When you say species of cat, you say exactly that, cat, not cats, the singular, don't you? There seems to be a mixture when it comes to bacteria. Some sources say species of bacteria, others speices of bacterium. Would you say "How many species of cats are there?" or "How many species of cat are there?". It seems, when it comes to bacteria, the plural is used in this case.... Peculiar! Any body got any ideas???