what are the differences in harmful and helpful bacteria?

Answer 1

Most bacteria are actually harmless. Some are even helpful. There's lactobacillus, which helps to make some dairy products like yogurt & cheese.
There are bacteria which aid the fermentation of wine & beer. There's even a genetically engineered strain which can digest petroleum. It can be used to help clean up oil spills.
We even have bacteria living in our intestinal tracts.
Of course there are also harmful bacteria which can make us sick or kill us.
Streptococcus causes "Strep throat" & can cause more serious problems. "Flesh-eating" Strep can infect wounds & can be fatal if gets into the bloodstream.
Staphylococcus causes "Staph" infections, which can also be really nasty.
Foodborne bacteria such as Salmonella, Listeria & E. Coli cause food poisoning, which can be very serious.
Then, there are the less-serious bacterial infections, like acne & tooth decay.
NOTE: Bacteria shouldn't be confused with viruses, which are completely different organisms.

Answer 2

As in any ecosystem, the most beneficial way to coexist is to be able to use your cohabitor's waste.

The difference between harmful and helpful bacteria lies within that fact.

Harmful bacteria feed either on you, or on limited resources that you need.

Helpful bacteria prefer to feed on your waste products, that you don't need.

Answer 3

In addition to what the guy above said, harmful bacteria sometimes produce poisonous by-products (waste) that make you sick.

Helpful bacteria help you digest your food. or produce helpful by-products (like vitamins) that you need.

Answer 4

Sometimes it is just, location, location and location.