1 . _____(bacterium) reproduce by dividing and making copies of themselves.  (所给词的适当形式填空)

2 . For example , pesticides can damage the land by killing not only harmful _______(bacterium) and insects , but also helpful ones . (所给词的适当形式填空)

3 . Until now, it is still unknown to the scientists which _____ (细菌) causes the infection.（根据汉语意思填空）