Answer all Review Questions 1. Before doing the experiment, would you expect S,

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Review Questions 1. Before doing the experiment, would you expect S, aureus to withstand higher salt concen trations than E. coli? Explain. 2. Based on what you know about the metabolic activities of yeast already, would you expect the yeast S. cerevisiae to withstand higher glucose concentrations than the two bacterial species? Explain. 3. Bacillus cereus is a common cause of food poisoning, particularly from rice in restaurants Why could this occur in cooked rice? 4. Relate the results observed in the UV light experiment to the possible effects of sunlight on human skin. IN

Explanation / Answer

Answer to Que 1:

Yes, we expect Staphylococcus aureus to tolerate more salt concentration than E. coli.

Explanation: Staphylococcus aureus is a halotolerant (a facultative halophile) bacteria which can fairly tolerate the salt concentrations from 1% to as much as 13%.

S. aureus is a gram positive, non motile, non sppore forming, non capsulated facultative anaerobe existing as cluster (bunch of grapes) of cocci. It gives positive reactions to coagulase and catalase test.

As it is a facultative halophile it can tolerate high osmotic pressure attributed due to the presence of Sodium chloride. These are also highly temperature resistant. These two reasons favor S. aureus to infect and one of the main reason for food poisoning. These properties also account for the various infections (like Skin infections, osteomyelitis, toxic shock syndrome, Pneumonia, acute endocarditis, infective arthritis, necrotizing fasciitis, sepsis) and the difficulty involved during the treatment

Reason for Halotolerance in the Staphylococcus aureus: Although there is no single factor which can be pointed out for halotolrance, it is due to the combination of various factors in the S. aureus like high teichoic acid in the cell wall, protein A (a surface protein which also inhibits the activation of complement in the host's immune system), various enzymes like staphylokinase, catalase contribute to the salt toleration in S. aureus

On the other hand Escherichia coli can tolerate salt concentration of upto 1% and not beyond that suggesting that it is not a halotolerant organism. E. coli is a gram positive rod shaped bacteria, which is a motile, facultative anaerobe and gives positive reactions for Indole test.

Due to the lipopolysaccharides content and the lesser periplasmic space, E. coli cannot tolerate high amount of salt concentrations

So, to conclude, S. aureus can tolerate 13 times more the concentration of salt than E. coli

Answer to Que 2:

Yes, Saccharomyces cerevisiae can tolerate more amount of glucose than E. coli and S. aureus.

Explanation:

S. cerevisiae are chemo-organotrophs and utilise carbon in the form of glucose or fructose for energy and also they are facultative anaerobes even though they cannot grow in complete anaerobic condition. Their growth is optimum in neutral or slightly acidic pH. Since hexose sugars are the main source of energy for yeast cells, their cellular metabolism is efficiently structured to handle even high concentrations of glucose beyond the normal concentrations they can utilize

The bacteria E.coli and S. aureus cannot tolerate high glucose concentrations, because, high glucose concentration in the medium leads to more uptake of glucose into the cells leading to higher flux of acetyl coA. High concentration of acetylcoA is then converted to acetyl phosphate instead of entering Citric acid cycle to yield energy.

Accumulation of acetic acid in the culture also leads to crabtree effect which down regulates the respiratory chain. Ultimately the growth rate is reduced.

Thus, high amount of glucose is not tolerated well in E.coli and S.aureus when compared to S. cerevisiae

Answer to Question 3:

Bacillus cereus is a gram positive, bacillus which is motile and facultatively anaerobic bacterium. It can form spores which can tide over the unfavorable conditions. When the rice, is cooked, B.cereus can form spores to withstand the high temperatures. When thus cooked rice is left at room temperature, spores germinate as the favorable temperature for the organism is 30-370C.

This may be the reason for the outbreaks of the food poisoning in restaurants or other bulk food serving stations. As the rice will be cooked in bulk and then allowed to stay at room temperature, it provides ample time for the growth of bacteria from the spores and can cause food poisoning.

Hence, it is preferable to maintain the temperature of the cooked rice at more than 63 0C or in refrigerator below 40C to restrict the growth of the microorganism.

It is also recommended to boil or cook the rice in little amounts to serve instantly and avoid storing rather than cooking in bulk and storing for longer periods of time which may attract the growth of microorganisms
Answer to Que 4:

UV light from the sun can be divided into UVA, UVB and UVC radiation. UVA is not absorbed in the ozone layer while UVB is not completely aborbed and UVC is completely absorbed by the ozone layer.

So, the harmful effects on the skin are attributed to the UVA and UVB radiation from the sun which can enter the earth's atmosphere.

Whatever may be the reason, ultraviolet radiation is harmful to the skin as it may even be absorbed into the deeper layers of the skin (upto dermis). Initial symptoms appear like reddening, itching, wheal (very rare unless the exposure is very severe) and darkening of the skin and long term effects may be much more severe like formation of wrinkles, early senescence of the skin and even may be responsible for development of cancer due to damage of DNA and mutations

They even penetrate the glass and hence it is advisable to use sunscreens to avoid the harmful exposure of skin to UV light

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