Strictly speaking, there are no 'good' and 'bad' bacteria existing in this world.

Hi Martin,

Good bacteria are organisms   useful  in some instances such as food industry (making yogurt or cheese); these organisms are very good lactose fermenters, which makes them ideal organisms for this purpose. These organisms also create desirable flavors in the milk while they are growing. However, E. coli, a well known pathogen can also ferment lactose. Pathogens are bad for human health, so in reality there are good ( useful)and bad ( harmful)bacteria .

Polysaccharides are complex sugars. Examples of polysaccharides include starch, cellulose, and glycogen. Polysaccharides do not occur naturally in milk. The only sugar found in milk is lactose, which is a disaccharide (double sugar) made up of the monosaccharides (simple sugars) galactose and glucose. Bacteria, whether they're ‘good' or ‘bad' can use many different types of sugars as energy sources. For example, many of the culture organisms used to make yogurt or cheese are very good lactose fermenters, which makes them ideal organisms for this purpose. These organisms also create desirable flavors in the milk while they are growing. However, E. coli, a well known pathogen can also ferment lactose, for example.

Thanks everybody

Thank you Dr. Ali