2. Which of the following are the biochemical features of S. typhi? They are: 1. lactose-negative H_(2)S-positive 3. glucose-positive 4. oxidase-positive

The correct answer is:<br /><br />1. lactose-negative<br />2. $H_{2}S$-positive<br />3. glucose-positive<br /><br />Explanation:<br />Salmonella typhi (S. typhi) is a Gram-negative bacterium that exhibits the following biochemical features:<br /><br />1. Lactose-negative: S. typhi does not ferment lactose, so it is lactose-negative.<br />2. $H_{2}S$-positive: S. typhi produces hydrogen sulfide ($H_{2}S$) when grown on certain media, such as Triple Sugar Iron (TSI) agar, making it $H_{2}S$-positive.<br />3. Glucose-positive: S. typhi can ferment glucose, so it is glucose-positive.<br /><br />The fourth option, oxidase-positive, is incorrect. S. typhi is oxidase-negative, meaning it does not produce the enzyme cytochrome c oxidase.