence the name of the species. Its made of poly-beta-hydroxybutyrate, a long lipid, used to store carbon and energy. See pg 84 of the textbook.
Do bacteria display "taxis" (movement) in response to pH or temperature?
Chemotaxis is the movement toward or away from a chemical stimulus. Turns out pH is indeed an environmental cue that bacteria can move up or down gradient. H. pylori, which causes peptic (stomach) ulcers, displays negative chemotaxis (ie moves away) from low pH. It likes to colonize the wall of the stomach, where the pH is slightly higher than in the lumen, or center, of the stomach. Moving to a higher pH brings the bacteria to the stomach wall where it can attach and grow. Here is the reference.
I havent found anything for temperature yet!
What happens to the bactoprenol during cell wall biosynthesis?
So it turns out the figure in the textbook (fig 6.5) is a little misleading. Bactoprenol, also called undecaprenol phosphate, is imbedded in the cytoplasmic side of the membrane, not floating around in the cytoplasm as the figure implies. Its a lipid, so that actually makes more sense, since thats where it would want to be based on its hydrophobicity. Peripheral membrane proteins on the cytoplasmic side catalyze the lnking of peptidoglycan to bactoprenol. While it is true what I said that the bactoprenol-peptidoglycan complex (called Lipid II) then goes throught the lipid bi-layer, it turns out that there are proteins involved too. Not channels, but some proteins seem to help flip the bactoprenol-peptidoglycan to the outside leaflet of the membrane. There, other enzymes catalyze the addition of the peptidoglycan to the wall and cleave it from the bactoprenol. The bactoprenol then flips back to the cytoplasmic side and is reused.
This process is targeted by a group of antibiotics called lantibiotics.
van Dam et al, 2007
Why did gas form in Clostridium sporogenes and E. coli deep agar tubes?
Energy conservation in bacterial is through the trapping of energy from a redox reaction as ATP. ATP holds that energy in phosphoanhydride bonds, which when broken, release the energy to be used in a desired biochemical reaction (such as membrane transport, extension/retraction of fimbriae, or metabolic reactions).
In respiration the redox reaction involves an exogenous electron acceptor, O2 if its aerobic respiration.
In fermentation, no exogenous electron acceptor is present, so an endogenous acceptor is used. An endogenous acceptor is a compound derived from the electron donor. Because of this, the difference in reduction potential (positions on the “electron tower”) is less, and therefore less energy is available.
Fermentation, like respiration, differs among organisms. Depending on the electron donor and acceptor, the products of fermentation differ. In the case of yeast (Sacchromyces cerevisiae, a eukaryote) fermentation of glucose yields ethanol and CO2….this is useful in making beer, wine, and bread.
Species of Clostridium lack a respiratory chain, therefore their only option is fermentation. See pages 380-383 in the textbook. C. sporogenes ferments amino acids (present in TSA from the soy and casein) producing CO2. Alanine is the electron donor, and glycine is the electron acceptor. See figure 12.59 in the textbook. Another species, C. perfringes, causes a very serious disease called gas gangrene, as the result of anaerobic growth and fermentation at a wound, producing CO2 and H2.
E. coli also produced gas in the deep agar tube. This bacterium is classified as a “mixed-acid” fermenter, meaning that unlike S. lactis, many different products are produced in fermentation including multiple different acids. CO2 and H2 are also produced. See pg 352-353 of the textbook. In fact, the production of gas during fermentation is one of many biochemical tests used to help identify E. coli.
Streptococcus lactis also grew anaerobically but did not appear to produce gas in the tube. The product of fermentation by this bacterium is lactic acid, and is through a pathway called homofermentation because only one product (lactic acid) is produced. No CO2 or other gas is produced. See pg 375-377, and figure 12.53. The production of lactic acid is important in food microbiology, as Streptococcus and related “lactic acid” bacteria are involved in production of buttermilk and other products.
What exactly is a megaplasmid?
It seems like there is no argeement on this. Some say it is a plasmid with 1000kb (1Mb) or more, mega meaning 1 million. By that definition, none of the plasmids listed on the table in your notes would be classified as a megaplasmid. Others seem to define it more losely as just big. However, its evolutionary origin may be a consideration too...is it a plasmid that has gained many genes thereby growing in size or was it a chromsome that has been reduced in size. According to some sources (Bently and Parkhill, 2004) it seems like what they consider megaplasmids are these genetic elements that cant quite be peffed down as a large plasmid or smallish chromosome. The determining factor is: is the megaplasmid essential? It is very difficult to cure a cell of plasmids that are so large so in most casses it has not been experiemntall demonstrated whether they are required. Bioinformatics can help though: knowing the sequence of the megaplasmid, we can determine whether any of the genes are likely to be essential (such as ribosomal RNA genes, DNA polymerases etc), and whether additional copies of the gene are found on the chromosome. Clearly, if an esential gene is carried only on the megaplasmid, it should be considered a chromosome.
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