Harmful bacterias are called pathogenic bacteria and can cause bacterial infection and diseases. They are single-celled little micro-organisms and can reproduce fast, so the germs can spread easily. An example of a bad bacteria is Salmonella. It is a type of food poisoning. Also it can cause illness.
There are different types of bacterias all around us. Bacterias like: lactobacillus is used as good bacteria, because it is used to preserve milk and plant products. The basic research says that the intestine's bacteria is SO numerous, that they make up 95% of the total cells in the human body. They help prevent bad bacteria from going near you and harming you. Lactobacillus is a bacteria that prevents you from having diseases. So the harmless bacteria are called non-pathogenic bacteria.
http://www.about-salmonella.com/http://www.drmirkin.com/nutrition/8966.html
http://www.buzzle.com/articles/helpful-and-harmful-types-of-bacteria.html
http://www.associatedcontent.com/article/51175/understanding_bacteria_harmless_and.html
