The singular of 'bacteria' is 'bacterium'.

bacteria的单数形式是bacterium。

《牛津词典》

Now, a decade later, the first direct evidence for the functioning of proteorhodopsin in native marine bacteria is presented, based on mutational analysis in a marine bacterium.

十年之后的现在，通过对海洋细菌的变异分析，首个直接证明变形菌视紫质功能的证据被发现。

youdao

Computer artwork of the end of a typical ciliated rod-shaped bacterium. Typical rod-shaped bacteria include E.coli and Salmonella.

典型纤毛杆状细的电脑绘图，典型的杆状细菌包括大肠杆菌和沙门氏菌。

youdao

So, you're familiar with some of these bacteria like salmonella, or the micro bacterium that causes tuberculosis are shown here.

所以 你们很熟悉像沙门氏菌这样的细菌,或者像这儿的引起结核的小细菌

耶鲁公开课 - 生物医学工程探索课程节选

The bacterium legionella pneumophila and related bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers.

BBC: The water fountain at Northampton's market square

DNA. Alternatively, it can be smuggled in inside a bacterium (splicing extra genes into bacteria for this purpose is easy).

ECONOMIST: Food fights

For the study, researchers from the University of Vermont analysed volatile organic compounds (VOCs) given off in exhaled breath by different bacteria as well as different strains of the same bacterium.

BBC: Lung infection identified using 'breath-print'