Four species of bacteria – three of them previously unknown to science – have been discovered on board the International Space Station (ISS), starting questions about how they got there, and how they have managed to survive.

Their discovery may also improve future efforts to cultivate crops during long spaceflight missions, since related species are known to promote the growth of plants and help them fight off pathogens（病原菌）.

Previous studies had suggested that certain bacteria could survive the hard conditions of space, which survived on the space station’s surface for three years. They were deliberately placed there to test the theory that life exists throughout the universe and may be transported between planets by space dust, asteroids（小行星）,or even polluted spacecraft.

The new bacteria were similarly identified from swabs（棉签）of various locations inside the ISS. One was discovered on a dining table; another on an overhead board in a research area used to study low gravity; the third in the Cupola observatory. The fourth species, which was already known of, was found on an old air-purifying filter（净化过滤器）, which had been returned to Earth. All of them are rod-shaped bacteria belonging to the   family – usually found in soil and fresh water, where they help to promote plant growth and defend against harmful bacteria.

They’re most likely to have been transferred to the ISS from Earth – rather than coming from outer space – and have either survived since the the beginning of the station, or were introduced when new astronauts or supplies arrived.

It is important to understand the types of microbes（微生物）that can accumulate, survive, and even grow well in this unique environment, because they may affect the health of crew, the spacecraft’s structure, or dirty the planets and astronomical bodies they visit.

【小题1】Why was the discovery of the bacteria important?

A．They are all well known species.	B．They are all previously unknown.
C．They are good for people’s health.	D．They are helpful with space planting.

【小题2】What have scientists found about the bacteria?

A．They can not live in hard conditions.	B．They can be transported by space dust.
C．Some kinds have a very strong life.	D．Some kinds exist throughout the universe.

【小题3】What can you infer from the last two paragraphs about the species?

A．They may strengthen the spacecraft.	B．They are unlikely to come from Mars.
C．They will not put other planets to danger.	D．They can hardly grow in the spacecraft.