Table of Contents
BEST IELTS Academic Reading Practice Test 171
IELTS ACADEMIC READING TEST 171 – PASSAGE – 2
IELTS ACADEMIC READING TEST – 171
READING PASSAGE – 2
A New Perspective on Bacteria
A. Microbes are organisms too small to be seen by the naked eye, including bacteria, blue-green algae, yeasts, fungi, viruses, and viroids.
A large, diverse group, almost all bacteria are between one and ten µ1 (larger ones reach 0.5 mm). Generally single-celled, with a distinctive cellular structure lacking a true nucleus, most bacterial genetic information is carried on a DNA loop in the cytoplasm2 with the membrane possessing some nuclear properties.
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There are three main kinds of bacteria – spherical, rod-like, and spiral – known by their Latin names of coccus, bacillus, and spirillum. Bacteria occur alone, in pairs, clusters, chains, or more complex configurations. Some live where oxygen is present; others, where it is absent.
The relationship between bacteria and their hosts is symbiotic, benefitting both organisms, or the hosts may be destroyed by parasitic or disease-causing bacteria.
B. In general, humans view bacteria suspiciously, yet it is now thought they partly owe their existence to microbes living long, Jong ago.
During photosynthesis, plants produce oxygen that humans need to fuel blood cells. Most geologists believe the early atmosphere on Earth contained very little oxygen until around 2½ billion years ago when microbes bloomed. Ancestral forms of cyanobacteria, for example, evolved into chloroplasts – the cells that carry out photosynthesis. Once plants inhabited the oceans, oxygen levels rose dramatically, so complex life forms could eventually be sustained.
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The air humans breathe today is oxygen-rich, and the majority of airborne microbes are harmless, but the air does contain industrial pollutants, allergens, and infectious microbes or pathogens that cause illness.
C. The fact is that scientists barely understand microbes. Bacteria have been proven to exist only in the past 350 years; viruses were discovered just over 100 years ago, but in the past three decades, the ubiquity of microbes has been established with bacteria found kilometres below the Earth’s crust and in the upper atmosphere. Surprisingly, they survive in dry deserts and the frozen reaches of Antarctica; they dwell in rain and snow clouds, as well as inside every living creature.
Air samples taken in 2006 from two cities in Texas contained at least 1,800 distinct species of bacteria, making the air as rich as the soil. These species originated both in Texas and as far away as western China. It now seems that the number of microbe species far exceeds the number of stars.
D. Inside every human being, there are trillions of bacteria with their weight estimated at 1.36 kg in an average adult, or about as heavy as the brain. Although tiny, 90% of cells in a human are bacterial. With around eight million genes, these bacteria outnumber genes in human cells by 300 times.
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The large intestine contains the most bacteria – almost 34,000 species – but the crook of the elbow harbours over 2,000 species. Many bacteria are helpful: digesting food; aiding the immune system; creating moisturiser; and, manufacturing vitamins. Some have highly specialised functions, like Bacteroides thetaiotaomicron, which breaks down plant starch, so an infant can make the transition from mother’s milk to a more varied diet.
Undeniably, some bacteria are life-threatening. One, known as golden staph, Staphylococcus aureus, plagues hospitals, where it infects instruments and devours human tissue until patients die from toxic shock. Worse, it is still resistant to antibiotics.
E. Antibiotics themselves are bacteria. In 1928, Alexander Fleming discovered that a mould in his laboratory produced a chemical he named penicillin. In 1951, William Bouw collected soil from the jungles of Borneo that eventually became vancomycin. Pharmaceutical companies still hunt for beneficial bacteria, but Michael Fischbach from the University of California believes that the human body itself is a ready supply.
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F. Scientific ignorance about bacteria is largely due to an inability to cultivate many of them in a laboratory, but recent DNA sequencing has meant populations can be analysed by a computer program without having to grow them.
Fischbach and his team have created and trained a computer program to identify gene clusters in microbial DNA sequences that might produce useful molecules. Having collected microbial DNA from 242 healthy human volunteers, the scientists sequenced the genomes of 2,340 different species of microbes, most of which were completely new discoveries.
In searching the gene clusters, Fischbach et al fund 3,118 common ones that could be used in pharmaceuticals, for example, a gene cluster from the bacterium Lactobacillus gasseri, successfully reared in the lab, produced a molecule they named lactocillin. Later, they discovered the structure of this was very similar to an antibiotic, LFF571, undergoing clinical trials by a major pharmaceutical company. To date, lactocillin has killed harmful bacteria, so it may also be a reliable antibiotic.
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G. Naturally, the path to patenting medicine is strewn with failures, but, since bacteria have been living inside humans for millions of years, they are probably safe to reintroduce in new combinations and in large amounts.
Undoubtedly, the fight against pathogens, like golden staph, must continue, but as scientists learn more about microbes, respect and excitement for them grow, and their positive applications become ever more probable.
Questions 14-18
Passage 2 has seven sections, A-G.
Which section contains the following information?
Write the correct letter A-G.
NB: Any section can be chosen more than once.
14. examples of bacteria as a patented medicine
15. a description of bacteria
16. gene cluster detection and culture
17. humans are teeming with bacteria
18. Fischbach’s hypothesis
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Questions 19-22
Choose the correct letter A, B, C, or D.
19. What do almost all bacteria share?
A. Their simple configurations
B. Their cellular organisation
C. Their survival without oxygen
D. Their parasitic nature
20. From the suffix ‘-bacillus’, what shape would you expect the bacterium Paenibacillus to be?
A. spherical
B. rod-like
C. spiral
D. amorphous
21. Why were ancient bacteria invaluable to humans?
A. They contributed to higher levels of oxygen.
B. They reduced widespread industrial pollution.
C. They protected humans from intestinal ailments.
D. They provided scientists with antibiotics.
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22. How prevalent are microbes?
A. Not at all
B. Somewhat
C. Very
D. Extremely
Questions 23-26
Choose NO MORE THAN THREE WORDS OR A NUMBER from the passage for each answer.
23. Which organ does the total weight of bacteria in a human body equal?
24. Roughly how many bacterial species live in a human’s large intestine?
25. In Fischbach’s view, where might useful bacteria come from in the future?
26. What do some scientists now feel towards microbes?
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ANSWERS ARE BELOW
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ANSWERS
14. E
15. A
16. F
17. D
18. E
19. B
20. B
21. A
22. D
23. THE BRAIN
24. 34,000/THIRTY-FOUR THOUSAND
25. THE HUMAN BODY/ HUMAN BEINGS
26. RESPECT AND EXCITEMENT / EXCITEMENT AND RESPEC