coccus vs staphylococci

coccus and staphylococci both are nouns.

Word	Noun	Adjective	Verb	Adverb
coccus	Yes	No	No	No
staphylococci	Yes	No	No	No

As nouns, staphylococci is a hyponym of coccus; that is, staphylococci is a word with a more specific, narrower meaning than coccus:

coccus: any spherical or nearly spherical bacteria
staphylococci: spherical Gram-positive parasitic bacteria that tend to form irregular colonies; some cause boils or septicemia or infections

Other hyponyms of coccus include staph, staphylococcus.

coccus (noun)	staphylococci (noun)
any spherical or nearly spherical bacteria	spherical Gram-positive parasitic bacteria that tend to form irregular colonies; some cause boils or septicemia or infections