Enterobacter aerogenes is a species of Enterobacteriaceae, which are small, Gram-negative intestinal bacteria. All Enterobacteriaceae have similar metabolic pathways and do not form spores, but the various genera and species of the family can differ in certain aspects, such as whether they are motile or remain in stationary colonies. These differences can help microbiologists identify the different species.
Enterobacter species have colony characteristics that are similar to those of Klebsiella (another genus of Enterobacteriaceae). Therefore, the two genera must be distinguished from each other in lab specimens. The primary way to do this is to realise that Klebsiella is non-motile, whereas Enterobacter is motile. In this context, motility refers to a bacterium's ability to move across a surface instead of remaining stationary in a colony.
Another way to distinguish Enterobacter from Klebsiella is to test for the presence of the H antigen. This flagellar structure is used for motility and is therefore absent in non-motile genera such as Klebsiella. Like most Enterobacteriaceae, Enterobacter species have O, H and K antigens. In contrast, Klebsiella only has O and K antigens.
Colony Morphology on Non-selective Media
Non-selective media is growth media that does not contain any inhibitors. Selective media contains inhibitors that will only allow certain types of bacteria (e.g., Gram-negative bacteria) to grow. When grown on non-selective media, E. aerogenes forms smooth, moist, grey colonies. The colour of the colonies differentiates it from Serratia marcescans (another member of the Enterobacteriaceae family), which forms red colonies.
Colony Morphology on Selective Media
When grown on MacConkey's agar, which contains lactose as well as a pH indicator that turns red in the presence of acid, E. aerogenes grows into pink or red mucoid colonies because of the fact that it ferments lactose and thereby produces acidic byproducts. This allows it to be differentiated from bacteria that cannot ferment lactose. On EMB agar, E. aerogenes grows into pink colonies, which often have a purple dot in the centre of each colony. This allows it to be distinguished from E. coli, which produces shiny green colonies.