Bacterium and Bacteria

Today I was watching a Tamil serial in TV by director Balachander .A character in the serial says there are bacteria's every where . The mother character who happens to be a nurse in the serial says it is not bacteria's ,say as bacilli .I was surprised to notice such a blooper in a KB serial. To my knowledge bacteria is the plural for the term bacterium .Nouns that end with ium has a plural form that ends with ia . Eg: Plural for millennium is millennia .Bacilli is the plural for Bacillus . Bacillus is a rod shaped bacterium and one of the three forms of bacteria .In a media like TV care should be taken to avoid bloopers like this,especially when it is emphatically said .