First things first – bacteria is an English word. But it is an uncountable one. Uncountable? That’s crazy! Yes, we know – the world is full of millions and millions of different ‘bacterias’ – but we never add an ‘s’. There is a lot of bacteria, many strands of different bacteria, or simply bacteria. We certainly don’t say ‘bacteries!’
Now, bacteria is good and bad as you know. But in English, if you want to highlight the bad bacteria – bacily – you need to use a different word entirely – germs. We say, ‘If you sneeze, germs spread everywhere’, we do not say, ‘bad bacteria’.
For good bacteria, we usually use the term ‘healthy bacteria’ – and that’s what appears in all those adverts for yoghurt!