what does it mean if a bacteria tested negative for H2S

Hydrogen sulfide test 

H₂S  or Hydrogen sulfide is produced during metabolism of sulfur containing compounds which is carried by some strains of bacteria for their growth and development. 

We can use the medium SIM which is the most sensitive medium for this test. Sulfur containing compounds can be sulphates, thiosulphates , sulphur containing amino acids like cysteine. 

This test is well known as hydrogen sulfide test . Some bacteria are able to reduce sulfur containing compounds into H₂S gas. It can be detected by growing them it a medium containing iron so that they can be identified by naked eyes.

H₂S  released during their metabolism combines with iron to form a black precipitate called ferric sulfide. Hence the bacteria is said to H₂S positive .

SIM ( sulphite indole motility) medium is the most sensitive medium for identification of members of certain bacterial family like Enterobacteriaceae