Bacteria
A thin bacteria is... well... a bacteria. I guess all bacteria are really thin. So thin you can't see them with your naked eye, in fact.
さいきん
This is a jukugo word that uses the on'yomi readings of the kanji. You've learned them already, so you should be able to read this on your own!
最近真剣に細菌について勉強をし始めました。
Recently, I started seriously studying bacteria.