Gram-negative rods

     MacConkey's agar

1) Lactose fermenter --- pink

-------fast lactose fermenter ---------

Klebsiella

E.coli

Enterobacter

-------slow lactose fermenter--------

Citrobacter

Serratia

2) Lactose non-fermenter-----white

A. ---------Oxidase negative--------

NO H2S production----------------------Shigella

H2S production--------------Salmonella

                          --------------Proteus

B. ---------Oxidase positive---------

                   P.aureginosa